What We Know, and Don't, and Why

By Sean Carroll | December 19, 2006 5:33 am

Yeah, I already used this title once before. It’s a good title, okay? Cut me a little holiday slack here.

By way of slightly-warmed-over blogging, I present to you the slides from a talk I gave a few weeks ago at Villanova, my undergrad alma mater. The original mandate was to talk about scientific literacy to a collection of undergrads, but I didn’t know how to make that fascinating. So I took it to the next level and went a bit meta, talking about the way science works. It was at a fairly abstract level — I didn’t go into building detectors, and error bars, or anything like that — but not too highbrow philosophy-of-sciencey — I didn’t get into Kuhn vs. Popper, much less Feyerabend or the Strong Programme, although you’ll find touches here and there.

To bring things down to earth (relatively speaking), most of the talk consisted of an extended look at the battle between “dark matter” and “modified gravity.” It goes all the way back to Leverrier and the discovery of Neptune, whose existence was inferred via its gravitational tug on the orbit of Uranus. Neptune was the first successful prediction of dark matter — some unseen substance whose existence is revealed by its gravitational influence. Leverrier tried again with the similarly-discrepant orbit of Mercury, positing a planet called Vulcan; but this time it turned out that gravity itself was the culprit, after Einstein showed that general relativity correctly accounted for the precession of Mercury’s orbit. So the lesson from history is — different ideas work in different circumstances. Keep an open mind until the data come down on one side or another. (And once they do, admit it.)

Today, of course, we’re dealing with an analogous problem, given that 25% of the universe is apparently some kind of dark matter that doesn’t fit into the Standard Model of particle physics, and 70% is some kind of dark energy that is even more mysterious. Modified gravity might be at work here as well, and I talked about the prospects.

Along the way, I drew out some of the lessons about how science works that these various investigations have taught us. I intentionally did not try to wrap it all up with a neat bow into a catch-all philosophy of science, as I think the reality is kind of messy, and it’s worth admitting that. The closest I came was the famous quote from Professor Rumsfeld, previously shared. This led to a series of cautionary homilies warning against misuse of the hypothesis-testing nature of scientific inquiry. The truth is, scientific knowledge is inevitably tentative, not metaphysically certain. But that doesn’t mean that anything goes — some things we really do understand! So I cautioned against various mistakes, using perpetual-motion machines, Intelligent Design, and What the Bleep Do We Know as good examples of what not to do.

CATEGORIZED UNDER: Science
  • Scott

    Very good, intellectually stimulating…

  • http://astrocath.blogspot.com Joe

    Sean,
    Yes, stimulating, but perhaps a little unsatisfying. I mean, haven’t you just “kicked the can down the road” a bit when you say “scientific knowledge is inevitably tentative”? Of course it is, but then you’ve begged the the question “What do we know and why?” with the answer “We know nothing for certain, ever”.
    Jacob Bronowski did a marvelous job addressing this in a book that preceded his PBS series (and I only wish I could find my copy and give you the title!).

    There’s so much (heated) discussion about this topic because we come so quickly to our core beliefs. Ego and arrogance steps in to stop the communication. I appreciate that here, at least, you’ve avoided that!

    Joe B. VU Astronomy ’76

  • http://theeternaluniverse.blogspot.com/index.html Joseph Smidt

    That Rumsfeld quote is great. Like you said in your previous post, if you step back and think about it it makes sense. But on the surface it is hilarious.

    So any bets on dark matter/modified gravity? In the spirit of the poker theorist mentioned on a previous post, I put my money on dark matter. I bet have at least 3:1 odds, especially with this cluster merger experiment.

  • George Musser

    Great stuff.

    To me, the astrology example is the weak link in the argument. The reason to reject the Venusian connection is not that theorists can’t think of a way to make it work, but that it doesn’t hold up under empirical scrutiny. If it did, I bet people would come up with mechanisms that are entirely consistent with present knowledge (new long-range forces, gravitational resonances, etc.).

    George

  • http://quasar9.blogspot.com/2006/12/europes-corot-launch.html Quasar9

    Hi Sean, doesn’t an expanding Universe
    make the Universe a perpetual motion machine.
    And doesn’t a Universe whose maths + physical laws
    can be ‘fathomed’ by intelligence make it ID.
    -
    note – ID as in it unquestionably has design
    and design requires ‘intelligence’ – else neither
    man (nor computers) can claim to have ‘intelligence’
    -
    I am not entering a theological debate about the existance of a creator or not – simply the logic of intelligent design in Nature, the understanding of which – its maths & laws of physics (and biology) – us humans measure as ‘intelligence’

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

    George, I have to utterly disagree, and this is one thing I think physicists don’t do a good enough job in explaining to the general public. The regime of “forces that act on ordinary matter with strengths greater than or equal to gravity over distances relevant in the Solar System” has been completely explored, there is absolutely no room for any new phenomena there. There is no need to do blind tests etc. to demonstrate that astrology (and telekinesis etc.) cannot possibly work. Sometimes we know enough about the universe to conclude that certain things just can’t happen, and this is one of those times.

