Relativity & Gravity: Resolving the Discrepancy

• B
• Hornbein
In summary, Sabine Hossenfelder says that gravity is not a force and has no effect on time. However, proper time can be affected by gravity even if an object is in free fall.
Hornbein
TL;DR Summary
I listened to and quite liked Sabine Hossenfelder's recent vid on relativity. Though one thing struck me as wrong. "Gravity is not a force," so it's effects does not cause true acceleration, so it has no effect on time. OK, but I'm pretty sure that satellites in different orbits have different timings. I once read that circular and elliptical orbits have different timing. How can this discrepancy be resolved or am I just wrong.

Hornbein said:
"Gravity is not a force," so it's effects does not cause true acceleration, so it has no effect on time.
Can you give a link to the video where she said that? I have difficulty believing that she would get a basic fact wrong. I just watched her video "Is Gravity a Force" and didn't see that statement about time. Did she say that somewhere else or is that your conclusion?

arturwojciechowicz

At 18:36 "It's not gravity that causes time to slow down because gravity is not a force. It's the acceleration that you need to not fall in the black hole that slows time down....The reason time slows down is acceleration."

It seems to me that this isn't exactly wrong but is highly misleading. She is talking about the context of being at rest relative to a gravitating body. OK, but neglects mentioning that proper time will be affected by said body even if you are in free fall. In such a case I would say that gravity is affecting proper time. The context has not been explicitly stated.

vanhees71 and FactChecker
I will bow out and leave this to the experts.

Hornbein said:
OK, but neglects mentioning that proper time will be affected by said body even if you are in free fall.
I think she talks about the local context, where space-time curvature is negligible and the gravitational field can be considered uniform. A free falling elevator here will accumulate the same proper-time for clocks at the floor and ceiling, while a hovering elevator won't.

But in general I find arguments about "causation" tricky. I prefer to say that the geometry of space-time "causes" both: the attraction we call gravity and the gravitational time dilation.

Dale, vanhees71 and PeroK
Oh wow. That's a mess. Just after 18.00 she says that clocks run slower at sea level than on a mountain top because the acceleration is higher there. But that's wrong. If it were right, gravitational time dilation would fall to zero at the center of the Earth, and that's not what GR predicts. GR predicts that differences in clock rates in gravitational fields (at least in cases where it can be defined) are due to differences in gravitational potential, not acceleration. Acceleration will fall to zero at the center of the Earth; potential difference between there and infinity will continue to increase; therefore clocks will tick slower even as the acceleration needed to hover falls to zero.

And "acceleration causes time to slow down" manifestly isn't true in general. I have a nasty suspicion that she justifies that earlier in the video with reference to the twin paradox. If so (I haven't watched it), she's wrong about that too.

Ugh. This is why I don't watch popsci about anything I know anything about. I always end up wanting to write ten page essays correcting it.

physicsworks, Sagittarius A-Star, LittleSchwinger and 9 others
Hornbein said:

At 18:36 "It's not gravity that causes time to slow down because gravity is not a force. It's the acceleration that you need to not fall in the black hole that slows time down....The reason time slows down is acceleration."

It seems to me that this isn't exactly wrong but is highly misleading. She is talking about the context of being at rest relative to a gravitating body. OK, but neglects mentioning that proper time will be affected by said body even if you are in free fall. In such a case I would say that gravity is affecting proper time. The context has not been explicitly stated.

This is why such videos are not suitable material for discussion of GR on here. You are attempting to understand physics by getting the right words in the correct order. Physics is only unambiguous when it gives a precise mathematical model and quantitative answers to specific questions.

hutchphd, vanhees71 and martinbn
PeroK said:
This is why such videos are not suitable material for discussion of GR on here. You are attempting to understand physics by getting the right words in the correct order. Physics is only unambiguous when it gives a precise mathematical model and quantitative answers to specific questions.
OK, then where am I supposed to discuss questions like this?

