Can Neutrinos Kill Their Own Grandfathers?

By Sean Carroll | September 24, 2011 12:54 pm

Building in part on my talk at the time conference, Scott Aaronson has a blog post about entropy and complexity that you should go read right now. It’s similar to one I’ve been contemplating myself, but more clever and original.

Back yet? Scott did foolishly at the end of the post mention the faster-than-light neutrino business. Which of course led to questions, in response to one of which he commented thusly:

Closed timelike curves seem to me to be a different order of strangeness from anything thus far discovered in physics—like maybe 1000 times stranger than relativity, QM, virtual particles, and black holes put together. And I don’t understand how one could have tachyonic neutrinos without getting CTCs as well—would anyone who accepts that possibility be kind enough to explain it to me?

The problem Scott is alluding to is that, in relativity, it’s the speed-of-light barrier that prevents particles (or anything) from zipping around and meeting themselves in the past — a closed loop in spacetime. On a diagram in which time stretches vertically and space horizontally, the possible paths of light from any event define light cones, and physical particles have to stay inside these light cones. “Spacelike” trajectories that leave the light cones simply aren’t allowed in the conventional way of doing things.

What you don’t see in this spacetime diagram is a slice representing “the universe at one fixed time,” because that kind of thing is completely observer-dependent in relativity. In particular, if you could move on a spacelike trajectory, there would be observers who would insist that you are traveling backwards in time. Once you can go faster than light, in other words, you can go back in time and meet yourself in the past. This is Scott’s reason for skepticism about the faster-than-light neutrinos: if you open that door even just a crack, all hell breaks loose.

But rest easy! It doesn’t necessarily follow. Theorists are more than ingenious enough to come up with ways to allow particles to move faster than light without letting them travel along closed curves through spacetime. One minor technical note: if some particle moves faster than light, it’s not “closed timelike curves” that we should be worried about, it’s “closed spacelike curves on which physical particles move.”

But we shouldn’t necessarily even worry about that. The usual argument that faster than light implies the ability to travel on a closed loop assumes Lorentz invariance; but if we discover a true FTL particle, your first guess should be that Lorentz invariance is broken. (Not your only possible guess, but a reasonable one.) Consider, for example, the existence of a heretofore unobserved fluid pervading the universe with a well-defined rest frame, that neutrinos interact with but photons do not. Or a vector field with similar properties. There are various ways we could imagine some background that actually picks out a preferred frame of reference, violating Lorentz invariance spontaneously.

If that’s true, the argument that FTL implies closed loops through spacetime no longer works. Even if neutrinos are able to sneak outside light cones, there may nevertheless be “neutrino cones” to which they are still confined. These neutrino cones could be a little bit broader than ordinary light cones, but they could still define a fixed notion of “going forward in time” that even neutrinos couldn’t violate.

There’s a nice (although technical) discussion of this in a short paper by Robert Geroch. Read Section 2 for the math, Section 3 for the words. From the discussion:

In short, the causal cones of special relativity, from this perspective, have no special place over and above the cones of any other system. This is democracy of causal cones with a vengeance. This, of course, is not the traditional view. That view — that the special relativity causal cones have a preferred role in physics — arises, I suspect, from the fact that a number of other systems — electromagnetism, the spin-s fields, etc — employ precisely those same cones as their own. And, indeed, it may be the case that the physical world is organized around such a commonality of cones. On the other hand, it is entirely possible that there exist any number of other systems — not yet observed (or maybe they have been!) — that employ quite different sets of causal cones. And the cones of these “other systems” could very well lie outside the null cones of special relativity, i.e., these systems could very well manifest superluminal signals. None of this would contradict our fundamental ideas about how physics is structured: An initial-value formulation, causal cones governing signals, etc.

The odds are still long against the OPERA result being right at face value. But even if it’s right, it doesn’t immediately imply that neutrinos are time-travelers.

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  • Thomas

    If they did travel faster than light, would we need to add more dimensions to our picture of the universe? Everybody seems to say that we have to “rewrite” Einstein, but why would you rewrite something that already works? Wouldn’t you just build on it, and add another term or two?

  • Moshe

    One good thing that may come of this result is more awareness that “ultimate speed in nature” (which is the foundation of special relativity”) is a logically distinct concept from the speed of light (or any other physical particle). If the ultimate speed of nature is saturated by neutrinos instead of light, I don’t see why this should necessarily change much the structure of special relativity. Of course, matching all we know about light is a different story.

    As for the idea that you can have two independent Lorentz symmetries, one for neutrinos and one for all the rest, this is great at the level of free particles. Going from free particles to interacting fields, it seems to be hard to reconcile this idea with constraints from effective field theory, which tell you there is a large number of relevant and marginal operators in any Lorentz violating version of the standard model. It seems a priori hard to imagine one can break LI in one sector while this breaking not leaking and contaminating all the rest of physics with large observable consequences.

  • http://youtube.com/user/integralmath Justicar

    Be honest, are you writing this from the future to throw us off the scent? =P

    Thanks for the food for the thought, Sean.

  • Ramanan

    Paul Krugman has a post on solving the financial crisis by going back in time ;-)

    http://krugman.blogs.nytimes.com/2011/09/24/solving-the-financial-crisis/

  • Dr. Morbius

    I was thinking about this yesterday. If the neutrinos are really traveling faster than light wouldn’t they have been detected before they were emitted?

  • Mark Weitzman

    Whats the big deal about CTC’s – GR already has many solution with CTC’s

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Moshe, it is hard to imagine, but we’re already imagining a hypothetical world in which we really did detect muon neutrinos moving faster than light, so the standards are a bit different.

    Mark, just because there are solutions doesn’t mean they describe reality. There are proofs that CTC’s can’t arise in nonsingular evolution from well-behaved initial data obeying the null energy condition.

  • http://www.scottaaronson.com Scott Aaronson

    Thanks, Sean — that helps!

    To clarify, I’m perfectly comfortable with the idea that, even if there’s a speed limit that plays the role of “c” in special relativity (i.e., the role of defining the causal structure), there’s no a priori reason for that limit to be identical with the speed of photons through the vacuum (it could be slightly greater, for example).

    But to whatever extent I was thinking this through at all, I thought that Einstein derived SR in the first place through careful consideration of Maxwell’s equations, and it was Maxwell’s equations that picked out the speed of photons as being special. So, if the true speed limit turned out to be the speed of certain high-energy neutrinos rather than photons, I suppose we’d say that something like SR might still be true, but Einstein’s original derivation of it from Maxwell’s equations only worked by a “happy accident”?

  • Michael Hennebry

    The c in special relativity is *the* reference-frame-independent speed of the universe.
    To define a causal cone different from that of special relativity,
    one needs to sacrifice reference-frame-independence.
    Note that actual light travels rather close to c:
    The upper limit on the rest energy of a photon is rather low.
    IIRC it allows photon frequencies as least as low as 60 Hz.
    Visible light frequencies are considerably higher.
    A visible light photon has an energy at least a million million times its rest energy
    Slow light was not the problem.

  • http://blogs.discovermagazine.com/cosmicvariance/sean/ Sean

    Scott, I think this idea would amount to abandoning the idea of one true speed limit for absolutely everything. Yes, Maxwell’s equations pick out c as special, although that speed could be special even if nothing moved at it (e.g. in a world where all particles were massive). This idea would be that there are different special speeds for different kinds of particles, abandoning the universality of Lorentz invariance (which came from Maxwell’s equations).

    It case it’s not clear, all this is incredibly unlikely, but should be kept in mind as a logical possibility.

  • http://www.scottaaronson.com Scott Aaronson

    Mark Weitzman #6: Just to elaborate on Sean’s answer, probably the biggest deal about CTCs is that, if they exist, then you need some way to deal with causal consistency problems (i.e., grandfather paradoxes)! Now, there are ways to deal with grandfather paradoxes—one elegant resolution, due to David Deutsch, uses quantum mechanics—but if you want those solutions, then not surprisingly you generally “pay a price elsewhere”! So for example, finding a consistent solution around the CTC could be an incredibly-hard computational problem—so you then need to accept either that CTCs would give us computational superpowers, or that there’s some mysterious meta-principle that prevents us from using CTCs for that purpose. (For more about this, see this paper by myself and John Watrous, or section 10 of my survey article “Why Philosophers Should Care About Computational Complexity.”)

