# Sabine Hossenfelder says time dilation is due to acceleration

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• Epic Mythology
In summary, Sabine Hossenfelder states in the video that time dilation in the twin's paradox is actually differential ageing caused by acceleration. However, looking at the equations for time dilation, it is clear that time dilation is actually caused by velocity, not acceleration. While acceleration is necessary for differential ageing to occur, the amount of differential ageing is directly related to the velocity profiles of the clocks involved, not the acceleration profiles. Therefore, it is not accurate to say that acceleration causes time dilation.
Epic Mythology
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?

At 12 minutes into this video ,

Hossenfelder states, "This is the real time dilation. It comes from acceleration."

Looking at the equations for time dilation, time dilation comes from velocity, not acceleration.

How can Hossenfelder state, "This is the real time dilation. It comes from acceleration."?

OmCheeto
Without having seen the video: take two observers A and B. As long as their velocities are constant (i.e. their mutual velocity is constant), the principle of relativity holds: as observers they are symmetric, and hence both will measure time dilation at each others clocks. If the situation wasn't symmetric, they could detect "who really moves", which is prohibited by the principle of relativity: all inertial observers are equivalent/no absolute motion exists. If you want to ask "who really ages the most", you have to break that symmetry. You do that by bringing A and B together to let them compare their clocks. At least one of them has to change her velocity for that. I.e.: she has to accelerate. Say, we choose for this observer A. Accelerating, her wordline will in general have a different length in spacetime as compared to B. And this worldline length is her elapsed proper time (and the same for B).

So in that sense I can understand Hossenfelder's statement.

dsaun777 and FactChecker
haushofer said:
Without having seen the video: take two observers A and B. As long as their velocities are constant (i.e. their mutual velocity is constant), the principle of relativity holds: as observers they are symmetric, and hence both will measure time dilation at each others clocks. If the situation wasn't symmetric, they could detect "who really moves", which is prohibited by the principle of relativity: all inertial observers are equivalent/no absolute motion exists. If you want to ask "who really ages the most", you have to break that symmetry. You do that by bringing A and B together to let them compare their clocks. At least one of them has to change her velocity for that. I.e.: she has to accelerate. Say, we choose for this observer A. Accelerating, her wordline will in general have a different length in spacetime as compared to B. And this worldline length is her elapsed proper time (and the same for B).

So in that sense I can understand Hossenfelder's statement.
So are you agreeing with Hossenfelder's statement that "This is the real time dilation. It comes from acceleration." What are the physical equations that show acceleration causes time dilation?

Epic Mythology said:
So are you agreeing with Hossenfelder's statement that "This is the real time dilation. It comes from acceleration." What are the physical equations that show acceleration causes time dilation?
We had a long discussion about this before. What Hossenfelder is calling "real time dilation" is what's usually called "differential aging". It isn't really due to acceleration - it's perfectly possible to construct scenarios where two objects undergo the same acceleration but end up with different ages.

jbriggs444, phinds and martinbn
Ibix said:
We had a long discussion aboutthis before. What Hossenfelder is calling "real time dilation" is what's usually called "differential aging". It isn't really due to acceleration - it's perfectly possible to construct scenarios where two objects undergo the same acceleration but end up with different ages.
So then you disagree with Sabine Hossenfelder who says time dilation is due to acceleration in the twin's paradox, as she states in the video: "This is the real time dilation. It comes from acceleration."

Epic Mythology said:
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?

At 12 minutes into this video ,

Hossenfelder states, "This is the real time dilation. It comes from acceleration."

Looking at the equations for time dilation, time dilation comes from velocity, not acceleration.

How can Hossenfelder state, "This is the real time dilation. It comes from acceleration."?

There are two concepts that are subtlely but significantly different: time dilation and differential ageing.

Time dilation is most definitely not caused by acceleration in any meaningful way.

There is, in fact, a version of the twin paradox without acceleration. It's on the Wikipedia page, I believe.