    To turn the argument around: if someone did find empirical support for astrology, the only reasonable implication would be that everything we know about physics is wrong. Which is of course logically possible, but a possibility that it’s perfectly reasonable to ignore.

  • Roman

    Sean – great presentation – I could understand everything!
    One thing though – as somebody from “the real world”, I tend to include mathematical theories in general term “scientific theories”. It may be incorrect, but believe me; a lot of us do this. They (mathematicians) can and even have to prove their theories, and as you said, physicists, astronomers etc basically can’t.
    Understanding why is that, why mathematical law has to be proven, and physical “law” isn’t even a law and can not be “proven”, was a great help for me in understanding the difference between “scientific approach” and “non-scientific approach”. I think somebody who understands this can’t argue about superiority of ID over this or that scientific theory – he/she can only believe in ID.

  • http://eskesthai.blogspot.com/2006/10/history-of-star-shine-to-now.html Plato

    Sean:So the lesson from history is — different ideas work in different circumstances. Keep an open mind until the data come down on one side or another.

    Without all the information “a little hint” of who should get credit in 1998?

    Discoverer of Neptune

    Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the “Neptune papers” (historical documents from the Royal Greenwich Observatory), which had apparently been misappropriated by astronomer Olin Eggen for nearly three decades and were only rediscovered (in his possession) immediately after his death. After reviewing the documents, some historians now suggest that Adams does not deserve equal credit with Le Verrier.

    So that might affect the underlying basis of your point, or not.

    So yes, the ideas about “dark energy” can be tantalizing, and different ideas can work in different circumstances. I was thinking about recent announcement on dark energy and the “ebb and flow,” of a expansionary universe?

    How do you explain it? So it becomes an open challenge for open minds.

    A “microperspective view” of the cosmos. Would that fall in line as an example of your lecture. I know you work form the top…….down:)

  • George Musser

    OK, strike what I said about a new long-range force. My point, though, wasn’t so much that astrology, if it were ever proved, would be a sign of new physics, but that it would be the hint of some new phenomenon.

    Years ago, pilots spoke of mysterious lights in the skies. Some called them alien spacecraft, and as a result, the reports got lumped in with the UFO nutcases and dismissed. It turned out that these lights were indeed a new phenomenon: high-altitude lightning. Thus we need to differentiate between observations and interpretations. The mysterious lights weren’t ETs, but they were something.

    To turn the argument around: if someone did find empirical support for astrology, the only reasonable implication would be that everything we know about physics is wrong.

    Really? To say this buys into the astrologers’ claims! Even if their observations are right, their interpretation need not be. acupuncture might work even if the qi theory is silly.

    A correlation between planetary positions and innate characteristics might have any number of explanations, beginning with the humans themselves. There might be a placebo effect. Or people might be psychologically affected by celestial cycles, which in turn might affect babies in utero. Or there might be some mechanism deeply rooted in our evolutionary heritage, whereby we adjust our physiology or embryonic development based on celestial cycles. Some people have claimed a connection between menstrual and lunar cycles; these claims may be dubious empirically, but they violate no law of physics or biology.

    George

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    Hi Sean, doesn’t an expanding Universe
    make the Universe a perpetual motion machine.

    No — no more than a smooth brick sliding on a lake of smooth ice (i.e. with no friction, continuing forever) is a perpetual motion machine, or a planet orbiting a star is a perpetual motion machine.

    They keep going, they don’t stop, so they seem to semantically fit “perpetual motion.” However, when somebody says “perpetual motion machine,” what they really mean is some machine that puts out more energy than is put into it. They also mean that it does this locally. Without Dark Energy, the Universe is expanding because of whatever it was that gave it the kick that got it started back during hte Big Bang or Inflation or whatever. Since then, it’s just been coasting, slowing down due to gravity. With Dark Energy… well, that’s scary.

    In GR, there’s no conservation of energy globally, and indeed the whole Dark Energy/accelerating Universe thing smells disturbingly like some sort of “free lunch” type perpetual motion machine. However, energy is still very much conserved locally; you couldn’t harness that to build some sort of machine to power a pendulum or some such, getting out more energy than you put in.

