Why don't photons experience time?

[ghwellsjr]

No. Photons don't interact with each other, but that isn't because they're massless; see below for further comment on that. There are massless particles that do interact with each other: gluons, for example.

Nothing. But that's not because they "don't experience time". It's because (a) photons don't interact with each other period; photons only interact with particles carrying electric charge, and photons don't carry any electric charge; and (b) the two photons are moving in the same direction at the same speed, so their worldlines will never intersect, so even if they could interact in principle, they wouldn't.

If only it were true, things would be so much tidier. However, see:

http://en.wikipedia.org/wiki/Two-photon_physics

 

[SysAdmin]

If only it were true, things would be so much tidier. However, see:

http://en.wikipedia.org/wiki/Two-photon_physics

So, tell me, even in this setup experiment, the saying "photon don't experienced time" is a true statement?

 

[SysAdmin]

Another way of looking at it is to ask: a photon gets emitted, and it gets absorbed. Are those two events the same event? Obviously not; they might be light-years apart. So the photon's worldline, which contains both of those events, can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).

Reading it again it make me conclude that, translating this math
\Delta \tau = \Delta t \sqrt{1 - \frac{v^2}{c^2}}
in which
\Delta \tau = 0
for "massless" particle (v=c) as "don't experienced time" is incorrect.

Instead, τ=0 in here has meaning that there is no delta time between event that endure by the particle (from relativistic point of view). The sequence of event it self can be view as "time" in English language.

 

[ghwellsjr]

If only it were true, things would be so much tidier. However, see:

http://en.wikipedia.org/wiki/Two-photon_physics

So, tell me, even in this setup experiment, the saying "photon don't experienced time" is a true statement?

No. Time does not apply to a photon. What applies to a photon is its speed which is defined to be c. This is Einstein's second postulate. You can't measure a photon's speed. We use the defined speed of a photon to define what remote time means and is fundamental to the concept of an Inertial Reference Frame in Special Relativity.

Did you read the link I referenced in post #71?

 

[Fredrik]

And if I'm not mistakenly, there is two particle that has massless, photon and gluon. At least two of the neutrino is suspected has mass. Since gluons are never observed as free particles, it left us using photon to define time "experiences" for massless particle. In other word, it doesn't have comparison for other particle. Is it?

What you're talking about here is what massless particles exist in the real world, or to be more precise, what quantum field theories that involve masssless particles have been found to make excellent predictions about results of experiments. I don't think that's relevant in this discussion. What's relevant is what SR says about classical point particles that move at the speed of light.

Also to clarifying, since this is phenomena for massless particle (that is v=c), it's analyzed using SR and GR. What other theory that can be used to analyzed a massless particle?

There's no other theory. However, SR at least, and maybe GR too, can be viewed as a mathematical framework in which both classical and quantum theories of matter can be defined. And we could consider a quantum field theory instead of a classical theory of point particles, but I think that would only make things much more complicated. I don't see how it could change any of the conclusions.

Reading it again it make me conclude that, translating this math
\Delta \tau = \Delta t \sqrt{1 - \frac{v^2}{c^2}}
in which
\Delta \tau = 0
for "massless" particle (v=c) as "don't experienced time" is incorrect.

Right, because "experience" is undefined.

The sequence of event it self can be view as "time" in English language.

I don't see a reason to view it as anything other than a set of events.

 

[SysAdmin]

No. Time does not apply to a photon. What applies to a photon is its speed which is defined to be c. This is Einstein's second postulate. You can't measure a photon's speed. We use the defined speed of a photon to define what remote time means and is fundamental to the concept of an Inertial Reference Frame in Special Relativity.

Did you read the link I referenced in post #71?

There is two thing in here
1. The statement it self "Why don't photons experience time?". Is the statement is true? In your reply, you give the correct statement (according to you) instead, that is "Time does not apply to photon".

2. In the link that you give, it simply saying, the event endure by "light" (or mass-less particle, i presume) is

It is in a class all by itself, the class that only applies to light

. So this event, can not be consider as equivalent of word "event" in English language. But as, PeterDonis said, emitted and re-absorb of photon

can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).

So how to make a conclusion from that kind of information? How should we view multiple event of photon, that is not just emitted and re-absorb, but something else?

 

[SysAdmin]

I don't see a reason to view it as anything other than a set of events.

So, a good Ph.D in physics, if jokingly saying "Photon don't experience time" in National Geographic channel, he or she should immediately explain set of event of photon? Same thing also, it should be said that "Time don't apply to photon, but set of event does."

 

[ghwellsjr]

No. Time does not apply to a photon. What applies to a photon is its speed which is defined to be c. This is Einstein's second postulate. You can't measure a photon's speed. We use the defined speed of a photon to define what remote time means and is fundamental to the concept of an Inertial Reference Frame in Special Relativity.

