The Biggest Ideas in the Universe | 16. Gravity

Gravity! This one’s sure to be a crowd-pleaser. We take advantage of some of our previous discussion of curvature and spacetime, but we also talk about Einstein’s physical motivations for inventing general relativity, and the origin of gravitational time dilation and the like.

The Biggest Ideas in the Universe | 16. Gravity

And here is the associated Q&A video. Distinguished mostly by a deeper dive into black holes, including the information-loss puzzle.

The Biggest Ideas in the Universe | Q&A 16 - Gravity
38 Comments

38 thoughts on “The Biggest Ideas in the Universe | 16. Gravity”

  1. Hello, Professor Carroll. I have enjoyed your books for several years—especially The Big Picture. There was a remark, attributed to Neil Young: rust never sleeps. I extended that to include: nor does gravity. Tree removal may take a nap—in the heat of the afternoon… Gravity is, of course an important matter. It is also our friend.

  2. You said the equivalence principle is correct over small distances of space-time. Is the term “small” dependent on how strong the gravitational field is? I assume that “small” means a different thing near a black hole than it does if you’re floating in the void between galaxy clusters?

  3. You are correct. Gravity is easier than QM. Such a clear presentation. I knew I should have gone to Cal Tech rather than MIT.

  4. This is a question about changes in the rate of time & a potential ‘further’ test of General Relativity.

    In 2010 NIST reported on their ground based precision clock Relativity tests comparing clocks at differing relative speeds (SR) & comparing clocks placed at differing positions within the gravity potential (GR).

    These NIST Relativity tests prove beyond any doubt that a clock moving at greater velocity ticks slower & that a clock ticks faster higher up within a gravity potential – BUT has it been experimentally proven that time ticks slower in the greater gravity field? (think bigger galaxy)

    Looking at the NIST 2010 experiments:
    For SR NIST gyrated the comparison clock to obtain relative speeds.
    For GR NIST raised the comparison clock 1 metre higher to obtain differing positions within the gravity potential 
    The SR experiment incurs a difference in centrifugal force for the gyrated clock as well as difference in speed.
    The GR experiment incurs a difference in both speed (centripetal) & centrifugal force for the raised clock as well as a difference in gravity.

    I have not been able to find any evidence of a clock comparison experiment conducted ‘anywhere’ that holds speed & centrifugal force equal for both clocks & ONLY a difference in gravity occurs.

    By isolating the variable of gravity from the variables of centripetal speed/centrifugal force & testing a difference in gravity ONLY* – 

    Q: Would a clock comparison experiment that measures ONLY a difference in gravity be a ‘further’ test of General Relativity?

    *(by utilising NIST portable clocks at differing locations of same longitude & height above sea level to equalise centripetal speed/centrifugal force, but of differing geological density – or via placing clocks at existing grav.wave detectors – or via these similarly oriented proposals:
    https://arxiv.org/abs/1506.02853
    https://arxiv.org/abs/1501.00996 )

  5. Did you mean to write -1 as the first coefficient in the metric for the expanding universe?

  6. You briefly mentioned Kruskal coordinates that are well-behaved on the complete manifold near a black hole and thus prevent awfull things from happening. Awful things did happen in the Big Bang. How can we be sure this isn’t just a consequence of a bad choice of coordinates?

  7. I noticed something you _don’t mention_, very, very much to my surprise.

    In every presentation of GR (except yours), a central place is given to the the concept of equivalence of inertial mass and gravitational mass. In newtonian mechanics that equivalence is a necessary supposition in order to have a theory of planetary motion at all.

    My favorite way of introducing the concept of equivalence principle is to present the case of pulling G’s onboard a slowly rotating Ehrenfest disk. Construct a space station that is large enough and with such an angular velocity that inhabitants of that Ehrenfest disk at a particular distance ‘r’ to the axis of rotation experience 1 G of centripetal acceleration.

    If you take the stairs to one level higher you find that for a clock on that higher level a bit more proper time is elapsing. Go to a shaft, and release an object. You know that once released the object will continue along an inertial trajectory. The motion of the object with respect to the Ehrenfest disk is accelerated motion.

    As we know: in terms of GR: as a matter of principle motion under any gravitational circumstances is truly _inertial motion_. Any gravitational-interaction-trajectory is truly an _inertial trajectory_. Logical implication: equivalence of inertial and gravitational mass.

