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## Are massless objects immune to time dilation?

Imagine there is a train moving at .9c with a person on the train and an observer on a platform watching the train go by.

The person on the train walks forward while shining a flashlight. To the observer on the platform, the person on the train is time dilated and walks forward in "slow motion," but the observer measures the light from the person's flashlight as c because c is constant and independent of an inertial frame of reference.

So my question is, is light (and other massless objects) immune to time dilation?

As always, thank you.

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 Recognitions: Gold Member Science Advisor Staff Emeritus One way of looking at it is that massless things are subject to infinite time dilation. Massless things travel at c. If you make an object with mass go very close to c, it's subject to very high time dilation. As you approach c, the time dilation approaches infinity. That means that time stops for that object. It's sort of valid to think of a photon as an object for which time has stopped. Another way of saying this is that you can't make a clock out of photons. A whole different way of getting at it is that you can't have a frame moving at c. Therefore it doesn't make sense to talk about how much time passes in the frame of a photon. When you talk about a photon's frequency, you're describing it in some other frame, not a frame associated with the photon.
 Recognitions: Gold Member Thanks, bcrowell. So talking about what the photon sees from its FoR is pretty useless because it's an impossible FoR for any observer with mass to have? I suppose a photon would see the world moving infinitely quickly since its infinitely dilated... How about from the FoR of the observer on the platform? He measures the velocity of the person walking and holding the flashlight as slowed down (relative to what the walker measures from his FoR), but not the speed of the light coming out of flashlight even though it's aboard a train moving at .9c. How does that work?

## Are massless objects immune to time dilation?

 Quote by bcrowell Another way of saying this is that you can't make a clock out of photons.
Could you elaborate on that a little further in the context of the following thought experiment...

There exist 2 focused pulses of light, such as short laser bursts. The energy content of each pulse is sufficient to cause gravitation which bends the path of the other to such an extent that the 2 pulses orbit a common point which is not traveling at C. An observer with suitable equipment should be able to count gravity waves and keep time by them.

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 Quote by coktail but not the speed of the light coming out of flashlight even though it's aboard a train moving at .9c. How does that work?
That's the significant thing about the universal speed limit (which light and other massless particles have to follow) ... it's not really a question of "how" does it work, it just IS, and is the basis of Special Relativity ... the universal speed limit is exactly the same in all inertial frames of reference. VERY non-intuitive.

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 Quote by coktail I suppose a photon would see the world moving infinitely quickly since its infinitely dilated...
Time dilation doesn't work like that. It's symmetric. If A sees B as slow, then B also sees A as slow.

 Recognitions: Gold Member I thought it depended each object's acceleration. If two objects accelerate away from each other symmetrically, they'll see each other as slowed down, but if one stays stationary and the ther accelerates away, the one that accelerated will see the stationary one as sped up, and the stationary one will see the acceleration one as slowed down. This is incorrect?

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 Quote by coktail I thought it depended each object's acceleration. If two objects accelerate away from each other symmetrically, they'll see each other as slowed down, but if one stays stationary and the ther accelerates away, the one that accelerated will see the stationary one as sped up, and the stationary one will see the acceleration one as slowed down. This is incorrect?
Totally incorrect. Which part of "it's symmetrical" did you not understand?

Also, time dilation is a function of speed, not acceleration. That is, fast-moving objects, relative to you, are seen as time dilated regardless of whether or not they are accelerating.

 Recognitions: Gold Member But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical. I understand that time dilation is about speed, but my understanding is that acceleration plays a role as well.

 Quote by coktail I thought it depended each object's acceleration. If two objects accelerate away from each other symmetrically, they'll see each other as slowed down, but if one stays stationary and the ther accelerates away, the one that accelerated will see the stationary one as sped up, and the stationary one will see the acceleration one as slowed down. This is incorrect?
There is a kernel of truth in there, but it is badly garbled.

In Special Relativity, the effect of acceleration on time dilation depends not just on the magnitude of the acceleration, but on the direction of the acceleration and the distance
to the clock that you are comparing from your own [accelerating] clock.

For a local clock accelerating away from a remote clock, the effect is an apparent slow-down in the remote clock. This slow-down is in addition to any slow-down from relative motion.

For a local clock accelerating toward a remote clock, the effect is an apparent speed-up in the remote clock. This speed-up is in addition to any slow-down from relative motion.

This slow-down and speed-up is because the inertial frame in which an accelerating object is momentarily at rest will change from one instant to the next. With the change in reference frame comes a change in synchronization convention. You can think of it as a line of synchronization sweeping forward (for acceleration toward) or sweeping back (for acceleration away) across the time line of the remote clock.

 Quote by coktail But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical. I understand that time dilation is about speed, but my understanding is that acceleration plays a role as well.
There are two types of time dilation:

- symmetrical from relative movement
- asymmetrical from the relative position in an gravitational field or an accelerating frame

In the twins scenario the acceleration breaks the symmetry, and the second time dilation type cancels and reverses the effect of the first, in the frame of the accelerated twin.