Did you read the link I referenced in post #71?

There is two thing in here
1. The statement it self "Why don't photons experience time?". Is the statement is true? In your reply, you give the correct statement (according to you) instead, that is "Time does not apply to photon".

Are you upset because I wouldn't answer these questions with a simple "yes" or "no"?

2. In the link that you give, it simply saying, the event endure by "light" (or mass-less particle, i presume) is

It is in a class all by itself, the class that only applies to light

. So this event, can not be consider as equivalent of word "event" in English language.

"This event"? What are you talking about? In my link, I was talking about the spacetime interval between two arbitrary events. How did you get from there to a single event?

But as, PeterDonis said, emitted and re-absorb of photon

can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).

So how to make a conclusion from that kind of information? How should we view multiple event of photon, that is not just emitted and re-absorb, but something else?

Do you understand that the word "event" in the context of Special Relativity refers to a point in space at an instant of time? It has no duration.

Do you understand what an Inertial Reference Frame (IRF) is in Special Relativity?

Do you understand how an event has a set of coordinates in an IRF?

Do you understand that that same event can have a different set of coordinates in another IRF moving with respect to the first one?

Do you understand how to use the Lorentz Transformation process to convert the coordinates of an event from one IRF to another IRF?

Do you understand that if two events in one IRF have a null spacetime interval, then they will have a null interval in all other IRF's?

What is your level of understanding of Special Relativity? Are you interested in increasing that level of understanding or is your only interest in increasing confusion? If it's the former, then I and others would like to help. If it's the latter, then I'm afraid you're going to get yourself banned.

 

[SysAdmin]

Are you upset because I wouldn't answer these questions with a simple "yes" or "no"?

Why must be so emotional? It's been several years since first time I see someone in documentary saying something about "photon has no time" and after that he explain no more explanation. So I came here, perhaps there an expert that have better saying in one or two paragraph.

What is your level of understanding of Special Relativity? Are you interested in increasing that level of understanding or is your only interest in increasing confusion? If it's the former, then I and others would like to help. If it's the latter, then I'm afraid you're going to get yourself banned.

After reading some reply, I think, simply saying "time doesn't apply to photon" must be follow by next explanation what we know about photon, for example

Another way of looking at it is to ask: a photon gets emitted, and it gets absorbed. Are those two events the same event? Obviously not; they might be light-years apart. So the photon's worldline, which contains both of those events, can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).

Just ask your self, how often simply saying "Photons doesn't experience time?" is followed by that kind of explanation.

Now I'm trying to understand what PeterDonis said. Just don't judge people easily ok. If you said explanation above is not exactly correct, than just say so. If time doesn't apply to photon, than how do I should understand photon? After common people understanding about photon is refute, shouldn't be scientist explain what is the better understanding?

If you want to test my understanding about SR, then I hope you genuinely want to help me, because I can recall that subject from my college time.

 

[nitsuj]

Draw one on a spacetime diagram, and it will be obvious; a photon's worldline is a *line* on the diagram, containing multiple points, just like other lines. The photon's worldline happens to have a Minkowski length of zero, but that's not the right measurement to use for "number of events on the line"; "what it looks like when you draw it on a spacetime diagram" is a much better measurement (there are still some technicalities, but they're minor for this case).

Another way of looking at it is to ask: a photon gets emitted, and it gets absorbed. Are those two events the same event? Obviously not; they might be light-years apart. So the photon's worldline, which contains both of those events, can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).

Yea was kinda my thinking as to what you meant. You seem to be saying that points in spacetime are the same as events in spacetime. All events are points but not all points are events.

Perhaps there is a definition for event as being an arbitrary point along a world line, but I think of an event as being something physical, a "happening" in a specific spacetime location. Not merely an arbitrary point in spacetime.

 

[WannabeNewton]

All events are points but not all points are events.

They are the same thing in a way although I see what you mean by "physically". You could think of a neighborhood of a point in space - time as being a set of events, possibly by situating an observer there.

 

[ghwellsjr]

Why must be so emotional? It's been several years since first time I see someone in documentary saying something about "photon has no time" and after that he explain no more explanation. So I came here, perhaps there an expert that have better saying in one or two paragraph.

OK, so you have gotten more explanation in this thread over and over again. Why do you continue to ask the same questions over and over again?

After reading some reply, I think, simply saying "time doesn't apply to photon" must be follow by next explanation what we know about photon, for example

The only thing that matters about a photon on the relativity forum in the context of an Inertial Reference Frame in Special Relativity is that it travels in a straight line at a speed of c from wherever it starts until it hits something.

Just ask your self, how often simply saying "Photons doesn't experience time?" is followed by that kind of explanation.

That's why I didn't just say "yes" or "no". I gave more explanation.