  • Count Iblis

    See also here:

    http://en.wikipedia.org/wiki/Tachyonic_antitelephone

    for a simple demonstration

  • jimthompson

    I KNEW the theorists wouldn’t let us down! (well done to Scott and Sean). On a slightly more serious note: is there any constraint (other than it hasn’t been observed before) on the speed of these neutrinos relative to c? Could the result be 1.5c say and still be plausible given the reasoning laid out above? With a special definition of “plausible” I suppose…..

  • Neil

    I thought Lorentz/CPT violations were pretty much ruled out with observations like the MINOS Far Detector.

  • Kaleberg

    On the other hand, Goldman Sachs should be interested in this, since they make a lot of money by running a “wire” con on other traders. They already pay extra to get their trading machines closer to the trading floor than anyone else so they can see and react to others’ bids before those others can react to theirs. (See The Sting or Queen of Hearts to see how this works.) If G&S could use a high speed neutrino link, they could beat anyone else relying solely on electronic data flow. Imagine the efficiencies and stability that this could introduce into the market. The mind boggles.

    If they discover that his lets them kill their own grandparents, that might be a good idea.

  • Hemo_jr

    Thiotimoline!

  • Luke

    Kaleberg, your inflammatory remarks are not appreciated.

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  • Terry Bollinger

    @ Sean Carroll said:

    “Consider, for example, the existence of a heretofore unobserved fluid pervading the universe with a well-defined rest frame, that neutrinos interact with but photons do not… There are various ways we could imagine some background that actually picks out a preferred frame of reference, violating Lorentz invariance spontaneously.”

    This is analyzable.

    The only obvious candidate for your fluid would be the CMB frame, which is accessible by moving at ~369 km/s towards the red shift pole (α, δ) = (23h 11m 57s, +7.22) [1][2]. In other words, you can access this frame by moving with a velocity of a bit over 0.1% of c towards a spot between Pisces and Pegasus. This is true even if the motion is contained (briefly) within a system residing on earth.

    First observation: If neutrinos are shifted based on some special relationship to the CMB frame, a reasonable inference (not strictly proven) is that Lorentz violation should maximize at the poles of the CMB dipole. Another reasonable inference is that neutrinos would appear slightly slower than c if directed towards one pole, and slightly faster than c only if directed towards the other pole. Finally, by symmetry the neutrino paths should have neutral behavior — a velocity of c — only for radial paths in the equatorial plane between the two poles, that is, at 90 degrees from each pole.

    Second observation: SN1987A seemed to limit neutrino velocities down to a range quite close to c. The implication for the Lorentz violation hypothesis is that SN1987A should reside close to the equatorial plane of the CMB dipole, else it could not have produced this seemingly ordinary result.

    I took the trouble to calculate that. To my surprise, the angle between SN1987A and the red shift pole (α, δ) = (23h 11m 57s, +7.22) is 98.861 degrees, or only about 9 degrees away from the optimum orientation. Thus the celestial location of SN1987A is at least approximately compatible with your Lorentz violation idea. Again, interesting, and statistically a bit unexpected.

    The inability to disprove your hypothesis via the singular but strong SN1987A data point means that it’s worth asking the next question: What are the exact timestamps for all of the detected neutrinos, and has anyone analyzed the OPERA data by indexing individual neutrino detections to their path orientations and directions relative to the CMB dipole? The conversion from time stamps to celestial vectors would require only standard astronomical tables and simple geometry.

    Hypothesis: If your idea of Lorentz violations is related to real superluminal neutrino results, then the celestial neutrino path vectors aligned most closely with the CMB poles should show the strongest deviations from c. One pole group should show consistent high-sigma sub-c velocities, and the other should show similarly high-sigma velocities that are slightly in excess of c.

    ———-
    [1] Lineweaver, C.H. et al: The Dipole Observed in the COBE DMR 4 Year Data. The Astrophysical Journal, 470, 38-42 (1996). Online: http://adsabs.harvard.edu/full/1996ApJ…470…38L

    [2] The blue shift pole of the CMB dipole is located at (α, δ) = (11h 11m 57s, -7.22)

  • lorantheon

    Looks like we need to discover a new line of physics.

  • Jorge

    It’s really funny to see how theorists as you try everything to save physical theories on which you “trust”. You always blame systematic errors, uncertainties on experiments, etc. And try to confort yourselfves by thinking “even if the experiment is right, it doesn’t change a thing” and then try to give some complicated explanations that not even you really believe.

    It is the same attitude physics took when some experiments were revealed such as those of Michelson and Morley and not to say about those leading to appearence of quantum mechanics.

    I’m not assuring that the conclusion that neutrions travel faster than light is truth. Indeed it may be wrong. I’m only saying that you haven’t learned a thing from past experience.

  • Avattoir

    Wow – up to this point, I was able to follow, not just the post and the linked post and all the related wikis, but almost every one of the reader replies here. But, now I read about “neutrions”, and that entire conceit just evaporates. Why aren’t we reading more about neutrions? Has the finding of faster-than-light-speed neutrions been confirmed to the same six-sigma level, or is there some shakiness in that? I can’t get this image out of my head of 65 billion neutrions per second transversing my left nostril, some of which are bubbling around at the edge of the detectable universe, none of which have enough mass to actually interact in any way beyond detection; I keep feeling something just has to blow.

  • Jorge

    Well the only thing that blows is that you prefer to do nonsense jokes about a grammar mistake instead of getting the point of a comment. NEUTRINOS. Are you happy now?

  • Jesse M.

    I’m trying to understand the basic concept of Geroch’s paper–when he talks about other physical systems having different causal cones, does he mean that these systems would no longer obey Lorentz-symmetric laws, but rather would have some different symmetry involving a different speed constant? It seems to that if you have the conditions 1) All physical systems obey Lorentz-symmetric equations, and 2) there are some physical systems that can be used to transmit FTL messages, then together these should automatically imply 3) it is possible to bounce FTL signals back and forth between two observers in such a way that the first observer receives the “answer” signal before he sent the original message (the “tachyonic antitelephone” mentioned by Count Iblis above).

  • Chris

    Well Jorge, your point was pretty silly to begin with. No where in Sean’s post was there any mention of systematic errors. This post was, in effect, saying: “Suppose it is true, what would happen? Does it necessarily imply time travel?” That is, your point claiming that Physicists are not trying to figure out the consequences of what FTL neutrinos are, was made about an article where a Physicist was trying to figure out the potential consequences of FTL neutrinos.

    So nonsense jokes are perfect reasonable way to respond to a completely nonsense comment.

  • http://backreaction.blogspot.com/ Bee

    The problem is that the ‘other cones’ are not invariant under Lorentz-transformations. You’d need a different transformation for each, which wrecks your notion of how a point transforms. (Been there, done that.) That’s the same problem you have in DSR.

    In any case, to maybe address Scott’s problem, note the following: The worldline of a tachyon (draw it) has no direction. If you boost it and it drops in the lower half of the Minkowski plane, there’s no sense in saying it goes backwards in time. You could equally well say it goes forwards in time, just into the other direction. There is no problem with that kind of propagation as long as you have a consistent time evolution. The problem with the grandfather comes along only if you add an arrow of time. It’s the arrow of time that necessarily has to single out a forward direction. That then however will also tell you which way the tachyon moves ‘forward.’

  • Neal J. King

    Viewpoint from a related angle:

    http://xkcd.com/660/

  • Ellipsis

    however, as Bee says, we know that neutrinos can interact (weakly) with other particles confined to normal light cones. So you still end up with problems that are essentially analogous to Einstein’s thought experiment showing that energetic photons must be affected by gravity: i.e. you send a neutrino off from Earth at its faster than light speed, then somewhere around alpha centauri or wherever it interacts weakly and a photon is generated, and that photon flies back to Earth, interacts electromagnetically, and your poor grandfather pays the price. so it takes more than just separating the fields to preserve causality — one would need to introduce time delays in interactions, and all sorts of other ad hoc things. not worth it for an undoubtedly erroneous experimental result!

  • http://vacua.blogspot.com Jim Harrison

    As I understand it, the theory that the universe will end in a big snap predicts that c will come to differ between photons of more or less energy in the run up to the conclusion. Does the possibility that certain neutrinos travel faster than light have any conceivable connection with this idea?

