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guitarphysics
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Why do we move (including the dimension of time) at the speed of light? I understand that when our velocity increases in a spatial dimension, it will decrease in time, but why is the initial, overall velocity c?
That, and Lorentz transformations keep the magnitude of (proper) 4-vectors invariant.Simon Bridge said:remembering that ##x_0=ct## and that, at rest, ##\gamma=1## should help.
when our velocity increases in a spatial dimension, it will decrease in time
bcrowell said:This is something that seems to have propagated by Brian Greene in his popularizations. Physicists in general do describe the four-velocity as having magnitude c, but do not typically describe objects as moving through spacetime with velocity c. The latter is just Greene's way of putting it. It's not wrong, it's just a nontechnical verbal description of an equation that every physicist agrees on. There is a distinction between "moving through space" and "moving through spacetime" (which only Greene talks about).
Don't worry, that was mostly just me trying to understand the question. Don't be afraid to post the context - it can be very helpful. Had you said "Brian Greene's book" or something earlier the replies would have been more understandable sooner.guitarphysics said:I couldn't really understand a lot of what Simon Bridge and mfb where talking about (as I said, I don't know much in the way of physics or math).
There is no such distinction in Epstein's idea. Both, light and massive objects, "advance" at c through space-propertime. The only special thing about light in this picture is that it advances only though space, and not through propertime, like massive objects do. Not sure if Greene meant the same thing.robphy said:Here is an important takeaway message concerning Greene's "moving through spacetime" at the speed of light idea.
(Actually, I think it was Lewis Epstein that first popularized this "moving through [space]time" phrasing.)
If you must use the "speed through spacetime" phrasing...
- A massive particle travels...
through space at speed less than c.,
but through spacetime with *speed* c [by choice of some convention].
- Light travels...
through space at speed c,
but through spacetime with *speed* zero.
guitarphysics said:Why do we move (including the dimension of time) at the speed of light? I understand that when our velocity increases in a spatial dimension, it will decrease in time, but why is the initial, overall velocity c?
I read some stuff in Kleppner & Kolenkow first, but I think of Schutz as the book that taught me SR, and also the basics of tensors (multilinear algebra, not differential geometry).WannabeNewton said:...or go the route I went (and IIRC Fredrik as well) and learn SR from Schutz's text "A First Course in General Relativity".
Focus on spacetime diagrams.guitarphysics said:By the way, for a year-long school project, I'm writing a (probably short) book for the layman on relativity (which is why I started learning it- besides it being extremely interesting), so if anybody has any suggestions before I start writing, they'd be greatly appreciated.
Purcell develops everything about the field of moving charges and magneto-statics as well as electrodynamics using special relativity, however; he doesn't develop it using the language of tensors, which is perfectly reasonable considering it is a first year text.guitarphysics said:Huh, I ordered Schutz two days ago :). It looked like a good book on SR and a good intro to GR. And I haven't gotten to it yet, but I'm pretty sure Purcell formulates some part of classical electromagnetism in terms of SR.
People who put an i there are using the standard dot product:guitarphysics said:A doubt I have though, is why Simon stated the four-velocity as [itex]\gamma[/itex](c,U) when (at least according to Kleppner) it should be [itex]\gamma[/itex](ic,U). (That's another doubt I have- why does Minkowski write it like that? Where did he derive it from?).
I read the responses above and got the gist of the conversation. Saying that we move at the speed of light is a very unfortunate way to describe the nature of what's going on. A person not fluent in the mathematics of spacetime could easily be deceived into believing that the term "motion" has the same meaning that it has in every day normal language, and it doesn't in this context.guitarphysics said:Why do we move (including the dimension of time) at the speed of light? I understand that when our velocity increases in a spatial dimension, it will decrease in time, but why is the initial, overall velocity c?
That's not what they're doing. They rewritePopper said:People who speak about things moving through spacetime have a totally different meaning for this “spatial distance”. They use what is known as the “spacetime interval” rather than “spatial distance.” When this is used in that manner you have to a accept that “light moving through spacetime” becomes undefined because the spacetime interval between two points on the trajectory of a photon is zero. If you divide by the coordinate time you get “the speed of light in spacetime is zero” or if you try to use what’s called “proper time” then the “speed of light in spacetime” is undefined.
guitarphysics said:Thanks everyone. Ben, you were spot on! I'm reading The Elegant Universe by Brian Greene. I don't know calculus and I've only been studying physics for the past month or so, but I'm very interested, so I decided to read his book.
Fredrik said:That's not what they're doing. They rewrite
$$-\left(\frac{dt}{d\tau}\right)^2+\sum_{i=1}^3 \left(\frac{dx^i}{d\tau}\right)^2=-1$$ as
$$\left(\frac{d\tau}{dt}\right)^2 +\sum_{i=1}^3 \left(\frac{dx^i}{dt}\right)^2=1,$$ and then they call the square root of the first term the "speed through time" and the square root of the entire left-hand side the "speed through spacetime". So the latter is by definition 1, even for massless particles.
This seems really pointless to me, but some authors like it.
See note 6 for chapter 2 (p. 392) of "The elegant universe" by Brian Greene. He also talks about this stuff towards the end of chapter 2, p. 47-51 in the same book, and around p. 49 in "The fabric of the cosmos".Popper said:Please post ths source of what you're referring to. I'd like to look at it myself. Thank you.
That is a different concept from what is being talked about here.Gadhav said:I am fully confused by moving at speed of light issue. I see that galaxies are moving away from each other probably into new space that is probably created at edge of universe.
Fredrik said:See note 6 for chapter 2 (p. 392) of "The elegant universe" by Brian Greene. He also talks about this stuff towards the end of chapter 2, p. 47-51 in the same book, and around p. 49 in "The fabric of the cosmos".
This post has a little more information than I included in my previous post in this thread.
In one of these threads, someone informed me that this viewpoint didn't start with Greene. It was used in the book "Relativity visualized" by Lewis Carroll Epstein, published in 1981. I had a quick look at it. This stuff is mentioned in chapter 5, but I didn't see any calculations there.
It's quite refreshing to see someone who understands that. All too many times, students learn SR from a text which defines 4-vectors as objects which transform in the same way asSimon Bridge said:I did it that way because of the position 4-vector (ct,x,y,z).