Do photons age? Do they remain stationary in x4?

[jtbell]

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.

 

[brunoeinstein]

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."

 

[DaTario]

[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]

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]

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]

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?

 

[brunoeinstein]

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 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.

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]

Enough already. Thread closed.