  • http://www.matthewputman.com Matthew Putman

    Fair enough that a neutrino is not a large mass object, let alone a person, but I would be careful. When a paradigm shift in physics occurs it often extends beyond current expectations. As a material scientist we saw this with the application of Quantum tunneling, first for semiconductors and then for polymers. What was small and experimentally proven , had large scale implications. We even see it for entanglement. It was first just electrons and photons et al. that could be entangled, which was big enough news in itself. Everyone assumed that it was a fundamental effect of the tiny however, which turned out not to be the case. We now entangle things as large as fullerenes. If the speed of light is not the limitation constant for anything, it is very possible it is not neutrino specific. In addition to that, while a person may not be able to travel to the past via this potential discovery, it is not unreasonable to think that we can carry information at this rate through an almost binary process. Thousands of neutrinos with one spin or another. Information to the past is still pretty important, and could result in the grandpa killing paradox being a real one.

  • Ellipsis

    on second thought, I’m not sure if my above comment is so simple. I should learn my lesson — an experimentalist should think twice before challenging a theorist on theory. anyway, my comment above can be ignored.

  • Jay Fox

    I’m having a hard time understanding why this is such a controversy. We are talking about a tiny amount of speed over the speed of light, which itself has been shown to vary under certain conditions.

    Einstein posited that as speed increases, so does mass (or energy). So how much does a photon weigh? Could it be that “light speed” is the fastest that something with the mass of a photon can travel, but something weighing less than that might go a bit faster?

    It would seem to me that gravitational lensing of light from distant galaxies implies some mass to those photons. Otherwise, what would the gravity be acting on?

    Do we have the ability to see if neutrinos are similarly steered by gravity? Is the effect the same, or is it slightly different?

    We know that photons carry some miniscule kind of weight, or a solar sail (shouldn’t that be a photon sail?) wouldn’t work. Still, a sail must be massive since each photon carries so little (weight/mass/energy). Neutrinos, being smaller, lighter, less massive, go right through the sail without even slowing down, presumably.

    We are not talking about speeds significantly greater than light speed, just something barely measurable. If you take mass into account, it seems to me that one should EXPECT higher speeds of matter that is lighter than a photon. Not huge increases, but something. None of this should prompt ideas of grandfather paradoxes, as getting to the speeds necessary would require shedding most of the mass.

    It is interesting to note that they had to do tests over a pretty big distance to find this anomaly. Even then, the time difference is miniscule. Certainly not enough to advantage traders, even if we could somehow devise communication methods using these particles. We’d first have to find a way to interact with them. Just detecting them seems to be hard enough.

  • Jesse M.

    @Jay Fox:
    I’m having a hard time understanding why this is such a controversy. We are talking about a tiny amount of speed over the speed of light, which itself has been shown to vary under certain conditions.

    If all laws of physics work the same in the different inertial reference frames given by the Lorentz transformation, then any faster-than-light signalling, no matter how small, can be used to send signals backwards in time. If A is the event of the signal being sent and B is the event of the signal being received, such that the signal would need to travel just a fraction faster than light to get from A to B, then it is always possible to find a different frame where A and B are simultaneous (the signal is sent at exactly the same time it’s received, even if the two locations are light-years apart) and also frames where B occurred before A. This is a technical consequence of something called the relativity of simultaneity in SR. And if the laws of physics work the same way in every frame, then if it’s possible in one frame to have a signal that received at the exact moment it’s sent, then this must be possible in every frame; this leads to the possibility that if two slower-than-light observers (call them Alice and Bob) are moving apart at slower-than-light speeds, then Alice can send a signal to Bob which is instantaneous in her frame but backwards in time in his, and Bob can send a reply to Alice which is instantaneous in his frame but backwards in time in hers, with the net result that she receives the reply before she sent the original signal! This is the “tachyonic antitelephone” Count Iblis mentioned in comment #12, and there are some spacetime diagrams showing how it would work on this page: http://www.theculture.org/rich/sharpblue/archives/000089.html

  • Jesse M.

    Oh, and I should add that while you’re correct that the speed of light “has been shown to vary” when light is traveling through a medium like air or water (due to the photons being continually absorbed and re-emitted by the medium), the constant c that appears in the equations of relativity refers solely to the speed of light in a vacuum, and there is no convincing evidence that this constant can vary.

  • http://www.thedelphicfuture.org Marc Fleury

    Sean,

    thanks for the discussion. It was a relief to find your blog. I read with great interest your ‘lorentz violation paper” and what you call the modern aether. I have been looking into this (purely by intellectual curiousity) and mostly because the lorentz invariance and all the ‘observer neutrality’ was just too repulsive to my brain.

    In any case. I do not understand why this would imply time travel. Again if lorentz falls then maybe we should stop hurting our brains with this space-time thing, in other words who cares what space-time diagrams say, they are irrelevant.

    I would like to hear comments on the fact that the main difference from 1987a and Opera is that in opera the neutrinos go through EARTH CRUST. A compressed ether. A compressed media has a higher velocity of waves. Aether would provide a simple approach to this problem.

    Why don’t you use the blog to pursue more sci-fi scenarios. The caution around the results is right and welcome but the speculation should be going wild. Bring the aether back. (Are you at all familiar with Beckmann’s work? do you give it any value?).

    As a side note, I enjoy the comments almost as much as the OP.

  • http://www.thedelphicfuture.org Marc Fleury

    Jesse M,

    thanks for the clear expose and the link to the minkowski diagram explaining the paradox. I think we can take this further.

    a/ Opera is wrong. Then move on it all holds there is no time travel.
    b/ Opera is right. According to minkowski diagrams then we can receive a message before sending it, which is of course a contradiction. We have proven by the absurd that SR is false.

    Or as is said very well in your original link
    “faster than light travel or communication, special relativity and causality cannot coexist.”

    I think the possibility that Special relativity is wrong should be seriously considered. Personally I can’t wait, I have a long standing beef with SR (which I never had with GR) that i could never swallow “relativity of simultaneity”.

    If FTL is true, SR has got to go. Causality is a much deeper philosophical hole. I know which one I sacrifice in a heartbeat. I can’t wait to see how this unfolds.

  • http://www.thedelphicfuture.org Marc Fleury
  • drm

    Maybe speed of light violations only count if you get caught. Neutrinos interact so weakly as to be barely detectable in the first place. Hence, they are a very weak channel for carrying information faster than light. In that case, the observational (and casuality) consequences are all practical purposes almost nil.

  • http://kem4kem.blogspot.com/ DAVID KEMMERLY

    So half spin nuetrino, nuetral one, jitter bugs boson while older photon, spin one slow dances taking longer till done?

  • Baby Bones

    Let’s say that the experiment is right; then, we should look closely at what the tiny number in excess of c is suggesting. I think it possibly suggests that c is minutely higher than we expected and that the speed of light that has been measured in experiments is a local speed wherein light is effectively slowed down by the medium it travels through (even if that medium is the “vacuum” on or nearby Earth). After all, the neutrino is hardly affected by any medium, even solid rock, but light is affected by almost every medium it travels through.

    Another idea is that light gets an effective mass through some sort of interaction with the vacuum.

    If the neutrino is really faster than light, and is a time traveller, we would have to combine that fact with the many world’s hypothesis about all possible futures within the tiny time difference and every one of those futures would have copies of neutrinos that would move through our present and exert a ghostly gravitational pull. That could add up to a lot of mass-energy coming from the future, and maybe that is dark energy or dark matter.

  • TimG

    Jorge (#21): On the contrary, they’ve learned that truly shocking, not-yet-confirmed results almost always end up being wrong, and only occasionally end up being right.

  • Elmer Fudd

    If neutrinos travel backwards in time, does that mean they will be created at some future point in time rather than during the big bang?

  • http://www.thoughtresults.com Saeed Neamati

    Needs more illustrations and pictures, for people who are not good at music, but interested in science and physics.

  • http://www.thedelphicfuture.org marc fleury

    Repeat after me, time travel TO THE PAST is impossible. Time travel to THE FUTURE, a trivial result of time dilation. There are ways to get at time dilation without all the machinery of minkowsky space. Causality is a deeper principle than special relativity. In short, do not talk ‘lightly’ about backwards time travel as it invalidates causality and I will always rule it as absurd. Opera, if true, is a proof by the absurd that special relativity is wrong.

