# Why don't photons experience time?

by la6ki
Tags: experience, photons, time
Physics
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P: 6,265
 Quote by Moonraker You should not mix up the observer’s view of the photon with the subject of the thread (what the photon is experiencing).

 Quote by Moonraker The problem might not be my ignorance of textbooks. The problem may be that this thread contains rather common questions in default of plausible solutions.
If you can show that you understand the standard usage of the terms of the discussion--"event", "worldline", "spacetime", etc.--then you are free to critique those terms and any assumptions that underlie them, and offer alternatives if you like. But you haven't shown an understanding of the standard terms yet.
C. Spirit
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P: 5,661
 Quote by nitsuj How big is a point in spacetime? You obviously understand that an event is a point in spacetime. In which case what I said literally makes no sense. To say it different a point is a set of coordinates of a specific location in spacetime. An event is idealized as having a specific point.
Mmmm I think this may be just a semantics issue in the end.
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 Quote by ghwellsjr If your interest is not really in Special Relativity but in learning about the quantum effects of photons, then you should be asking on the Quantum Physics forum because here we are not concerned with the quantum effects of photons, only their speed. We could just as easily be talking about billions of photons all traveling together as a burst of bright light. In fact, that is almost always what anyone actually means when they say "photon".
People need time to think, Sir. It's Physics.

You may see that I'm asking it again and again, but each expert has different way to express their understanding about relation of time and photon.

Yes now I realized, when an expert say "photon don't experience time" the context of that saying only the explanation photon using SR and GR. That is why I'm asking what else branch of Physic that need to be used to analyzing mass-less particle. More over, relation between mass-less particle and time. Perhaps like you said, added by Quantum Physics explanation.

 Quote by ghwellsjr There are an infinite number of events describing the path of a photon in any IRF, any two of which specify the same spacetime interval, a null interval that has nothing to do with time or distance. And when you transform the coordinates of any two of those events from one IRF to another, you continue to get null intervals. Do you understand what I'm talking about?
Off course I understand it, even though our course is not in English, therefore we have Physics term in our language. Now after I clarify my understanding, my interest is move on to how we apply relativity for mass-less particle in other branch of physics. So I'm not asking it here.

It just in my opinion, after explaining that the two event of emitted and re-absorb of photon are said to be separated by a lightlike, or null interval, it should be explain also that this two event can not be call as single event. As ordinary people, this kind of explanation is lot more make sense.
 P: 29 Events "An event is a given place at a given time. Einstein, and others, suggested that we should think of space and time as a single entity called spacetime. An event is a point p in spacetime. To keep track of events we label each by four numbers: p = (t,x,y,z), where t represents the time coordinate and x, y and z represent the space coordinates (assuming a Cartesian coordinate system)." In coordinate (x,y,z) if we got 2 point with null interval, then it the same point. In coordinate (t,x,y,z) if we got point with null interval, will it be the same point also?
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Can you write more? Because in my understanding, the word experience is include interaction. That is, if an object interact with something else, than it experienced something. That is why, I ask before does Photon interact through out gravity interaction?

Or it just has meaning, we should avoid the word "experience" all together?
Mentor
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 Quote by SysAdmin In coordinate (t,x,y,z) if we got point with null interval, will it be the same point also?
No. (Although there are four-dimensional spaces in which that would be true, the four-dimensional Minkowski spacetime is not one of them. Note also that this isn't just a property of the (t,x,y,z) coordinate system; I'd get the same results in Minkowski spacetime using the (t,r,theta,phi) cordinates).

Consider two points in spacetime: a spot on the the surface of the sun at a particular time; and a spot on the retina of my left eye about nine minutes later as measured by my watch. That's the path of a photon from the surface of the sun to my left eye, and the spacetime interval between those two events is zero.

But they are most certainly NOT the same point: they are nine minutes and 100,000,000 miles apart from each other as far as I'm concerned... and a good thing too.
P: 29
 Quote by Nugatory NOT the same point: they are nine minutes and 100,000,000 miles apart from each other as far as I'm concerned... and a good thing too.
I think that is the source of the confusion, null interval at four-dimensional Minkowski spacetime got translated to null interval in three dimensional ordinary space.

That is, to answer the original question
 Quote by la6ki If yes, then why can't we extend the same logic to a photon? It is moving at 100% of c and if it... had a clock attached to it, we would say that the clock is stopped. But won't the photon still perceive the clock as ticking at its regular rate?
If (the really really if) we can attach a clock to photon, then according to photon, during emitted and re-absorb, the photon will see its clock is not ticking at all. But in four-dimensional Minkowski spacetime understanding, we can not say that the photon still in the same point. The photon simply has null interval.