  • http://infralever.blogspot.com/ Peter Fred

    In his post Sean writes The truth is, scientific knowledge is inevitably tentative, not metaphysically certain.

    Say we are in a situation that the Scholastics had to face and the theory that we have embraced for centuries rests on an artifact that only gives appearance of predicting the observations. There is an artifact that might do this and that is the infrared luminosity emanating from a central body, which like the gravitational force, varies inversely as the square of the distance from that body. Since there is also a high correlation between the luminosity of a star and its mass maybe we should reexamine the pressure studies before we so confidently conclude that light can only be repulsive. This is especially indicative since there are so many observations confirming the Tully Fisher.

    Then there is the problem of the possible role that the tremendous pressure found at the center of all orbiting planets and stars might play in the centripetal acceleration of that orbiting body, which we so confidently believe has already been explained by Newton’s Law of gravity. We have this confidence despite the fact that we have yet to specify, as Newton asked us to do, that inherent property of mass that varies as 1/r^2 that “causes” the centripetal acceleration.

    Say, the sunlight causes earth’s day side surface gravity to be less than the night side surface gravity by 0.06% which is equal to 0.006 m/s^2 its centripetal acceleration, then the formula I get, using well understood physics. for the net force between the day and night hemispheres that would propel the earth around the sun is

    F_net = GMm/r^2*3/4

    where GM/r^2 = v/r^2 is the Newtonian formula that supposedly predicts the centripetal acceleration of an orbiting body.

  • Scott

    You don’t believe in astrology!–must be one of those Scorpio people.

  • anon

    Well done, Peter Fred! Nothing can orbits a dark object. I guess satellites don’t actually orbit objects like the earth, moon, Mars, …

    Oh. Wait. Hmm.

  • Ijon Tichy

    A suggestion for Sean: next time give them a reference to a good, meaty description of how real, messy science is done, like Gravity’s Shadow: The Search for Gravitational Waves by Harry Collins. Now that’s a book for grown-ups.

  • http://quasar9.blogspot.com/ Quasar9

    Rob Knopp said: when somebody says “perpetual motion machine,” what they really mean is some machine that puts out more energy than is put into it.

    That must have been one hell of an ‘energy’ kick at the beginning to give rise to the universe in motion – and if it was bigger than the combined energy that we see coming out of the Sun in our Solar system and all the energy that keeps them galaxies and stars buzzing, and all the energy still coming out of said galaxies and stars, don’t you think.

    Rob, had you said the Universe is NOT a perpetual motion machine because one day it will STOP – I may have understood.

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    Since there is also a high correlation between the luminosity of a star and its mass maybe we should reexamine the pressure studies before we so confidently conclude that light can only be repulsive.

    Dude, red giants.

    My A102 students knew half-way through the semester that the correlation you just described doesn’t uniformly exist.

    -Rob

  • http://quasar9.blogspot.com/ Quasar9

    Rob Knop said: the Universe is expanding because of whatever it was that gave it the kick that got it started back during the Big Bang or Inflation or whatever. Since then, it’s just been coasting, slowing down due to gravity

    Rob, A Universe which over time produces enough energy to give birth to Suns, new Stars and galaxies, is coasting – like a brick sliding on a smooth lake of ice?

  • Jack

    Attach a cable to a sufficiently distant galaxy. As the Universe accelerates, you will get an unlimited amount of energy for free from this gadget. [When the cable snaps, attach it to a nearer galaxy. Repeat.]

    Energy is not conserved in an accelerating universe. All the talk about local vs global is irrelevant.

  • http://brahms.phy.vanderbilt.edu/~rknop/blog/ Rob Knop

    Rob, A Universe which over time produces enough energy to give birth to Suns, new Stars and galaxies, is coasting – like a brick sliding on a smooth lake of ice?

    I realize I really shouldn’t be feeding the trolls, but just in case somebody else is reading who might learn something….

    The expansion of the Universe itself, without Dark Energy, is coasting, and indeed slowing down from the mutual gravity of all the mass in the Universe. However, Dark Energy is in fact there, and is speeding up. The coasting of the Universe expanding is much like any other kind of coasting — smooth block on ice, baseball thrown in intergalactic space continuing it’s Newton’s 1st Law motion, etc.

    The formation of the stars, galaxies, etc., all comes not from any of the energy of the Big Bang itself. Rather, the energy of all of that comes out of (initially) gravitational potential energy lost (and, yes, energy is conserved there– a galaxy is “local” when you’re talking the GR of the Robertson Walker metric), and then, later, energy through fusion of Hydrogen. (Also, some fusion of other elements.) Large scale structure formation has pretty much stopped in our Universe; we’re not building new clusters, just condensing the ones we have. Give it a few hundred billion years, or whatever, and eventually stars will have used up all of the Hydrogen available, and they will no longer shine. Stars shine for a long time, but they’re not perpetual motion machines; they are using up their fuel.