    I opened the transcript and I searched for the word ‘inertial’. You mention ‘inertial trajectory’ once in your presentation, and never again.

    Of course, the whole thrust of your presentation is that objects subject only to gravitational interaction move along a _geodesic_ of GR spacetime.

    I wonder (this is sheer speculation), is it the case that in your opinion GR does not need the concept of inertia?

  8. 1:24:48 can you explain why the bending of light is two times larger when calculating with GR compared to Newton Gravity?

  9. Why do things fall „downwards“ and not „upwards“ and what has time diletation to do with it?

  10. William H Harnew

    Wonderful talk as always. For those who might want to dig into Einstein’s GR journey and thinking (he first produced a theory that had real problems, for instance) from historical research: “The Genesis and Transformations of General Relativity” by Jürgen Renn, Director, Max Planck Institute for the History of Science

    https://www.youtube.com/watch?v=Qq7Wi_gVzdw

  11. Re: the Einstein-Hilbert action, since it is a Lagrangian, it should be Kinetic Energy – Potential Energy.
    So, R (curvature scalar) is the Kinetic Energy of the field, and the distribution of matter and energy is the potential. Except the sign is wrong, meaning that matter and energy are really negative. What is the significance of that?

  12. I have read that GR predicts the existence of the graviton.
    So I ask…..
    Where is the connection field that predicts the graviton?

  13. I have read that GR predicts the existence of the graviton.
    So I ask…..
    Where is the connection field that predicts the graviton?

  14. Any chance you could talk a bit more about singularities? They seem like one of the wackiest and most interesting things in physics, but it’s also hard to know how to think about them correctly.

    (sample questions: Does an infalling object have a future beyond the moment it hits the singularity? I guess all bets are off at that point? What do you think may happen there? You’ve said the singularity doesn’t have a point in space, but it seems like you could attribute a point in spacetime to where a particular in falling particle hits the singularity? Is it still meaningful to suggest that the singularity is a point of infinite (or nearly so) density? Is there any way to say ‘where’ the singularity is in space? )
    Thanks!

  15. Is it true that Gravitational potential energy is not defined in GR? Can you elaborate more on this?

    Thanks.

  16. Could you please explain gravity as a gauge theory in more detail. If the Poincarre group is the gauge group, why do we not have 10 spin-1 gauge bosons? Is there an intuitive understanding why space-time curvature can also be understood in terms of gravitons?

  17. I have two questions about black holes:

    1. Gravity generally works in a way that is symmetric with respect to time. The event horizon seems like an exception to this, because it is possible for things to take a trajectory where they cross the event horizon going into the black hole, but if you reverse the trajectory it is impossible to cross the event horizon from inside. How is this reconciled with time symmetry?

    2. Since gravity waves propagate at the speed of light, does that mean they also cannot escape a black hole? If a large mass falls into a black hole, is it done affecting the gravitational field around the black hole once it crosses the event horizon?

  18. Thank you Dr. Carroll for this quarantine largesse. How you present these very fascinating and very difficult topics in physics is a usefully different supplement to what else is out there. Wrt to #16, the connection between small regions of spacetime and the infinitesimals of calculus was especially illuminating. I have a few questions which may or may not be pertinent to the forthcoming gravity q & a as I’m only now catching up in the series.

    You didn’t discuss gravitons, but in previous videos you seemed to say that quantum physicists act as if they are real just not yet confirmed. Suppose I understood that correctly. Then why was the Higgs regarded as the final missing piece of the standard model? Where would the graviton go on the table of particles, or would it even?

    Also, about geodesics, could you clarify what you meant by “longest time path”? That sounds counterintuitive to me.

    In this video, and in recent ones, you referred to how the designation of “forces” is really just a vernacular shorthand and that the deeper you go into studying physics the less meaningful the term “force” becomes, that quantum specialists in particular talk in terms of fields, not forces. So, then, what are GUTs unifying exactly, just those alpha constants?

    Finally, and this is neither here nor there, and as a physicist maybe you don’t care, but as I presume you value naturalness and elegance I wonder if you take a position on the pi vs. tau issue? I see you using pi without comment, but maybe that’s just by convention, not wanting to add needless complication in an explanatory setting?

