Category: Time

Time-Reversal Violation Is Not the "Arrow of Time"

By Sean Carroll | November 20, 2012 10:01 am

Looks like the good folks at the BaBar experiment at SLAC, feeling that my attention has been distracted by the Higgs boson, decided that they might be able to slip a pet peeve of mine past an unsuspecting public without drawing my ire. Not so fast, good folks at BaBar!

They are good folks, actually, and they’ve carried out an extremely impressive bit of experimental virtuosity: obtaining a direct measurement of the asymmetry between a particle-physics process and its time-reverse, thereby establishing very direct evidence that the time-reversal operation “T” is not a good symmetry of nature. Here’s the technical paper, the SLAC press release, and a semi-popular explanation by the APS. (I could link you to the Physical Review Letters journal server rather than the arxiv, but the former is behind a paywall while the latter is free, and they’re the same content, so why would I do that? [Update: the PRL version is available free here, but not from the PRL page directly.])

The reason why it’s an impressive experiment is that it’s very difficult to directly compare the rate of one process to its precise time-reverse. You can measure the lifetime of a muon, for example, as it decays into an electron, a neutrino, and an anti-neutrino. But it’s very difficult (utterly impractical, actually) to shoot a neutrino and an anti-neutrino directly at an electron and measure the probability that it all turns into a muon. So what you want to look at are oscillations: one particle turning into another, which can also convert back. That usually doesn’t happen — electrons can’t convert into positrons because charge is conserved, and they can’t convert into negatively-charged pions because energy and lepton number are conserved, etc. But you can get the trick to work with certain quark-antiquark pairs, like neutral kaons or neutral B mesons, where the particle and its antiparticle can oscillate back and forth into each other. If you can somehow distinguish between the particle and antiparticle, for example if they decay into different things, you can in principle measure the oscillation rates in each direction. If the rates are different, we say that we have measured a violation of T reversal symmetry, or T-violation for short.

As I discuss in From Eternity to Here, this kind of phenomenon has been measured before, for example by the CPLEAR experiment at CERN in 1998. They used kaons and anti-kaons, and watched them decay into different offspring particles. If the BaBar press release is to be believed there is some controversy over whether that was “really” was measuring T-violation. I didn’t know about that, but in any event it’s always good to do a completely independent measurement.

So BaBar looked at B mesons. I won’t go into the details (see the explainer here), but they were able to precisely time the oscillations between one kind of neutral B meson, and the exact reverse of that operation. (Okay, tiny detail: one kind was an eigenstate of CP, the other was an eigenstate of flavor. Happy now?)

They found that T is indeed violated. This is a great result, although it surprises absolutely nobody. There is a famous result called the CPT theorem, which says that whenever you have an ordinary quantum field theory (“ordinary” means “local and Lorentz-invariant”), the combined operations of time-reversal T, parity P, and particle/antiparticle switching C will always be a good symmetry of the theory. And we know that CP is violated in nature; that won the Nobel Prize for Cronin and Fitch in 1980. So T has to be violated, to cancel out the fact that CP is violated and make the combination CPT a good symmetry. Either that, or the universe does not run according to an ordinary quantum field theory, and that would be big news indeed.

All perfectly fine and glorious. The pet peeve only comes up in the sub-headline of the SLAC press release: “Time’s quantum arrow has a preferred direction, new analysis shows.” Colorful language rather than precise statement, to be sure, but colorful language that is extremely misleading. Read More

CATEGORIZED UNDER: arxiv, Science, Time, Top Posts

Feynman on Initial Conditions, Evolving Laws, and What We Consider Physics

By Sean Carroll | October 18, 2012 8:02 am

We’ve mentioned before that Richard Feynman was way ahead of his time when it came to the need to understand cosmological initial conditions and the low entropy of the early universe. (Among other things, of course.) Feynman actually wrote three different books in the early 1960’s — in his way of “writing books,” which consisted of giving lectures and having others transcribe them — all of which made a point of discussing this problem. The Character of Physical Law was aimed at a popular audience, the Feynman Lectures on Physics were aimed at undergraduate physics majors, and the Feynman Lectures on Gravitation were aimed at advanced graduate students — and in every case he emphasized that we can only account for the Second Law of Thermodynamics by assuming a low-entropy boundary condition in the past, for which we currently have no reliable explanation. (These days we have a larger number of speculations, but still nothing reliable.)

