# Do photons age? Do they remain stationary in x4?

Practice what you preach much?
I hope so! Where do you think I am falling short?

I will make amends to correct any shortcomings.

Thanks! :)

I think Greene's explanation of SR is really bad, so I wouldn't recommend anyone to try to learn SR from his writings.
Hello Fredrick,

I did some research on Brian Greene. He is one of the most famous physicists of our era, and too, he is one of the most famous teachers of physics.

http://en.wikipedia.org/wiki/Brian_Greene
http://www.columbia.edu/cu/physics/fac-bios/Greene/faculty.html

Please elaborate on why you say that, "Greene's explanation of SR is really bad, so I wouldn't recommend anyone to try to learn SR from his writings."

Greene is a world-renown scientist with degrees from Oxford in theoretical physics.

Why do you think that he is not good?

The New York Times Book Review wrote:

The New York Times Book Review - George Johnson
Greene...explor[es] the ideas and recent developments with a depth and clarity I wouldn't have thought possible. He has a rare ability to explain even the most evanescent ideas in a way that gives at least the illusion of understnding.He developes one fresh new insight after another....In the great tradition of physicists writing for the masses, The Elegant Universe sets a standard that will be hard to beat.
--http://search.barnesandnoble.com/The-Elegant-Universe/Brian-Greene/e/9780375708114#TABS

Scientific American - Chris Quigg

Beautifully told...The Elegant Universe presents the ideas and aspirations — and some of the characters — of string theory with clarity and charm...a thoughtful and important book.
The London Review of Books - Ellis

...I can only say that Greene's book is an explanatory tour-de-force...It would be hard to imagine anyone producing a clearer account than this of the difficult ideas involved, and Greene even brings out something of the actual excitement of scientific discovery...
Are you saying, Fredrick, that Greene as these sources are not to be trusted? What are your credentials, if we may ask? Perhaps that will help us to put your opinion in proper perspective? Thanks!

Given the unanimous praise for Brian Greene & his book, as well as for Einstein, coming from everywhere, I am going to have to keep with this explanation & interpretation:

More from Brian Greene & Einstein,

"Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light. . . We now see that time slows down when an object moves relative to us because this diverts some of its motion through time into motion through space. The speed of an object through space is thus merely a reflection of how much of its motion through time is diverted." -- p. 50, THE ELEGANT UNIVERSE (read it for yourself if you log in @ https://www.amazon.com/Elegant-Univer...der_0375708111)

1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene

2. Fact: A photon travels at c through the three spatial dimensions.

3. "The speed of an object through space is thus merely a reflection of how much of its motion through time is diverted." --Brian Greene

4. Because a photon moves at c through the three spatial dimensions, a photon's "motion through time must be entirely diverted."

5. Ergo, a photon has no velocity component in the fourth dimension.

6. Ergo a photon stays at the same place in the fourth dimension, as it has no motion in the fourth dimension, all its "motion through time being diverted," as Brian Greene and Einstein state.

Best,

B.E.

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Bruno...

The time measured on a clock or watch relies on the emission and propagation of photons, be it in the context of an unwinding clock spring or an oscillating quartz crystal, or even the beating of a heart. And photons are matter that surf the fourth expanding dimension.
really humorous....nice try!!! but insofar as current science has determined, purely speculative...

Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light
..

that's not quite the quote..... ....you mean "...traveling through spacetime.." at one fixed speed.....so it's the spacetime interval that IS invarient....

I do agree with the rest of your post immediately above including " Please elaborate on why you say that, "Greene's explanation of SR is really bad, so I wouldn't recommend anyone to try to learn SR from his writings."......for a largely non mathematical discussion of relativity Greene does a good job..and is consistent from what I have seen with several other prominent physicsts.

Bruno...

really humorous....nice try!!! but insofar as current science has determined, purely speculative...

..

that's not quite the quote..... ....you mean "...traveling through spacetime.." at one fixed speed.....so it's the spacetime interval that IS invarient....

I do agree with the rest of your post immediately above including " Please elaborate on why you say that, "Greene's explanation of SR is really bad, so I wouldn't recommend anyone to try to learn SR from his writings."......for a largely non mathematical discussion of relativity Greene does a good job..and is consistent from what I have seen with several other prominent physicsts.
Yes, I agree. Above I quoted: "1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene

Best,

BE :)

thanks all! well, seems like we're making some progress on a consensus!

at this point, is there anyone who does not agree with this, or finds fault with the following:

More from Brian Greene & Einstein,

"Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light. . . We now see that time slows down when an object moves relative to us because this diverts some of its motion through time into motion through space. The speed of an object through space is thus merely a reflection of how much of its motion through time is diverted." -- p. 50, THE ELEGANT UNIVERSE (read it for yourself if you log in @ https://www.amazon.com/Elegant-Univer...der_0375708111)

1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene

2. Fact: A photon travels at c through the three spatial dimensions.

3. "The speed of an object through space is thus merely a reflection of how much of its motion through time is diverted." --Brian Greene

4. Because a photon moves at c through the three spatial dimensions, a photon's "motion through time must be entirely diverted."