The different with the twin paradox, the twin still can see through telescope the other clock. In the photon case, we can not look at photon clock. Out clock ticking normally and tell us, about around 9 minute, the photon arrive from the sun. When we check the clock of the photon, it will be not ticking at all. The same thing with the twin paradox, when we check the clock from the twin that travel at nearly the speed of the light (after he is arrive), his clock nearly not ticking at all, which is agree with the his experience. He will said, "I've been gone for 0.00000000001 second, but why you get older nine minute"?
Physics
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 Quote by SysAdmin In coordinate (t,x,y,z) if we got point with null interval, will it be the same point also?
No. Consider the two points (0, 0, 0, 0) and (1000, 1000, 0, 0). These two points are separated by a null interval, but they are obviously not the same point.
 P: 29 "If E1 is the emission of photon at one point in space-time, event E2 is the absorption of that photon at the same other point in space time, then the interval between them is always zero" So how to write down E1 and E2? is that still in E1=p1=(t1, x1, y1, z1) and E2=p2=(t2, x2, y2, z2) ? Than how to calculate the null interval of it? ds^2 = (cdt)^2 - dl^2 since ds=0 is because dt=0 and dl=0 or because c.dt=dl ?
Physics
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 Quote by SysAdmin If (the really really if) we can attach a clock to photon
We can't. It's physically impossible. That means there's no point in even considering such hypotheses; it is like asking what would happen if 2 were an odd number.

 Quote by SysAdmin In the photon case, we can not look at photon clock.
Because there's no such thing. Objects that move on null worldlines are fundamentally different, physically, from objects that move on timelike worldlines. Trying to understand photons by making analogies with objects that move almost at the speed of light is not a good strategy; it focuses attention on the wrong things.
P: 29
 Quote by PeterDonis We can't. It's physically impossible. That means there's no point in even considering such hypotheses; it is like asking what would happen if 2 were an odd number. Because there's no such thing. Objects that move on null worldlines are fundamentally different, physically, from objects that move on timelike worldlines. Trying to understand photons by making analogies with objects that move almost at the speed of light is not a good strategy; it focuses attention on the wrong things.
Then pardon the analogy, It just attempt to extent the OP logic,
 Quote by la6ki If yes, then why can't we extend the same logic to a photon?
What I'm trying to say is, particle moving at 0.9999c will see it's clock ticking slower (after it return to earth) than 0.99c. If the travel is 0.999999999999...9c it will see its clock nearly stop ticking when it return. By that analogy only then when it 1c the ticking will be no longer nearly stop, but stand still.

The same way $$\lim_{x\to a^-} f(x)\ = 0$$ when right left limit in not same, we still draw empty circle at the end of the line, that is at y=0

But then off course the saying is different when v=c. I get it now.

Btw, saying "Time stand still" is better than saying "There is no time" or "Time do not apply", imho.

Can you answer my previous question, how to write down the event of emitted and absorb of photon and why it's null interval is zero, is because dt=0 and dl=0, or because c.dt=dl
Physics
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 Quote by SysAdmin What I'm trying to say is, particle moving at 0.9999c will see it's clock ticking slower (after it return to earth) than 0.99c.
But for this to happen, the moving particle has to turn around. If it just keeps moving at .99c or .9999c, it will never come back to Earth. It's the turning around and coming back, so that it meets up again with a clock that stayed on Earth, that produces the difference in elapsed time.

My point is that, by focusing on the speed the particle is traveling, instead of the fact that it goes out, then turns around and comes back, you're focusing on the wrong thing.

 Quote by SysAdmin Btw, saying "Time stand still" is much more acceptable than saying "There is no time" or "Time do not apply", imho.
Not if saying "time stands still" gives the wrong impression and leads to incorrect inferences.

 Quote by SysAdmin Can you answer my previous question, how to write down the event of emitted and absorb of photon and why it's null interval is zero, is because dt=0 and dl=0, or because c.dt=dl
Because c dt = dl. Look at the two events I wrote down:

(0, 0, 0, 0) and (1000, 1000, 0, 0)

I used units where c = 1, so we have dt = 1000 and dl = 1000; the interval is null because the two are equal.
P: 29
 Quote by PeterDonis Not if saying "time stands still" gives the wrong impression and leads to incorrect inferences.
The "time stands still", is after I read the text book. As reader it less confusing for me.

 Quote by PeterDonis But for this to happen, the moving particle has to turn around. If it just keeps moving at .99c or .9999c, it will never come back to Earth. It's the turning around and coming back, so that it meets up again with a clock that stayed on Earth, that produces the difference in elapsed time. My point is that, by focusing on the speed the particle is traveling, instead of the fact that it goes out, then turns around and comes back, you're focusing on the wrong thing.
I'm trying to see the OP point of view, that is why I edit the comment and add the limit notation, you know, if the limit goes to zero, than it will no surprise if you draw the blank circle in the end of the line, that is y=0

 P: 1,162 just to play Lucifer's Lawyer in the photon cause: I submit that there is ample reason to consider the passage of photon time. Consider: Our concept and measure of time is based on change, Periodic fluctuation. In this regard photons are intrinsically endowed. SO the photon proper time interval between two points is simply the number of cycles of EM phase transition. $\Delta$t=(D/c)f The time difference between two photons of varying frequency is obviously relative Time Dilation Lite. I rest my case
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PeterDonis

 Because c dt = dl. Look at the two events I wrote down: (0, 0, 0, 0) and (1000, 1000, 0, 0) I used units where c = 1, so we have dt = 1000 and dl = 1000; the interval is null because the two are equal.

very helpful insight....I had forgotten the distinction between a null interval in space versus a null interval in space-time.......

a nice brief discussion here for others who may be learning:

http://en.wikipedia.org/wiki/Spaceti...time_intervals

and also 'null events of a photon' trace out a lightcone....illustration here,
in flat space-time

http://en.wikipedia.org/wiki/Light-cone

This kind of light-cone has some characteristics of null surfaces in cosmological,
also accelerated, horizons, right??
Physics