Does a photon know how fast to leave its emitter?

[stever]

Does a photon "know" how fast to leave its emitter?

Does a photon "know" how fast to leave its emitter, which is moving relative to the photon's receiver, so as to be traveling at c when it reaches the receiver?

 

[Bill_K]

The photon leaves at a speed of c in the emitter's frame. This is also c in the receiver's frame, or in any other frame. The speed of a beam of light is the same in every frame.

 

[atyy]

The photon knows by traveling not relative to the emitter or receiver, but by traveling on the "null geodesic" of spacetime.

 

[Elroch]

c is the speed at which the electromagnetic field propagates through the vacuum. The quanta of this field are individually entirely ignorant of this fact (I presume).

[On the other hand, every chemical and electron signal in our brains is based entirely on the electromagnetic force, so the processes that make us conscious are entirely due to the interactions of real and virtual photons. Fortunately they all travel at the right speed without us worrying about it.]

 

[arabianights]

the photons also knows to take the shortest path when it crosses the boundary between matters with different densities. it's as if it knows the path it's going to travel is the most efficient one and less time-consuming, perfect example of effect before cause

 

[phinds]

the photons also knows to take the shortest path when it crosses the boundary between matters with different densities. it's as if it knows the path it's going to travel is the most efficient one and less time-consuming, perfect example of effect before cause

Hm ... I don't believe simple refraction is effect before cause. I'd be interested to hear comments from folks who know more about this than I do. Any optics gurus out there?

 

[stever]

Unfortunately I don't know what a null geodesic of spacetime is. I googled it but it looks like it will take a while to study it.

Basically I am looking for some explanation of how light speed gets "adjusted" to the motion of emitters or receivers so as to remain the same.

I am confused by the possibility that the one way speed of light, which cannot be measured, may vary while the round trip is at c. If one way speed does vary, it seems a little strange that light could be traveling at different speeds in different directions (for example within the same object).

Another problem is that I see no explanation in the slowing of time and contraction of length of the moving object, as the two mechanisms (slowing and contraction) affect equally both oncoming light and catching up light which, if they have different speeds, would continue to have them.

 

[phinds]

Basically I am looking for some explanation of how light speed gets "adjusted" to the motion of emitters or receivers so as to remain the same.

Light does not adjust itself to emitters and receivers. It is utterly indifferent to them. When a photon is created, it immediately travels at c and is seen to do so in all reference frames.

I am confused by the possibility that the one way speed of light, which cannot be measured, may vary while the round trip is at c. If one way speed does vary, it seems a little strange that light could be traveling at different speeds in different directions (for example within the same object).

The problem is NOT that the one-way speed of light is different than any other speed of light; there is only one speed of light in a vacuum, but what IS a problem is that we can't properly MEASURE the one-way speed of light.

Another problem is that I see no explanation in the slowing of time and contraction of length of the moving object, as the two mechanisms (slowing and contraction) affect equally both oncoming light and catching up light which, if they have different speeds, would continue to have them.

These effects do not affect light. Light travels at c. Period.

 

[stever]

I read on Wikipedia that the Lorentz theory of relativity uses one-way speeds of light that vary, and that Lorentz's theory is indistinguishable from Einstein's otherwise.

 

[atyy]

I read on Wikipedia that the Lorentz theory of relativity uses one-way speeds of light that vary, and that Lorentz's theory is indistinguishable from Einstein's otherwise.

It's very unclear what Lorentz's theory of relativity was, and probably of interest only from a historical viewpoint.

There is a view of special relativity which is Lorentzian in spirit, eg. Bell's "Lorentzian pedagogy". It is special relativity described in any particular Lorentz inertial frame. It is equivalent to special relativity.

Special relativity is a theory that spacetime and all matter have a particular symmetry called Lorentz symmetry (more strictly, Poincare symmetry). Lorentz symmetry means that spacetime at each event is divided into past, future, and elsewhere. A null geodesic forms the boundary between elsewhere and the past or future. A wave traveling on a null geodesic will be seen to have the same "privileged speed" in all Lorentz inertial frames. Lorentz symmetry and the fact that electromagnetic fields have an additional symmetry called conformal symmetry imply that electromagnetic waves travel on null geodesics, which is the same as saying that light travels at the "privileged speed" in all Lorentz inertial frames. The "privileged speed" is therefore usually called the speed of light. In contrast to light, the fields describing a massive emitter and massive receiver have Lorentz symmetry, but not conformal symmetry.