In flat spacetime, differential ageing requires some sort of real (proper) acceleration. The amount of differential ageing, however, is directly related to the velocity profiles of the clocks involved. And only indirectly relates to the acceleration profiles. Saying acceleration causes differential ageing is a It's a bit like saying acceleration earns you a speeding ticket!

The Bill
PeroK said:
There are two concepts that are subtlely but significantly different: time dilation and differential ageing.

Time dilation is most definitely not caused by acceleration in any meaningful way.

There is, in fact, a version of the twin paradox without acceleration. It's on the Wikipedia page, I believe.

In flat spacetime, differential ageing requires some sort of real (proper) acceleration. The amount of differential ageing, however, is directly related to the velocity profiles of the clocks involved. And only indirectly relates to the acceleration profiles. Saying acceleration causes differential ageing is a It's a bit like saying acceleration earns you a speeding ticket!

So then you disagree with Sabine Hossenfelder who says time dilation is due to acceleration in the twin's paradox, as she states in the video: "This is the real time dilation. It comes from acceleration."

Epic Mythology said:
So then you disagree with Sabine Hossenfelder who says time dilation is due to acceleration in the twin's paradox, as she states in the video: "This is the real time dilation. It comes from acceleration."
Yes. In flat spacetime someone has to accelerate for twins to meet twice, but the effect depends on the velocity and how much time is spent at different velocities, not on acceleration. Working in inertial coordinates, the elapsed time for one twin is ##\int\sqrt{1-v^2(t)/c^2}dt##. Do you see any acceleration term in there?

lightarrow
Epic Mythology said:
So are you agreeing with Hossenfelder's statement that "This is the real time dilation. It comes from acceleration." What are the physical equations that show acceleration causes time dilation?
There are other versions of the Twins paradox that makes it hard to say that acceleration causes time dilation.
It's probably better to say that acceleration violates the assumptions that allow the simplest calculation. In the usual scenario of the Twins paradox, it is possible to take acceleration into account in a more complicated calculation and get the answer that agrees with the simple calculation of the "stationary" twin.

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Epic Mythology said:
Is Sabine aware that she is wrong? Her videos reach millions.
She must understand relativity. The problem is an "explanation" that happens to give the right answer in one simple example and not thinking through all the implications for other circumstances. Which we do think through because we spend our spare time fielding questions inspired by such bad explanations...

A huge problem with all popsci is oversimplification.

PhDeezNutz and martinbn
Epic Mythology said:
How can Hossenfelder state, "This is the real time dilation. It comes from acceleration."?
She can state it because she is making a pop-sci video which has no peer review and so she can get away with saying whatever she feels like.

It is not a coincidence that such videos are not considered suitable for answering questions on this site (although they certainly produce many questions)

Imagine that you have a triangle with vertices ##A##, ##B##, and ##C##. Now, we know from the triangle inequality that ##AB+BC>AC## but we do not say that the extra length “comes from” the bend at ##B##. The fact that the path ##ABC## is bent at ##B## allows us to distinguish between it and path ##AC##, it breaks the symmetry, but the extra length comes from the whole path, not just the bend at ##B##.

Similarly, with relativity proper acceleration is a bend in a worldline, and proper time is the length of a world line. So the extra length (time) doesn’t come from the bend (acceleration).

lightarrow, haushofer and martinbn
Dale said:
She can state it because she is making a pop-sci video which has no peer review and so she can get away with saying whatever she feels like.

It is not a coincidence that such videos are not considered suitable for answering questions on this site (although they certainly produce many questions)

Imagine that you have a triangle with vertices ##A##, ##B##, and ##C##. Now, we know from the triangle inequality that ##AB+BC>AC## but we do not say that the extra length “comes from” the bend at ##B##. The fact that the path ##ABC## is bent at ##B## allows us to distinguish between it and path ##AC##, it breaks the symmetry, but the extra length comes from the whole path, not just the bend at ##B##.

