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Why don't photons experience time?

by la6ki
Tags: experience, photons, time
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Naty1
#55
Jan28-13, 08:54 AM
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Quote by andrien

photons always travel at speed c.After entering a medium it does not change
This is false and the lack of it traveling at c in generality after entering a medium is a very well known fact.
The individual photons DO travel at c; but as they progress thru the material, delays are encountered so the overall, effective transmission rate is slower than c:

From the wiki reference above:

In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few meters per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances.
WannabeNewton
#56
Jan28-13, 09:08 AM
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Quote Quote by Naty1 View Post
The individual photons DO travel at c; but as they progress thru the material, delays are encountered so the overall, effective transmission rate is slower than c:
I was talking about light as a wave traveling through the medium. If you want to talk about the individual photons then it is much more subtle than that. This is not related to the thread so for now take a look at: http://physics.stackexchange.com/que...-through-glass
Naty1
#57
Jan28-13, 09:16 AM
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La6ki:
By the way, I hope you won't mind if I ask a question which is not directly related to the thread title, but one for which I don't want to start a new thread: .... From the frame of reference of our stationary electron the density of the electrons in the wire will increase due to length contraction and hence it will look like the wire is negatively charged. ....
Short answer: yes, that apparently works.....

there have been discussions in these forums about it...search if you want more.
I had not seen such before the discussions here and found the concepts worthwhile.

Check out here: http://en.wikipedia.org/wiki/Relativ...ectromagnetism


I searched...and got sidetracked......found BenCrowell posted about a text he likes on the subject here:

http://www.physicsforums.com/showthr...ectromagnetism
A.T.
#58
Jan28-13, 09:36 AM
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Quote Quote by PeterDonis View Post
I would say that SR says there is no such thing as "the rest frame of a photon", not that it says nothing at all about it.
It simply doesn't make any predictions about physics in the rest frame of a photon.
PeterDonis
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Jan28-13, 09:55 AM
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Quote Quote by A.T. View Post
It simply doesn't make any predictions about physics in the rest frame of a photon.
I hate to keep nit-picking about language, but the way this is phrased implies (at least, I expect it will imply to a lot of newbies) that there *is* something called "the rest frame of the photon", when the whole point is that there isn't. SR says there is no such thing as "the rest frame of the photon". IMO that's the way to phrase it.
Naty1
#60
Jan28-13, 11:16 AM
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la6ki:
In case you haven't seen it, there are some closely related perspectives here:

Light doesn't travel through time?
http://www.physicsforums.com/showthr...ht=photon+time

where you should check the posts from Fredrik and Dalespam...and follow the few links provided.
Naty1
#61
Jan28-13, 11:27 AM
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Can somebody tell me what "CTC"s are [from post #26]:

Quote by Naty1
I started, then stopped, a search in Google for
"who said 'Eternity is no time at all for a photon'....because I have forgotten....
and what turns up....THIS THREAD>> OMG We ARE being watched!!!!!!!!!!!!
BCrowell:

If CTCs are going to turn up on PF, the relativity subforum would be the logical place.
Fredrik
#62
Jan28-13, 11:44 AM
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CTC = Closed Timelike Curve.

If there's a CTC in spacetime, you could in principle move as described by it and meet a younger version of yourself. BCrowell was joking. It got a LOL out of me, so the joke works on some nerds at least. I wouldn't try it as a pick-up line though.
dm4b
#63
Jan28-13, 12:08 PM
P: 317
It was my understanding that the logic (put in laymen's terms, perhaps) that the reason a neutrino cannot be massless is because it can undergo neutrino oscillations, which was the solution to the solar neutrino problem. Neutrino Oscillations are a time-dependent phenomenon and since a massless particle does not experience the passage of time, it would not be able to experience these oscillations. Since it has been shown neutrinos do undergo the oscillations they therefore cannot be massless.

There was a show on the science channel that said as much too.

Also, when you analyze the time dilation formula in the limit of v=c, while the equations are still valid, as you get infinitesimally close to v=c, deltaT moves infinitesimally close to zero.
Naty1
#64
Jan28-13, 12:23 PM
P: 5,632
Minor Notes:
1]Closed Timelike Curve....haven't seen that one in a long time!

