What is the velocity of a photon?

[PAllen]

I would not say this and neither did Einstein. Rather, you can use the assumed (not measured) speed of photons (if you can figure out exactly how to do it) to synchronize your remote clocks to a local clock.

What I am getting at is simply that if you treat light as photons, a two way measurement is a measure of photon speed - the average speed of outgoing and incoming photons. The fact that it may not be the same photon is not really relevant.

It's not merely the most practical way, it's the only way.

Some people think that Einstein never thought of this supposed alternate way to synchronize clocks but he wrote about this in his 1905 paper and rejected it. In fact he gave the formula to show how much time, as defined in a Frame of Reference, a clock loses during the course of transporting it at a given velocity, v, over a period of time, t, as ½tv²/c² (near the end of section 4). The only way this could work is if the clock travels at a velocity of zero or takes zero time to get there meaning it is not possible.

Of course I know this. The point (which I did not bother to elaborate because I thought it was understood) is that if you slow transport a clock in a round trip you can (in principle) measure time difference. You then still have to assume you can cut this in half for a one way slow clock trip synchronization (however, this is a different assumption than isotropy of light speed; it is an assumption of isotropy of time shift for clock transport). However, this remains an independent measurement, because in measuring the slow clock round trip shift, you have nowhere used light. You have measured c as a feature of spacetime. Then, in my view (with the noted isotropy assumption), you can measure that light travels at this c (if you synchronized clocks thus, and measured bullets, you would obviously not find they travel at c).

Thus, another independent synchronization convention is possible: measure slow clock round trip shift, and assume isotropy of clock transport shift. There is no part of this convention that assumes light speed (one way or two way).

Did I say something like that? Where is this quote from?

I was referring to the following (in your post #32), which is the same observation I and several others made:

"You ask about the point (x1=0, x2=0, x3=0, x4=0) but that is not a point, it's an event because it includes x4, the time coordinate, the part of the Frame of Reference that uses the speed of light in its definition. So in any Frame of Reference, light travels at c because that is how we define it and how we define time in the Frame of Reference."

 

[PAllen]

I define velocity in the same way Einstein does in The Meaning of Relativity. Einstein, unlike you, doesn't provide three different definitions of velocity.

No, you have presented no definition. The section you quoted is not definition of velocity. ghwellsjr has shown, in detail, how other sections of this book are consistent with other papers and books by Einstein, so I will not belabor this.

PAllen--do you agree with George that, "I'm just saying that we can't measure thee speed of photons."

I agree with George that you can't measure the one way speed of photons without a bunch of additional assumptions. Thus, I definitely agree you cannot directly measure the one way speed of photons - because of the assumptions you have to make.

Perhaps I have a slight disagreement on whether a two way speed of light measurement constitutes a two way speed of photon measurement - I think it does.

Do you agree with George that, "No, it's not that time stops at the speed of light, rather, there is no such thing as time at the speed of light?"

Yes, I agree with this. I would normally put it as: there is no such thing as the reference frame of a photon; or you can talk about speed of light relative to a material body, but you cannot talk about speed (or anything else) relative to light.

As for the following, loose snippets of popular books are quite commonly misleading as authors try to poetically emphasize one aspect of a situation. You cannot treat these as serious science.

Do both of you disagree with Dr. Brian Greene who wrote, "Time stops when traveling at the speed of light through space." --http://books.google.com/books?id=MfQDBKHgMxkC&pg=PA49&dq=time+stops+speed+of+light&hl=en&ei=InzYTofYJoHciQKQ1amLCg&sa=X&oi=book_result&ct=result&resnum=1&ved=0CDUQ6AEwAA#v=onepage&q=time%20stops%20speed%20of%20light&f=false

Do you disagree with Nigel Calder?
http://books.google.com/books?id=_8...ook_result&ct=result&resnum=6&ved=0CE4Q6AEwBQ
"Just as time stops completely on the very edge of a black hole, so time would stop if you could travel at precisely the speed of light. "

This is an example of misleading poetic license. Time does not stop at a black hole horizon for a clock passing through it. A clock stationary at the black hole horizon is an impossibility. I guess, you call this sort of true if you took the sense to mean: it is no more possible to have a stationary clock at the event horizon than it is to have a clock moving at the speed of light.

Do you disagree with National Geographic?
http://books.google.com/books?id=BB...k_result&ct=result&resnum=2&ved=0CDMQ6AEwATgK
" At the speed of light itself, time stops. "

National geographic is not a source for physics.

