# Do photons experience time?

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• YoungPhysicist
In summary, according to the author, a massive object will experience time slower when its speed is close to the speed of light. However, photons themselves move at the speed of light, so does that mean that they experience no time? Unfortunately, this statement is not accurate and is essentially a common misconception.

#### YoungPhysicist

As far as I know, a object will experience time slower when its speed is close to the speed of light.
But photons themselves moves at the speed of light, does that mean that they experience no time?

Well, fundamental particles don’t really experience anything. However, the time that human beings experience (and clocks and other animals and ...) is called “proper time”, and photons do not have any proper time.

Young physicist said:
As far as I know, a object will experience time slower when its speed is close to the speed of light.

This statement is not accurate and is essentially a common misconception. Motion is relative and a massive object can only ever be said to be moving relative to something. There is no sense in Which an object can be said to be moving near the speed of light, or at any specific absolute speed.

What is true is that if one clock is moving relative to another clock, then the time they record is dilated as measured by the other clock. Or, more precisely, as measured in the rest frame of the other.

Note that this time dilation is symmetric, so neither can be said to be absolutely time dilated. Time dilation, like motion, is relative.

Tom Atkinson and Alfredo Tifi
How would you measure the time rate "experienced by" a photon?

Demystifier
Dale said:
photons do not have any proper time.
They do, but it's equal to zero.

Dale
Demystifier said:
They do, but it's equal to zero.
I actually prefer to say that proper time is only defined on timelike worldlines. So it has a spacetime interval which is equal to zero, but no proper time.

Demystifier and jbriggs444
Dale said:
I actually prefer to say that proper time is only defined on timelike worldlines. So it has a spacetime interval which is equal to zero, but no proper time.

I agree with this. The justification for interpreting the arc length along timelike worldlines as "proper time" is that you can use arc length as an affine parameter. You can't use arc length along a null worldline as an affine parameter, so the justification for interpreting arc length as proper time (of zero in the case of a null worldline) does not hold.

vanhees71 and Dale
A further observation is that if you try to view null interval as a limit of timelike interval , then it is also a limit of spacelike interval. So is it zero time or zero length? Neither seems the best answer to me.

Dale and PeroK
This has got quite technical.

@Young physicist - space and time form one 4d whole. You can measure "distance" along a path through spacetime, just as you can measure distance through space - and the "distance" along your path through spacetime turns out to be c times the elapsed time on your wristwatch.

An odd feature about the paths light follows is that this "distance" is zero. So time-for-a-pulse-of-light isn't a useful concept because it doesn't distinguish between points along the path. Advanced topic: you can use a slightly more complicated concept called an affine parameter to do the same mark-distance-along-the-path job as proper time with light. But these things don't confer a sense of time for light, really.

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vanhees71, YoungPhysicist and PeterDonis
Ibix said:
This has got quite technical.

@Young physicist -
An odd feature about the paths light follows is that this "distance" is zero. So time-for-a-pulse-of-light isn't a useful concept because it doesn't distinguish between points along the path.
So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.

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Young physicist said:
I mean at its perspective.
A pulse of light doesn’t have a perspective, if by perspective you mean a rest frame.

Young physicist said:
So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.

If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions.

For example, in post #3 I pointed out that all motion is relative, so nothing can "tell if it's moving or not". An insight, by the way, that goes back to Galileo.

YoungPhysicist
Young physicist said:
So do you mean that photons can’t tell if it’s moving or not?

I mean at its perspective.
No one can tell if they are moving or not. You can only tell whether or not you are moving relative to something (typically the surface of the Earth in every day life).

But what I mean is that you can't ask questions about the perspective of light. The question cannot be framed coherently. This kind of thing is very common as you move away from every day experience. Apparently perfectly reasonable questions are based on assumptions that don't apply. Asking about the perspective of light is like asking which direction is north - while standing at the north pole. The only possible answer is that there isn't one.

You will find people (notably Brian Greene) saying that time stops at the speed of light. He doesn't do it in his professional publications, I gather, only in his pop-sci stuff. So apparently even he doesn't think that's a useful answer...

PeroK said:
If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions.

For example, in post #3 I pointed out that all motion is relative, so nothing can "tell if it's moving or not". An insight, by the way, that goes back to Galileo.

Do you mean things like this?

https://web.stanford.edu/~oas/SI/SRGR/notes/srHarris.pdf

Young physicist said:
Do you mean things like this?

This is simpler.

YoungPhysicist
I mean this part:
PeroK said:
If you really want to learn SR, you need a proper textbook. At the moment, you're just fishing in the dark. You need to get rid of these fundamental misconceptions..

And yeah, that part too.

OK you got me, but IMO Galileo is much underrated ;) I see a lot of questions here that were answered comprehensively 500 years ago.

As for textbooks, I learned my stuff from the WWW . . . The Wikipedia page here is not too bad, but the shortest canned overview of SR that I know of is here (it is not strictly a textbook, but describes special relativity in a modern way, derives the Lorentz transform from Einstein's postulates, and from it the spacetime interval - you know, the one that is zero for light).

Both of these contrast SR with Newtonian/Galilean relativity.

Sorcerer and YoungPhysicist
Thanks. I’ll check it out.

Young physicist said:
Do you mean things like this?

https://web.stanford.edu/~oas/SI/SRGR/notes/srHarris.pdf

That looks quite good, but there are some questionable statements in there. For example:

"Einstein wrote two theories of relativity; the 1905 work is known as “special relativity” because it deals only with the
special case of uniform (i.e. non-accelerating) motion."