    Also I yearn for a simplification of physics on the basis of aether approaches. It is no mean feat given the Michelson-Morley type of experiments, which have “exquisite precision” according to Sean. I always like to remind people that while we observe time dilation for the muons coming down to earth, the reciprocity, that muons observe muons on earth dilating slowly has never been observed. The “equisite precision” has not been for ‘every observer’. It makes me sad to already read about short cuts in higher dimensions and what not, I want aether, a 19th century understanding of “mechanics” with all the modern math flavor a la Sean, to be taken seriously.

    I think in 20 years we will ask each other, where were you when they announced Opera?

  • http://quantummechanics.mchmultimedia.com/ Bryan Sanctuary

    Neutrinos are governed by weak interactions and gravitation. I do not think it unreasonable that the mediators of all forces travel at light speed. Do not know if that might affect neutrino speed.

  • Paul Stankus

    I like to start lectures on relativity by putting the idea of “proper time” or subjective time in a central role. A simple statement of SR, which bridges over to GR, is that proper time experienced by any object traveling along a path is purely a function of the path itself; ie all objects (particles, physicists) following the same path experience the same increment of subjective time passing.

    With the “everyone gets their own cone” conception, this appealing picture of proper time as a function of path is one of the things that has to be surrendered. For a path which is outside the photon cone but inside the neutrino cone, those two types of particles obviously experience different amounts of proper time (technically, the photon would have negative proper time squared, while the neutrino would have positive proper time squared).

    So, don’t be too sanguine thinking that the “each their own cone” is some kind of simple fix. Accommodating superluminal neutrinos will require erasing a lot of stuff, waaaaay up the page, and starting over to re-define even such basic concepts as proper time or rest frames.

    (I think this is similar to Bee’s remark above; BTW, note to Bee: regarding “(Been there, done that.),” what work does this refer to? Links, references?)

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  • Mike

    Just heard a great joke:

    Neutrino. Who’s there? Knock knock.

    ;)

  • Jesse M.

    Mike, I heard a different version:

    The bartender says, “we don’t serve neutrinos in here.” A neutrino walks into a bar.

    Of course even before the FTL study, there were other such neutrino jokes:

    A neutrino passes through a bar.

  • Justin Loe

    The following is worth noting: “Luca Stanco, a senior member of the Opera collaboration (who also worked on the ZEUS experiment with me several years ago). He points out that although he is a member of Opera, he did not sign the arXiv preprint because while he supported the seminar and release of results, he considers the analysis “preliminary” due at least in part to worries like those I describe… Four other senior members of Opera also removed their names from the author list for this result.” source: http://www.guardian.co.uk/science/life-and-physics/2011/sep/24/1?newsfeed=true

  • Count Iblis

    Another issue may also be worth looking into. Neutrinos have a finite magnetic moment (of the order of 10^(-19) times the Bohr maenton according to the Standard Model, but it could be much larger), therefore you would expect superluminal neutrinos to emit Cherenkov radiation.

    But unlike Cherenkov radiation emitted by particles in a medium, the radiated power will be infinitely large, because the index of refraction of the vacuum won’t become larger than unity at some high frequency, so there is no high frequency cut-off for the Cherenckov radiation, making the total radiated power divergent.

    Of course, if neutrinos really can travel faser than light, then there must be other effects making the Cherenkov radiation finite.So, if there exists a field with some vacuum expectation value that effectively breaks Lorentz invariance for neutrinos, then this must also act as a medium that affects the effective index of refraction of vacuum

  • relativelydumb

    Why doesn’t the temperature of the CMB provide an “absolute clock” (at least in principle) such that “relativity of simultaneity” questions can never arise? Yes, I know the temperature varies depending on one’s motion but can’t one correct for that? Surely someone here can explain that in simple words that a non-theorist can understand?

  • Alex

    Sean, wouldn’t time travel to the past itself be an example of Lorentz violation? Imagine time travel was possible, and you ended up in the past. By looking around you see you’re in the past (e.g. Elvis on the TV), so wouldn’t this information itself tell you that you are/were travelling faster than light – therefore breaking the idea that an observer should not be able to measure their own velocity?

    Neutrino. Who’s there? Knock knock.

    We already knew that neutrinos move faster than sound through air before this experiment, though.

  • Mike

    “We already knew that neutrinos move faster than sound through air before this experiment, though”

    Actually, you’re right about that ;)

  • Jesse M.

    @relativelydumb:
    Why doesn’t the temperature of the CMB provide an “absolute clock” (at least in principle) such that “relativity of simultaneity” questions can never arise?

    Because the CMB is just a collection of physical particles, whereas when physicists talk about all reference frames being equal, they mean that the basic equations of physics work the same way in each frame. If you could create a region of space where all the CMB photons were first absorbed and then replaced by a new collection of photons that had the same spectrum but a different average rest frame, an observer in this region at rest relative to this frame wouldn’t see these photons behaving any differently than an observer in a region with CMB photons in the CMB rest frame.

  • Jesse M.

    @Alex:
    Sean, wouldn’t time travel to the past itself be an example of Lorentz violation? Imagine time travel was possible, and you ended up in the past. By looking around you see you’re in the past (e.g. Elvis on the TV), so wouldn’t this information itself tell you that you are/were travelling faster than light – therefore breaking the idea that an observer should not be able to measure their own velocity?

    Relativity just says the laws of physics work the same way in all the sublight inertial frames given by the Lorentz transformation. So, for two objects moving at sublight speeds which are at rest in different frames, there can be no frame-independent sense in which one has a greater speed; but all these inertial frames still all agree about whether something is moving slower than light, at light speed, or faster than light (and the Lorentz transformation would not allow an FTL observer to have his own inertial frame).

  • HP

    I think you’re all missing the larger point of all this: If these results are confirmed, we may soon have a way to reliably predict when CERN is just about to turn on their proton beam.

  • relativelydumb

    @Jesse M. Thanks but doesn’t get to the root of what I was trying to ask: why doesn’t the temperture of the CMB provide a clock against which all questions of simultaneity can be resolved? Which is another way (I think) of asking shouldn’t all observers in inertial frames measure the same temp of the CMB and thus be able to know WHEN they are, relative to the time of the universe becoming transparent to the CMB? (note the name I’m using here!).

  • Jesse M.

    Thanks but doesn’t get to the root of what I was trying to ask: why doesn’t the temperture of the CMB provide a clock against which all questions of simultaneity can be resolved?

    Observers could certainly use this as an agreed-upon universal standard if they chose, but it would be an arbitrary human convention, it would not be a “physically preferred” definition of simultaneity in the sense that physicists are talking about. As I said, a preferred definition of simultaneity would mean that the fundamental laws of physics pick out one frame as “special”, not the particular pattern of matter/energy which is a result of historical contingencies.

  • Eric Mertz

    Forgive me if this has already been discussed as I am a relative layman in Quantum Mechanics and have not taken the necessary math classes to take the necessary physics classes, but wouldn’t the time necessary for the (relatively) prime causal event to take place, and the time necessary to compose the message to be sent if one is to be sent, mean that by the time the message is sent, any chance of an Antitachyonic Telephone to occur would have passed?

    Let’s say that we could send a message at 500c, at what distance and speed of communication after the event, would you have to be to actually create an Antitachyonic Telephone effect? Would you still generate an Antitachyonic Telephone if you achieved FTL Communication/Travel by shortening the distance to be traveled instead of increasing velocity?

    I have tried to read up on the nature of FTL causing time travel, but most of the articles I have found were either too technical for my level of knowledge in the subject, or ignored the time necessary for the event to take place and the information to reach Alice so she can tell Bob about it.

    From what I have been able to understand about the technology of this, it sounds like you would have to use a Wormhole of some kind to attain causal violations and time travel, but that may be my own lack of understanding of the subject matter.

    Any explanations geared towards a Freshman in College would be greatly appreciated, thanks. And again, sorry if the answers should be obvious and I just can’t see them from my vantage point.

  • Alex

    Thanks, Jesse, that makes some sense.