5. Ergo, a photon has no velocity component in the fourth dimension.

6. Ergo a photon stays at the same place in the fourth dimension, as it has no motion in the fourth dimension, all its "motion through time being diverted," as Brian Greene and Einstein state.

Best,

B.E.
if there is anyone who does not agree with the above, please share your opinion and characterize why you do not agree with the above. thanks all!

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Dale
Mentor
But too, a photon does not age.
I agree, but that has nothing whatsoever to do with its passage through coordinate time (your x4).

Regarding Brian Greene, he is a good physicist, but you are quoting a pop-sci book of his. Like all pop-sci books it is necessarily very sloppy. It is not a useful reference, nor is it intended to be.

Dale
Mentor

1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene
This is sloppy useage. He is refering to the norm of the four-velocity, which is only defined for massive particles. Not photons. You cannot extend conclusions or reasoning based on this to photons or other massless particles.

4. Because a photon moves at c through the three spatial dimensions, a photon's "motion through time must be entirely diverted."
Meaning that its proper time is 0.

5. Ergo, a photon has no velocity component in the fourth dimension.
Incorrect. You are confusing coordinate time (the fourth dimension) with proper time (the spacetime interval along a worldline).

"1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene"
^^^^
This is sloppy useage. He is refering to the norm of the four-velocity, which is only defined for massive particles. Not photons. You cannot extend conclusions or reasoning based on this to photons or other massless particles.
So are you saying that Einstein's statement, "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." does not apply to photons?

Are you saying that light (photons) doth not travel at the speed of light?

You do realize, that that is entirely ridiculous. Right?

It doth appear that you are the one who is being sloppy, sir.

Best,

BE :)

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Dale
Mentor

Are you saying that light (photons) doth not travel at the speed of light?
This is sloppy useage. He is referring to the norm of the four-velocity, which is only defined for massive particles. Not photons. You cannot extend conclusions or reasoning based on this to photons or other massless particles.
Green's quote (misattributed to Einstein) can either refer to the norm of the four-velocity or the norm of the three-velocity. If it refers to the norm of the three-velocity then it is simply not true, and if it refers to the norm of the four-velocity then it does not apply to photons since the four-velocity is undefined for them.

You seem to be confusing three-velocity and four-velocity as well as confusing proper time and coordinate time. Your confusion is not surprising if your sole source of information on relativity is a pop-sci book.

Green's quote (misattributed to Einstein) can either refer to the norm of the four-velocity or the norm of the three-velocity. If it refers to the norm of the three-velocity then it is simply not true, and if it refers to the norm of the four-velocity then it does not apply to photons since the four-velocity is undefined for them.
I read EXACTLY what you said and I responded to it.

Everyone can see that I am responding to EXACTLY what you said, as I quote EXACTLY what you said.

"1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene"
^^^^
This is sloppy useage. He is refering to the norm of the four-velocity, which is only defined for massive particles. Not photons. You cannot extend conclusions or reasoning based on this to photons or other massless particles.
So are you saying that Einstein's statement, "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." does not apply to photons?

Are you saying that light (photons) doth not travel at the speed of light?

You do realize, that that is entirely ridiculous. Right?

It doth appear that you are the one who is being sloppy, sir.

Best,

BE :)

jtbell
Mentor

So are you saying that Einstein's statement, "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." does not apply to photons?
That's Greene's statement, not Einstein's.

That's Greene's statement, not Einstein's.
Yes it is Greene stating what Einstein's statement was in these exact words from Greene's book The Elegant Universe, "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light."

[putting Fredrik's response in different words]

For a given type of particle, the probability that it will decay during the next second does not depend on the time that the particle has already "lived." It is constant. As far as we know, there is no difference between a muon that was created one microsecond ago, versus one that was created one second ago, or one that was created one hour ago. (assuming of course that it still exists)
It seems wrong to think this way, as a given dsitribution of relative frequencies in a series of events (decays) can be described in general by more than one probabilistc model. If you have an exponential decay it means that the number of decays decrease with the number of active entities. But it does not mean that the only possible model describing this statistical result is a markovian process (without memory).

Best wishes

DaTario

Fredrik
Staff Emeritus
Gold Member
What, specifically, do you find "really bad" about Greene's explanations of SR?
The claim that everything moves through spacetime at speed c is just saying that the Minkowski square of the four-velocity is equal to -c2 (or +c2 if you use the +--- version of the metric instead of -+++). As DaleSpam already mentioned, this isn't even true for photons; the Minkowski square of a photon's four-velocity is 0, not -c2. For massive particles, the claim is true, but it's just a normalization convention. It's like saying "If Mike goes completely bald, he won't have any hair on his head". It tells us something about how we have chosen to define certain words, but it doesn't tell us anything about the physics.