You are telling me that I am not welcome in this subgroup. What is the majority opinion on this? Is attempting to understand physics without math banned here?

Feynstein100 and fresh_42
Ibix said:
Oh wow. That's a mess. Just after 18.00 she says that clocks run slower at sea level than on a mountain top because the acceleration is higher there. But that's wrong.
Yes, that part is clearly wrong.

If you had two local elevators, each with ceiling and floor clocks, at the mountain top and bottom, then you could draw a connection between proper acceleration of an elevator and the proper time delta between its two clocks.

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Dale and vanhees71
Hornbein said:
Is attempting to understand physics without math banned here?
PeroK said:
"Whoever then has the effrontery to study physics while neglecting mathematics must know from the start that he will never make his entry through the portals of wisdom."

Roger Bacon (1214-84)

vanhees71 and haushofer
You did not answer my question. I find your communications patronizing to a degree that I find insulting. Do you understand what I am saying?

malawi_glenn, Motore, weirdoguy and 2 others
Hornbein said:
You did not answer my question. I find your communications patronizing to a degree that I find insulting. Do you understand what I am saying?
It's impossible to understand GR without mathematics. There's nothing patronising about that. It's a plain fact.

vanhees71, weirdoguy and Frabjous
Hornbein said:
Is attempting to understand physics without math banned here?
It's doomed to failure, certainly. There's simply no way to express all the interactions and limitations in things you think you know without the maths. Ultimately, that's why physicists use maths to describe these phenomena - it's not really a matter of choice if you want to actually understand, rather than know some disconnected facts.

I mean, I'm pointing out errors in the video which wouldn't even be possible if she had used maths. I'm not using maths myself, so there are limitations in things I'm saying from other factors that come in to play if you change the scenarios (sometimes very little) from the one I'm thinking of. I can't even begin to tell you what they all are because I can't list every possible variation of a scenario - but it's all encoded in the maths.

vanhees71 and PeroK
Fundamentally what "causes" time dilation is different path lengths for different clocks between a pair of specified spacelike planes, always, no exceptions. The only real difference between different circumstances is whether there are zero, one, or many ways to specify those hyperplanes in a reasonable non-arbitrary manner and the details of what those planes are and what paths clocks take, and how many unexamined assumptions there can be hidden in your analysis.

Gravitational time dilation is a useful special case where there's only one sensible way to specify planes, which allows you to separate out gravitational time dilation from kinematic time dilation. We often use this model for spacetime near Earth. But it's dangerous to generalise statements about "gravity" made near Earth to other scenarios without understanding (a) why we use this model near Earth, (b) why it's not correct to do that, (c) why you can get away with it anyway, and (d) a rough understanding of when and why it's all going to go wrong. And it's hard-to-impossible to express all those limits without maths.

Noting @A.T.'s post just above, I've seen visualisations of things from Epstein's book (I believe A.T. has animated a few himself). I certainly find such visualisations useful, but would have little confidence in my ability to actually manipulate anything more complex than a Minkowski diagram without understanding the maths.

Hornbein said:
You did not answer my question. I find your communications patronizing to a degree that I find insulting. Do you understand what I am saying?
I understand your frustration. And personally I think you can come far understanding these conceptual issues without diving too far into the mathematics. So if I take your quote,

""Gravity is not a force," so it's effects does not cause true acceleration, so it has no effect on time."

I would say the following:

- Indeed, gravity is not considered to be a force in GR. But that's mainly a matter of speaking terms. In GR, particles follow the shortest path (geodesics) between two events when there are no forces acting on it. In curved spacetime these geodesics curve along.
- I'm not sure what "true acceleration" means, but I think she says that in GR it's ambiguous to talk about "the acceleration due to gravity" because of the equivalence principle: locally, an acceleration can be interpreted as a gravitational field. So that makes the notion of "the acceleration due to gravity" a bit complicated and is the reason why gravity is not considered to be a force in GR: a force is something which results in an acceleration.
- The last conclusion, "hence gravity has no effect on time", I do not follow. I'd say that yes, curved spacetime certainly does result in time dilation, and I don't get how she draws this conclusion in the first place from the premisse.

