- #71
ghwellsjr
Science Advisor
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Post #57 by DaleSpam in this thread and post #59 by me might be helpful at this point.
PeterDonis said: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.
PeterDonis said:But that doesn't cover the case v = c, only v < c but getting closer and closer.
PeterDonis said: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.
dm4b said:Just because something leads to confusion doesn't necessarily mean it is fundamentally incorrect.
dm4b said: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.
dm4b said:Exactly, that's the point of a limit. Plot that up and tell me the trend you see.
Exactly, combine that with the trend above and what does that suggest.
dm4b said: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"
PeterDonis said:So I would say that the statement "neutrino oscillation requires neutrinos to have non-zero invariant mass" is, while technically correct, a little misleading since it invites the false implication that *any* kind of "oscillation" requires a non-zero invariant mass.
dm4b said:virtual particles. I really wish they invented a different way to talk about those guys! Even graduate level QFT physics texts could do a better job here.
there is no subtlety of individual photon speed.It is always c.The refractive index concept applies to phase speed of light which has nothing to do with photon's speed.refractive index was used when one has no picture of electrons etc.Also in more modern treatment classical theory of refractive index does agree with quantum explanations.Also the retarding of light in a medium of refractive index n is overall written with a factor c/n.But still it is wrong to say that light in a medium light is retarded at speed c/n.WannabeNewton said: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/questions/1909/how-does-a-photon-travel-through-glass
Naty1 said: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...
I don't understand what you mean by saying that we don't have experimental evidence at v=c.
I'll start a new thread...that may enable you guys to help me understand "what happens when a null like path [a photon] intersects a null like surface [an event horizon]. [just a first thought as a problem statement]The description of event horizons given by general relativity is thought to be incomplete. When the conditions under which event horizons occur are modeled using a more comprehensive picture of the way the universe works, that includes both relativity and quantum mechanics, event horizons are expected to have properties that are different from those predicted using general relativity alone.
In a universe full of particles that can only move at lightspeed (i.e. gauge boson) there should be no possibility of interaction from the particles' point of view because time has stopped for them, according to SR
SysAdmin said:Is it true according current physics knowledge?
SysAdmin said:If two photon travel parallel in empty universe, what will happen?
Naty1 said:This below seems to be one example which I had not seen before...I just stumbled across it...but it conveys the concept I am attempting to describe already:
PeterDonis said: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.
SysAdmin said:Photon live in a instant, it's emitted than re-absorb instantly (according to itself),
SysAdmin said:it doesn't decay, not even at the Schwartzschild Horizon
SysAdmin said:and not interact each other in gravitational force.
SysAdmin said:Does all boson behave like this?
SysAdmin said:Now I understand time dilation is 0 for v=c under SR. Will it be also 0 under GR?
PeterDonis said:You are saying that a photon's worldline contains only a single event; that's not correct, photon worldlines contain multiple events.
nitsuj said:What are the multiple events on a photon's worldline?
PeterDonis said:No, this is not correct. You are saying that a photon's worldline contains only a single event; that's not correct, photon worldlines contain multiple events. You can't use proper time to label the events, but you can use other affine parameters; and the fact that you can't use proper time to label the events does *not* mean that "they all happen at the same time".
(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
Yes, these things are essentially the same in GR. (The main difference is that the coordinate systems we're talking about are defined on proper subsets of spacetime instead of on spacetime). Note that the correct conclusion (in both SR and GR) isn't that photons experience zero time. It's that there's no natural way to assign a meaning to statements about what a massless particle "experiences".SysAdmin said:in the popular media, they always refer to SR as for explanation. That is why i wonder, have they consider GR before saying that "photon don't experienced time"?
SysAdmin said:The question about interaction is just because you said
Fredrik said:It's that there's no natural way to assign a meaning to statements about what a massless particle "experiences".
If only it were true, things would be so much tidier. However, see:PeterDonis said: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.
ghwellsjr said:If only it were true, things would be so much tidier. However, see:
http://en.wikipedia.org/wiki/Two-photon_physics
PeterDonis said: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).
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.SysAdmin said:So, tell me, even in this setup experiment, the saying "photon don't experienced time" is a true statement?ghwellsjr said:If only it were true, things would be so much tidier. However, see:
http://en.wikipedia.org/wiki/Two-photon_physics
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.SysAdmin said: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?
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.SysAdmin said: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?
Right, because "experience" is undefined.SysAdmin said:Reading it again it make me conclude that, translating this math
[tex] \Delta \tau = \Delta t \sqrt{1 - \frac{v^2}{c^2}} [/tex]
in which
[tex]\Delta \tau = 0[/tex]
for "massless" particle (v=c) as "don't experienced time" is incorrect.
I don't see a reason to view it as anything other than a set of events.SysAdmin said:The sequence of event it self can be view as "time" in English language.
ghwellsjr said: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?
. So this event, can not be consider as equivalent of word "event" in English language. But as, PeterDonis said, emitted and re-absorb of photonIt is in a class all by itself, the class that only applies to light
can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).
Fredrik said:I don't see a reason to view it as anything other than a set of events.
Are you upset because I wouldn't answer these questions with a simple "yes" or "no"?SysAdmin said:There is two thing in hereghwellsjr said: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?
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".
"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?SysAdmin said:2. In the link that you give, it simply saying, the event endure by "light" (or mass-less particle, i presume) is
. So this event, can not be consider as equivalent of word "event" in English language.It is in a class all by itself, the class that only applies to light
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.SysAdmin said:But as, PeterDonis said, emitted and re-absorb of photon
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?can't be just a single event; it must contain multiple events (the two endpoints, plus all the ones in between).
ghwellsjr said:Are you upset because I wouldn't answer these questions with a simple "yes" or "no"?
ghwellsjr said: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.
PeterDonis said: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).
PeterDonis said: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).
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.nitsuj said:All events are points but not all points are events.
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?SysAdmin said: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.
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.SysAdmin said: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
That's why I didn't just say "yes" or "no". I gave more explanation.SysAdmin said:Just ask your self, how often simply saying "Photons doesn't experience time?" is followed by that kind of explanation.
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?SysAdmin said: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?
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.SysAdmin said: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.
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 said: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.
ghwellsjr said: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.
SysAdmin said:I see someone in documentary saying something about "photon has no time" and after that he explain no more explanation.
SysAdmin said:So I came here, perhaps there an expert that have better saying in one or two paragraph.
SysAdmin said:Just ask your self, how often simply saying "Photons doesn't experience time?" is followed by that kind of explanation.
SysAdmin said:If time doesn't apply to photon, than how do I should understand photon?