Now I'm trying to understand what PeterDonis said. Just don't judge people easily ok. If you said explanation above is not exactly correct, than just say so. If time doesn't apply to photon, than how do I should understand photon? After common people understanding about photon is refute, shouldn't be scientist explain what is the better understanding?

I thought in my previous post I agreed with what PeterDonis said. 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?

If you want to test my understanding about SR, then I hope you genuinely want to help me, because I can recall that subject from my college time.

It's not that I want to test your understanding to pass or fail you, I just don't know if when I talk about an IRF or an event or doing a Lorentz Transformation or any of a number of other aspects of SR, it is making sense to you.

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".

 

[ghwellsjr]

Yea was kinda my thinking as to what you meant. You seem to be saying that points in spacetime are the same as events in spacetime. All events are points but not all points are events.

Perhaps there is a definition for event as being an arbitrary point along a world line, but I think of an event as being something physical, a "happening" in a specific spacetime location. Not merely an arbitrary point in spacetime.

In any given scenario in Special Relativity, there is something "happening" at every arbitrary point, not just along a world line but every where else and at all times. We just focus our attention on certain ones in order not to be overwhelmed with all the data that would actually be happening in the real world.

 

[nitsuj]

In any given scenario in Special Relativity, there is something "happening" at every arbitrary point, not just along a world line but every where else and at all times. We just focus our attention on certain ones in order not to be overwhelmed with all the data that would actually be happening in the real world.

Never mind use of the word "happening", the definition of event encompasses that idea, along with the specific point in spacetime. Classically, what event is happening in empty space?

We are talking about events for a photon. which don't include all other events in the universe.

Can a photon be observed without interacting with it?

 

[PeterDonis]

I see someone in documentary saying something about "photon has no time" and after that he explain no more explanation.

The lesson to be learned from this is that "documentaries" like this are not intended to actually teach you physics, even when physics professors appear on them.

So I came here, perhaps there an expert that have better saying in one or two paragraph.

This is a good place to come to get better information, but part of the better information you get is going to be learning better questions to ask, not just learning answers to the questions that you thought of when you were watching the documentary.

Just ask your self, how often simply saying "Photons doesn't experience time?" is followed by that kind of explanation.

Part of the problem, as ghwellsjr pointed out, is that we don't know what knowledge you already have, or why you are asking these questions. That's why we don't always know how much additional explanation to give.

If time doesn't apply to photon, than how do I should understand photon?

By looking at the equations that apply to massless particles. For example, the equation that says \tau = 0. There is nothing wrong with that equation; the only problem is trying to interpret \tau as "time experienced by a photon". That doesn't work because the case \tau = 0 is fundamentally different from the case \tau > 0.

In general, if you really want to understand photons, or anything else in physics, you have to learn the math. Any explanation in ordinary language is going to be limited, and is eventually going to lead you down the wrong path. But if you learn the math, you can use it to get answers without having to worry about how to "interpret" it in ordinary language. You can use the equation \tau = 0 without having to worry about whether \tau is or is not "the time experienced by a photon". It doesn't matter; the math is the same either way.

 

[Moonraker]

A photon gets emitted, and it gets absorbed. Are those two events the same event? Obviously not; they might be light-years apart.

Obviously? I don’t see any evidence that this is obvious. It rather seems to be the reason why we cannot close this thread. Admitting only one event (emission = absorption) would comply with all SR principles of time dilation, length contraction and spacetime interval. Also, quantum physics are not concerned.

 

[PeterDonis]

Obviously? I don’t see any evidence that this is obvious.

An event is a single point in spacetime. If a photon is emitted and absorbed at the same point in spacetime, it doesn't exist in the first place.

 

[nitsuj]

An event is a single point in spacetime. If a photon is emitted and absorbed at the same point in spacetime, it doesn't exist in the first place.

This is false, In reality an event is not a specific point in spacetime. The event can be point "a" (emission) through to point "b" (absorption). At least as far as I understand the linguistic definition of event in the context of relativity.

There must be some duration, and that's to your own point "If a photon is emitted and absorbed at the same point in spacetime, it doesn't exist in the first place."

 

[Naty1]

experts: this is a great thread... very insightful...but not so easy to grasp
in short order...so SysAdmin is struggling a bit, I think, as am I, to understand your perspective...what the math tells us and what it doesn'.

Here are a few excerpts from posts you experts have made which I noted for myself: [no particular order]

...that the concept of "passage of time" does not apply to a photon.

we can't define a unit vector that points along its worldline, because its worldline is null--any vector that points along the worldline has length zero,

...But light from the sun does appear magically on Earth …We cannot observe {a photon} ... emission from the sun as a separate event from its arrival on earth. We cannot observe its progress as it travels the space between the sun and the Earth. All we can observe is the instant it actually arrives on Earth.]

...the word "event" in the context of Special Relativity refers to a point in space at an instant of time? It has no duration

...the case τ=0 is fundamentally different from the case τ>0.

