Traveling at Light Speed: Is a Photon Stationary?

In summary, the conversation discusses the concept of traveling alongside a train and a photon of light, and the idea that time and motion are relative in relativity. It is explained that it is not possible to travel at the speed of light and therefore, the question of what light would look like if traveled alongside it cannot be answered. The conversation also addresses the misconception that light can be seen as stationary or from a specific perspective, and emphasizes that this is not a coherent concept in relativity.
  • #1
Joe
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If you were to travel alongside a train, as fast the train, to you the train would seem stationary. I read that if you were to travel along a photon of light, as fast as the speed of light, that photon would not seem stationary. Is this true? If so, why?
 
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You are misunderstanding something. The key point is that you cannot travel at the speed of light - so the question of "what would light look like if you traveled alongside it" can't be answered.

Possibly what you misunderstood was that Einstein's second postulate is that the speed of light is always ##c## in all inertial frames. A frame traveling at the speed of light would therefore lead to a contradiction - that light is stationary (because the frame is moving at the same speed) and also moving at ##c## (because that's part of the definition of an inertial frame in relativity). Since there's a contradiction, you can't have an inertial frame moving at the speed of light. Edit: just to be clear, this does not mean that light isn't stationary if you travel alongside it. The (self-contradictory) conclusion that light wouldn't be stationary even when it must be shows that being stationary with respect to light isn't a coherent concept in relativity.
 
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  • #3
Ibix said:
You are misunderstanding something. The key point is that you cannot travel at the speed of light - so the question of "what would light look like if you traveled alongside it" can't be answered.

Possibly what you misunderstood was that Einstein's second postulate is that the speed of light is always ##c## in all inertial frames. A frame traveling at the speed of light would therefore lead to a contradiction - that light is stationary (because the frame is moving at the same speed) and also moving at ##c## (because that's part of the definition of an inertial frame in relativity). Since there's a contradiction, you can't have an inertial frame moving at the speed of light. Edit: just to be clear, this does not mean that light isn't stationary if you travel alongside it. The (self-contradictory) conclusion that light wouldn't be stationary even when it must be shows that being stationary with respect to light isn't a coherent concept in relativity.
Thank you. From what I understood - from your edit - light can seem to be stationary? Two photons traveling alongside each other will seem not moving from each other's perspective?
 
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Joe said:
Thank you. From what I understood - from your edit - light can seem to be stationary? Two photons traveling alongside each other will seem not moving from each other's perspective?
There is no such thing as the perspective of a photon.
 
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Joe said:
Thank you. From what I understood - from your edit - light can seem to be stationary? Two photons traveling alongside each other will seem not moving from each other's perspective?
You still misunderstand. There IS NO "point of view" (or perspective) of a photon.

EDIT: I see jbriggs beat me to it :smile:
 
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Joe said:
Thank you. From what I understood - from your edit - light can seem to be stationary? Two photons traveling alongside each other will seem not moving from each other's perspective?
As jbriggs444 and phinds have noted, it's simply not possible to describe the perspective of a thing traveling at the speed of light. Attempting to do so leads to the contradiction I mentioned.

Unfortunately, something you find as you move away from every day experience is that questions that seem perfectly sensible turn out to be nonsense. You don't even have to go that far outside the every day. Could you tell me which way is north where you are now? Could you tell me which way is north if you were at the north pole? Asking what anyone or anything would see at the speed of light is like that second question. You can't answer because the question has hidden assumptions that are not valid in the case it's talking about.

Note that you do find pop-sci sources (notably Brian Greene) that say things like "time stops at the speed of light". They're the result of forcing an answer at (metaphorical, I hope) gunpoint and don't make coherent sense, but usually satisfy non-physicists enough that they shut up and go away. It's worth noting that I gather that Greene himself does not make this claim in professional publications, only his pop-sci stuff.
 
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Ibix said:
As jbriggs444 and phinds have noted, it's simply not possible to describe the perspective of a thing traveling at the speed of light. Attempting to do so leads to the contradiction I mentioned.

Unfortunately, something you find as you move away from every day experience is that questions that seem perfectly sensible turn out to be nonsense. You don't even have to go that far outside the every day. Could you tell me which way is north where you are now? Could you tell me which way is north if you were at the north pole? Asking what anyone or anything would see at the speed of light is like that second question. You can't answer because the question has hidden assumptions that are not valid in the case it's talking about.

Note that you do find pop-sci sources (notably Brian Greene) that say things like "time stops at the speed of light". They're the result of forcing an answer at (metaphorical, I hope) gunpoint and don't make coherent sense, but usually satisfy non-physicists enough that they shut up and go away. It's worth noting that I gather that Greene himself does not make this claim in professional publications, only his pop-sci stuff.
Thanks again. Nicely explained!
 

1. What is light speed and how is it measured?

Light speed, also known as the speed of light, is the fastest speed at which energy can travel in a vacuum. It is measured at approximately 299,792,458 meters per second, or 670,616,629 miles per hour. This is the maximum speed at which all matter and information can travel in the universe.

2. Is a photon stationary when traveling at light speed?

Yes, a photon is considered stationary when traveling at light speed. This is because photons have no mass and therefore do not experience time or distance. From the perspective of a photon, it is emitted and absorbed at the same time, making it seem as though it is stationary.

3. Can anything travel faster than the speed of light?

According to the theory of relativity, nothing can travel faster than the speed of light. This is because as an object approaches the speed of light, its mass and energy increase exponentially, making it impossible to reach or exceed the speed of light.

4. How does traveling at light speed affect time and distance?

Traveling at light speed causes time to slow down and distances to contract. This is known as time dilation and length contraction, both of which are predicted by the theory of relativity. As an object approaches the speed of light, time and distance become relative and are no longer constant.

5. Can humans ever travel at light speed?

Currently, it is not possible for humans to travel at light speed. This is due to the immense amount of energy and technology required to reach such a high speed. However, scientists continue to study and develop theories that could potentially make light speed travel a reality in the distant future.

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