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 Quote by coktail I thought it depended each object's acceleration. If two objects accelerate away from each other symmetrically, they'll see each other as slowed down, but if one stays stationary and the ther accelerates away, the one that accelerated will see the stationary one as sped up, and the stationary one will see the acceleration one as slowed down. This is incorrect?
 Quote by coktail But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical. I understand that time dilation is about speed, but my understanding is that acceleration plays a role as well.
The easiest way to understand time dilation is to pick a single inertial reference frame that includes all the objects/observers under consideration.

The reference frame defines coordinate time (and space) and the positions, speeds and accelerations of all objects/observers are specified according to this arbitrarily selected reference frame.

It's important to also understand that time dilation is never something that any observers can actually see. Rather, it is something that is calculated. Furthermore, it is always a slowing down of time for moving objects/observers based only on their instantaneous speed according to the selected inertial reference frame and has absolutely nothing to do with acceleration, the direction of acceleration or the direction of velocity.

Remember, different reference frames will assign different speeds, and therefore different time dilations, to the objects/observers, and will have no bearing on what the observers actually see.

So getting back to your examples:
 If two objects accelerate away from each other symmetrically, they'll see each other as slowed down
Your two objects will always be at the same speed so they will always be time dilated by the same amount according to the initial inertial reference frame in which they started out at rest. But this is not what they will see. (They will actually both see the other ones clock running slower than their own.) What they see is called Doppler and is a different subject.
 if one stays stationary and the ther accelerates away, the one that accelerated will see the stationary one as sped up, and the stationary one will see the acceleration one as slowed down. This is incorrect?
The one that stays stationary will not be time dilated but the one that accelerates away will be time dilated. However, this again has nothing to do with what they will actually see. (They will actually both see the other ones clock running slower than their own.)
 But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical.
There are many ways to set up the twin paradox but it will always require a non symmetrical situation if the twins end up at different ages when they finally reunite. So let's start with your first situation but have both objects accelerate in such a way that they reunite:
 If two objects accelerate away from each other symmetrically, they'll see each other as slowed down
Now there are many ways that they could continue to accelerate symmetrically, thus maintaining the same speed according to the reference frame and have them come back together. One would be for them to both reverse their acceleration so that they trace back the same paths they left on. Another would be for them to accelerate in curving paths so that they can eventually meet again. But as long as they continually accelerate symmetrically, their speeds will be always identical and their time dilations will be always identical and so their clocks will always have the same time on them and whenever they get together, they will have aged by the same amount.

Now we could have broken the symmetry in this example and had one of the objects accelerate differently than the other so as to bring them back together and depending on the speeds and therefore their time dilations, we could determine the time difference on their clocks when they reunite.

Now let's take your second example:
 if one stays stationary and the ther accelerates away
Now how do you want to bring them back together? If you leave the first one stationary and accelerate the second one so that he eventually returns to the stationary one, then obviously the first ones clock will never be time dilated while the accelerated one will have less time on it when they return.

But you could have chosen instead for the stationary one to accelerate even more than the second one so that he can catch up to him. In this case, because of his greater speed, he would experience even more time dilation and be the one with a younger age when they reunite. But it's important to work out all the specific details in an actual situation.

 But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical.
Hopefully by now you can see that if one twin ages more slowly than the other then it cannot be symmetrical but I think maybe the point you may have overlooked is the aspect that they must reunite in order to compare the difference in their two clocks.

 Recognitions: Gold Member Thank you, everybody. There's a lot of great and helpful info here. So, a photon is infinitely time-dilated, and from a photon's perspective, the world is infinitely time-dilated, not sped up. Correct?

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 Quote by coktail Thank you, everybody. There's a lot of great and helpful info here. So, a photon is infinitely time-dilated, and from a photon's perspective, the world is infinitely time-dilated, not sped up. Correct?
I think it's more correct to say that there IS no such thing as a "photon's perspective", at least that's the way I always see it here on this forum.

But as a limit as you approach c, I think your statement holds (substituting "approaches" for "is")

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 Quote by coktail But what about the twin paradox where one twin ages more slowly than the other? Clearly this is not symmetrical. I understand that time dilation is about speed, but my understanding is that acceleration plays a role as well.
Yeah, it would have been more correct of me to say that it doesn't HAVE to be accelerating in order for you to see it as time dilated, and as others have explained, the turn-around in the twin paradox is a game-changer.

 Recognitions: Gold Member But the turn-around scenario could never apply to a photon because light never accelerates or decelerates. Is that correct?
 Recognitions: Gold Member Science Advisor As phinds said, 'there IS no such thing as a "photon's perspective"'. See our FAQ: Rest frame of a photon .