# Velocities in space relative to other objects

1. Jun 2, 2013

### schonovic

Forgive me if I sound stupid here but is there anything that we could call Dead Space? An essence or quantum material that is traveling at zero velocity to everything? Something that we could say we are all traveling at x velocity relative to. I know we can all say we are traveling at such and such velocity relative to light but Guth said that the speed of light is relative to the compact density of the universe after the Big Bang. So if space-time has different densities in different areas of the universe; how can we say we are traveling at such and such velocity relative to anything at all? There's got to be some concept here that I'm not getting.

2. Jun 2, 2013

### marcus

Sure, in cosmology there is the idea of being at rest relative to the soup of ancient light (the Cosmic Microwave Background or CMB) that we are immersed in. And we can say that the solar system is moving at a definite speed in a definite direction relative to CMB.

this was measured rather accurately back in 1990s, the keyword would be "CMB dipole" I think.
the solar system speed turned out to be 370 km/s and the direction is marked in the sky by the constellation Leo.

the way they tell is like this: the CMB light is amazingly uniform, same thermal mix of wavelengths, same "temperature" in all directions to within one part in 100,000 that is to within one thousandth of one percent.

except for a doppler hotspot around the constellation Leo! Because we are running towards the light it is detected as warmer in that direction! By how much?

Well, what percent of the speed of light is 370 km/s? The temperature is raised by exactly that percentage because of the doppler effect of moving towards the light.

speed of light is 300,000 km/s so the percentage is 370/3000= 0.12 percent
about a tenth of a percent warmer in the Leo direction

our motion increases temp by 1/8 of a percent. Once our own motion is subtracted from the data the variation is only 1/1000 of a percent. That is the way it would look to an observer at CMB rest.

So there is that idea of absolute rest and absolute motion, relative to the universe. It gets used a lot in cosmology.

Last edited: Jun 2, 2013
3. Jun 3, 2013

### Staff: Mentor

Perhaps Marcus misunderstood your question. (Or I might be) There is no single frame that is at rest with respect to every other frame. Not even the CMB frame. That is just used as a convenient frame to measure everything against. We can ONLY say that we are moving at X velocity with respect to Y frame. Since Y frame is any frame of your choosing, your velocity X will be different depending on what you measure against.

Also, we do not measure our velocity with respect to light, as it will always be measured at the same speed, c.

4. Jun 5, 2013

### chasrob

What if you took the reference frame of a man sitting on the earth's equator--

you find his velocity vector in 3-space resulting from the earth's rotation

add to that the velocity vector of the earth in its orbit

add to that the velocity vector of the sun in its orbit in the Milky Way

add to that the velocity vector of the MW in its orbit with M31

add to that the velocity vector of the Local Group towards the Great Attractor

Take the sum of all these vectors and from his chair, launch him at an equal and opposite vector in 3-space (thru the earth ). Could he now be considered to be at rest with regard to all above, or what am I doing wrong?

5. Jun 5, 2013

### Staff: Mentor

The man is only at rest relative to his own frame of reference or another one on Earth. After doing all of the above you would find the velocity vector of the man with respect to the Great Attractor.