# What did Al mean?

1. Jul 8, 2004

In Brian Greene’s book, “The Elegant Universe” Greene says (but not quotes) that Einstein once said that EVERYTHING in the universe is traveling at the speed of light. Greene didn’t elaborate on this and I have yet to find anything else on this supposed statement of Einstein’s. Can anyone fill me in on how Einstein viewed that ‘everything’ in the universe is traveling at the speed of light?

In case you are reading this and, like me, are ignorant as to what Einstein meant please don’t give any theories of your own or just plain BS. I want to hear from people who know what this is really about. Thanks

2. Jul 8, 2004

### jcsd

He menat that the magnitude of the 4-vector velocity for any object is always c.

3. Jul 8, 2004

Jcsd

You seem to know what you are talking about because at that point in the book Greene was talking about vectors and this lost me. It has been a while since I read the book and I understood most of the concepts he was trying to project. I remember him trying to use compass headings as a way to explain this.

I am ignorant in even simple geometry. In a nutshell, is everything in the universe truly moving at c or is this just a mathematical abstraction? Your answer will influence my next reply because if I have to take a JC college course in basic geometry I will. I really want to understand this and I greatly appreciate everyones help. Bob

Last edited: Jul 8, 2004
4. Jul 8, 2004

### jcsd

5. Jul 8, 2004

JS: I appreciate your reply. I read up on vectors and the concepts seem rather simple. If need be I will at least understand this mathematically but I still want to conceptulize that everything in the universe is traveling at the speed of light. Question as opposed to what is the frame of reference or is it in directions adjacent to each other? Your answer looks good on paper but still cannot visualize this. Can anyone help?

6. Jul 8, 2004

### VantagePoint72

Greene elaborates on this considerably in another of his books, "Fabric of the Cosmos: Space, Time, and the Texture of Reality". Basically, he means that every object is moving through spaceTIME at the speed of light. Since space and time are unified under the theory of relativity, movement through one affects movement through the other. In the special theory of relativity, you can calculate that from that from any observer's perspective (since the speed of light is constant) that time is standing still for the photons of light. It's travelling through space at a rate of 299,792,458 m/s, and it's travelling through time at a rate of 0 m/s. The theory predicts that your velocity through space AND time together is always the same, so it logically follows that from my perspective, an object at rest to me is travelling through space at 0 m/s and is travelling through time at 299,792,458 m/s. As you increase your spatial velocity relative to me (since only relative velocities matter), then I would obseve your velocity through time to decrease, in order for me to observe your total velocity as being c. This is the basis of relative time, and is another way of explaining why as you increase your speed, an outside observer sees time appear to slow down for you. That's what Greene meant. You can divert your velocity through time, relative to another observer, by increasing your spatial velocity, relative to that observer, but your net velocity is always c, never faster, never slower.

Hope that helps!

Last edited: Jul 8, 2004
7. Jul 8, 2004

### robphy

We should be careful here.

Observers (particles with non-zero mass) have a [unit] 4-velocity with magnitude c [defined by the square-root of (c^2-times-the-squared-temporal-part minus the squared-spatial-part)].

However, their 3-velocities in any reference frame [defined by the ratio of the spatial part and temporal part] is always less than c.

Light (particles with zero mass) have a 4-velocity with magnitude zero
[defined by the square-root of the sum of the c^2-times-the-squared-temporal-part minus the squared-spatial-part].

However, their 3-velocities in any reference frame [defined by the ratio of the spatial part and temporal part] is always c.

8. Jul 8, 2004

### yogi

Its a very useful concept that has been advanced by a number of authors - Hawking, Green, Epstein and others. In fact, it obviates the need for Einstein's isotropic light hypothesis since one can directly obtain the Lorentz transformations for the spacetime intervals directly from this idea. Every isotropic light frame has a velocity c along the time axis - and when an object moves with respect to that frame, the composite velocity is still c, but one component of the interval is composed of a (vt)^2 term. Epstein makes the following observation: "The reason you cannot go faster than the speed of light is that you can't go slower - everything travels at c... clocks moving through space are perceived to run slower ... because a clock properly runs through time, not through space...if you compel it to run through space, it is able to due so only be diverting (his words) some of the speed ...."

9. Jul 9, 2004

Thanks

Lastone I want to thank you for referring me to other Greene Books. I will check them out. I checked out a math book yesterday and and had to look up vectors (its been a while since school). With the help of your replies, I am starting to get an idea of what this is all about. I know what time distortion is and I am beginning to see that this part of relativity ties in very closely with time distortion. Thank guys!

10. Jul 9, 2004

### Hurkyl

Staff Emeritus
That's not quite right; 4-velocity is simply ill-defined for light. Recall that 4-velocity is the derivative of 4-position with respect to proper time; since proper time is zero along any lightlike path, the derivative does not exist.

If you observe an object approaching the speed of light, you will see the components of 4-velocity diverge towards infinity.

11. Jul 10, 2004

### robphy

Thanks.
I should have said that "a light ray has a tangent vector which is null (i.e., has zero norm)". Being null, that vector can't be normalized into a unit-vector. (4-velocities are the unit-timelike tangent 4-vectors of massive particles.)