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Is light time dilated?

ynot

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ynot
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If not - Why not?

If so - How can it travel at all relative to an observer, let alone travel at c?
 
ynot said:
If so - How can it travel at all relative to an observer, let alone travel at c?
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Why should there be any problem with something that is time dilated travelling relative to an observer? Remember that the velocity of an object with respect to an observer is determined in the coordinate system of the observer - it's the rate of change in location (in observer's coordinates) vs. change in time (again, observer's coordinate time). It doesn't involve the object's proper time (the one that is dilated).

MetalPig said:
Same here. As far as I understand, photons don't age.
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While that is true, travel at the speed of light is not necessary for that - electrons, for example, don't age either.

So it would be better to say that photons can't age - or more precisely, can't undergo spontaneous changes depending only on the time they have been travelling. (Note that they can and do undergo other changes - they interact with particles, they get redshifted as they travel through curved spacetime of the expanding universe, etc.)
 
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Seems to me that it's not so much that photons don't age as that in their reference frame time does not pass. They have zero age.

How "zero age" differs from "not existing" is something best left for the philosophers.
 
Wouldn’t light experience it’s own “proper time” but be time dilated to the point of stopping relative to the “proper time” of an observer?
 
Thabiguy said:
Why should there be any problem with something that is time dilated travelling relative to an observer? Remember that the velocity of an object with respect to an observer is determined in the coordinate system of the observer - it's the rate of change in location (in observer's coordinates) vs. change in time (again, observer's coordinate time). It doesn't involve the object's proper time (the one that is dilated).
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Wouldn’t the travelling of the light itself be time dilated to the point that it would stop relative to the “proper time” of the observer? (if time stops at c)

ETA - I guess I’m asking that if relative motion causes time dilation wouldn’t that time dilation also dilate the very motion that causes the dilation?
 
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Reality Check said:
No because light is not an observer.
Proper time is not dilated according to an observer travelling with the clock.
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But I’m talking about light being observed by a “stationary” observer that light is travelling relative to at c. How can anything be observed to be moving at c (or any speed) if time stops at c?
 
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ynot said:
But I’m talking about light being observed by a “stationary” observer that light is travelling relative to at c. How can anything be observed to be moving at c (or any speed) if time stops at c?
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I do not know what you mean by this.
All observers see light travelling at the speed of light even if they were to be travelling at the speed of light.
There is not such thing as a “stationary” observer -unless you state what they are stationary with respect to.
 
ynot said:
But I’m talking about light being observed by a “stationary” observer that light is travelling relative to at c. How can anything be observed to be moving at c (or any speed) if time stops at c?
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I believe Reality Check is saying that time dilation is defined as the relative rates as measured by two clocks in relative motion. Since no clock can be at rest in a photon's frame of reference it is not meaningful to speak of the rate at which time flows in a photon's frame of reference. If that's not what Reality Check is saying then it's what I'm saying.
 
Let’s agree for now that light is “special” and it’s speed isn’t effected by either relative motion or time dilation. But what about how time dilation effects the speed of material things moving relative to each other? Is the observed relative speed of all material things time dilated and their “actual” speed is faster than what is observed?

As I asked earlier - “If relative motion causes time dilation wouldn’t that time dilation also dilate the very motion that causes the dilation?” (discounting light).
 
ynot said:
Let’s agree for now that light is “special” and it’s speed isn’t effected by either relative motion or time dilation. But what about how time dilation effects the speed of material things moving relative to each other? Is the observed relative speed of all material things time dilated and their “actual” speed is faster than what is observed?

As I asked earlier - “If relative motion causes time dilation wouldn’t that time dilation also dilate the very motion that causes the dilation?” (discounting light).
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Time dilation does not case length contraction - it is caused by the relative velocity of the observers.
Thus time dialation will not "dilate the very motion that causes the dilation" if that is what you mean.

Let there be 2 observers Alice and Bob who have clocks. Bob is moving at a different velocity from Alice.
Alice will measure that Bob's clock is dilated (ticking slower).
Alice will measure that Bob's length is contracted.
Alice will measure Bob's velocity according to her reference frame, i.e. she will measure Bob's position at a point in time, measure Bob's position again to get his change in position and then divide by the amount of time that has passed. This velocity is not affected by her measurements of Bob's time dilation or length contraction.
 
ynot said:
As I asked earlier - “If relative motion causes time dilation wouldn’t that time dilation also dilate the very motion that causes the dilation?” (discounting light).
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Let's make sure you've understood the basics. You set up a series of markers in your lab, spaced evenly a meter apart in a straight line. Then you stand nearby holding a timer as a clock whizzes by. You measure at what time the clock passes each marker (taking into account the time it takes its light to reach you), and dividing a meter by the time interval you measured gives its velocity in your rest frame.

When you do this you'll also notice the clock is running a bit slowly, and that it's a bit contracted in length. The degree of dilation and contraction is given by the relativistic gamma factor, as a function of the velocity you measured in the way I just described.

Now, is in some sense the velocity you measured a "dilated" version of the "real" velocity? Yes, I suppose so... you could think of the reason nothing can exceed the speed of light that way (the dilation gets arbitrarily large there, preventing any increase in the velocity you measure). But there's no need to think of things like that - the formula takes as its input the speed you measure, and it's much simpler to express things in those terms.
 
The answer is no, because a single photon or wave of light can't experience an event or anything else upon which to base a calibration of time. One of the implications of Special Relativity is that the word "time" doesn't really mean anything until you specify a context for the discussion, that is, a frame of reference within which an observer can exist. To be clear, it's OK for that observer to be purely theoretical, but you can't talk about how long something takes to happen, or whether any two events can be thought of as simultaneous, or when any event happened relative to any other event, until you know how fast your frame is moving relative to the other things around it. That is why it's called "The Theory of Relativity" in the first place.

Under normal conditions, things work the way Newton said they did. If you could go fast enough for relativistic effects to come into play, you would have to expend enormous amounts of energy to get to .9 times C, then .99, then .999. But at each point, the speed of light remains constant from your frame, light doesn't play by the same rules, and you can't get to C. You just can't, and forming a thought experiment that postulates that you can is beyond discussion, although perhaps not entirely beyond meaning. Einstein himself supposedly conducted that thought experiment many times.

So the best possible answer is that the behavior of time for a theoretical observer traveling along at the speed of light is undefined. For example, we know the light from the Sun takes around 8 minutes to get to us, but if we could ride that beam of light, would it take some brief number of seconds, or would it take no time at all and seem instantaneous?

No one knows the answer because there is no way to define or calibrate time in that context. Science doesn't have an answer. It's a mystery.
 
ynot said:
If not - Why not?

If so - How can it travel at all relative to an observer, let alone travel at c?
Click to expand...


Simply stated, the photon from its frame of reference does not experience time. According to relativity, this is because the speed it moves at has stretched time till it no longer experiences it pass. If we are to take relativity theory seriously, it then means that as a mathematical implication, it does not move through space either.

Now, conceptually understand that, and you can pretty much believe anything the relativistic theories provide.
 
Singularitarian said:
Simply stated, the photon from its frame of reference does not experience time. According to relativity, this is because the speed it moves at has stretched time till it no longer experiences it pass. If we are to take relativity theory seriously, it then means that as a mathematical implication, it does not move through space either.

Now, conceptually understand that, and you can pretty much believe anything the relativistic theories provide.
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Correctly stated, the photon from its frame of reference does "experience" time. According to relativity, this is because the photon is observing itself and thus its relative velocity is zero. Therefore the photon "experiences" no relativistic effects such as time dilation or length contraction.
 
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