Which is not right. You can study any motion in SR, accelerated or not. What SR does not deal with is Gravity and curved spacetime.

So, I'd be careful with that pdf. There are a few things I saw like that that made me raise an eyebrow.

If you want a free source, I would recommend:

http://www.lightandmatter.com/sr/

If you want to buy a book, I would recommend:

vanhees71, Dale and YoungPhysicist
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?

sqljunkey said:
So if I was traveling at the speed of light

You can't travel at the speed of light. Only light (and other massless things) can.

vanhees71
sqljunkey said:
So if I was traveling at the speed of light my clock would not be moving relative to me?
Don’t worry, you won’t be traveling at c no matter what, so the question never arises.

vanhees71 and m4r35n357
sqljunkey said:
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?
It isn't possible to describe what it looks like when traveling at c, any more than it is possible to describe which way is north at the north pole. The question makes no sense.

vanhees71
sqljunkey said:
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?
You can't. Thinking about that (at age 16) was what got Einstein started towards SR in the first place.

vanhees71
sqljunkey said:
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?

1. You can travel at ANY speed with your clock, and your clock will not be moving relative to you. That's the definition of things being in the same reference frame. Presumably, the device that you typed with to post on this forum wasn't moving with respect to you while you were using it. So why not the clock?

2. There is a fundamental problem with the statement "... if I was traveling at the speed of light..." and then, applying Special relativity concepts of time and space to conclude what will happen. The problem comes in with the postulates of SR. These are the starting points of SR, whereby all other consequences, such as time dilation and length contraction, are derived from. One of the postulates is that the speed of light is a constant in all inertial reference frame. This means that no matter what reference frame that you choose, the speed of light remains at "c".

So already your scenario has a problem, because if you are traveling at the speed of light, then light must be having a speed of zero in your reference frame. If this is so, you cannot use the equations derived from SR, because they are not valid! It breaks one of the postulates of SR to begin with. So your concept of time (i.e. your ability to measure time in your frame and in another frame) is unknown and undefined. You will have to use another concept of time from a theory that has yet to be consistently formulated AND verified that goes beyond SR.

This type of question has appeared repeatedly in this forum, and it might be educational to do a search on it, because all the responses that we are providing here are almost repetition of what have already been mentioned several times.

Zz.

I know you can't go at C, because of the mass. I'm just asking a what if question. What if you were traveling at C, what would the clock show. I'm not planning on traveling at C anytime soon ibix.

But it seems it's not important "what if" question because you will never reach C.

It is NOT about the mass, and your question still can't be answered. This is the most fundamental thing to understand about SR, and denying it will not help.

Dale and m4r35n357
sqljunkey said:
So if I was traveling at the speed of light my clock would not be moving relative to me? I would be frozen in time? I wouldn't notice it but to others that's how it looks like?
No. A material body can’t travel at the speed of light, and a reference frame at light speed is impossible in SR. See:

sqljunkey said:
I'm just asking a what if question

You can't ask even a what if question based on a self-contradictory premise.

sqljunkey said:
What if you were traveling at C, what would the clock show.

A clock can't travel at c either, so again, the question is based on a self-contradictory premise and is meaningless. As @ZapperZ has already pointed out, there are plenty of previous threads, plus a FAQ article, here on PF explaining this.

jbriggs444
PeroK said:
That looks quite good, but there are some questionable statements in there. For example:

"Einstein wrote two theories of relativity; the 1905 work is known as “special relativity” because it deals only with the
special case of uniform (i.e. non-accelerating) motion."

Which is not right. You can study any motion in SR, accelerated or not. What SR does not deal with is Gravity and curved spacetime.

So, I'd be careful with that pdf. There are a few things I saw like that that made me raise an eyebrow.

If you want a free source, I would recommend:

http://www.lightandmatter.com/sr/

If you want to buy a book, I would recommend:

It's also using the outdated concept of "relativistic mass". Here's my attempt for a modern introduction to SRT, starting right away with the covariant formalism a la Minkowski (however with a real "metric" as it should be):

https://th.physik.uni-frankfurt.de/~hees/pf-faq/srt.pdf

weirdoguy and YoungPhysicist
Young physicist said:
As far as I know, a object will experience time slower when its speed is close to the speed of light.

That can't possibly be true. It would provide a way to distinguish between a state of rest and a state of uniform motion, violating the foundation upon which the entire theory is based.

What's confusing here is that when an object moves relative to you, time passes more slowly for that object relative to you.

Likewise, when you move relative to an object, time passes more slowly for you relative to that object.

Note that because of this symmetry, there is no way to claim whether it's you, the object, or both, that are in motion.

Another part of the foundation is that a beam of light in a vacuum will always travel at the same speed ##c## relative relative to both you and that object. Imagine trying to chase after that beam. No matter how fast you travel in your attempt to catch it, it will always recede from you at speed ##c##. Thus you can never reach speed ##c##.

YoungPhysicist
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?

Tom Rathz said:
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?

Quantum entanglement applies to all types of particle.

Ibix
Tom Rathz said:
From what PAllen said that a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon, then why is the concept of quantum entanglement so mysterious?

What does "... a photon of light does not have a reference frame and another Forum I read saying that the concept of space or time don't apply to a photon ... " have anything to do with "... quantum entanglement... " that makes it so "mysterious"?

Zz.