  • http://www.thedelphicfuture.org marc fleury

    Eric,

    I am biased anti-SR. After a Ph.D. in physics the only time I had to read, re-read, shake my head in disbelief, read again, give up trying to understand and just apply the math, and 20 years later, go back, re-read, re-disbelief, finally understand what the math is saying (captured in minkowski diagrams) and STILL not believe a word of it (relativity of simultaneity? really?) was with SR. The innocuous statement of “same laws of physics according to every observer” does lead to all the minkowski diagram machinery. I do recommend you read up on that because outside of that mathematical machinery there is NOTHING that will explain to you why you need to give up classical thinking and embrace ‘relativity of simultaneity’, curved mixing time and space (different concepts) and all the mental gymnastic the full SR picture requires. I consider it, to this day, a monster.

    When people claim SR ‘has been fully tested’ as Sean in OP. Remember only ONE observer has been fully tested, the one usually at rest in the lab frame or tethered to the earth gravity, there are aether based theories that do not depend on SR. The ‘relativity of observers’ has never been tested, and until we get lab equipment going at the speed of light we will not test it. It does lead to all the philosophical monsters.

    Back to your question: the minkowski diagram will show you that if you have FTL, you can easily build time travel see links above in discussion on great ways to see it. OF COURSE THERE ISN”T ANY WAY TO EXPLAIN THIS TO A LAYMAN, BECAUSE IT DOESN”T MEAN ANYTHING (imho) OUTSIDE OF THE LORENTZ MATH. Just read this (http://www.thedelphicfuture.org/2011/09/causality-ftl-opera-and-special.html) which reuses the link above and see that SR with FTL leads to time travel backwards.

    I am amused to see people seriously entertaining the idea in this forum, ‘what would it mean?”. It doesn’t mean anything. Time travel backwards is a philosophical monster because it violates causality (receiving a message before it is sent etc) and why this is not dismissed out of hand is bizarre to me. Causality must remain whole. Unfortunately the physics profession, from my standpoint, has tortured itself so much that it is ready to accept just about anything the math models tell it. I vividly remember deciding I would get out of the field of theoretical physics the day I swallowed without questioning that the universe had 23 dimension (string theory class) and the teacher said “that can be problematic for a few folks”. I turned around to my neighbor and asked why, he just rolled his eyes at pointed at the space around us, “what do you see?”. I had been brainwashed. I did finish my PhD without any joy and just got out as fast as I could.

    So it is with a certain glee that I hope this result will hold. Until then I will gladly repeat what I consider fundamental philosophical truth, namely that causality is a deeper principle than SR. I don’t care what SR says, there are plenty theories that account for the phenomenology of SR without all the suspension of disbelief. Backward time travel doesn’t mean anything to a human brain and should not be entertained on the basis that ‘the math tells us so’ in violation of causality.

    I want to see some sanity restored to the field of physics.

  • Jesse M.

    @marc fleury:
    When people claim SR ‘has been fully tested’ as Sean in OP. Remember only ONE observer has been fully tested, the one usually at rest in the lab frame or tethered to the earth gravity, there are aether based theories that do not depend on SR.

    It’s true that experiments are done on Earth, but it’s simply a matter of doing a mathematical transformation to see how the same events would be described in a coordinate system moving at high speed relative to the Earth; conversely, any theory of physics that violated Lorentz-symmetry should have effects that could be measured (and found to disagree with relativity) in the Earth frame.

    Not sure what you mean by “aether based theories that do not depend on SR”. Do you mean theories that actually make different predictions about experimental observations than SR, or do you mean those that make the same predictions but interpret them differently, saying that there is a true aether rest frame but we can’t detect it because rulers moving relative to this frame shrink and clocks moving relative to it slow down, in just the right way so that all ruler/clock systems measure things to be obeying the same laws of physics? If the latter, see the discussion here about why such interpretations are considered highly contrived.

    Time travel backwards is a philosophical monster because it violates causality (receiving a message before it is sent etc) and why this is not dismissed out of hand is bizarre to me.

    Because the idea of causality violation, while very implausible, need not actually lead to any logical contradictions, one could postulate that the laws of physics only allow self-consistent histories, of which there will always be some possible–see the discussion of the Novikov self-consistency principle along with some of the scientific papers in the notes.

    Backward time travel doesn’t mean anything to a human brain and should not be entertained on the basis that ‘the math tells us so’ in violation of causality.

    Speak for yourself ;) I would bet a lot of money that backwards time travel will not end up being possible according to the most fundamental laws of physics, but I don’t find the idea meaningless to my brain (and I would bet nearly as much that this neutrino result doesn’t portend a preferred frame of reference…speaking of which, have you seen this cartoon?)

  • Dan T. Benedict

    Whether or not the OPERA results hold, this discussion is a perfect example of how FLT does not necessarily imply backward time travel; only that for particles to travel FTL, they must be non-electromagnetic in nature. In order for backward time travel to be demonstrated (as the knock-knock joke demonstrates) the effect, i.e. the detection of neutrinos in the lab at Gran Sasso would have to be recorded *before* the event at CERN (cause), i.e. the creation of the neutrino beam. This is not what is being measured and therefore cause and effect are not being violated.

    As Sean correctly points out, if the OPERA results hold, it only demonstrates that Lorentz invariance is broken for these particular particles (neutrinos are non-EM in nature and not necessarily subject to this invariance). Since Lorentz invariance would be non-applicable to neutrinos, so would the math and logic that leads to the antitachyonic telephone effect.

    What is not clear is how these results affect our basic understanding of the fundamental nature of mass-energy and whether there exists a similar Lorentz-type invariance(s) to which all non-EM particles must adhere. For example,

    Sean wrote: “Even if neutrinos are able to sneak outside light cones, there may nevertheless be “neutrino cones” to which they are still confined. These neutrino cones could be a little bit broader than ordinary light cones, but they could still define a fixed notion of “going forward in time” that even neutrinos couldn’t violate.”

    I cannot agree with the “neutrino cone” since they have non-zero mass and therefore their velocity should vary wrt their kinetic energy. This velocity may be truly tachyonic in nature and therefore may vary yet must always be greater than c. However, as illustrated above, this does not necessarily imply any violations of causality.

  • Jesse M.

    @Dan T. Benedict:
    Whether or not the OPERA results hold, this discussion is a perfect example of how FLT does not necessarily imply backward time travel; only that for particles to travel FTL, they must be non-electromagnetic in nature.

    Lorentz-invariance is not supposed to apply only to electromagnetism! Relativity’s prediction is that it applies to all fundamental laws of physics, and it is indeed true that both quantum chromodynamics (which deals with the strong force) and the Standard Model (which unites electromagnetism with the weak force, and also accounts for the strong force although it doesn’t unite it with the other two) are Lorentz-invariant, and of course general relativity (which deals with gravity) is locally Lorentz-invariant. If any of the fundamental laws of physics fail to be locally Lorentz-symmetric, this would be understood as a falsification of relativity.

    I cannot agree with the “neutrino cone” since they have non-zero mass and therefore their velocity should vary wrt their kinetic energy.

    But unless you assume relativity is correct (in which case FTL neutrinos imply causality violation), you can’t assume anything about how their energy varies with velocity, it’s possible the energy would approach infinity at some finite velocity greater than c (just as an electron’s energy goes to infinity as it approaches c) and give a “neutrino cone”. It’s also possible it wouldn’t, but I think Geroch’s paper was just giving this as a possibility.

    Whether an FTL neutrino would have a maximum finite speed or could travel arbitrarily fast but not violate causality, I think this would imply a preferred reference frame. In the case where they could travel arbitrarily fast but not back in time, there would have to be a preferred definition of simultaneity implying a preferred frame. In the case of a finite maximum speed, there would have to be a unique frame where light cones and neutrino cones had their central axes aligned, meaning that in this frame light would travel at the same speed in all directions and so would neutrinos…in other frames of the type assumed in SR, light would still have the same speed in all directions but neutrino speed would have to be direction-dependent).

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  • http://quantumnonsense.blogspot.com/ Qubit

    Did get there before it set of though? did it? I think not!
    It’s not like CERN was built without such things in mind! FLT; CERN are counting on it.

  • Jesse M.

    FLT; CERN are counting on it.

    Did you mean to write “FTL” (faster than light)? If so I don’t know what you mean when you say they were counting on it, I think nearly all physicists would say the likelihood of FTL particles is extremely low.

