B Light Speed Traveling: Cosmic Background Radiation Effects

[David Lewis]

TL;DR

Could the frequency of cosmic background radiation blue-shift?

If a spaceship travels near the speed of light, could the frequency of cosmic background radiation blue-shift?

 

[PeroK]

That's one of the practical problems of fast interstellar space travel. The CMBR gets blue shifted to x-rays and eventually gamma rays.

 

[PAllen]

It is also how we know that the sun is not a comoving body. We see anisotropy of the CMB. Modeling earth’s motion around the sun, we know that the sun would not see CMB isotropy. Nor would the milky way galaxy as a whole. Nor would the local group of galaxies.

 

[Grasshopper]

It is also how we know that the sun is not a comoving body. We see anisotropy of the CMB. Modeling earth’s motion around the sun, we know that the sun would not see CMB isotropy. Nor would the milky way galaxy as a whole. Nor would the local group of galaxies.

Does this mean we're not either?

 

[PAllen]

Does this mean we're not either?

Of course. I thought that was too obvious to state, e.g. if the milky way was comoving, the Earth could not be. Actually I said we, on earth, see anisotropy. That, by definition, means we are not comoving.

[post originally said isotropy instead of anisotropy]

 

[jbriggs444]

Does this mean we're not either?

Yes.

One can Google (one moment while I do). "speed of Earth relative to cmb" and get an answer of

about 627 km/sec

Edit: One can also Google for "speed of Earth relative to sun in km/s" and see that our velocity with respect to the sun is only about 30 km/sec. So no, we can't just wait six months and wind up momentarily at rest relative to the CMBR. Our velocity relative to the Milky Way is only about 220 km/sec. So we can't wait for half of a galactic year (100 million years or so) and expect that to work either.

 

[Grasshopper]

Of course. I thought that was too obvious to state, e.g. if the milky way was comoving, the Earth could not be. Actually I said we, on earth, see anisotropy. That, by definition, means we are not comoving.

[post originally said isotropy instead of anisotropy]

Yes.

One can Google (one moment while I do). "speed of Earth relative to cmb" and get an answer ofEdit: One can also Google for "speed of Earth relative to sun in km/s" and see that our velocity with respect to the sun is only about 30 km/sec. So no, we can't just wait six months and wind up momentarily at rest relative to the CMBR. Our velocity relative to the Milky Way is only about 220 km/sec. So we can't wait for half of a galactic year (100 million years or so) and expect that to work either.

Good. Because that would be an insane coincidence.

 

[David Lewis]

If the wavelength of background microwave radiation is the same in all directions, does that imply a preferred frame of reference?

 

[PeterDonis]

If the wavelength of background microwave radiation is the same in all directions, does that imply a preferred frame of reference?

"Preferred" in the sense of being picked out by a particular symmetry in the distribution of stress-energy, yes.

"Preferred" in the sense of being picked out by the laws of physics, no.

 

[Ibix]

If the wavelength of background microwave radiation is the same in all directions, does that imply a preferred frame of reference?

Like the local rest frame of the surface of the Earth is a preferred frame for Earthbound physics, yes. The formulae are the same in any frame, but terms often simplify or drop out in the frame where most of the mass is at rest.