 

[nitsuj]

Unfortunately I don't know what a null geodesic of spacetime is. I googled it but it looks like it will take a while to study it.

I find it is made overly complicated for a layman like myself.

Know that c is the fastest speed, anything that travels this fast, i.e. 299,792,458 meters in one second is following a "null" geodesic path.


In the image below the yellow line is a null geodesic path, this is the path a photon would travel since they travel at c.
any coordinate on the graph that is on the right side of the null path is spacelike, timelike is on the left side.

This is a very generalized description, but is enough to give the term a context. I came across the null geodesic term and googled it like you did. It wasn't until I started reading about intervals that it became clear what a "null geodesic path" is. Said differently reading about (and asking questions here) intervals made clear what the terms spacelike, timelike and null mean in this context.

for a mathless approach check out spacetime diagrams.


c is always the same speed;
So something traveling at 0.5c emits a stream of photons, those photons are traveling at the same speed the always do, they're not "adjusted" in anyway.

When the dude traveling at 0.5c calculates the speed of those photons it will be the same as everyone elses calculated value of c.

It is the measurements that are "adjusted".

Specifically time dilation and length contraction "adjust" accordingly so that the calculated value of c is always the same, no matter your relative speed to those photons.

(image below is from wikipedia)

 

[PAllen]

Hm ... I don't believe simple refraction is effect before cause. I'd be interested to hear comments from folks who know more about this than I do. Any optics gurus out there?

Not at all an optics guru, but a simple mathematical observation may demystify this. When one minimizes an integral quantity (by path variation), the result (Euler-Lagrange equation) is a strictly local differential criterion. This criterion amounts to following the local 'path of least resistance' or 'straightest possible path - in a possibly abstract phase space'. It happens that this strictly local condition is a necessary (but not sufficient) condition for an overall extremum.

In my view, it is better to think about the light, system, or whatever directly following this local condition rather than 'magically' discovering a global extremum.

 

[phinds]

Not at all an optics guru, but a simple mathematical observation may demystify this. When one minimizes an integral quantity (by path variation), the result (Euler-Lagrange equation) is a strictly local differential criterion. This criterion amounts to following the local 'path of least resistance' or 'straightest possible path - in a possibly abstract phase space'. It happens that this strictly local condition is a necessary (but not sufficient) condition for an overall extremum.

In my view, it is better to think about the light, system, or whatever directly following this local condition rather than 'magically' discovering a global extremum.

I don't follow that, but let me ask a slightly different question which may already be answered by your statement. Would it be fair to say that refraction is caused ENTIRELY because of what happens at the BOUNDARY between materials (a very local effect) and NOT because of anything outside the boundary (which would be a global effect and COULD imply effect before cause). This seems very reasonable to me because once the light leaves the boundary, it does not change direction again.

 

[PAllen]

I don't follow that, but let me ask a slightly different question which may already be answered by your statement. Would it be fair to say that refraction is caused ENTIRELY because of what happens at the BOUNDARY between materials (a very local effect) and NOT because of anything outside the boundary (which would be a global effect and COULD imply effect before cause). This seems very reasonable to me because once the light leaves the boundary, it does not change direction again.

Yes, you could say that, and I meant to imply that mathematically. Further, if you consider a material with continuously varying index of refraction, the light is really (oops, there's that problem word again..) responding only to local conditions.

 

[phinds]

Yes, you could say that, and I meant to imply that mathematically. Further, if you consider a material with continuously varying index of refraction, the light is really (oops, there's that problem word again..) responding only to local conditions.

Thanks

 

[Elroch]

I think if Feynman was asked this question, he would say that a photon takes every path simultaneously, including those that are much faster or slower than the speed of light. Then all the amplitudes for all the paths are added up (a huge integration). If you do this calculation, the only paths that get left with amplitudes different to zero are the equivalent of straight lines in curved spacetime at a speed of c. This is truly a wonderful thing, and may be a hint why the Universe behaves the way it does.

