Gravity is Not Intuitive

I agree. The idea when I jump off a chair I am moving across curved space-time is not intuitive at all. Newton’s description was much more intuitive yet did not account for planetary orbits.

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You don’t have to jump off a chair to move across curved space-time. Just sitting there you are moving in the “time” direction at the speed of light. Indeed, you are always moving in spacetime, and it is impossible for you to stop!

Everything moves at the speed of light in spacetime. Light just uses as much of this velocity as possible to move in space, while typically objects use most of this speed to move in the time direction instead.

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Does that have to do with why time slows down as you approach the speed of light, traveling within space?

Yup, it absolutely has something to do with time dilation!

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Isn’t this question (about whether gravity is really a force or not) something of an interpretation of the data over and above what the experimental evidence, and even what the mathematics, says? Sure, the orthodox interpretation of GR is that gravity is nothing more than spacetime curvature. But GR hasn’t been reconciled with quantum theory, and it may well turn out that the correct theory of quantum gravity makes gravity out to be a force (in the same way as electromagnetism and the weak and strong forces).

There’s even a reformulation of GR (or at least, GR of a restricted class of manifolds) by Julian Barbour that breaks spacetime back down to space (as a 3-dimensional entity) evolving over time, so that “spacetime” would just be a mathematical description, not corresponding to what actually exists. (Barbour actually thinks this formulation is “timeless”, but the same mathematical structure can be easily used by someone who is a philosophical “A-theorist” about time.)

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Of course! I am answering from the point of view of currently known physics. Bear in mind that it might also be possible that everything is spacetime curvature and nothing is “force” in the traditional sense. Indeed this avenue seems to be promising from the point of view of string theory.

I am aware of Barbour’s formalism, but have never actually read any of its mathematical details, so I can’t say much about it.

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By the way, I know time from the physics perspective but don’t really follow the philosophical literature. Therefore, I never grasp the issue of the A/B-theory in the philosophy of time, or their philosophical context and the debates between their proponents. Perhaps this is something that @structureoftruth can clarify/discuss in a different post. I am sure there are others in this forum that would find this interesting.

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Hey Dr. Josh and all, sorry for the radio silence from the forum the past couple of months, some family illness issues, but thankfully things are better.

I love the cross-disciplinary science questions… we need more of this in the sciences, a bit like it used to be when scientists were generalists.

It’s a great question and see some great answers below. At the same time, scientists have questioned whether gravity does work the same at all scales:
https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

As some thought MOND might be an alternative to Dark Matter to explain galaxy behavior.

But support for these theories has mostly evaporated with new experiments and data, including some that show that gravity is the same down to .1 nanometers:
https://www.iflscience.com/physics/scientists-have-measured-gravity-at-an-extremely-small-distance-and-it-could-reveal-the-secrets-of-extra-dimensions/

“Based on the scattering, they found that no unexplained force was acting on the neutrons below 0.1 nanometers. In other words, this appeared to be the lower limit at which gravity works.”

Love to see the cross-disciplinary questions coming! Due family health issues, would also love to see more focus on this forum to the role Christian scientists play in coming up with practical solutions to problems facing modern humanity, including health issues (not just discussions about the past and/or theological battles).

Often it seems solutions come out of nowhere when geneticists start talking more with physicists and vice versa =)

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Unless there is an anti-graviton out there to be found? =)

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Interestingly enough, there are a couple of studies going on at places like CERN at the moment that are trying to find out whether or not matter and antimatter are affected in the same way by gravity.

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Yep, this is what I was referring to. It’s quite amazing how hard it is to create antimatter particles, trap them, release them and then just see whether they fall down to the ground. In my lab (some of whom are working on ATRAP, which is a competitor to the ALPHA experiment described in the article), just making atoms of antihydrogen has been a 30-year quest.

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Isn’t this more of an issue of semantics? Why can’t you identify “force” with “curved spacetime”? This seems to me like the question of whether fields are real or just useful calculational tools to approximate particles mediating interactions. Yes, gravity is different that there is no verified theory of quantum gravity, many people believe that such a theory should exist. If it didn’t, then why do we still keep thinking of gravity as one of the four interactions in nature?

