I'm glad y'all are finding the chat useful. Tim did a great job sticking in pictures and editing the conversation to stay on topic.
Frank, as always, is up for another chat. If I can convince my wife that the amount of time for the last chat was an aberration, I might have one more chance to chat before my month-long trip to Rome ( ). Maybe last weekend in April, or first weekend in May.
Joined: Sat Mar 21, 2015 10:47 pm Posts: 45 Location: Norway
Edit: I found the vault! Great stuff and many new hours will be spent!
Great thing with this saved chat!
Are there more of them?
I like learning by listening to people discuss.
G.A
_________________ There is a theory which states that if ever anyone discovers exactly what the Universe is for and why it is here, it will instantly disappear and be replaced by something even more bizarre and inexplicable. There is another theory which states that this has already happened...
A *very* interesting chat. It is, of course, impossible to do this kind of thing without minor errors and approximations.
One I noted (which probably should go without saying) and which does not affect any of the conclusions drawn, is that while light has no rest mass, it does indeed have mass in the sense that it gravitationally attracts things.
Here is a kind of funny reference...
Einstein said that light is deflected by a massive object - but is light gravitationally deflected by light? Tolman, Ehrenfest and Podolsky discovered that in the weak field limit, two light beams moving in the same direction do not interact gravitationally, but two light beams moving in the opposite direction do.
It is, of course, impossible to do this kind of thing without minor errors and approximations.
One I noted (which probably should go without saying) and which does not affect any of the conclusions drawn, is that while light has no rest mass, it does indeed have mass in the sense that it gravitationally attracts things.
Did you just write this to irk me? There are approximations suitable for a given audience. Lots of them, I'm sure. But my saying "light has no mass" is no error and no approximation.
In contrast, saying light has no rest mass is an error of nonsense, because light is never at rest in any inertial frame.
Similarly implying light "does indeed have mass" is just wrong. In your reference, no one ever says light has mass. That light interacts with gravity fields is quite true, but it does so through its E and B fields' energy density terms; it does not have a mass density.
No, I didn't say it to irk you. Actually I tried pretty hard to make it clear that I wasn't. I am really honest in saying you can't make a presentation like that and be totally precise. You couldn't cover what you did precisely in less than several semesters. And you did the best job of explaining it without being WRONG anywhere that I have seen short of Feynman.
But I was also trying to be precise in saying it has mass "in the sense of exerting a gravitational force".
The Podalsky quote was fun, because when two photons are moving in the same direction, the would be (in some funny sense) motionless relative to one another, and hence have no mass - and exert no gravitational force- and that is what the equations came out to - they did not attract one another. I thought that was funny - perhaps my sense of humor is merely warped.
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