# Objects faster than light because space is expanding

1. Jul 11, 2006

### MeJennifer

I have seen the argument many times that objects can travel faster than the speed of light because space is expanding.

Now, what I understand from SR and GR is that no object with mass can travel at or above the speed of light.

So how can this be?

Moderator: feel free to move this to the SR/GR section if it is more appropriate there.

Last edited: Jul 11, 2006
2. Jul 11, 2006

### EL

Try this one: http://www.astro.ucla.edu/~wright/cosmology_faq.html#FTL
Check out the link "many distance definitions".

3. Jul 12, 2006

### Jorrie

A short and sweet answer is that as space expands, it carries motionless (relative to space) bodies with it. If you have a big enough piece of space expanding, the two 'ends' can move apart faster than the speed of light, while the speed of all bodies through space can be zero.

4. Jul 12, 2006

### DaveC426913

Projecting that upward, you can have bodies moving apart at relativistic speeds AND that are far enough apart to have significant spatial expansion between them, resulting in a relative motion that exceeds c.

SR does not forbid this.

5. Jul 12, 2006

### George Jones

Staff Emeritus
For my take on what this means, see my post in this thread.

What happens in cosmology is well explained in the link given by the OP in this other thread.

6. Jul 12, 2006

### MeJennifer

Well what I know is true is that one cannot simply add up the speeds of two objects moving away at relativistic speeds.
Spatial expansion?
So are you suggesting that there is some ether or absolute space that expands? What is actually expanding in your view?

So where does all this space expanding idea come from? Can it be derived from GR or is it simply postulated by some cosmologists as true?

I mean I can understand that when obects move apart we can consider that the space between them increases. But we seem to talk about space as some property of nature, some "balloon" that is getting bigger. Now that is a nice idea but is there any shred of evidence that this is true?
Isn't space simply the distance between objects?

Could it not be that this whole "space is expanding" is really a mistunderstanding. For starters we cannot take space by itself to measure distances in GR. We have to use space-time. The distance between two points is not just dependant on the spatial coordinates. In a curved space-time the time part can play interesting "tricks" on that distance, isn't that true?
So are we saying that space-time is expanding?

In order for us to consider space by itself we would need to take a slice of the 4-dimensional Minkowsi manifold from our particular frame of reference in the present right?

And then there is the question what would cause an expansion. Assume a certain mass distribution which will cause a particular curvature in space-time. Then some force perhaps expands this space-time, or does it just expands by itself or what?

Then presumably if space-time expands so do our measuring rods correct? So how would we even be able to measure or detect it?

Last edited: Jul 12, 2006
7. Jul 12, 2006

### Jorrie

Quote: "So where does all this space expanding idea come from? Can it be derived from GR or is it simply postulated by some cosmologists as true?"

Solutions to Einstein’s GR tell us that the fabric of space itself must either expand or contract. Observation tells us that it expands.

BTW, our measuring rods are not space, so they do not expand! That is unless we take light as our "measuring rod" - the wavelength of light that comes to us over cosmological distances does expand with space.

You should read some cosmology books or websites - that way this forum will be more profitable for you :)

8. Jul 12, 2006

### MeJennifer

Well what I get from this is that everything that was built upon the development of GR is broken down.

Absolute space and time are reintroduced by the hubble flow, and cosmologic time, c is no longer constant and distances are no longer measured with geodesics. And I suppose cosmologists consider that progress.

9. Jul 12, 2006

### EL

10. Jul 13, 2006

### pervect

Staff Emeritus
GR allows one to deal with ANY coordinate system, one tailors the coordinate system to take maximum advantage of the symmetries of the problem.

Cosmologists have taken full advantage of this, tailoring their coordiante system to take full advantage of the most obvious symmetries (such as the Hubble flow) in order to make solving problems as easy as possible.

This is great for problem-solving, and there is no particular problem with GR at all in doing this. The main issues that arise are compatibility with SR, which is pretty much non-existent. Unfortunately this confuses a lot of people.

While nice SR compatible coordinate systems do exist for empty universes, unfortunately they can't be applied to universes with matter in them - SR applies globally only in flat space-time, and the universe is not "flat" in the required sense when it has matter in it. It's not clear that given a universe with matter in it that there are necessarily any better choices than the cosmologisits have made.

11. Jul 17, 2006

### Rake

From what I understood there is the motion of actual space as it "stretches" and "drags" any embedded objects along with it. General Relativity attributes this expansion in part to a non-zero vacuum energy.

Then there is relative motion of an object as it moves through local space. When defining velocity of an object, SR only considers the distance travelled within local space by the object and the time that it took to do so. The distinction is that as an object moves through space, space itself is stretching, but the object is not considered to be travelling across this newly added space, merely that new space is being added, so then this new distance is not taken into account when calculating the speed of the object. Hence the reason why there is no conflict with SR.

However, the special case of relativity does state that the velocity of an object within the previously defined context cannot reach or exceed the speed of light.

Last edited: Jul 17, 2006
12. Jul 18, 2006

### Chronos

There is no logical contradiction. As space expands, so does time [redshift]. Keep in mind that SR does not factor in gravity [as already pointed out].

13. Jul 18, 2006

### Brinx

As far as I understand it, if you were to draw a spacetime diagram representing a part of (one dimension of) our universe on a piece of paper and place two worldlines in it, both corresponding to inertially moving observers spaced far apart from eachother, they would show up as lines gently curving away from eachother - even though they experience no acceleration themselves and remain inertial observers all the time. This, to me, is the difference between the relative velocity of objects arising from the expansion of space itself, which doesn't involve any acceleration of the objects (they can start out at rest relative to eachother, although far apart), and the 'classical' relative velocity of objects (in non-expanding space, or at least negligibly so) ) which can never reach the speed of light if the objects 'try' to attain that speed by acceleration.

This probably sounds rather convoluted, but for the people who can make sense of this: is this the correct view according to GR?

14. Jul 18, 2006

### Jorrie

As I understand it, cosmologists usually use co-moving coordinates, in which case the worldlines would not diverge - they would remain straight and parallel, because the "rulers" of co-moving coordinates expand with space. If you use light travel time as a distance measure, your picture is correct, I think.

Last edited: Jul 19, 2006