 

[phinds]

I think if Feynman was asked this question, he would say that a photon takes every path simultaneously, including those that are much faster or slower than the speed of light. Then all the amplitudes for all the paths are added up (a huge integration). If you do this calculation, the only paths that get left with amplitudes different to zero are the equivalent of straight lines in curved spacetime at a speed of c. This is truly a wonderful thing, and may be a hint why the Universe behaves the way it does.

I think you are overextrapolating Feynman. I don't think he believed in FTL.

 

[Buckleymanor]

Does a photon "know" how fast to leave its emitter, which is moving relative to the photon's receiver, so as to be traveling at c when it reaches the receiver?

I don't know what you mean by "know" photons know nothing.

 

[phinds]

I don't know what you mean by "know" photons know nothing.

The word was put in quotes and in context the meaning seemed quite clear.

 

[yoron]

Well, define SpaceTime as one wave equation, define interactions as the wave equation 'falling out' locally, relative interference as in the Feynman definition and you have one answer :) maybe. Or .. .. ..

Refraction is a index defined relative 1, as I understands it. Where one is 'space' always being 1. Defining 'space' globaly as 'densities', for example relative 'gravity', should then invalidate the index of refraction as it no longer have a valid definition to be measured relative.

A photon 'exists' in two ways to us. Due to the 'recoil' observed in a 'source' as demanded by the conservation laws, and in its annihilation at your retina. There is nowhere in between it can be said to 'exist' without a measurement.

But it also has to do with what you 'think'. Do you expect a propagation? Then you expect a 'path'.

 

[Elroch]

I think if Feynman was asked this question, he would say that a photon takes every path simultaneously, including those that are much faster or slower than the speed of light. Then all the amplitudes for all the paths are added up (a huge integration). If you do this calculation, the only paths that get left with amplitudes different to zero are the equivalent of straight lines in curved spacetime at a speed of c. This is truly a wonderful thing, and may be a hint why the Universe behaves the way it does..

I think you are overextrapolating Feynman. I don't think he believed in FTL.

I checked what Feynman wrote and found it supported my statement completely. For example he discussed an example where a photon was emitted at one point and detected at another after reflection from a mirror, and considered the amplitudes for a range of paths between these two points in space-time. Obviously almost all of these paths are longer than the straight line path, implying a faster speed, but the amplitudes were all included.

 

[PAllen]

I checked what Feynman wrote and found it supported my statement completely. For example he discussed an example where a photon was emitted at one point and detected at another after reflection from a mirror, and considered the amplitudes for a range of paths between these two points in space-time. Obviously almost all of these paths are longer than the straight line path, implying a faster speed, but the amplitudes were all included.

This agrees with my understanding of what Feynman wrote on this. I have no idea whether this was the only way he looked at the question. However, I like that, when thinking more classically, it is completely unnecessary to invoke global influences to explain any least action principles, e.g.:

- path of light
- evolution of a system defined by a Lagrangian

 

[phinds]

I checked what Feynman wrote and found it supported my statement completely. For example he discussed an example where a photon was emitted at one point and detected at another after reflection from a mirror, and considered the amplitudes for a range of paths between these two points in space-time. Obviously almost all of these paths are longer than the straight line path, implying a faster speed, but the amplitudes were all included.

Wow ... I'm surprized. Thanks for that correction.

 

[Buckleymanor]

The word was put in quotes and in context the meaning seemed quite clear.

It has no meaning the photon does not know how fast to leave it's emitter it is an inate object it knows nowt.

 

[phinds]

It has no meaning the photon does not know how fast to leave it's emitter it is an inate object it knows nowt.

Which is why the word was put in quotes. You are clearly being WAY too literal here and ignoring the conventions of the English language.

 

[Buckleymanor]

Which is why the word was put in quotes. You are clearly being WAY too literal here and ignoring the conventions of the English language.

Which is why I asked.

I don't know what you mean by "know" photons know nothing.

And included the quotes and you replied.

The word was put in quotes and in context the meaning seemed quite clear.
Y 01:34 AM

Which is as clear as mud.

