Thought experiment on photon entanglement

[Steve Blandford]

TL;DR Summary

entangled photons and spooky action at a distance.

This is a thought experiment I had with myself a couple of weeks ago. I have never seen it proposed. But please be kind I am not physicist but just a hobbyist. So this experiment is specifically for entangled photons. So here goes.

I was thinking of something going faster and faster and time slowing down until one gets to the speed of light. At that point time stops. So then I thought of the photon. This particle spends it's life at speed of light so it experiences no time. From it's perspective it exists at both it's beginning and end of life.

If this is the case then why couldn't it (when measured) just go back to the beginning and tell its other half to become what is measured. It does exist in both places at once after all. This thought led me to another thought. Could the lowly photon be just a 3 dimensional representation of a 4th dimensional particle.

I have no way of going at this experimentally, but as a though experiment it seems plausible.

Thanks for your time.

 

[phinds]

I was thinking of something going faster and faster and time slowing down until one gets to the speed of light.

Time does not slow down at all in the frame of reference of a moving object. The "time slows down" is a very misleading way of expressing what really happens, but it is always used by pop-sci presentations.

Time dilation and length contraction never happen to an object itself but are only perceived to be happening by an observer who is not at rest in the frame of reference in which the object is at rest.

At that point time stops. So then I thought of the photon. This particle spends it's life at speed of light so it experiences no time. From it's perspective it exists at both it's beginning and end of life.

No, it does not. Your misunderstanding is in not realizing that there IS no such thing as "time" for a photon.

Your confusion about this is SO common that there's a FAQ entry on this somewhere on the forum but I don't have a link to it at hand.

 

[Steve Blandford]

OK. I understand now. And that is what I should have seen. The time stop is only from the perspective of the stationary entity. But the photon which is the moving thing still experiences time as it always has from its perspective. thank you for clearing that up. And I will have to start reading that FAQ. Sorry for the confusion.

 

[phinds]

OK. I understand now. And that is what I should have seen. The time stop is only from the perspective of the stationary entity. But the photon which is the moving thing still experiences time as it always has from its perspective. thank you for clearing that up. And I will have to start reading that FAQ. Sorry for the confusion.

No, you still misunderstand. A photon does NOT experience time at all. There IS no time for a photon. It is not well defined. YOU can measure the time it takes for light to get somewhere but a light ray can't. Neither can a gravity wave. Things without mass travel at the universal speed limit and do not "experience" time at all, so you can't legitimately say anything at all about how time acts for such an object because there is no definition of time for such objects.

@berkeman, do you happen to have a link to our FAQ entry about that?

 

[berkeman]

@berkeman, do you happen to have a link to our FAQ entry about that?

Oh sure, put me to work...

Searching now...

 

[PeterDonis]

there's a FAQ entry on this

It's in the relativity forum, because this misconception usually comes up in that forum:

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/

 

[phinds]

Thanks. I've made a note of it in my Physics Forums file so I won't need to bother you next time (unless I'm feeling lazy and decide you need bothering )

EDIT: OOPS I thought I was answering Berkeman.

 

[Johan0001]

https://www.physicsforums.com/threads/rest-frame-of-a-photon.511170/ I read in the above link and some of its references , that a photon cannot have a rest frame with v = 0, since it travels at c in all universal frames of reference.

If for example 2 photons leave the sun and travel towards Earth at exactly the same time side by side.
Would the one photon not be at rest with respect to the other photon ?
Would this not be a contradiction?

I think it was Einstein that asked , if he could hitch a ride on a photon what would he see?

Has anybody tried to answer that question since?

I would appreciate any references

 

[PeterDonis]

If for example 2 photons leave the sun and travel towards Earth at exactly the same time side by side.
Would the one photon not be at rest with respect to the other photon ?

The article you were given a link to says photons do not have a rest frame. In other words, they cannot be at rest in any inertial frame.

That is not the same as saying a photon cannot be "at rest" with respect to another photon. In the example you give, the two photons can be said to be "at rest" with respect to each other. But there is no inertial frame in which they are at rest. The sense of "at rest" in which the photons are "at rest" with respect to each other is a different sense of "at rest", which turns out not to be useful for anything in relativity.

I think it was Einstein that asked , if he could hitch a ride on a photon what would he see?

Yes, and his answer was that it was impossible to hitch a ride on a photon. Another photon, emitted at the same time from the same place in the same direction, could be said to do so, but a photon doesn't "see" anything; a photon can't be an observer because an observer has to be at rest in some inertial frame, and a photon can't be.

 

[Johan0001]

Thanks Peter
I understand the point you make.

One more question

The article you were given a link to says photons do not have a rest frame. In other words, they cannot be at rest in any inertial frame.

Theoretically speaking, my thought experiment.

If we enclosed my two photons in a large empty vacuum cube with neutrinos at each Corner , all traveling in the same direction.
Why can we not define this as an Inertial frame of reference where the photons are at rest, by definition?

This is my confusion.

 

[phinds]

Thanks Peter
I understand the point you make.

One more question
Theoretically speaking, my thought experiment.

If we enclosed my two photons in a large empty vacuum cube with neutrinos at each Corner , all traveling in the same direction.
Why can we not define this as an Inertial frame of reference where the photons are at rest, by definition?

This is my confusion.

Neutrinos don't travel at the speed of light so either the neutrino or the photons would not remain contained, but that's pretty much beside the point anyway.

Your mistake is that you are taking the rest frame of the box (apparently) as the rest frame of the photons but the box cannot travel at c and the photons have to so in the rest frame of the box, the photons are traveling at c, not at rest.

 

[Johan0001]

Neutrinos don't travel at the speed of light so either the neutrino or the photons would not remain contained, but that's pretty much beside the point anyway.

Apologies , i thought that Neutrinos were massless.

So in essence are you saying there is no "box" that can be defined with massless particles ( or any other for that matter) other than photons.
That could travel at c with the two enclosed photons?

 

[berkeman]

So in essence are you saying there is no "box" that can be defined with massless particles ( or any other for that matter) other than photons.
That could travel at c with the two enclosed photons?

I threw my mother from the train. A kiss.

 

[Johan0001]

I threw my mother from the train. A kiss

The sandpipers

Thanks guys .

 

[PeterDonis]

If we enclosed my two photons in a large empty vacuum cube with neutrinos at each Corner , all traveling in the same direction.

If, as you believed when you wrote this, neutrinos are massless, then they are all moving at the speed of light along with the photons, so none of them are at rest in any inertial frame.

If, as you have been corrected, neutrinos are not massless, then it is impossible for them to be moving along with the photons.

Why can we not define this as an Inertial frame of reference where the photons are at rest, by definition?

Because there already is a definition of an inertial frame, and that definition makes it clear that photons cannot be at rest in any inertial frame.

It sounds like you need to go work through a textbook on relativity and learn the definition of an inertial frame.