What is the velocity of a photon?

[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).