Similarly, with relativity proper acceleration is a bend in a worldline, and proper time is the length of a world line. So the extra length (time) doesn’t come from the bend (acceleration).
You write, "She can state it because she is making a pop-sci video which has no peer review and so she can get away with saying whatever she feels like."

Do you believe that influential and popular public science educators have a duty to speak the truth and be correct?

Epic Mythology said:
Do you believe that influential and popular public science educators have a duty to speak the truth and be correct?
The problem is that "being correct" means using maths. And then nobody reads it.

PhDeezNutz
Ibix said:
The problem is that "being correct" means using maths. And then nobody reads it.
So basically it is better to say things that are not true than to say things that are true, just as long as one uses no maths?

Epic Mythology said:
You write, "She can state it because she is making a pop-sci video which has no peer review and so she can get away with saying whatever she feels like."

Do you believe that influential and popular public science educators have a duty to speak the truth and be correct?
I do. And I think that she and Brian Greene in particular frequently fail to fulfill that duty.

Other pop sci authors do make mistakes or poor explanations, but those two in particular are notoriously bad.

Anyway, perhaps we can focus on the actual physics rather than the personal criticisms?

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jbriggs444
Epic Mythology said:
So basically it is better to say things that are not true than to say things that are true, just as long as one uses no maths?
I don't write popsci. And there's plenty of room for people giving poor explanations, or ones of only limited validity, without the ethical failing you seem to be implying.

dsaun777, hutchphd and vela
Epic Mythology said:
Is Sabine aware that she is wrong? Her videos reach millions.

You can try to make her aware for only 2€/Month (plus VAT):
Sabine said:
Just here to let me know you like my videos and you want to see more? Then that's the tier for you.
https://www.patreon.com/Sabine?fan_landing=true

A similar, old discussion on her (free) backreaction blog was closed.
Sabine said:
COMMENTS ON THIS BLOG ARE PERMANENTLY CLOSED. You can join the discussion on Patreon.
Source:
https://backreaction.blogspot.com/2...omment=1378658543377&m=1#c1341223111415144705

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Epic Mythology said:
So then you disagree with Sabine Hossenfelder who says time dilation is due to acceleration in the twin's paradox, as she states in the video: "This is the real time dilation. It comes from acceleration."
I don't have time to look at the video. In any case, the twin paradox is technically about differential ageing; not time dilation.

Epic Mythology said:
Do you believe that influential and popular public science educators have a duty to speak the truth and be correct?
It makes no difference what I consider to be "correct". They write what they write and say what they say. Nothing I think or say can change that.

But, if you want to learn science as an academic subject (or to the point where you can solve problems yourself), you have to put aside the popular science books and videos. Note that in this case, if you understood SR, then you would be able to form your own opinion about SH's video.

Epic Mythology said:
Is Sabine aware that she is wrong? Her videos reach millions.
Give her a call and see if she listens to you.

Epic Mythology said:
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?
To pile on in agreement w/ what has already been said, NO, it is not true and you would do well to stop paying attention to pop-sci presentations and study the actual physics.

Pop-sci presentations at their best are attempts to INTEREST people in science, not TEACH them science. There is a huge difference. And that's pop-sci at its best. At the more normal end, the goal is simply to make a buck by selling books or whatever.

malawi_glenn said:
Give her a call and see if she listens to you.

She will probably listen:
Sabine said:
Talk To A Scientist
...
To set up an appointment, please send an email with the topic(s) that you are interested to discuss to expert.science.consult@gmail.com and wait for a response email containing a formal introduction of the service terms. Typical response times are 1-5 business days.

Live session terms vary with each consultant in our network. Fees can vary accordingly, but are approximately US\$50 per 20 minutes. The final offering depends on the format of the consultation.
Source:
https://backreaction.blogspot.com/p/talk-to-physicist_27.html

PeroK
OP, there seems to be a lot of hostility in this and your recently closed thread. What exactly is your goal?

If it;s to tell us that somebody is wrong on the internet, this is not a surprise. That ship sailed a long time ago.