  • Dan T. Benedict

    Jesse M. #65,

    You wrote: “But unless you assume relativity is correct (in which case FTL neutrinos imply causality violation), …”,

    and yet the OPERA results shows that the neutrinos were recorded at Gran Sasso *after* the creation of the neutrino beam at CERN, which implies causality has *not* been violated even with a neutrino velocity measured slightly greater than c.

    You wrote: “it’s possible the energy would approach infinity at some finite velocity greater than c (just as an electron’s energy goes to infinity as it approaches c) and give a “neutrino cone”.”

    and yet a “neutrino cone”, if analogous to a light cone, implies that the neutrinos have a constant velocity (presumably slightly greater than c, if OPERA holds) regardless of the reference frame, otherwise the analogy would not hold (i.e., if neutrino velocity varies, there cannot be a “neutrino cone”).

    Finally, you wrote: “Whether an FTL neutrino would have a maximum finite speed or could travel arbitrarily fast but not violate causality, I think this would imply a preferred reference frame.”

    and yet a preferred reference frame does exist wrt to *time* (time is not a fundamental law of physics) since all local reference frames are subsets of a cosmological frame that connects all events into a single cosmos that is bounded in time (in the past) by the singularity. This is how we can even designate the universe to be 13.7 billion years old. This preferred frame does not violate relativity, as its laws act as a rule to transform between one frame and another depending on the local conditions that exist in each frame wrt the others.

  • Jesse M.

    @Dan T. Benedict:
    and yet the OPERA results shows that the neutrinos were recorded at Gran Sasso *after* the creation of the neutrino beam at CERN, which implies causality has *not* been violated even with a neutrino velocity measured slightly greater than c.

    I didn’t mean that any FTL signal transmission would involve a causality violation (you could in fact find a different inertial coordinate system where the signal was received earlier than it was sent, but “causality violation” means a cause having an effect in its own past light cone and that wouldn’t be the case for a one-way signal transmission). I just meant that if relativity is correct, then the existence of FTL signals implies it is guaranteed to be physically possible to design an experiment involving these signals where causality is violated, see my comment #33 above for details.

    and yet a “neutrino cone”, if analogous to a light cone, implies that the neutrinos have a constant velocity (presumably slightly greater than c, if OPERA holds) regardless of the reference frame, otherwise the analogy would not hold (i.e., if neutrino velocity varies, there cannot be a “neutrino cone”).

    No, under the inertial frames given by the Lorentz transformation, it’s easy to show that anything going faster than light must have different speeds in different frames, that’s just a property of the coordinate transformation. The “neutrino cone” idea implies that you could design a different coordinate transformation involving a different set of coordinate systems than those given by the Lorentz transformation, and in these new coordinate systems, neutrinos would have a constant speed while the speed of light rays would vary in different frames. Regardless of which set of coordinate systems you chose, there would be one unique frame where the two sets of cones aligned in the way I described in comment #65.

    and yet a preferred reference frame does exist wrt to *time* (time is not a fundamental law of physics) since all local reference frames are subsets of a cosmological frame that connects all events into a single cosmos that is bounded in time (in the past) by the singularity.

    There is no unique “cosmological frame”–a “frame” is just a coordinate system, and in the curved spacetime of general relativity you can use absolutely any crazy coordinate system on any given spacetime and the equations of general relativity will still work (see the comments in this article about “diffeomorphism invariance”), so there are an infinite number of different coordinate systems you could use to describe the entire history of the universe. But in any case, as I said earlier physicists would define a “preferred frame” in terms of the fundamental laws of physics working differently in one coordinate system than another, so specific physical events including the Big Bang aren’t really relevant to whether a preferred frame exists or not. Also, a “preferred frame” normally means a preferred inertial frame, and no coordinate system covering a large region of curved spacetime is considered “inertial”. But general relativity is “locally Lorentz-invariant” which basically means if you pick smaller and smaller patches of spacetime to look at, the laws of physics observed by a freefalling observer in that region get arbitrarily close to those seen in an inertial frame in special relativity (see this article on the equivalence principle). So when physicists talk about a preferred frame they really mean a breakdown in this local Lorentz-invariance, so even if you restricted your experiments to a very small region of space and time where the effects of gravity were negligible, you’d be able to pick out one locally inertial frame in which the laws of physics take a “special” form.

  • http://www.thedelphicfuture.org marc fleury

    Jesse,

    thanks for the reply and links.
    1/ On Navikov. I was familiar with some of these ideas (the billiard balls I had studied briefly) and they are part of the gobbledigook I refer to. In a way self-consistency is a way of saying time travel without paradoxes by definition of ‘no paradoxes’. Time travel (back) is possible but if you get there you can’t touch”. Of course this makes for great sci-fi, as in the smoke monster in Lost, and back to the future, but I find it silly when it comes to physics. Are we to believe there is a completely separate branch of physics that limits your actions (of which we have no inkling today) just to satisfy the ‘self-consistent’ principle. Which means that yourself in the past are severely constrained (by what?) in what you can do. Again while intellectually amusing it is an example of the suspension of disbelief and complicated complications (which I enjoy) when “no causality violation” takes about 0.30s for my brain to compute as truth at a simple philosophical level. Causality is a deeper principle than SR (or symmetry since you seem to equate the two).

    2/ Thanks for the link on SR/Aether BTW it is very informative and i intend to read 1 and 2 later. While I have developed a taste for aether research, I have a training in theoretical physics and renormalization of gauge theories felt normal at some point. I have left the field in disgust after my PhD mostly owning to a distaste for all the math based approaches, which while powerful and I could do them left me profoundly unhappy by being devoid of ‘common sense’ interpretation. I couldn’t connect the whole symmetry thing with ‘reality”. Of late, I have grown to appreciate that these symmetries, limit the possible oscillation and embrace aether as the medium for that oscillation. I have been looking into NYE crystal tensor formulation as with 23 degrees of liberty in the oscillations you can easily build a ‘particle/wave’ picture that has enough variables to address advanced problems like the standard model and gravity. Smolin’s approach to quantum loop gravity is a similar type of approach.

    On predictive power. Here is the thing, since then, i have dabbled in biology, finance, macro economics, computers and have found that most science is profoundly messy. I too find profoundly puzzling that “math has such a predictive power” in physics, the link you give gives a great overview.

    That is also to say that I don’t relate to the search for ‘beauty and unification’ in the math that so many people seem to get off on. I find the search for beauty in the models as distracting from the search for truth. There is no reason why truth should be beautiful, it is in fact quite ugly in biology and the math only gets you so far, I think it is an affliction of physics as exemplified in the smolin book for example.

    But enough meta bla bla. Back to FTL neutrinos, from your comments I gather that you agree that FTL neutrino would imply a violation of lorentz invariance and thus SR locally, almost by definition. I think the question of time travel is void since it is an artefact of the lorentz transform and minkowski diagrams. It would be a great example of “truth doesn’t need to be pretty”, FTL violating neutrinos do not care what Einstein thinks.

    I am with you on the cartoon, I will bet you 20 :)

  • Wilhelmus de Wilde

    Going faster then light c, means that the object that is speeding faster as c, is going to catch up with signals that were emitted at c, so it can “see” the past, it is not going into the past !!!

    keep on thinking free

    Wilhelmus

  • http://www.thedelphicfuture.org marc fleury

    I agree with you and that is my first ‘interpretation’ of FTL from a classic standpoint. However once you throw in lorentz transformation FTL DOES lead to time travel, hence proof by the absurd as far as I am concerned.

  • gnome

    Most laymen don’t realize that the crux of the issue is not simply the speed-of-light (they think this news is great, faster spaceships! Star Trek here we come!) Not so fast! The issue at stake here is the very geometry of space (as we understand it).

  • rational

    I think sean is taking a very reasonable approach.
    I would add that it is very likely that there is some systemic issue in how the neutrino speed was measured. It is very unlikely that we would not have noticed this already if it were true, or that we wouldn’t be able to find data that confirmed the observation independently.

    The discover would mean that we would be able to determine if a supernova would occur before seeing it and other things. It would also mean that our cosmic horizon may have two frontiers that are of different distances but the exact same age.

  • http://quantumnonsense.blogspot.com Qubit

    Hi Jesse m,

    Of course I meant faster than light, the detectors on the LHC look to me like they have been designed in order to have a dual purpose; one one them is manipulate FLT plasma (QGP) in order to create a type of time travel. Its amazing what you can do when you use all of the detectors in entangled unison; in order to create complex manifolds.