Here’s a video clip from about ten years afterward, in 1973, where Feynman raises a similar point in a conversation with Fred Hoyle, the accomplished astronomer and a pioneer of the Steady State cosmology. (Thanks to Ronan Mehigan.) They don’t go into details, but Feynman introduces the idea as a kind of meta-issue in physics:

“What, today, do we not consider part of physics, which we may ultimately be part of physics?”

His answer (which should be cued up here at the 7:10 mark) is the initial conditions of the universe, as well as the possibility that the physical laws themselves evolve with time. (Conversation continues for a tiny bit in the followup video. Listen on to hear Feynman explain how he doesn’t like to speculate about things.)

What’s interesting is that now, four decades later, it’s commonplace to address the issue of initial conditions in a scientific context, and even to consider the evolution of local physical laws, as we do with the multiverse and the string theory landscape. I’m not sure what is the precise history of this endeavor, but in the very same year this interview was aired, Collins and Hawking wrote an early paper asking why the universe is isotropic. In 1979, Dicke and Peebles published “The Big Bang Cosmology — Enigmas and Nostrums,” which set out many of the puzzles that Alan Guth would attempt to address with the inflationary universe scenario. When we marry inflation with the idea of a landscape of vacua (whether from string theory or elsewhere), we naturally are led to the idea of an evolving set of local physical laws, raising the possibility that we might be able to actually explain (using the anthropic principle or simple probability arguments) why we observe one set of laws rather than some other. Not that we have, or even seem very close, but the scientific agenda is clear.

So how could we answer Feynman’s question today? What do we not consider part of physics, which someday we might?

CATEGORIZED UNDER: Science, Time, Top Posts

Mysteries of Time: New Teaching Company Course

By Sean Carroll | October 1, 2012 8:24 am

Just in time for the holidays (Halloween totally counts as a holiday), the Teaching Company (a/k/a “The Great Courses) is releasing a new course I recorded — Mysteries of Modern Physics: Time. For those of you who aren’t familiar with the format (and my previous course, Dark Matter and Dark Energy), this is a set of 24 lectures, each half an hour each, modeled on an undergraduate college course for non-scientists. Note that both are hugely discounted at the moment, by 70% off the ordinary price, which isn’t always the case. Unlike the previous course, this new one is available in an audio-only format as well as on video. But in the last few years they have upgraded their graphics and animation considerably, so the video version might be worth a look. Here’s the teaser:

Unsurprisingly, a lot of the course follows the outline in From Eternity to Here. But it’s also pretty different; the organization has been switched around, and the course has a lot more emphasis on time in everyday life and the psychology of time, less on cosmology and the multiverse. Here are the lecture titles: Read More


Time Talk

By Sean Carroll | September 18, 2012 2:14 pm

With The Particle at the End of the Universe scheduled to come out in November, most of the popular-level talks I’ll be giving in the near future will have to do with the LHC and the Higgs boson — and quantum field theory, as part of my secret agenda to get QFT accepted as part of the mainstream pop-sci vocabulary. So I’ll be giving fewer talks about the arrow of time, at least near-term. I thought I’d commemorate the occasion by sharing the slides I used for a recent version of this talk: “The Origin of the Universe and the Arrow of Time.” Not that I’m by any stretch done talking about it, but hopefully the next time the occasion arises I’ll have the energy to make up new slides from scratch.


Time the Destroyer

By Sean Carroll | July 17, 2012 10:37 am

Andy Albrecht of UC Davis gave an entertaining TEDx talk on entropy — or as he calls it, “destruction” — and the arrow of time. I especially like how he is willing to look clumsy in the cause of greater pedagogy!

CATEGORIZED UNDER: Science, Time, Top Posts

Time Travel via YouTube

By Sean Carroll | July 7, 2012 12:39 pm

Via everywhere on the internet, here’s Jeremiah McDonald, who used a 20-year-old videotape of his younger self to carry on a conversation across time. (Seems legit at a casual glance, but I suppose it could be faked.)

Sadly we can’t actually transfer information into the past. If we could, I would have started writing this book a bit earlier.


Does This Ontological Commitment Make Me Look Fat?

By Sean Carroll | June 4, 2012 8:39 am

3:am magazine (yes, that’s what it’s called) has a very good interview with Craig Callender, philosopher of physics at UC San Diego and a charter member of the small club of people who think professionally about the nature of time. The whole thing is worth reading, so naturally I am going to be completely unfair and nitpick about the one tiny part that mentions my name. The interviewer asks:

But there is nothing in the second law of thermodynamics to explain why the universe starts with low entropy. Now maybe its just a brute fact that there’s nothing to explain. But some physicists believe they need to explain it. So Sean Carroll develops an idea of a multiverse to explain the low entropy. You make this a parade case of the kind of ontological speculation that is too expensive. Having to posit such a huge untestable ontological commitment to explain something like low entropy at the big bang you just don’t think is worth it.