 

[phinds]

Which to me is as clear as mud.

Then I'd say you have a poor understanding of the niceties of the English language. I had no trouble understanding what he meant.

 

[Buckleymanor]

Then I'd say you have a poor understanding of the niceties of the English language. I had no trouble understanding what he meant.

Well if you would care to enlighten me with some of the niceties of the English language I would be more than gratefull.Maybe with the definition of know in brackets or without, literal or conventional.
It would be most enligtening and might shed some light on my misunderstanding.

 

[phinds]

I have a feeling that you are being sarcastic, but on the off chance that I'm wrong about that:

It's very common in English to attribute to an inanimate object, for the purpose of discussion, an attribute of volition and/or intelligence even though the speaker knows full well that is is not actually the case and that he does not want this to be interpreted literally, which would be moronic, so he puts the attribute in quotes exactly as was done in this case.

When, in English, you see a word in quotes which would be nonsensical if NOT in quotes, then there's a good chance it is being used in this way.

 

[Buckleymanor]

I have a feeling that you are being sarcastic, but on the off chance that I'm wrong about that:

It's very common in English to attribute to an inanimate object, for the purpose of discussion, an attribute of volition and/or intelligence even though the speaker knows full well that is is not actually the case and that he does not want this to be interpreted literally, which would be moronic, so he puts the attribute in quotes exactly as was done in this case.

When, in English, you see a word in quotes which would be nonsensical if NOT in quotes, then there's a good chance it is being used in this way.

No sarcasm intended.
Often words are put in quotes that are not used in that way.
In context and balance you are probably right and I thank you for your tolerence and explanation.

 

[phinds]

No sarcasm intended.
Often words are put in quotes that are not used in that way.
In context and balance you are probably right and I thank you for your tolerence and explanation.

Happy to help, and yes you're right that you will often see words in quotes used in other ways, and sometimes in nonsensical ways, but intelligent writers of English will get this one right, I think. I'm a bit of a bear on grammar and language usage so I tend to notice such things.

 

[nitsuj]

...yes you're right that you will often see words in quotes used in other ways, and sometimes in nonsensical ways, but intelligent writers of English will get this one right, I think. I'm a bit of a bear on grammar and language usage so I tend to notice such things.

A perfect opportunity to point this out,

Would you have been satisfied with the OP's question if the analogues term fabric were "fabric" instead?

In the OP i'd say the question is clearly independent of the analogy.

As you've pointed out; the use of the word know in the title of this thread, and it at least being clear what isn't implied. lol spacetime a fabric

Opps wrong thread, I was referring to this one, specifically this post which appears particular, ummm, noneducational.

 

[stever]

I thought someone above said that length contraction and time dilation would make the corrections required to bring light speed for a moving object to c . I tried to make those corrections and could not get it to work. It seems to me that both length contraction and time dilation, if applied to the light in my frame which flies into the moving object from behind and from in front, will raise the speeds of these two lights. (contraction means the moving measuring rod gets shorter and takes less time to pass the light (although no one can actually measure it); time dilation means less time elapses on the moving object for a given distance of the light passage, hence more apparent speed for the light. Correct me if I'm wrong on either of those) The increased light speed is helpful for light trying to catch up from behind the moving object, because it was too slow to begin with, but it makes the light from in front even faster, and it was already faster than c. So the corrections do not resolve the inequalities of light speed for a moving object. This assumes that one subscribes to the idea of the inequality of light speeds that would exist when the one-way speed of light is different from the average round trip c speed.

I would be interested in hearing about the implications of light traveling through an object at speeds that vary with the direction.

What about speeds that vary regardless of direction?

 

[A.T.]

I thought someone above said that length contraction and time dilation would make the corrections required to bring light speed for a moving object to c

Length contraction, time dilation and relativity of simultaneity are consequences of the Lorentz transformation, which ensures that light speed is the same for all observers:

https://www.youtube.com/watch?v=C2VMO7pcWhg

 

[stever]

How does the animation answer or change my question?
Einstein I guess thought the one-way speed of light was the same for all frames. Lorentz did not. I can't understand the Einstein conception. I'm trying to make sense of Lorentz's conception.