2]It is a lot more relaxing to read you experts picking each other apart than reading when you pick ME apart!!!!

3] la6ki: You lucked out getting all these experts to offer perspectives!! Great discussion.


la6ki: I hope after reading the posts from a number of the most knowledgeable people here you have a perspective now on WHY I posted early on:

Well, nobody knows for sure what photons experience.....blah,blah....Whatever the exact meaning, I hope eventually some part of the FAQ explanation above will be found incorrect.
Now I have been in these forums enough to know better....I should have known THAT wording would get some riled up...far better had I said something like "I hope eventually some part of the FAQ can be revised as a result of new discoveries."

Some of the ways the experts phrase it in this discussion:

"....there are no inertial coordinate systems that are comoving with a massless particle"

That means we can't even define concepts like "rate of time flow" for a photon.
...Photons don't have to "experience time" to interact, either with a gravitational field or with anything else
....we can't define a meaningful concept of "velocity of one photon relative to another photon".
.... We are talking about classical point particles whose world lines are null geodesics in Minkowski spacetime
.... I would say that SR says there is no such thing as "the rest frame of a photon",
You can decide for yourself if you think those are conclusive answers to your original question about time for a photon. Such answers, which I think ARE completely accurate within GR, make me SUSPECT we have more to learn. They just seem inadequate to me. I say "we can do better.' Now if quantum theory offered more precise answers I'd be more comfortable...but that is another bag of worms worse than this one!!

Another way to express my concern is that I think most of these posters would agree QM and GR have some problems at what we call singularities, apparent infinities....like the center of a black hole and at the big bang. Most probably don't think we have the full answers at those points; my question is whether we should consider that maybe we don't have a full understanding at v = c.

You can decide for yourself what you make of all this.
PeterDonis
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Jan28-13, 12:48 PM
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Quote Quote by Naty1 View Post
Another way to express my concern is that I think most of these posters would agree QM and GR have some problems at what we call singularities, apparent infinities....like the center of a black hole and at the big bang. Most probably don't think we have the full answers at those points; my question is whether we should consider that maybe we don't have a full understanding at v = c.
I'm not sure exactly what aspects of QM you're referring to, but the singularities at the center of a black hole and at the Big Bang have nothing to do with any understanding (or lack thereof) at "v = c".

IMO we understand perfectly well what happens at v = c; the behavior of null curves, and how it differs from the behavior of timelike curves, is well understood. The fact that it's difficult to describe this behavior to lay people in English is because English is not well suited to describing physics, not because the physics is not well understood.
PeterDonis
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Jan28-13, 12:51 PM
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Quote Quote by dm4b View Post
(put in laymen's terms, perhaps)
Exactly: that's the point. Putting things in laymen's terms distorts them.

Quote Quote by dm4b View Post
There was a show on the science channel that said as much too.
Which show? I'd be interested to see if it is on the list of "usual suspects" that tend to generate these PF threads.

Quote Quote by dm4b View Post
when you analyze the time dilation formula in the limit of v=c, while the equations are still valid, as you get infinitesimally close to v=c, deltaT moves infinitesimally close to zero.
The Lorentz transformation is *not* valid at v = c; the factor that goes to zero is in the denominator, and you can't divide by zero.
Naty1
#67
Jan28-13, 01:56 PM
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I'm not sure exactly what aspects of QM you're referring to, but the singularities at the center of a black hole and at the Big Bang have nothing to do with any understanding (or lack thereof) at "v = c".
not yet! [I have high hopes!!]

You seem to think relativity is more complete than I..you may be right. We haven't any experimental evidence I can think of at either [the 'infinities', nor at v= c] yet, so a discussion seems moot,maybe that's your point, and that's ok by me....

Perhaps I have an inflated hope for science?
PeterDonis
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Jan28-13, 02:24 PM
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Quote Quote by Naty1 View Post
You seem to think relativity is more complete than I..you may be right.
I think my comments were more in the nature of clarifying exactly where the incompleteness is.

Quote Quote by Naty1 View Post
We haven't any experimental evidence I can think of at either [the 'infinities',
True.