    None of these processes “suck energy” out of the expansion of the Universe or any such. It’s all just local stuff on the expansion of the Universe. It’s akin to a few ants having a campfire on the top of that block sliding on the lake of ice. Yeah, they’re burning up energy, but it has nothing to do with the sliding of the block on the ice.

    -Rob

  • http://quasar9.blogspot.com/ Quasar9

    Rob knop said: Stars shine for a long time, but they’re not perpetual motion machines; they are using up their fuel.

    Yes Rob, nuclear reactors are not perpetual motion machines, they are machines built by man using up their fuel (like stars do?).
    But though it may appear clear to you in your head, you have not shown that the Universe is not a perpetual motion machine inside which there are many galaxies and solar systems, and on one of these there is a planet we (humans) call earth, and on which ants live.

  • Charon

    you have not shown that the Universe is not a perpetual motion machine

    Go back and read his definition of perpetual motion machine. He’s pointed out that the expansion of the universe doesn’t give the energy that creates stars, etc. (violating one condition to be a “perpetual motion machine”). He’s also pointed out that perpetual motion machines need to be local. Even with the generous definition of “local” that GR with FRW gives, the universe as a whole is not local.

    This has become trolling. Stop.

  • http://www.einemillioneurohomepage.de Swetlana Maßat

    Great and excellent article t’s realy helpful. Thanks again.

  • Pingback: My comments to Sean Carroll’s post about his Villanova talk « the luminous universe

  • Paul Valletta

    As Sean clearly stated, there is a narrow band of what we actually know, this is really embedded into a wide-band spectrum of learning. There must be some stastistics, a graph plot or:

    http://en.wikipedia.org/wiki/Frequency
    diagram showing how often throughout history science moves away from the norm ?

    Obviously, a similar plotted graph for say Astrology?, should have a “fixed straight line” within their criteria, showing no deviation away from the narrow band, because of their steadfast beliefs.

    Every aspect of human “knowledge” history can be plotted using available data, as example, Religion can have certain truths, for instance Moses existed, along with many other Humans. What needs to be addressed is the factual course of “probable” events leading to current understanding, did Moses go up to the mount and communicate with a “God”..or did Moses as an old man (therfore exert himself physically) travel from a sea level location, climb a mountain and experience:Altitude sickness, physical trauma,with delusional events being interpeted by some as mystical?

    What we know NOW, must, as the course of learning evolution expects, leads one to conclude that Humans are suseptable to have their environment experience, alter their fragile fear of the Unknowns!

    Had Moses, or anyone else at the time been aware of exposure to Altitude sickness, Deprevation of Oxygen, or even the prospect of High Energy Cosmic Ray particle’s frying the Human brain at certain altitudes, then we could be experiencing a totally different “beliefs” ?

    Data, when collected is open to intepretation, anything can happen, and some say “will” happen, but until that time, stay within the known knowns !

  • http://infralever.blogspot.com/ Peter Fred

    just testing

  • http://quasar9.blogspot.com/ Qiasar9

    Charon,

    1) I have read Rob knops’s definition of a perpetual motion machine: when somebody says “perpetual motion machine,” what they really mean is some machine that puts out more energy than is put into it.
    -
    2) Charon you claim: the universe doesn’t give the energy that creates stars, etc. (violating one condition to be a “perpetual motion machine”).
    The Universe produces all the energy within it. If there is more energy in the Universe today than yesterday, or say 13.7 billion years ago +/- 0.03% what does that say in relation to Rob knop’s definition of perpetual motion machine.
    -
    3) You and Rob knop claim: The Universe is not local?
    Are you speaking from another ‘pocket’ universe somewhere else?
    -
    Though it may appear clear to you both inside your heads, neither you or Rob knop have shown that the Universe is not a perpetual motion machine.
    -

  • http://quasar9.blogspot.com/ Quasar9

    Charon said: the universe doesn’t give the energy that creates stars, etc. (violating one condition to be a “perpetual motion machine”).

    Priceless – The Universe does not produce the stars inside it?
    Where did the energy of the first cause of motion come from?
    From a power source (or big foot) kicking the universe into motion?
    -

  • http://eskesthai.blogspot.com/2006/12/cosmic-ray-spallation.html Plato
  • Adam S

    There is a relativistic version of MOND–it’s called TeVeS and it was formulated by another Israeli Prof named Bekenstein. I think it’s a recent innovation.