So what Greene does is to take a statement that's vacuously true for massive particles and wrong for massless particles, and insinuate that it's a deep and profound statement about all particles.

Who do you suggest we learn SR from?
I like the SR section of Schutz's GR book. Taylor & Wheeler is the book that gets the most recommendations from competent people here in the forum, so I assume that's a good choice too, but I haven't read it myself.

Fredrik
Staff Emeritus
Gold Member
Let's look at some Greene quotes that were posted in another thread some time ago.

...Einstein found that precisely this idea - the sharing of motion between different dimensions - underlies all of the remarkable physics of special relativity...

...Einstein proclaimed that all objects in the universe are always traveling through space-time at one fixed speed - that of light...
I don't believe Einstein said any of these things. These are Greene's own thoughts about SR, not Einstein's.

The claim that all objects are traveling through spacetime at speed c, is true for massive particles, if "speed through spacetime" is defined as $$\sqrt{-u^T\eta u}$$ where u is the four-velocity, but then it isn't true for massless particles. So he's already in trouble here.

...If an object does not move through space all of the objects motion is used to travel through time...
No objection here. Note that he seems to be talking about motion such that every point on the world line has the same spatial coordinates in some inertial frame.

...Something traveling at light speed through space will have no speed left for motion through time. Thus light does not get old; a photon that emerged from the big bang is the same age today as it was then. There is no passage of time at the speed of light.
The first claim is only valid if what he said in the first quote is correct, and it isn't. The claim also doesn't make sense, because when he talked about no motion through space, he must have meant that all points on the world line have the same spatial coordinates in some inertial frame, so no motion through time should mean that all points on the world line has the same time coordinate in some inertial frame. But such a world line represents infinite speed, not light speed.

The last claim is nonsense for the reasons I explained earlier in this thread.

Dale
Mentor

I read EXACTLY what you said and I responded to it.

Everyone can see that I am responding to EXACTLY what you said, as I quote EXACTLY what you said.
Instead of going in circles why don't you respond to the substantive criticisms above. Specifically regarding your confusion between the three-velocity and the four-velocity and your confusion between proper time and coordinate time. Do you even know what those terms mean? Probably not if your only source of information is a pop-sci book. We can help explain these terms, but we need to know what level of detail to present.

Do you understand that proper time is not a dimension? Do you realize that the four-velocity of a photon is not well-defined? Do you understand how the chain of logic you laid out above is flawed and does not lead to the conclusion you assert?

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The claim that everything moves through spacetime at speed c is just saying that the Minkowski square of the four-velocity is equal to -c2 (or +c2 if you use the +--- version of the metric instead of -+++). As DaleSpam already mentioned, this isn't even true for photons; the Minkowski square of a photon's four-velocity is 0, not -c2. For massive particles, the claim is true, but it's just a normalization convention. It's like saying "If Mike goes completely bald, he won't have any hair on his head". It tells us something about how we have chosen to define certain words, but it doesn't tell us anything about the physics.

So what Greene does is to take a statement that's vacuously true for massive particles and wrong for massless particles, and insinuate that it's a deep and profound statement about all particles.

I like the SR section of Schutz's GR book. Taylor & Wheeler is the book that gets the most recommendations from competent people here in the forum, so I assume that's a good choice too, but I haven't read it myself.
Well, Greene is right.

Photons travel at c--the velocity of light. I'm still not sure why you have a problem with Greene and why you call him sloppy. Everything he says is 100% true, whereas you earler stated:

Originally Posted by DaleSpam View Post

"1. "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." --Brian Greene"
^^^^
This is sloppy useage. He is refering to the norm of the four-velocity, which is only defined for massive particles. Not photons. You cannot extend conclusions or reasoning based on this to photons or other massless particles.
So are you saying that Einstein's statement, "Einstein proclaimed that all objects in the universe are always traveling at one fixed speed--that of light." does not apply to photons?

Are you saying that light (photons) doth not travel at the speed of light?

You do realize, that that is entirely ridiculous. Right?

It doth appear that you are the one who is being sloppy, sir.

This is not going in circles, sir, but you still have not retracted your statements.

Until you retract your statements, it will be hard to take you seriously, as you stipulate that light and photons do not travel at c.

Would you be prepared to write a letter to Greene and his publisher that telling him that he is wrong and that photons do not travel at c as you say above?

I am quite amazed that you have never seen the Taylor and Wheeler book on relativity as they say the same things as Greene.

Both Einstein and Greene are right, sir, as are Taylor and Wheeler.

It is you who are wrong.

Einstein, Greene, Taylor, and Wheeler are all accomplished physicists who are to be trusted.

Dear sir--what degrees might you have? What have you published? Perhaps this will help us put your erroneous opinion in perspective.

Thanks!

BE

Doc Al
Mentor