Quote Quote by Naty1 View Post
nor at v= c]
Here I disagree: we've studied the behavior of light in great detail. That counts as evidence of "v = c".
dm4b
#69
Jan28-13, 03:00 PM
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Quote Quote by PeterDonis View Post
Exactly: that's the point. Putting things in laymen's terms distorts them.
I think it boils down to the same thing. Massless particles do not sense the passage of time, or however else one may prefer to say that.


Quote Quote by PeterDonis View Post
The Lorentz transformation is *not* valid at v = c; the factor that goes to zero is in the denominator, and you can't divide by zero.

Um, that's why I said analyzed in the limit as v=c. Perhaps better wording would have been as v goes to c.
PeterDonis
#70
Jan28-13, 03:12 PM
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Quote Quote by dm4b View Post
I think it boils down to the same thing. Massless particles do not sense the passage of time, or however else one may prefer to say that.
But how one prefers to say it has a huge effect on what inferences lay people draw from it. Say that massless particles are fundamentally different physically from massive ones, so the concept of "passage of time" doesn't even apply to massless particles, and you get questions about why that is, which leads to a fruitful discussion about the behavior of timelike vs. null vectors or worldlines and the way that Lorentz transformations separately take each of those subspaces of Minkowski spacetime into itself.

But say that massless particles do not sense the passage of time, and you get interminable threads about how this means photons don't move in time at all, only in space, how a photon can see the entire Universe all at once, etc., etc., leading to all sorts of further inferences that are just false. Then you have to patiently go back and explain how, when you said massless particles do not sense the passage of time, you didn't really mean that, but something else.

Quote Quote by dm4b View Post
Um, that's why I said analyzed in the limit as v=c. Perhaps better wording would have been as v goes to c.
But that doesn't cover the case v = c, only v < c but getting closer and closer. Also, the statement as you gave it is frame-dependent: an object can be moving at v = .9999999999999999c in one frame but be at rest in another, and its "deltaT" changes in concert with that. But an object that is moving at v = c in one frame is moving at v = c in every frame. The two kinds of objects (timelike vs. lightlike) are fundamentally different.
ghwellsjr
#71
Jan28-13, 03:30 PM
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Post #57 by DaleSpam in this thread and post #59 by me might be helpful at this point.
dm4b
#72
Jan28-13, 03:45 PM
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Quote Quote by PeterDonis View Post
But how one prefers to say it has a huge effect on what inferences lay people draw from it. Say that massless particles are fundamentally different physically from massive ones, so the concept of "passage of time" doesn't even apply to massless particles, and you get questions about why that is, which leads to a fruitful discussion about the behavior of timelike vs. null vectors or worldlines and the way that Lorentz transformations separately take each of those subspaces of Minkowski spacetime into itself.

But say that massless particles do not sense the passage of time, and you get interminable threads about how this means photons don't move in time at all, only in space, how a photon can see the entire Universe all at once, etc., etc., leading to all sorts of further inferences that are just false. Then you have to patiently go back and explain how, when you said massless particles do not sense the passage of time, you didn't really mean that, but something else.
Just because something leads to confusion doesn't necessarily mean it is fundamentally incorrect. A more technical and exact discussion can alleviate the chances of that and be more fruitful, but that doesn't mean the same kind of confusion can't happen there too.

Quote Quote by PeterDonis View Post
But that doesn't cover the case v = c, only v < c but getting closer and closer.
Exactly, that's the point of a limit. Plot that up and tell me the trend you see.


Quote Quote by PeterDonis View Post
Also, the statement as you gave it is frame-dependent: an object can be moving at v = .9999999999999999c in one frame but be at rest in another, and its "deltaT" changes in concert with that. But an object that is moving at v = c in one frame is moving at v = c in every frame. The two kinds of objects (timelike vs. lightlike) are fundamentally different.
Exactly, combine that with the trend above and what does that suggest.

Combine that with the fact that neutrinos would not able to undergo neutrino oscillations if they had zero mass and what does that suggest.

It all suggests that "massless particles do not sense the passage of time"


In short, I think that saying the phrase in quotes is dead wrong would be as misleading as saying it is technically exact.


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