Why do we move at the speed of light?

[Fredrik]

Please post ths source of what you're referring to. I'd like to look at it myself. Thank you.

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.

 

[Gadhav]

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. What no one talks about it the space in between and everywhere. Space cannot just get created at the edge. It is created everywhere. Is it likely that galaxies are not moving apart but space in between is expanding. Also it can get created at any speed even faster than speed of light since as "Nothing can travel faster than speed of light" Here if you replace "Nothing" by space or vacuum, we have an answer that says that vacuum can be expanded at speed higher than light Why is it that always mass appears in all equations of physics but no vacuum?

 

[Simon Bridge]

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.

That is a different concept from what is being talked about here.
above - they are talking about the magnitude of the four-velocity and the different ways we can use the 4-velocity to make sense of the concept of "speed" in GR.

There are lots of ways that things can be observed to be moving faster than the speed of light ... relative motion of galaxies due to the expansion of space is an example.

There are lots of misunderstandings in your description though. You should put that post in a separate thread for discussion, if you haven't already. You will discover that the ideas you say nobody is talking about have been considered long ago and are accounted for or disproved in established theory.

 

[Popper]

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.

Thanks Fredrik. I'll take a look at it the next time I'm at the library. Much appreciated.

 

[Popper]

I did it that way because of the position 4-vector (ct,x,y,z).

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 as

$$\Delta X = (c\Delta t, \Delta x, \Delta y, \Delta z)$$

but which doesn't explain that (ct,x,y,z) is exactly the same expression when the initial point of the 4-vecrtor $$\Delta X$$ is the origin of coordinates. It's so nice to see that at least some people understand how and why (ct,x,y,z) is a Lorentz 4-vector in flat spacetime.

 

[Popper]

Sorry to bring this post back after such a long time, but I've been learning relativity (from Kleppner and Feynman) for the past month or so, and I understand this all a lot more now :).
A doubt I have though, is why Simon stated the four-velocity as $\gamma$(c,U) when (at least according to Kleppner) it should be $\gamma$(ic,U). (That's another doubt I have- why does Minkowski write it like that? Where did he derive it from?).

The purpose of the "i" is so that the magnitiude of a 4-vector uses the Eucliean metric, i.e. so that

|R| = sqrt[ (X⁰)² + (X¹)² + (X²)² + (X³)² ]

There are some textbooks who still use it. Basic Relativity by Richard A. Mould (1994) is one such example.

 

[Simon Bridge]

It's quite refreshing to see someone who understands that.

It was hard won!

There are some textbooks who still use [x0=ict].

... some lecture still use it too, as well as older papers and texts and books - so students still need to know about it.

 

[Popper]

I'm confused. What does It was shard won! mean? Perhaps my confusion is a cross cultural thing?

 

[Simon Bridge]

I'm confused. What does It was shard won! mean? Perhaps my confusion is a cross cultural thing?

I had to pry it out of my professor using a shard of glass ... the physics course requiring improvised weapons skills as a prerequisite. What you learn in combat stays with your for the rest of your life.

Or it was a typo...

 

[WannabeNewton]

I don't like the concept of a position 4-vector because you will frequently see threads asking "what happens to position vectors when we go to GR" creating quite a confusion since position vectors are meaningless in GR. It fully exploits the fact that Minkowski space-time is just the vector space $\mathbb{R}^{4}$ with a pseudo metric so the concept of a position 4-vector is far from general.

 

[HomogenousCow]

I had to pry it out of my professor using a shard of glass ... the physics course requiring improvised weapons skills as a prerequisite. What you learn in combat stays with your for the rest of your life.

Or it was a typo...

After doing some Jackson's problems I'm ready to use an improvised weapon on myself.

 

[phyti]

guitarphysics post 1:
The Minkowski spacetime graphic is an historical record of an object in motion in one spatial dimension (x), with a correlation between the x position and the (‘time’) ct position. This is sufficient info to eliminate ‘motion in time’ and motion at c.

If you decompose the constant ct light vector into a spatial component c1 (object motion) and c2 (object time), the components form a right triangle. This is the origin of ‘moving through space and time’ analogies used by Brian Greene and others .

Einstein developed the invariant interval in the form of an equality, X^2 - (ct)^2 = 0, with X the 3D spatial interval. Minkowski manipulated ct using complex variable notation to achieve a homogenous general form s^2 = ∑x^2, with ct treated as a mathematical ‘4th dimension’.