    Sorry, but that just how it looks to me.

    For light time stopped, for the neutrino; it had its first tick! It covered the whole of the first second and it never travelled back in time.

  • fco.

    I have the same question as Ellipsis (#28).
    For neutrinos to be confined to their own cones, in a way that we can’t use them to mess with our timeline, shouldn’t we not be able to even detect them?

    (better read #28, he asked it better than I)

  • Glen

    Traveling faster than light could be traveling without light and it could be extra dark.

  • Terry

    I have thought about time travel and have considered many different theories. I came to my ultimate theory and conclusion that time is neither linier nor conical in space or time. Time is helical; therefore there is no speed at which time will encounter itself in the past because there is no past relevant to the position of time on the helix.

  • gnome

    http://arxiv.org/abs/1109.6562 — anybody want to comment on this?

  • FB36

    The relativistic mass equation implies mass must be an imaginary number for FTL. It is also known that neutrinos oscillate to different varieties as they move and they have a tiny mass.
    Since the experiment measured the speed of Tau neutrinos only, I think it is possible that oscillation to Tau neutrino phase switches mass to an imaginary number and only the neutrinos born in that phase has FTL property. (And also the ones born in other phases never reach FTL even when they switch to Tau phase because they still would need to gain energy to speed up also. That still maybe possible if they pass through high energy cosmic ray etc. zone in space while in that phase. So maybe the 1987a supernova neutrinos never passed such a zone so they never had a chance to speed up.)

  • FB36

    I also have a simple explanation why FTL does not mean going back in time for neutrinos.
    Imagine an an FTL spaceship going from Earth to Mars and the astronaut watching a movie broadcast from Earth. The movie would really run in backwards because the ship is moving faster than the radio waves. This implies time is also running backwards for the astronaut.
    But this analogy does not apply to an FTL neutrino because neutrinos does not interact w/ EM fields. So the neutrino can never watch the broadcast.

  • Aaron Sheldon

    Actually the big problem with FTL particles (and other Lorentz violating theories) is that it violates local stress-energy conservation. To an observer that crosses path with an FTL particle, they would see a pair of identical particles spontaneously created, and then traveling in opposite directions from the point in space-time where the FTL particle crossed the observers path.

    Simple put there is no way you can “see” a FTL particle coming at you, because it always out races its own signal, leaving the observer “surprised” by its sudden appearance out of thin air. So if you subscribe to the information theoretic interpretation of entropy, even one FTL particle would contribute an infinite amount of entropy to any local neighborhood through which it travels.

    Unfortunately even if you violate local conservation of stress-energy just a tiny smidge, it would add up to huge consequences in things like solar and galactic evolution. Now don’t quote me on this because this is not even a back of the envelop calculation, but if FTL neutrinos where produced with the density to be observed at the rate they were claimed to have been observed, then the sun should have collapsed into a black hole within the first couple hundred million years of its formation, due to the observed spontaneous creation of extra mass. And a careful reading of their paper will reveal that the MLE used assumed no dispersal in the neutrino speeds, so the conclusion from the estimator is that every neutrino produced is an FTL neutrino.

    In some sense particles travel backwards in time everywhere in space-time (just always slower than the speed of ligth): they are called their anti-particle partner. Its at least a mathematical nicety.

    In the end the analysis needs to be rerun to include MLEs for both the speed of the neutrinos, and the dispersal of velocities

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  • Jesse M.

    @FB36–you’re misunderstanding the reason people say FTL leads to time travel in relativity, it has nothing whatsoever to do with visual appearances, you really need to understand the concept of the relativity of simultaneity in different inertial frames given by the Lorentz transformation, and the idea of relativity that the laws of physics should work the same way in all these different inertial frames. See my comment #33 above for some more info.

  • walter

    Please help. I am having a difficult time figuring something out. When we speak of a “spacelike” trajectory I take that to mean a trajectory through the fabric of space/time. The shortest distance between A and B through the fabric of space time is a straight line, but said straight line can be curved since the fabric of space time is warped. Any signal faster than c would violate causality, unless we have a different causality cones for different particles. But why is this necessary? If space/time is curved then we could have short cuts (wormholes) or particles could use extra dimensions traversing a “straighter” line between A and B than the one permitted by the space/time fabric. So, a signal could travel faster than c without actually moving faster than c. My question is; would this violate causality or the Lorentz covariance? Could particles be temporarily trapped in this “subspace” arriving at their destination before c would permit it? Would this be a violation of causality, and if so why? I have not been able to find an answer to the question, please help.

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  • Jesse M.

    @walter: When we speak of a “spacelike” trajectory I take that to mean a trajectory through the fabric of space/time.

    All trajectories are understood to be trajectories though spacetime, but they are divided into three categories: “timelike”, “lightlike” and “spacelike”. All you really need to know for this discussion is that a “timelike” trajectory is a path that could be taken by a particle moving slower than light, a “lightlike” trajectory is one that could be taken by a particle moving at the speed of light, and a “spacelike” trajectory is one that could only be taken by a particle moving faster than light (of course even if no such particles exist, we can still define such trajectories in terms of whatever coordinate system we use). There’s a little more on the distinction between these three types in the wiki article on spacetime (see in particular the section “spacetime intervals” for how timelike, lightlike and spacelike intervals are defined in special relativity, and also the section “spacetime in general relativity”).

    If space/time is curved then we could have short cuts (wormholes) or particles could use extra dimensions traversing a “straighter” line between A and B than the one permitted by the space/time fabric.

    A particle traveling through a wormhole or any other such “short cut” allowed by general relativity is still traveling through the same spacetime, just a strangely curved region of it (see the diagram of a 2D space with a wormhole in it on this page), so it’s still “locally” moving slower than light which travels through the same region even if it gets somewhere faster than a light beam which took the “long way”. Things might get more complicated if we were talking about “short cuts” allowed by string theory, like the idea that 3D space might be a kind of membrane in a higher-dimensional space, so if it wasn’t perfectly “flat” in this higher dimensional space a particle might take a shortcut by leaving the brane, traveling through the extra dimension and then returning to the brane. I think this probably would avoid the possibility of time travel though I’m not perfectly sure.

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  • http://www.smartstop.weebly.com Aaron

    You do realize that this leaves a possibility that photonic mass has a greater impact on the overall mas of the universe? I think dark matter could be in hot water….

  • Jim

    All these discussions of trim-travel – in the sense of “killing you grandfather” and reverse causality since the whole concepts violates everythingwe know about space-time. A particle cannot mee themselves in the past because a particle can occupy more than one position in space-time.Thie simple, elegant fact has been around for a long time yet most physicists who dream about time travel seem to willfully ignore it.

  • Jim

    “Consider, for example, the existence of a heretofore unobserved fluid pervading the universe with a well-defined rest frame…” Aether theory makes a comeback!

  • Robert Hanson

    Suppose a neutrino can indeed travel faster than light. It does not mean that it travels backwards in time. If a photon takes 4 years to travel from the sun to Alpha Centauri, and a neutrino travels at an amazingly fast speed of 110% of C, then it takes slightly less than 4 years to reach Alpha Centauri. It does not arrive before it left (which is what would happen if it could travel backwards in time.

    What happens is that the neutrino conveys information from the sun to alpha centauri faster than a photon can.

  • Pingback: Neutrinos spark wild scientific leaps « sciblogging

  • http://www.smartstop.weebly.com Aaron

    actually, if these results are verified it would mean that there must be more than one particle that can travel faster than light, and the cosmic speed barrier is broken.

    introduce particle “polly”. polly is similar to a photon or neutrino in that it is very very small and is emmited by star “a”. it reaches relativistic speeds instantaneously and begins its journey through space to our receiver “b”.

    traditionally we would say that this particle moves from point a to point b. it takes a measurable time depending on the distance. this time can be measured and thus we derive the speed.

    in our 4th deminsional world, this makes sense as this is how we percieve things. we watch them move from a to b. the event of emmision happens at a defineable point sometime prior to the event of receiving and there is some measurable time between the two, and these events will be in that order, no matter what because we cannot go back in time right? event a precedes event b because event a is a cause of event b and so on.