    MOND proponants claim that TeVeS changes the way gravitaitonal lensing works, so the bullet cluster results may not seal the deal (they did convince me, although I was rooting for MOND)

    Along the lines of what George was saying, the freakonomics people wrote an article in the NYTimes about correlations between birth-month and success in life–it was societal, of course, no astrology. But still…
    http://www.nytimes.com/2006/05/07/magazine/07wwln_freak.html?ex=1166763600&en=abb6f6ba07d89cf3&ei=5070

  • KundryVolare

    Thank you folks for such excellent brainfood once again…

    The nonlocality question is important and I found this book quite elucidating>>

    ‘The NonLocal Universe: The new physics and matters ofthe mind’
    -Robert Nadeau & Menas Kafatos; Oxford UP 1999!

    Re:astrology v. “science” see: ‘Cosmos and Psyche: Intimations of a new world view’
    -Richard Tarnas; Viking 2006.
    There is quite a difference between Rob Brezhny and Dane Rudhyar, but i’ve heard that those born with grand crosses in Aries in the fifth house are geniuses or serial killers…

  • Haludza

    No-one’s looked at the bullet cluster with TeVeS properly yet, but I think it may already be excluded by solar system etc.. anyhow.

  • http://quantumfieldtheory.org nc

    The idea that 70% is dark energy and 25% dark matter is the best mainstream model, the lambda-CDM model.

    However, the lack of slowing down of supernovae recession wasn’t predicted by the cosmological constant, but it was predicted and published two years ahead by a dynamics for gravity which is entirely factual. There is 100% bias in favour of mainstream and 0% objectivity towards new work, especially where it’s based entirely on facts which are well established (spacetime, Hubble recession, Newton’s 3rd law etc.).

    Any statement of the constructive facts get ignored, any criticism of the mainstream gets responded to with a request for constructive facts, which then get ignored. What you never get is any interest in fact based science.

  • http://quantumfieldtheory.org nc

    The creation-annihilation loops in the vacuum are limited to the range of the IR cutoff (~1 fm) so they don’t justify “dark energy” which is a hoax due to a faulty use of general relativity uncorrected for redshift of quantum gravity mediating gauge bosons, which get redshifted when exchanged between receding masses (gravitational charges), weaking gravity and preventing the big bang expansion from slowing down due to gravity as Friedmann’s [N*O*T E*V*E*N W*R*O*N*G solution to general relativity] suggested.

    (I respect the fact that a comment here is not the best place, but with Jacques Distler and others on arXiv deleting my papers in 2002 within 45 seconds, without reading them, it isn’t possible to put information where it should be. Red tape gets in the way, not to mention elitism, hatred of so-called “pet theories”, etc. Even if you use entirely established facts, you’re still censored because they don’t have the time to read anything not in their carefully censored journals, which are biased in favour of M-theory and related speculations. To publish facts that actually predict gravity with that stuff would be embarrassing and thus impossible not only to them, but to me. I published in Electronics World.)

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

    nc, you’ve said all this before. More along the same lines will be deleted — we’re insufferably mainstream around these parts.

  • Pingback: the luminous universe » On Newton’s 1666 discovery of the linkage between earth’s surface acceleration and the moon’s centripetal acceleration

  • whywhy

    OK I always thought a perpetual motion machine is a machine which is capable of doing an infinite amount of net work without any new input of energy. This does not necessarily mean that it outputs more energy than is put into it as said by Rob Knopp.

    There are two basic kinds of PMMs (perpetual motion machines): those that violate the first and second laws of thermo (which are Rob Knopp type machines that output more than they put in) and those that only violate the second law (machines that are 100 percent efficient). A PPM requries that work be done since a machine is some device that does work. What is work? The physics definition is that work is the dot product of force and displacement. Essentially this means that work = forcexdistance but also that the force and the distance have to be in the same direction (they should be parallel and not perpendicular). A block sliding on ice or better still a body moving through space is not an example of a PPM because there is no force being exerted. A planetary body does have a force being exerted on it over a distance but the displacement and the force are always perpendicular (not in the same direction) so the work done is zero. I am not sure about the expanding universe but my feeling is that there is not force involved in expanding the universe and therefore there is no work. However I don’t really understand GR and I am not sure how things work when you are talking about space itself expanding.

    The reason why PPM don’t work is heat. Whenever any machine does work it produces heat. So in order for the machine to keep going it must somehow recover the heat energy it loses. However you basically end up producing even more heat if you try to recover the heat energy you lose and so you can never have a machine with 100 percent efficiency.

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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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