Is Michio Kaku wrong too?
http://books.google.com/books?id=ub...v=onepage&q=time stops speed of light&f=false
"In fact, if the rocket were traveling at the speed of light, time would apparently stop inside the rocket, the rocket would be compressed to nothing, and the mass of the rocket would be infinite. Since none of these observations make. . ."

To me, this is just an argument about why you cannot talk about rockets or clocks moving at c, and why it is not meaningful to talk about such a frame. The sentence you chopped off suggests this is the beginning of a classic reductio at absurdum argument. You are taking the 'absurd' part of the argument and pretending it is meant as not absurd.

http://books.google.com/books?id=87...&q=time stops speed of light einstein&f=false
"Time stops at the speed of light."

There are many, many more books stating thusly.

Are you prepared to write letters to all the authors and editors, stating they are wrong?

Are you going to help George correct all those errant wikipedia pages too?

Your exercises in sophistry never end ...

 

[ghwellsjr]

This is not an accurate statement of Einstein's second postulate because of your inserted first phrase, "As measured in any inertial frame of reference".

I wouldn't go so far as this. I wouldn't say that it is not inaccurate, rather it is tautological. In any inertial frame of reference light goes at c because an inertial frame of reference is one in which light goes at c.

What I was objecting to was the insertion of the word "measured" in the second postulate. If the postulate was: "As measured in any inertial frame of reference, the speed of light in empty space is c and is independent of the state of motion of the emitting body", then I would agree this would be accurate, but redundant because that postulate is covered by the first postulate and does not need its own special enunciation. But it is not accurate to include the word "measured" in a postulate that refers to the propagation of light because we can neither observe nor measure the progress of light. If we could, we wouldn't need a second postulate. And remember, Einstein specifically made a point that his second postulate seemed to be at odds with the first and if he meant that it was referring to something about light that could be measured, there wouldn't be any issue of reconciliation.

 

[ghwellsjr]

It's not merely the most practical way, it's the only way.

Some people think that Einstein never thought of this supposed alternate way to synchronize clocks but he wrote about this in his 1905 paper and rejected it. In fact he gave the formula to show how much time, as defined in a Frame of Reference, a clock loses during the course of transporting it at a given velocity, v, over a period of time, t, as ½tv²/c² (near the end of section 4). The only way this could work is if the clock travels at a velocity of zero or takes zero time to get there meaning it is not possible.

Of course I know this. The point (which I did not bother to elaborate because I thought it was understood) is that if you slow transport a clock in a round trip you can (in principle) measure time difference. You then still have to assume you can cut this in half for a one way slow clock trip synchronization (however, this is a different assumption than isotropy of light speed; it is an assumption of isotropy of time shift for clock transport). However, this remains an independent measurement, because in measuring the slow clock round trip shift, you have nowhere used light. You have measured c as a feature of spacetime. Then, in my view (with the noted isotropy assumption), you can measure that light travels at this c (if you synchronized clocks thus, and measured bullets, you would obviously not find they travel at c).

Thus, another independent synchronization convention is possible: measure slow clock round trip shift, and assume isotropy of clock transport shift. There is no part of this convention that assumes light speed (one way or two way).

This is the first time I ever heard of anyone claiming that the slow transport of clocks involved splitting the difference in a round-trip. No matter, it still cannot be used to synchronize remote clocks or Einstein would have suggested it since he obviously knew about the possibility. It is only because Einstein has previously synchronized remote clocks that he can make a claim about how a clock loses time when it is transported from one location to another and that claim only holds true in that one frame. As viewed by another frame in motion with respect to the initial rest frame of the two clocks in question, the times on the two clocks are not any where near being synchronized when they remain separated even though when they are brought back together all frames agree on their difference.

 

[PeterDonis]

Are you prepared to write letters to all the authors and editors, stating they are wrong?

Are you going to help George correct all those errant wikipedia pages too?

Oh, great, this again. News flash: pop science books and articles sometimes mis-state things because the authors think it will somehow make it easier for lay people to understand. Sometimes this belief is mistaken. Also, Wikipedia is not an authoritative source, and I tend to agree with those who, having tried to improve various articles and gotten embroiled in "edit wars", have concluded that it's often not worth the effort to try to make Wikipedia more accurate.

Okay, enough grumping. The actual physics is straightforward, and I gave it in a previous post. To recap briefly:

(1) All objects (including both timelike objects, with non-zero rest mass, and photons, with zero rest mass) have a 4-momentum vector at any given event on their worldlines.

(2) Objects with non-zero rest mass move on timelike worldlines, and have a 4-momentum vector whose length is positive (and equal to their rest mass). Since the 4-momentum has positive length, we can define a "4-velocity" vector by dividing the 4-momentum by its length. In any given inertial frame, we can then form a "3-velocity" vector from the 4-velocity, by taking its spatial components as a spatial 3-vector, and dividing by its time component. The length of this 3-velocity vector will be the "speed" of the object in the given inertial frame.