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This is a good question.

Here is the easy answer: In GR, force refers to the stuff you put on the right hand side of the geodesic equation to make the particle motion deviates from a geodesic. A particle that is just following its geodesic is not under the effect of any force, and is just moving in accordance to the geometry of spacetime. In this way there is a canonical way to separate things that are “force” and things that are “curved spacetime”.

Here is the more complicated answer: The “forces” in QFT are really “interactions”. In this picture, “forces” are built up out of interacting fields in the Lagrangian. GR can be derived this way: it was proven that interacting spin-2 fields in the Lagrangian produces exactly the Einstein Field Equation if the fields are massless. In this sense, GR has been reduced to “interactions” the same way as E&M, weak, and strong forces. One could then say that GR is a “force” because it is an “interaction” much like the other things we call forces. Crucially, note that in this view the background spacetime remains flat.

In the end, as you pointed out, it boils down to semantics and really depends on what you call “forces”. A relativist will surely say that gravity is not a force, but a QFT person might say that it is, as it is an interaction much like the other forces.

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Thanks, this was useful and made me reread Caroll’s book (section 3.4) where he introduces the geodesic equation and yes, he actually does say that

the geodesic equation can be thought of as the generalization of Newton’s law f = ma, for the case f = 0, to curved spacetime. It is also possible to introduce forces by adding terms to the right-hand side…

He gives as an example the Lorentz force, which is a non-gravitational interaction, so I guess you can indeed differentiate non-gravitational and gravitational interactions formally. My reflex response was, “Why can’t you keep the right hand zero and subsume the force into the metric instead?” But I guess this is precisely what you were saying in that some string theorists are trying to reinterpret E&M in that way!

Yup! Note that the fact that E&M can be written as a metric is not a string theory phenomenon, but was known in Einstein’s days (circa 1920s). The question of whether the other forces can be interpreted this way was a big motivation for string theory.

I find it remarkable that there seems to be quite a bit of debate about Gravity, even among @physicists. Yet another way that gravity is not intuitive…

Haha, is gravity special in this? After studying physics for 10 years, I feel that my intuition is super useless :rofl:

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Ah - this was Kaluza-Klein theory right? Now this is starting to make more sense…

I think it’s a reflection that physics is hyper-specialized, so with the exception of having taken a graduate class in GR two years ago (which I never ever used again), my intuition about gravity is not much different than as a physics major in college. I’m sure that the general physics intuition I’ve developed in graduate school does help, but the technical aspects of GR I have mostly forgotten. Someone who actually uses GR in their research everyday will have better intuition on what gravity exactly is.

Also even in these graduate classes, we often focus too much on solving problems and understanding many technical details, which are probably of interest to budding theorists wanting to specialize in the area, but not so much for physicists in general. There is little time spent on the big picture - how the content of different sub-fields of physics (e.g. E&M and gravity in this case) can connect in a deep way. Or even a big picture view of GR itself. Instead I remember spending a lot of time in class doing very heavy-duty calculations of Einstein tensors from the metric by hand, which is mentally taxing but not very insightful about understanding gravity at a conceptual level.

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That’s right. Note that while the wikipedia article is historically correct in referring to the E&M+gravity spacetime theory as the Kaluza-Klein theory, physicists these days have taken to calling any gravity+(other interactions/forces) spacetime theory as a Kaluza-Klein theory.

I think this is a big problem in physics education. I believe that the issue is that most physicists do not enjoy teaching, and it is much easier to teach students by giving them a bunch of problems to solve, especially if the professor can just take the problems from a textbook, than composing a compelling and clear lecture.

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Is there a reason for your interest to focus the forum to the role Christian scientists are playing in coming up with practical solutions to problems facing modern humanity? Are the secular scientists not doing enough? or not to your liking? Perhaps this forum can be interested in what role ALL scientists are playing in helping humanity independently of whether they are Christian or not.

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