Can Some Galaxies Travel Faster Than Light?

[narrator]

Amateur with questions ;)

It seems possible (probable even?) that some galaxies are moving away from us faster than light, which means we'll never get to observe them. That may only be a relative speed (like 2 cars doing 60mph in opposite directions, meaning 120mph relative to each other), but is there a theory that some galaxies are actually traveling faster than light? And what would a theoretical observer near say a sun see? Would it be like a comet, a totally dark sun, trailing light behind it?

Just thinking about this, Could light from a faster than light mass even escape that mass?

 

[DaveC426913]

Nothing can move faster than light locally, but over cosmological distances, space is expanding and carrying galaxies with it. Nothing in relativity prevents this.

Locally, those galaxies are effectively at rest, just like we are here.

 

[bcrowell]

FAQ: What does general relativity say about the relative velocities of objects that are far away from one another?

Nothing. General relativity doesn't provide a uniquely defined way of measuring the velocity of objects that are far away from one another. For example, there is no well defined value for the velocity of one galaxy relative to another at cosmological distances. You can say it's some big number, but it's equally valid to say that they're both at rest, and the space between them is expanding. Neither verbal description is preferred over the other in GR. Only local velocities are uniquely defined in GR, not global ones.

Confusion on this point is at the root of many other problems in understanding GR:

Question: How can distant galaxies be moving away from us at more than the speed of light?

Answer: They don't have any well-defined velocity relative to us. The relativistic speed limit of c is a local one, not a global one, precisely because velocity isn't globally well defined.

Question: Does the edge of the observable universe occur at the place where the Hubble velocity relative to us equals c, so that the redshift approaches infinity?

Answer: No, because that velocity isn't uniquely defined. For one fairly popular definition of the velocity (based on distances measured by rulers at rest with respect to the Hubble flow), we can actually observe galaxies that are moving away from us at >c, and that always have been moving away from us at >c.[Davis 2004]

Question: A distant galaxy is moving away from us at 99% of the speed of light. That means it has a huge amount of kinetic energy, which is equivalent to a huge amount of mass. Does that mean that its gravitational attraction to our own galaxy is greatly enhanced?

Answer: No, because we could equally well describe it as being at rest relative to us. In addition, general relativity doesn't describe gravity as a force, it describes it as curvature of spacetime.

Question: How do I apply a Lorentz transformation in general relativity?

Answer: General relativity doesn't have global Lorentz transformations, and one way to see that it can't have them is that such a transformation would involve the relative velocities of distant objects. Such velocities are not uniquely defined.

Question: How much of a cosmological redshift is kinematic, and how much is gravitational?

Answer: The amount of kinematic redshift depends on the distant galaxy's velocity relative to us. That velocity isn't uniquely well defined, so you can say that the redshift is 100% kinematic, 100% gravitational, or anything in between.

Let's take a closer look at the final point, about kinematic versus gravitational redshifts. Suppose that a photon is observed after having traveled to Earth from a distant galaxy G, and is found to be red-shifted. Alice, who likes expansion, will explain this by saying that while the photon was in flight, the space it occupied expanded, lengthening its wavelength. Betty, who dislikes expansion, wants to interpret it as a kinematic red shift, arising from the motion of galaxy G relative to the Milky Way Malaxy, M. If Alice and Betty's disagreement is to be decided as a matter of absolute truth, then we need some objective method for resolving an observed redshift into two terms, one kinematic and one gravitational. But this is only possible for a stationary spacetime, and cosmological spacetimes are not stationary. As an extreme example, suppose that Betty, in galaxy M, receives a photon without realizing that she lives in a closed universe, and the photon has made a circuit of the cosmos, having been emitted from her own galaxy in the distant past. If she insists on interpreting this as a kinematic red shift, the she must conclude that her galaxy M is moving at some extremely high velocity relative to itself. This is in fact not an impossible interpretation, if we say that M's high velocity is relative to itself *in the past.* An observer who sets up a frame of reference with its origin fixed at galaxy G will happily confirm that M has been accelerating over the eons. What this demonstrates is that we can split up a cosmological red shift into kinematic and gravitational parts in any way we like, depending on our choice of coordinate system.

Davis and Lineweaver, Publications of the Astronomical Society of Australia, 21 (2004) 97, msowww.anu.edu.au/~charley/papers/DavisLineweaver04.pdf

 

[Pengwuino]

Well first you need to realize that there is nothing special about two objects receding at speeds faster than the speed of light. They can also communicate to each other, there is no problem there. The deal with relativity is that each galaxy can't see the other galaxy moving at a speed faster than the speed of light.

