Why don't photons experience time?

[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).

 

[Nugatory]

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?

Your statement ("here must be some duration") is equivalent to saying that there must be some points between the emission event and the absorption event - hence two different events.

 

[nitsuj]

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).

No I'm not asking why they can't be the same event. I see two and only two events for a photon.

 

[nitsuj]

Your statement ("here must be some duration") is equivalent to saying that there must be some points between the emission event and the absorption event - hence two different events.

I agree.

 

[nitsuj]

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

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.

 

[Moonraker]

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.

You should not mix up the observer’s view of the photon with the subject of the thread (what the photon is experiencing).

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.

 

[PeterDonis]

How is what I said equivalent to saying a photon must have two events to have ever existed?

Your earlier post basically said that an "event" is a piece of spacetime that we *model* as a single point. This may be because our model is coarse--i.e., what we model as a "point" is actually a region of spacetime the size of, say, the Bohr radius of a hydrogen atom, or the Compton radius of an electron, because we don't model the details on any finer scale than that.

In the case of a photon, that means a single "event" on the photon's worldline is a piece of spacetime that is small enough compared to the photon's wavelength that we can't distinguish any smaller piece of the photon; as far as our model is concerned, a single event is the smallest piece of the photon we can distinguish. The emission of the photon is one such piece--the first piece where we can distinguish the photon as an object separate from whatever emitted it--and the absorption of the photon is another such piece--the last piece where we can distinguish the photon as an object separate from whatever absorbs it. If those two pieces of spacetime are the same piece, then we can't distinguish the photon as a separate object at all, so as far as our model is concerned it doesn't exist--we can't detect it. So for the photon to be a distinguishable object in our model, its worldline must consist of at least two events: emission and absorption.

 

[PeterDonis]

You should not mix up the observer’s view of the photon with the subject of the thread (what the photon is experiencing).

We've already answered that: photons don't experience anything.

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.

 

[WannabeNewton]

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.

 

[SysAdmin]

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.

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.

 

[SysAdmin]

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?

 

[SysAdmin]

We've already answered that: photons don't experience anything.

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?

 

[Nugatory]

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.

 

[SysAdmin]

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

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"?

 

[PeterDonis]

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.

 

[SysAdmin]

"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

?

 

[PeterDonis]

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.

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.

 

[SysAdmin]

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,

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

 

[PeterDonis]

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.

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.

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.

 

[SysAdmin]

Not if saying "time stands still" gives the wrong impression and leads to incorrect inferences.

The "time stands still", is after I read the textbook. As reader it less confusing for me.

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

Thank you for your answer. :)

 

[Austin0]

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. \Deltat=(D/c)f
The time difference between two photons of varying frequency is obviously relative Time Dilation Lite.

I rest my case

 

[Naty1]

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/Spacetime_interval#Spacetime_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??

 

[PeterDonis]

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

Light cones are null surfaces, and all null surfaces have properties in common, yes.