(3) Photons, with zero rest mass, move on null worldlines, and their 4-momentum has length zero. So we can't form a "4-velocity" from their 4-momentum the way we can for timelike objects, as above. However, we *can* use their 4-momentum the same way as we used the 4-velocity for a timelike object, to form a "3-velocity" vector for the photon: take the spatial components of the photon's 4-momentum in a given frame, as a spatial 3-vector, and divide them by the time component of the photon's 4-momentum. (Note that we could also do this directly with the 4-momentum of a timelike object instead of the 4-velocity, since the rest mass will cancel out when we divide through by the time component.) We will find (in flat spacetime, or in a local inertial frame in curved spacetime) that the length of the photon's 3-velocity vector, formed in this way, is always c.

That's the physics. Now for those pop science quotes:

(A) Pop science books and articles sometimes use the phrase "time stops for photons" to refer to the fact that photons move on null worldlines, so the spacetime interval between any two events on a photon's worldline is zero. This is based on an analogy with timelike worldlines, for which the spacetime interval between any two events is the proper time elapsed between those events for an object moving along the worldline. IMO, this analogy is not a good one, because it invites confusion, as you have demonstrated; but I don't expect pop science presentations, even those by reputable physicists, to change any time soon, because IMO they are not really interested in pedagogy; they're interested in generating good quick sound bites that will convince the media to publish their stuff.

(B) The fact that the length of a photon's 3-velocity in any inertial frame, determined as I described above, is always c, is commonly expressed as "the speed of light is c in all inertial frames". But as ghwellsjr has explained several times now, this is a matter of how we *define* inertial frames; it's not a result of measurements in inertial frames that we defined independently of how light behaves.

 

[PeterDonis]

If a lightning bolt strikes, this causes an event in spacetime. The photons will travel away from where the bolt struck at the rate of c to an observer.

The event of the lightning bolt striking is *not* the same thing as "where the bolt struck". The latter refers to the location in *space* of the bolt striking, not the event in *spacetime*. So your second sentence is talking about something different than your first: it's talking about the relative velocities of the photon's worldline and the worldline of the point in space where the bolt struck. It is *not* talking about the "relative velocity" of the photon and the *event* where the bolt struck, which doesn't make sense.

 

[PAllen]

This is the first time I ever heard of anyone claiming that the slow transport of clocks involved splitting the difference in a round-trip. No matter, it still cannot be used to synchronize remote clocks or Einstein would have suggested it since he obviously knew about the possibility. It is only because Einstein has previously synchronized remote clocks that he can make a claim about how a clock loses time when it is transported from one location to another and that claim only holds true in that one frame. As viewed by another frame in motion with respect to the initial rest frame of the two clocks in question, the times on the two clocks are not any where near being synchronized when they remain separated even though when they are brought back together all frames agree on their difference.

If you want to eliminate second order effects without extrapolating from a series of measurements with ever slower clocks towards zero, you would do this (measure round trip time dilation and assume isotropy). Or simply move the clock slow enough so second order effects are below measurement precision.

I never said or implied that clocks synchronized this way are synchronized in a frame independent way. Of course they aren't. If another (relatively moving) observer synchronized clocks with slow transport and compared his to yours, he would find yours are not syncrhonized. I am just pushing that there are more than one set up assumptions by which you can arrive at relativity. Further, that slow clock transport does constitute an independent synchronization convention. With one set of assumptions, you measure a universal constant c independent of light (by measuring round trip time dilation), then measure lightspeed. With Einstein's assumptions you predict and measure time dilation and constant two way speed of light. I claim they are both reasonable approaches, and in that sense, you can talk about measuring one way light speed with a different set of assumptions than Einstein used (but not with Einstein's assumptions).

 

[gbfmgbfm]

Each of those books is a pop-sci book, not a mainstream scientific book.

So are you saying that Dr. Brian Greene publishes falsehoods? Would you be willing to write him a letter asking him to retract his falsehoods?

Are you saying that Dr. Brian Greene is not a mainstream scientist publishing books for the mainstream?

Finally, what "mainstream scientific books" do you swear by?

 

[gbfmgbfm]

The event of the lightning bolt striking is *not* the same thing as "where the bolt struck". The latter refers to the location in *space* of the bolt striking, not the event in *spacetime*. So your second sentence is talking about something different than your first: it's talking about the relative velocities of the photon's worldline and the worldline of the point in space where the bolt struck. It is *not* talking about the "relative velocity" of the photon and the *event* where the bolt struck, which doesn't make sense.