And in light of previous posters that just popped up, I suppose I should clarify that I'm not taking into account the whole General relativistic effects of expanding space-time into account.

 

[russ_watters]

It seems possible (probable even?) that some galaxies are moving away from us faster than light, which means we'll never get to observe them.

Just like we can hear planes moving faster than the speed of light, we can see galaxies receding from us faster than the speed of light.

...but is there a theory that some galaxies are actually traveling faster than light?

Some galaxies are observed to move away from us at faster than the speed of light.

And what would a theoretical observer near say a sun see? Would it be like a comet, a totally dark sun, trailing light behind it?

Not sure what you are asking.

Just thinking about this, Could light from a faster than light mass even escape that mass?

Objects do not travel faster than the speed of light/light always travels at the speed of light away from them. Galaxies receding faster than the speed of light only appear that way because of the expansion of space.

 

[narrator]

Thanks bcrowel, that helped my understanding a lot :)

Russ, as I tried to explain, my questions were not about velocity relative to us or other galaxies, but their velocity relative to themselves, though I'm learning that's not as straight forward as I once thought, especially in spacetime.

And I've always found it intriguing that objects (except tachyons?) cannot move faster than light. I know it's accepted science, but I still like to play with the idea of "what-if".. lol. "What-ifs" are sometimes a way to highlight "what-is".

 

[alt]

Another (south side of) amateur here. I too often try to get my mind around these things and usually end up more confused. May I ask a couple of simpler questions.

A dark rectagular room (very long) and two lasers.

1) Stand at one wall, and fire both at the same time toward the opposite wall. They arrive at the same time. Were they traveling at zero c relative to each other ?

2) Place each at opposite walls. Fire at the same time. Are they traveling at 2c relative to each other ?

Thanks.

 

[alt]

Just like we can hear planes moving faster than the speed of light, we can see galaxies receding from us faster than the speed of light. Some galaxies are observed to move away from us at faster than the speed of light. Not sure what you are asking. Objects do not travel faster than the speed of light/light always travels at the speed of light away from them. Galaxies receding faster than the speed of light only appear that way because of the expansion of space.

I think you meant planes / speed of sound ? (otherwise, I'm even more confused).

Also, if distant galaxies are observed to be moving away from us at faster than light speed, could it be as true to say that they may be at rest, or moving at any sub light speed, and WE are traveling at faster than light speed ?

 

[GDogg]

Another (south side of) amateur here. I too often try to get my mind around these things and usually end up more confused. May I ask a couple of simpler questions.

A dark rectagular room (very long) and two lasers.

1) Stand at one wall, and fire both at the same time toward the opposite wall. They arrive at the same time. Were they traveling at zero c relative to each other ?

2) Place each at opposite walls. Fire at the same time. Are they traveling at 2c relative to each other ?

Thanks.

1) & 2) There's no frame of reference associated with something traveling at c. So you can't really say what they do relative to each other.

 

[DaveC426913]

I concur with GDogg. c is not a valid frame of reference.

You cannot ride a beam of light and measure how fast other things are moving relative to you.

You can do it with less than c objects.

Two objects traveling at .99c in parallel are statinoary wrt each other.

When traveling in opposite directions it's a bit trickier; it depends on what FoR you are measuring from.

If you are stationary between them, you will measure each object as moving away from you at .99c. (No object is moving wrt you at > c)
If you are riding on one of the objects, then the v of the other object is calcuated using the Lorentz Transform; it will work out to less than c (like ~.999c). (no object is moving wrt you at > c)

 

[narrator]

no object is moving wrt you at > c

I'm a little confused about that. Unless I read it wrong, I thought there were galaxies moving away from us at >c but are moving at <c wrt themselves?

Like in Russ's reply earlier: "Just like we can hear planes moving faster than the speed of light, we can see galaxies receding from us faster than the speed of light."

And bcrowell's faq quote: "we can actually observe galaxies that are moving away from us at >c, and that always have been moving away from us at >c"

 

[DaveC426913]

I'm a little confused about that. "

I was simply addressing alt's scenario, not the initial galaxy scenario.

alt's scenario is a local phenomenon and does not involve the expansion of space.

 

[narrator]

ahh cool.. now I understand.. thanks