Experience Travel on a Photon: Hitch-Hiker's Guide to the Universe

[Karl24]

TL;DR Summary

What's the experience of travel like for a photon?

We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are all locations and all times along his path of travel an instantaneous 'everywhere' and 'everywhen' for him?

And a subsidiary question: since the universe is finite, presumably there is eventually a frontier between space and no-space. Depending on where his photon was emitted, it could be that the hitch-hiker will never reach that frontier, because the accumulating expansion of space itself may exceed his rate of travel. But assuming the journey began somewhere already near the edge of space and the frontier was reachable, what happens when he reaches it? Does his mere presence extend the universe, and he continues merrily on his way? Or does he bounce back? Or cease to exist? Just wondering, before I book my ride!

 

[Nugatory]

We know that as speed increases, time slows, and at the speed of light time apparently ceases

You’ll often hear time dilation described that way, but it’s not right. One way of seeing the problem is to consider that right now you (along with the Earth and the entire solar system) are moving very close to speed of light relative to some cosmic ray particle zipping through - but is time slowed for you?

Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are all locations and all times along his path of travel an instantaneous 'everywhere' and 'everywhen' for him?

It’s not possible to draw any conclusions from this hypothetical, for about the same reason that any thought process that starts “Imagine an integer that multiplied by one is not equal to itself….” must lead to bogus conclusions - the premise is inconsistent. The problem here is that the light moves at speed $c$ relative to everything so we’re trying to imagine a situation in which our flash of light is both at rest relative to the hitchhiker and moving at speed $c$ relative to the hitchhiker - internally contradictory from the start. (This question comes up often enough that we have a FAQ: https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/).

And a subsidiary question: since the universe is finite,

Is it? That’s still an open question. But even if the universe is finite it does not follow that

presumably there is eventually a frontier between space and no-space.

Consider the two dimensional surface of the spherical earth. It’s finite, but there is no edge, no frontier between surface and non-surface. No matter how far you walk in any direction, there’s always room in front of you for your next step.

If the universe is finite it will be curved in such a way that it has no edge.

 

[ergospherical]

If the universe is finite it will be curved in such a way that it has no edge.

This is a physically reasonable assumption but I don't believe you can state it with certainty. The formalism does not exclude finite spacetimes which are manifolds with boundaries, no?

 

[Mister T]

Summary:: What's the experience of travel like for a photon?

There is no way to answer this. By the way, I recommend that the question be changed so that you refer to a beam of light instead of a photon. Even then the question can't be answered, but a discussion of why will be more direct because you won't have to enter into the realm of quantum physics.

We know that as speed increases, time slows, and at the speed of light time apparently ceases.

What do you mean by "ceases"? Do you mean that there's a transition from a scenario where time exists to a scenario where it doesn't? You cannot make a transition from a speed less than $c$ to a speed of $c$. That's a consequence of the same postulates that lead to the notion of time dilation.

 

[hutchphd]

For the OP
https://www.nytimes.com/2015/11/01/...stein tried to,the wave would seem stationary.

 

[DPH]

Summary:: What's the experience of travel like for a photon?

We know that as speed increases, time slows, and at the speed of light time apparently ceases. Imagine a hypothetical hitch-hiker sitting on a photon and speeding through the universe at -- well, the speed of light. If time has stopped for him (her?), what is his experience of his journey? Are all locations and all times along his path of travel an instantaneous 'everywhere' and 'everywhen' for him?

And a subsidiary question: since the universe is finite, presumably there is eventually a frontier between space and no-space. Depending on where his photon was emitted, it could be that the hitch-hiker will never reach that frontier, because the accumulating expansion of space itself may exceed his rate of travel. But assuming the journey began somewhere already near the edge of space and the frontier was reachable, what happens when he reaches it? Does his mere presence extend the universe, and he continues merrily on his way? Or does he bounce back? Or cease to exist? Just wondering, before I book my ride!

We can see back in space just short of 13.8 billion years when the Big Bang begin. At the 13.8 limit space is expanding at the speed of light and creates an effective edge to what we can know. If we do the calculations that edge is now out 46 billion light years since it immitted light at the beginning of time.

 

[PeterDonis]

Moderator's note: A post has been deleted and the thread, which was temporarily closed for moderation, has been reopened.

 

[phinds]

At the 13.8 limit space is expanding at the speed of light

No, not at all. The stuff at the outer edge of our observable universe is receding at about 3c.

And, even aside from that, saying that "space is expanding at the speed of light" is an utterly meaningless statement. Expansion is a RATE, not a speed.