There is an interesting issue here, namely that Craig likes to make the case that the low entropy of the early universe might not need explaining — maybe it’s just a brute fact about the universe we have to learn to accept. I do try to always list this possibility as one that is very much on the table, but as a working scientist I think it’s extremely unlikely, and certainly it would be bad practice to act as if it were true. The low entropy of the early universe might be a clue to really important features of how Nature works, and to simply ignore it as “not requiring explanation” would be a terrible mistake, even if we ultimately decide that that’s the best answer we have.

But what I want to harp on is the idea of “ontological speculation that is just too expensive.” This is not, I think, a matter of taste — it’s just wrong. Read More

CATEGORIZED UNDER: Philosophy, Science, Time, Top Posts

Time and Marshmallows

By Sean Carroll | March 12, 2012 5:42 am

“Perhaps no one comprehends the roots of depravity and cruelty better than Philip Zimbardo.” At least, that’s what it says here. They’re referring to the fact that Zimbardo — a psychologist who long ago supervised the notorious Stanford Prison Experiment (chilling video here) — is an expert on the psychology of “evil” behavior. But he’s also an expert on the psychology of time, which we can all agree is much more interesting.

I recently got to hear a talk by Zimbardo, in which among other things he discussed the Stanford Marshmallow Experiment — a rather more adorable experience than the prison experiment, from what I understand. The Marshmallow experiment, originally conducted by Walter Mischel in 1972, was aimed at understanding how we think about different times — the future vs. the present. Children were asked to do some easy tasks, and then were rewarded by being given a marshmallow. But! They were told that the experimenter had to step outside for a few minutes, and if they could just sit tight and not eat their marshmallow until he came back, they could have that and also an additional marshmallow.

It’s a matter of future vs. present rewards. It’s natural (and totally rational) to discount rewards that are promised in the future — after all, the future is hard to predict, and anything can happen. If I offered you a choice between $4 today and $5 ten years from now, you’d be sensible to take the lower amount today — depending on how much you trusted me, of course. But if there is a good reason to trust, and the future isn’t that far off, it makes sense to delay gratification a bit. So what happens when some four-year-olds are put to the test?

Read More

CATEGORIZED UNDER: Science, Time, Top Posts

The Arrow of Time in a Restless Universe

By Sean Carroll | March 2, 2012 9:21 am

A group of philosophers and scientists interested in cosmology have started a new project, funded by the Templeton Foundation, imaginatively titled the Rutgers Templeton Project on Philosophy of Cosmology. It’s a great group of people, led by David Albert and Barry Loewer, and I’m looking forward to interesting things from them. (Getting tiresome questions quickly out of the way: like the Foundational Questions Institute or the World Science Festival, I’m totally in favor of this project even though I’m not a big fan of the Templeton Foundation. This isn’t the place to talk about that, okay?)

They also have a blog, because blogs are awesome. It has a humble title: What There Is and Why There Is Anything. They have a new post up, by Eric Winsberg, that brings up the issue of whether the multiverse can help explain the arrow of time. The post is basically a pointer to this paper by Eric, which is a close analysis of the kind of scenario I’ve been pursuing since my 2004 paper with Jennifer Chen. If this kind of thing is your bag, consider going over there and commenting on Eric’s paper.

I am working on a real science paper about some of these issues myself, but going has been admittedly slow. Let me just lay out a couple of the major issues here. Read More

CATEGORIZED UNDER: Philosophy, Science, Time, Top Posts

A Minute of Time

By Sean Carroll | November 14, 2011 7:54 am

For you arrow-of-time freaks who have been looking for a quick and engaging intro to the issues (maybe to show your friends to get them to appreciate your obsession), here’s a guest spot I did for the terrific Minute Physics series illustrated by Henry Reich. If you’re not already familiar with them, check out the entire series.

Previously I did one on dark energy. It came out right after the Nobel Prize announcement, but don’t let that trick you into thinking I won the Prize myself. (Some people were tricked.)

Meanwhile, in a parallel universe, instead of writing Spacetime and Geometry, I wrote a massive tome on Cosmology. This parallel universe was featured on this week’s episode of Fringe. Here’s Walter Bishop retrieving his copy from Peter.

I helped with some of the equations on the episode. Thanks to Glen Whitman and Rob Chiappetta for the shout-out.

CATEGORIZED UNDER: Science, Time, Top Posts

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