  19. William H Harnew

    Wonderful video. I very much appreciate you walking through the concepts and related equations. It is a great teaching. At least one very good thing has come from the present difficulties. Now for some confusions.
    1. “The more you move in space, the less you move in time”… I think this is a key concept that I’m failing to grasp. Perhaps everyone else got it. Any other discussion would help.
    2. Related (I think) is when you say ” As Minkowski taught us, Tau^2 = t^2 – x^2″ where Tau is the “time elapsed.
    3. Also related (I think) is the Spacetime Interval Equation that has a first term of -dt^2… then, the -1 in the Metric g mu, nu matrix…. etc. Any further discussion of the relation between the two would be helpful. (I very much enjoyed the Schwartzschild explanations.)
    4. Finally, in an expanding universe, that matrix term turns positive +1…. Yikes!
    5. I’m assuming you will do a video on Quantum Gravity? Does the GRW approach best adapt GR (because it’s a “gravitational collapse” model?) And is there any useful discussion/explanation for the mismatch of the QM prediction for Lambda and the currently measured values?

  20. Excellent talk.
    About the 22 minute point, you comment that gravity is not really a force it is a feature of the curvature of spacetime. I agree, and would be interested in hearing your response to a question about other “forces”. If quantum fields are considered fundamental why can the other “forces” not be considered as due to curvature (or whatever term is appropriate) of the corresponding quantum field? Why, for example, does the electromagnetic force need the photon as a force carrier rather than just the result of the curvature of the field?

  21. One thing I’d like to understand better about GR is its relation to Mach‘s principle, which states that there is no absolute motion, so in particular the effect of rotation on an object (which we can measure as a centrifugal force) would only exist because the object is rotating relative to other objects (like the earth, or distant galaxies). I think Einstein himself was a big fan of this idea, and even used it as a guiding principle to find GR. But I think that it is actually not fully satisfied in GR, because the metric itself is a „thing“, relative to which we can e.g. observe rotation, even if there are no other masses creating the background metric (like e.g. Minkowski spacetime). Also, I don’t think (but am not sure) that setting a distant matter distribution into rotating motion would at some later point lead to the same centrifugal force for a test object at the origin that would be identical to the one you would measure if only the object was rotating, but not the distant masses. (There might be partial effects due to frame dragging, but I suspect they would still be different.)
    Would you agree, and what are your thoughts about GR & Mach’s principle?

  22. Najdan Arpadzic

    Hello, I have somewhat connected questions (i appoligize if they are nonsensical):

    1. Would it be wrong to use the term “streched” instead of “curved” for space? For example: object that is traveling near some planet is affected by planets gravity field, thus it is experiencing pull towards its center of mass. Could that be interpreted as if the object is just following the path of least action because the space is more streched (less dense) in the direction pointing to the center of planet then in the opposite direction?

    2. Is the gravity just the effect of the (localy) weakened higgs field? To clarify: if higgs field is “slowing down” matter (while giving it mass), could it be that it is weakened in that particular area of space? So, because it (higgs field) has distributed much of its local energy to slow massive particles, the rate at which it is slowing them down gets lower localy, making the space “less dense” the more we get to center of mass, giving arise to an effect which we call curvature of space – gravity? P.S. As in the example from the first question, the object affected by gravity is just following the path of least resistance/action, as higgs field is weaker in the direction facing the center of mass then in the opposite direction.

  23. I know that you have recently become more interested in statistical mechanics aspects of physics. I wonder if you would spend an hour or so on what may or may not turn out to be a great idea: induced gravity. By this I refer to Sakharov’s idea of a shift in the zero point energy of quantum fields, Ted Jacobson’s thermodynamic derivation of GR, and the entropic gravity ideas of Padmanabhan and Vilekin.

  24. The effects of gravitational waves from unimaginably distant black hole collisions that LIGO detects are minuscule but what effect would they have close to the source.
    Theorists are looking for the way to quantize gravity in order to understand its effects on the tiny scales that QM deals with (aren’t they?) but if gravity is so very weak compared to the forces at work in QM, then aren’t its effects negligible on that scale?

  25. William H Harnew

    I really enjoyed the Schwarzschild explanations. Could you do something similar for FRIEDMANN (open, closed, flat spacetime) and the expanding universe ? That would be fantastic.

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