Hope this helps.

PeroK
A. Arguing with people by proxy is futile.
B. Some people believe that nobody really disagrees with them. Everybody secretly agrees with them - they're just too dishonest to admit it. Arguing with them is futile. Arguing with them by proxy is especially futile.
C. Bloggery anf Youtubery does not make one an expert.

PhDeezNutz, vanhees71 and martinbn
Sabine is not a snake-oil seller, and I would be very cautious about underestimating the Youtube video.

But also, she speaks to the general public, and perhaps she has to use some artist license to get to the general public, and would use other wording if she had to write a post in this forum.

Hornbein said:
then where am I supposed to discuss questions like this?
Here is fine. The issue is not where you posted this thread.

Hornbein said:
You are telling me that I am not welcome in this subgroup.
No, he is telling you, correctly, that the video you referenced, even though it is by a physicist, is still a pop science source and is not a good source from which to learn the actual physics.

Hornbein said:
Is attempting to understand physics without math banned here?
No. But I think you will find it a lot more difficult and the understanding you will be able to achieve will be a lot more limited than if you did it with math.

To make just one obvious point: even if you give a qualitative, non-mathematical explanation for something, how do you know it's right? How did the physicists who built the theory you are using know it was right? The only way to do that is to make detailed, quantitative predictions and test them using detailed, quantitative experiments. That requires math.

Dale, Vanadium 50, vanhees71 and 2 others
PeterDonis said:
To make just one obvious point: even if you give a qualitative, non-mathematical explanation for something, how do you know it's right?
Prestige, and she also sings

haushofer and malawi_glenn
Lluis Olle said:
Sabine is not a snake-oil seller, and I would be very cautious about underestimating the Youtube video.
This is actually the problem. She's well known and widely respected - yet completely wrong in at least one thing she says in the one minute of her video that I watched.

PhDeezNutz, Dale, aaroman and 1 other person
Ibix said:
But that's wrong. If it were right, gravitational time dilation would fall to zero at the center of the Earth, and that's not what GR predicts. GR predicts that differences in clock rates in gravitational fields (at least in cases where it can be defined) are due to differences in gravitational potential, not acceleration. Acceleration will fall to zero at the center of the Earth; potential difference between there and infinity will continue to increase; therefore clocks will tick slower even as the acceleration needed to hover falls to zero.

If the Earth is considered a perfect sphere, the Gravitational potential at the center of the Earth will be the same as the Gravitational potential at the surface. The time dilation from a clock at the center of the Earth to infinite will be the same as the clock at the surface to infinite.

Sabine is right.

jbriggs444, Dale, PeroK and 2 others
You think the earth is a shell with vacuum inside?

Dale, Vanadium 50, member 728827 and 2 others
Lluis Olle said:
If the Earth is considered a perfect sphere, the Gravitational potential at the center of the Earth will be the same as the Gravitational potential at the surface.
If that were the case you'd be in free fall inside the Earth, since ##\vec g=\vec\nabla\phi## would be zero everywhere.
Lluis Olle said:
The time dilation from a clock at the center of the Earth to infinite will be the same as the clock at the surface to infinite.
Feel free to look at the Schwarzschild interior metric (here's my Insight on the topic). It's trivial to see from the ##g_{tt}## term that you are wrong.

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Jarvis323 said:
Sorry for posting ChatGPT stuff here, just found it an interesting case study:
ChatGPT is not an acceptable source for PF. Do not post further ChatGPT content as it will just continue to get you warned, and eventually banned.