...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

perhaps my favorite:

...If you have a null interval then why would you identify that with proper time rather than proper length?

...photons cannot couple directly to each other since they carry no charge, but they can interact through higher-order processes...

[I think that last one is more QM than SR.]

In summary, lots to consider, especially for those of us who have not studied this in all it's mathematical glory!

 

[Moonraker]

An event is a single point in spacetime. If a photon is emitted and absorbed at the same point in spacetime, it doesn't exist in the first place.

The photon is not submitted to our spacetime. We are observing it as a lightbeam phenomenon, not as an IRF. If a photon does not follow the same rules as IRFs this does not mean it doesn’t exist.

 

[Nugatory]

The photon is not submitted to our spacetime. We are observing it as a lightbeam phenomenon, not as an IRF. If a photon does not follow the same rules as IRFs this does not mean it doesn’t exist.

Any reference frame, whether inertial or not, is a convention for assigning coordinates to points of spacetime. You can use one to assign coordinates to the endpoints of a photon's path just as easily and effectively as any other points in spacetime.

 

[Nugatory]

In reality an event is not a specific point in spacetime.

Do you have a source for that statement? I have never seen "event" defined as anything except a specific point in spacetime.

 

[PeterDonis]

In reality an event is not a specific point in spacetime. The event can be point "a" (emission) through to point "b" (absorption). At least as far as I understand the linguistic definition of event in the context of relativity.

You understand it incorrectly. Check any relativity textbook. An event is a single point in spacetime; if you are using a coordinate chart, an event is mapped to a single 4-tuple (t, x, y, z) of coordinates.

There must be some duration, and that's to your own point "If a photon is emitted and absorbed at the same point in spacetime, it doesn't exist in the first place."

Yes, but this is equivalent to saying that for a photon to exist, its worldline must consist of more than one event.

 

[WannabeNewton]

Do you have a source for that statement? I have never seen "event" defined as anything except a specific point in spacetime.

I doubt there is a source for that because what you said is a textbook definition so it is probably just a misconception on the part of nitsuj

 

[Moonraker]

Any reference frame, whether inertial or not, is a convention for assigning coordinates to points of spacetime. You can use one to assign coordinates to the endpoints of a photon's path just as easily and effectively as any other points in spacetime.

Right, the lightlike movement is the view of observers. But the photon might not experience the same kind of spacetime as we do. It is acting as a remote momentum which is skipping (vacuum) space. I don’t see how to exclude that this was only one event.

 

[PeterDonis]

But the photon might not experience the same kind of spacetime as we do.

Huh? A photon's worldline is a curve in spacetime, just like the worldline of a timelike object. The only difference is that a photon's worldline has a Minkowski length of zero. That doesn't change the fact that it's a *line*, composed of multiple points, and therefore multiple events. Please check a relativity textbook for the proper definition of "event", "worldline", "spacetime", etc.

It is acting as a remote momentum which is skipping (vacuum) space.

This doesn't make sense. Are you saying that a photon doesn't have a worldline--that it just somehow "skips" from emission to absorption without traversing the spacetime in between? That's false; again, check a relativity textbook.

I don’t see how to exclude that this was only one event.

Once more, check a relativity textbook for the proper definition of an "event".

 

[nitsuj]

Do you have a source for that statement? I have never seen "event" defined as anything except a specific point in spacetime.

Nope no source. In reality, an event is not merely a specific point in spacetime. You can say it is specific point and need to calculate as if it were. An event is treated/considered as a point, but occupies spacetime necessarily larger than a specific point.

 

[nitsuj]

You understand it incorrectly. Check any relativity textbook. An event is a single point in spacetime; if you are using a coordinate chart, an event is mapped to a single 4-tuple (t, x, y, z) of coordinates.
Yes, but this is equivalent to saying that for a photon to exist, its worldline must consist of more than one event.

Well of course you need to assign coordinates for the event, or in other words a point.I don't see how to figure that last statement. How is what I said equivalent to saying a photon must have two events to have ever existed?

 

[WannabeNewton]

Nope no source. In reality, an event is not merely a specific point in spacetime. You can say it is specific point and need to calculate as if it were. An event is treated/considered as a point, but occupies spacetime necessarily larger than a specific point.

This literally makes no sense. Please clarify your last sentence.

 

[Nugatory]

I don't see how to figure that last statement. How is that saying a photon must have two events to have ever existed?

There's an emission event and and an absorption event. That's two events.

So are you asking why they can't be the same event? Well, the photon only exists between the two events, and if they're the same then there's no "between".

(This last bit is probably more intuitive if you treat the light signal as an electromagnetic wave. I have a nagging suspicion that you are thinking of a "photon" as a "particle" as a "little teeny grain of sand" and picking up more distortion at each step).