If it's to learn relativity, the best way is to stop watching Youtube videos, pick up a copy of Taylor and Wheeler and start working through it.

PhDeezNutz, martinbn, PeroK and 1 other person
Sagittarius A-Star said:
I sometimes tell my students that if they want a reply for free, they can send me an email - but I will only respond when I have time.
If they want a quick reply, they should send money to me via bank-transfer and ask the question in the message box for the transfer - then I will answer right away!

PhDeezNutz and Sagittarius A-Star
I agree that she's wrong to say that the twin effect is solely due to acceleration, but I think what Sabine might be saying is that:
Given two time-like separated events, where observers are present at both events, then the proper time measured along the world-line of a non-inertial observer is generally less than the proper time between the events as measured along the world-line of an inertial observer.
Just my 2 pennies

PeroK
DAH said:
Given two time-like separated events, where observers are present at both events, then the proper time measured along the world-line of a non-inertial observer is generally less than the proper time between the events as measured along the world-line of an inertial observer.
That would be correct if she said it, yes (although you could say "always" rather than "generally"), but I don't recall her saying that...

DAH
DAH said:
I agree that she's wrong to say that the twin effect is solely due to acceleration, but I think what Sabine might be saying is that:
Given two time-like separated events, where observers are present at both events, then the proper time measured along the world-line of a non-inertial observer is generally less than the proper time between the events as measured along the world-line of an inertial observer.
Just my 2 pennies
That may indeed be what Hossenfelder intended. I'm even inclined to think that it is, in which case I will criticize it as poor pedagogy (a matter of personal taste) rather than bad physics or an outright false statement.

The problem in this thread is that she wasn't speaking with that level of precision (this is an occupational hazard of writing popularizations) so what she actually said can be interpreted in different ways, and some of these are indeed bad physics or just plain wrong.

DAH, martinbn and PeroK
Epic Mythology said:
Do you believe that influential and popular public science educators have a duty to speak the truth and be correct?

You can find a nice explanation what time dilation is in this post https://www.physicsforums.com/threa...e-experiences-which-time.1051231/post-6870973

All other "effects" of time discrepancies should imo NOT be called time dilation in order not to be confusing.

phinds and Dale
Epic Mythology said:
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?

At 12 minutes into this video ,

Hossenfelder states, "This is the real time dilation. It comes from acceleration."

Looking at the equations for time dilation, time dilation comes from velocity, not acceleration.

How can Hossenfelder state, "This is the real time dilation. It comes from acceleration."?

The confusion starts when accelerating reference frames (which can have clocks going at different rates at different locations) are conflated with the proper acceleration of the clocks themselves (which doesn't affect the clock rate).

Sabine Hossenfelder's statements at timestamp 18:00 are clearly contradicting the clock hypothesis:
https://en.wikipedia.org/wiki/Time_dilation#Clock_hypothesis

Dale, malawi_glenn, PeroK and 1 other person
Epic Mythology said:
So are you agreeing with Hossenfelder's statement that "This is the real time dilation. It comes from acceleration." What are the physical equations that show acceleration causes time dilation?
There isn't one in the sense you want. The clock hypothesis states that time dilation only depends on velocity.

It's like calculating dL of a line segment, integrating them to find L between two fixed points a and b, and noticing that L depends on the change of direction of the line segments dL. This change of direction is reflected in the dx and dy parts in every dL.

Similarly, an acceleration changes v(t), and hence the elapsed proper time. But this elapsed proper time does not explicitly depend on the acceleration.

Edit: plus what Dale says in #13.

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malawi_glenn
Yes, and the clock hypothesis has been confirmed by observation for amazing accelerations. I don't know, why you make such a fuss about some pop-sci youtube video...

PeroK, russ_watters and malawi_glenn
vanhees71 said:
I don't know, why you make such a fuss about some pop-sci youtube video...
If I had a coin for each time a pop-sci article or video were... let's say "not accurate" - I would be rich!

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