Dale, aaroman, Ibix and 2 others
Lluis Olle said:
If the Earth is considered a perfect sphere, the Gravitational potential at the center of the Earth will be the same as the Gravitational potential at the surface. The time dilation from a clock at the center of the Earth to infinite will be the same as the clock at the surface to infinite.
No, it won't. For the case of a constant density Earth the exact solution is known and does not have this property--the clock at the center runs slower than the clock at the surface. But even for the case of non-constant density, the Einstein Field Equations for any static, spherically symmetric object is sufficient to prove that gravitational potential continues to decrease as you go deeper inside the object.

Lluis Olle said:
Sabine is right.
I don't think Sabine is making the same wrong claims you are making.

Dale, member 728827, PeroK and 2 others
Hornbein said:
Is attempting to understand physics without math banned here?
It is not banned, it is doomed. Physics is explaining and understanding nature using mathematics. The languange of physics is math.

aaroman, Dale, pinball1970 and 2 others
I like that she begins by saying popular science descriptions of relativity left her confused as a teenager. Then, she went on to get doctorate in theoretical physics. Now it is her turn to try a popular exposition, she says. I guess we see that she could no more avoid all the pitfalls than anyone else.

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PhDeezNutz, Dale, dsaun777 and 6 others
There is a link to transcript, references and discussion on her blog. I thought it would be easier and faster to read it than to watch the video. But the link takes me to the subscription page and expects me to pay. And it is not the only thing that is for money. I get the impression that she tries to sell everything. For instance she has "talk to a scientist" option for \$50 per 20min. I understand that she wants to be paid for the her efforts, but it is very off putting.

dextercioby, malawi_glenn and weirdoguy
martinbn said:
I understand that she wants to be paid for the her efforts, but it is very off putting.
perhaps it is language and cultural, after all, she is german

malawi_glenn said:
perhaps it is language and cultural, after all, she is german
Well, there are other germans that have blogs, vlogs, websites where the put there writings/videos (popular or not) etc. but don't ask for a credit card for every other click.

Hornbein said:
You did not answer my question. I find your communications patronizing to a degree that I find insulting. Do you understand what I am saying?
I was in your position. I bought all the wrong books and watched all the wrong videos.
I learned ABOUT some physics and about the history of physics but that's it.
I still yearn for non mathematical explanations sometimes but I try and keep them at bay.
That will not get me anywhere.

Dale, PeroK, weirdoguy and 1 other person
I find relativity to be a tricky one to understand even for other non relativity physics tutors. I have seen various youtube physics channel's which are generally reputable reporting on the twins paradox and claiming the "time difference" all occurs during the deceleration / acceleration phase when they turn around at the other end instead of being caused by the change of reference frame. Had it not been for reading this forum I would have believed that was the case also. Sadly hearing Sabine make the same reference is not at all surprising.

I have learnt basic concepts from watching youtube video's from "reputable" channels but had it not been for the use of this forum to get some "meat on the bones" of the claim and the finer details I would never have gained the understanding I now have.

berkeman, PeroK, pinball1970 and 1 other person
Ibix said:
I always end up wanting to write ten page essays correcting it.
You can do this for only 2 € / Month (+ VAT). But I don't think, that she published this error intentionally to get money from you.

Unfortunately, I see the same error also in other publications, even in otherwise good textbooks.

Example page 32, chapter 3.5.2 Time dilation (my translation from German to English):
A clock that moves with the satellite runs slower than a clock that is stationary on the earth. On the other hand, it runs faster than this one because it is in a weaker gravitational field.
Source - book "Spezielle Relativitätstheorie" (Schröder):
https://www.amazon.de/dp/3808556536/

Ibix
She definitely isn't alone in making the mistake. But in your textbook example you could at least argue that "weaker gravitational field" is talking about depth in the potential well, unless that's stated to mean the acceleration due to gravity elsewhere (or it's unambiguous in German). Depth in the potential well would be correct, if not clearly stated as "strength" of a gravitational field. The original video explicitly equates acceleration with time dilation, which is wrong.

vanhees71

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