Actually this makes perfect sense, "If a lightning bolt strikes, this causes an event in spacetime. The photons will travel away from where the bolt struck at the rate of c to an observer."

Where can refer to space and time, for we live in a 4d manifold where they are inseperable.

Many of you want to work the word "worldline" into your discussions like little boys flexing their muscles at the swimming pool, muddying the waters so as to appear deeper like Nietzsche said, but Einstein makes NO MENTION of worldlines in his seminal book on Relativity--The Meaning of Relativity.

https://www.amazon.com/gp/product/0691120277/?tag=pfamazon01-20

Above someone accuses Dr. Brian Green of Columbia University of being non-mainstream and unscientific. Is Einstein also a non-mainstream, non-scientist because he doesn't use the word "worldline" in defining relativity?

Does anyone on this forum have a Ph.D. in physics? If so, where from?

A lot of the assertions here, such as "It is meaningless to talk of the velocity of light," and "one cannot measure the velocity of light" are quite laughable. And then, to add insult to injury, some are insulting Dr. Brian Greene (even though you would never man up and write him a letter accusing him of being wrong) and Albert Einstein--the very Father of Relativity.

lol

 

[PeterDonis]

Actually this makes perfect sense, "If a lightning bolt strikes, this causes an event in spacetime. The photons will travel away from where the bolt struck at the rate of c to an observer."

Where can refer to space and time, for we live in a 4d manifold where they are inseperable.

What "where" refers to depends on context. English is often ambiguous, which is why we prefer to express things mathematically in physics when we want to be precise. Once again, in the sentence "the photons will travel away from where the bolt struck at the rate of c", the word "where" does *not* refer to the event of the bolt striking; it refers to the worldline of the point in space where the bolt struck; the event of the bolt striking is one single point on that worldline.

Many of you want to work the word "worldline" into your discussions like little boys flexing their muscles at the swimming pool, muddying the waters so as to appear deeper like Nietzsche said, but Einstein makes NO MENTION of worldlines in his seminal book on Relativity--The Meaning of Relativity.

So what? What matters is not the words we use; what matters is the physics we're describing. Einstein didn't use spacetime either, but we have found that using spacetime is often a much better way of describing the physics, so we use it; evidently you agree since you said that the lightning bolt striking is an event in spacetime. (Edit: Actually, IIRC Einstein did refer to the spacetime description in the book you refer to, even though he didn't use it in his original 1905 papers. The book was written after he finished General Relativity, by which time he had become convinced of the usefulness of the spacetime description.) The term "worldline" is part of the spacetime description, which is why we use it today even though Einstein didn't.

Above someone accuses Brian Green of being a non-mainstream scientist. Is Einstein also a non-mainstream scientist because he doesn't use the word "worldline" in defining relativity?

See above. But also, you are misinterpreting what DaleSpam said. He didn't say Brian Greene wasn't a mainstream scientist; he said the book by Greene you were quoting from was a pop science book, not a mainstream science book. A mainstream science book is a book that is written for scientists, in which the object is to describe the science precisely and correctly, whether or not it is easy to understand. A pop science book is a book that is written for non-scientists, in which the object is to describe the science in a way that's easy to understand, whether or not it is precisely correct. For an example of the difference, read my post #65 and compare it with what Greene said in the book you quoted from.

 

[gbfmgbfm]

What "where" refers to depends on context. English is often ambiguous, which is why we prefer to express things mathematically in physics when we want to be precise. Once again, in the sentence "the photons will travel away from where the bolt struck at the rate of c", the word "where" does *not* refer to the event of the bolt striking; it refers to the worldline of the point in space where the bolt struck; the event of the bolt striking is one single point on that worldline.
So what? What matters is not the words we use; what matters is the physics we're describing. Einstein didn't use spacetime either, but we have found that using spacetime is often a much better way of describing the physics, so we use it; evidently you agree since you said that the lightning bolt striking is an event in spacetime. The term "worldline" is part of the spacetime description, which is why we use it today even though Einstein didn't.
See above. But also, you are misinterpreting what DaleSpam said. He didn't say Brian Greene wasn't a mainstream scientist; he said the book by Greene you were quoting from was a pop science book, not a mainstream science book. A mainstream science book is a book that is written for scientists, in which the object is to describe the science precisely and correctly, whether or not it is easy to understand. A pop science book is a book that is written for non-scientists, in which the object is to describe the science in a way that's easy to understand, whether or not it is precisely correct. For an example of the difference, read my post #65 and compare it with what Greene said in the book you quoted from.