    But if you think about it, the particle really exists at all points between point a to point b.

    follow with me. if you were to percieve the world with a 5th deminsional attitude, we would see a bunch of elongated snakes of humans. well really, we would see the universe is stuffed full of stuff, stretched from its birth or origin to the end or death. the universe would be observed to be infinitely dense and infinitely full.

    of course this doesnt make sense to us, because just as 2 dimensional creatures cannot make sense of a 3rd dimensional depth, and a 3rd dimensional creature cannot understand the fluidity of time, we do not readily grasp the concept of seeing all time at once.

    What happens to photons when we slow them down? they decay, very rapidly. but if we leave a photon at c, it will go on for billions of years. that is how we see such far distances in space. or so we understand. but if we think about it in a 5th dimensional way, we are just moving down a road along our 5th dimensional path. and if we go further, to encompass all possible paths at once, then we are thinking even higher, sixth dimensional. (i know i am rambling, i dont want to post a novel and i am sure lots of you are breaking out your crack pot stamps, sorry but this does make sense.)

    this 6th dimensional idea means that not only does polly our particle exist at all points from a to b at once, but also all possibilities of pollys fate do as well. polly does not choose (for kicks, polly is sentient…) her path, but rather navigates her way through a maze of possible paths by making decisions. (this gets back to neutrinos being ftl… i swear!)

    now reckon this to a jet fighter traveling at mach 2 in a world of beings that can only sense thier surroundings using their ears. if you were to ask them what the ultimate speed is… they would tell you mach 1. even though the jet is travelling faster, our blind friends cannot measure it because the only means of observation are thier ears and thus they would only observe the jet to move at mach 1.
    so back to polly and us…

    we only had our eyes to observe the particles of our universe until recently. hence it would only make sense that we would assume that the fastest visible thing that we can see is actually where our speed barrier lay.

    string theory tells us through math that our imagined 5th and 6th and beyond deminsions are there, so this shouldnt be science fiction to anyone. but if we had whatever 5th deminsional entities may use to observe thier surroundings, i think that the speed of light would seem rediculously slow, seeing as how everything happens all at once, and as close as infinately fast as could be reasoned. so who cares if the neutrinos move faster than photons. if you stop thinking about c as being some barrier that we cannot cross and start thinking of it as simply the fastest that something can go that we can watch from beginning to end, then it really should be no big deal.

    so back to polly and her journey. can we not model her as existing at all points a to b with us only being able to observe certain features of her existance, and those features only being able to be seen as moving speed “x”?

    this way, when we encounter FTL particles, they are just more excited. we cant track a neutrino and its path from a to b. so it would stand to reason that the bits we can observe just happen faster, slightly above our observable parameters?

    everything happening all at once when we put ourselves in its shoes, a much different world indeed.

    i dunno, you all prolly think i am a nutcase, but it makes sense to me. i sincerely hope someone would be willing to address this idea with me or at least shoot it down somehow.

    -Aaron

  • http://axitronics.blogspot.com/ DR BDO Adams

    I have a possible cause for the extra speed of the neutrino. The Scarnhorst effect, which noted as early as 1994, that QED and other quantum field theories, modified the speed of the force carrier accorded to amount of energy in near by fields, the effect is rather small for light, but it scales as the inverse four power of the mass of the fermion in the theory, so for it neutrinos its some billions of billions times bigger. To generate the effect we require that neutrino feel some fifth force, and that there is a binding background field (that doesn’t scatter) in ordinary matter. I’ve been investigating the existance of such a field for the last five or so years, and it seems, not only perfectly allowable, but it could also solve dark energy. Given an additional U(1) force for neutrino (an axial force, left hand-right hand), a background sea to cancel the additional charges on nucleons, and the Scarnhorst effect, we can generate an faster than light speed for neutrinos.

    Each field has its only speed of propagator (light etc), which must be the same in the vacuum, however depending on the energy in any background field (of the same type), the speed of propagator can vary. Neutrinos would still travel at the speed of light in the vacuum, but would closely approach the increased speed of the new forces propagator in a background field with negative (i.e. binding) energy. Since the neutrino is still technically slower than the relavistic speed no breaking radiation would be emitted.

    See my blog Axitronics for more details.

  • Conrad F

    I’m no physicist, but I don’t buy the idea of time-related problems with neutrinos – it just lends credence to some whackier ideas that have little empirical evidence at present to comfort their theorizers.

    It is wholly an assumption that nothing moves faster than light.

    Einsteins’ thought experiment merely suggested the appearance of time travel by viewing light from an earlier period – not actual time travel! The fact that you can view something ‘out of time’ does not mean time travel has taken place – only the illusion of it. In actuality you would be in the same time viewing a past event and unable to interract with it.

    Now, this assumption that nothing moves faster than light is upended with the neutrino speed measurements. I prefer to assume a different view on the matter. That is, I see that the speed of light is the maximum speed of a photon *given its particular mass*. Another particle’s limit may be something different. I think this in no small part because a particle’s velocity is related both to its mass and the energy it has been given. Light may not be able to go beyond a certain speed, but, in my hypothesis, a smaller particle would be able to. The corollary of this is that the smaller the particle, the faster it can move and at zero mass we have infinite energy – which is exactly what we have *before* the formation of subatomic particles in the ‘big bang’: infinite energy at zero volume. Einstein proposed that E=mc^2 may not be quite right. Probably, for us, we should find the speed limit of the smallest particle and rework the equation by replacing ‘c’ with a variable for whatever that something is, be it a neutrino or something else. In summary, I postulate that a neutrino has a slightly lower mass than a photon and that this accounts for the recent measurements.

    I am not alone in this view: http://usersguidetotheuniverse.com/?p=2181

  • Guthrie Prentice

    Sean, I posted on your other entry regarding faster than light neutrinos that in certain frameworks, namely those of 6 dimensions, neutrinos and gravity might be able to travel backwards in time:

    http://arxiv.org/abs/gr-qc/0603045

    However, this is only likely if the dimensional shortcut comprises of a 6 dimensional framework instead of 5 dimensions, and according to this paper, gravity and neutrinos would take the path of least resistance to avoid forming a closed timelike curve.

  • Mr Big Stones

    I agree with Conrad F.! and this article’s primary point in general is exactly what I have been thinking whenever I saw a report on this experiment.

    Getting from an event to a receiver faster than light does not mean that the object is going back in time. The object will be a significant distance away from its Grandfather (the source). When it arrives it could look back and witness a time prior to when the source occurred due to the light having been overtaken by the object as it traveled but if it were to return to the same space it came from, it would have to travel in the opposite direction, reversing this effect so that once it arrived, time would have passed at the source. So no killing the grandfather.

    To suggest that it would be possible would be like claiming that because light reaches us from an explosion before the sound does, we would be somehow capable of preventing that sound from being created.

    Perhaps realtivity considers time as being stuck too closely to light’s speed for how it really is. Perhaps certain objects can squeeze into a gap between light speed and infinite time.

  • Jeff Garvey

    Point 1: Has anyone given any thought to the idea that the measured speed of the nutrinos may be correct, but the conventional value for the speed of light may be wrong? 7.5 km/sec isn’t much. How do we know with such certainty that the speed of light isn’t 300,007.5 km/sec and not 300, 000? Did any of those “speed of light” measurement experiments receive the level of scrutiny that this one will?

    Point 2: Suppose it is so. The Einstein theory was an explanation of the other famous experiment by Michaelson/Morely, which merely established that the speed of light is anisotropic, not setting a specific value for it. The whole structure of Relativity derives from the anisotropy, not the value. If it is true that these nutrinos are going faster than photons, then someone ought to be preparing a setup to test whether the speed of neutrinos is anisotropic; if it is, the results of Relativity still remain.

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  • Pingback: Faster-Than-Light Neutrinos Confirmed? In One Way, Yes; In Another, No | The Crux | Discover Magazine

  • Pingback: Faster-Than-Light Neutrinos Confirmed? In One Way, Yes; In Another, No | My Blog

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Cosmic Variance

Random samplings from a universe of ideas.

About Sean Carroll

Sean Carroll is a Senior Research Associate in the Department of Physics at the California Institute of Technology. His research interests include theoretical aspects of cosmology, field theory, and gravitation. His most recent book is The Particle at the End of the Universe, about the Large Hadron Collider and the search for the Higgs boson. Here are some of his favorite blog posts, home page, and email: carroll [at] cosmicvariance.com .

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