Hey Peter--either Brian Greene is right or wrong. So are you saying that he is wrong? Because if so, we need to send him an email and have him correct future editions of his book.

 

[PeterDonis]

Hey Peter--either Brian Greene is right or wrong. So are you saying that he is wrong?

Did you read my post #65 and compare it with what he said in his book? Please do so and then decide whether you think he was right or wrong.

 

[gbfmgbfm]

Did you read my post #65 and compare it with what he said in his book? Please do so and then decide whether you think he was right or wrong.

Peter--please just let us all know if you are saying Brian Green is right or wrong. In science it is important to be precise and state what you mean. So please, be precise and tell us--is Brian Greene *right* or *wrong?" We don't really have time for your convoluted, giant-walls-of-text semantics games. Please stop muddying the waters so as to appear deeper, and please keep it simple, and tell us, Peter, "Is Brian Greene right or wrong?"

Please be simple and precise--like physics.

 

[Dale]

So are you saying that Dr. Brian Greene publishes falsehoods? Would you be willing to write him a letter asking him to retract his falsehoods?

No. I agree with Peter Donis' post 65.

Are you saying that Dr. Brian Greene is not a mainstream scientist publishing books for the mainstream?

No. Just that the referenced books are not mainstream. He has other peer-reviewed work. If you want to find an authoritative quote from Brian Green you need to look at his peer-reviewed work, not his pop-sci work.

Finally, what "mainstream scientific books" do you swear by?

A good list is here: http://science.thomsonreuters.com/cgi-bin/jrnlst/jlresults.cgi?PC=MASTER

 

[gbfmgbfm]

No.

No. Just that the referenced books are not mainstream. He has other peer-reviewed work.

A good list is here: http://science.thomsonreuters.com/cgi-bin/jrnlst/jlresults.cgi?PC=MASTER

Cool! Then, since you are admitting that Dr. Brian Green does not publish falsehoods, then he is publishing the truth!

QED :)

 

[ghwellsjr]

A lot of the assertions here, such as "It is meaningless to talk of the velocity of light," and "one cannot measure the velocity of light" are quite laughable.

Where do these quotes exist on this thread?

 

[PeterDonis]

Peter--please just let us all know if you are saying Brian Green is right or wrong. In science it is important to be precise and state what you mean. So please, be precise and tell us--is Brian Greene *right* or *wrong?" We don't really have time for your convoluted, giant-walls-of-text semantics games. Please stop muddying the waters so as to appear deeper, and please keep it simple, and tell us, Peter, "Is Brian Greene right or wrong?"

Please be simple and precise--like physics.

I already have. Once again, have you read my post #65? That is my simple and precise explanation of the relevant physics. There are no "semantics games" there. I even explained how the physics relates to the statement by Brian Greene that you quoted, that "time stops for photons" (or words to that effect). In short, there is more than enough information in that post for you to decide for yourself whether Brian Greene is "right" or "wrong", if you care about that so much.

Having said that, let me ask this: why *do* you care whether Brian Greene is "right" or "wrong"? If you understand the physics, then you understand the physics; you don't need Brian Greene or any other authority to pronounce on whether a given statement is "right" or "wrong". If you don't understand the physics, then going around buttonholing people and asking them whether Brian Greene is "right" or "wrong" is not, IMHO, a good way to learn it. You asked a question in the OP of this thread, and it has been answered. If you don't understand the answer, then ask about what you don't understand--for example, if there's something in my post #65 you don't understand, ask about it.

 

[Dale]

Cool! Then, since you are admitting that Dr. Brian Green does not publish falsehoods, then he is publishing the truth!

Sorry about the lack of clarity in my previous post. My "No" is referring to the second question. I.e. "No I am not willing to write a letter...". The truth or falsehood of those statements is irrelevant since they are not mainstream scientific references. It is pointless to write letters requesting retractions of irrelevant statements.

Again, if you want to find an authoritative quote from Brian Green you need to look at his peer-reviewed work, not his pop-sci work.

 

[D H]

All: Look at post #75, or any other post by that user. Notice the strike-thru on the name.I have two questions for those of you who have been participating in this discussion:

• Is there any point in keeping this thread alive?
• Is there any point in keeping this thread around?

 

[PAllen]

All: Look at post #75, or any other post by that user. Notice the strike-thru on the name.I have two questions for those of you who have been participating in this discussion:

• Is there any point in keeping this thread alive?
• Is there any point in keeping this thread around?

I would suggest lock it and keep it around. The discussions by several contributors on different types of velocity, and what it takes to measure them, are potentially useful.

 

[D H]

Good enough. Thread is closed (but not deleted).