Galilean Relativity, the CMB and the definition of 'at rest'

• Galilean Relativity, the CMB and the definition of 'at rest' by nicp on 05 Apr, 2021 17:46
• Bit of a science question here, hope that's ok for a spaceflight forum. There is an (imaginary) spaceship!
  
  I may be travelling in a car, and someone sitting at a bus stop will see me moving at 60 mph, but I can argue they are travelling at 60 mph and I am at rest. We are both right. This is Galilean invariance (or relativity). https://en.wikipedia.org/wiki/Galilean_invariance
  
  Now imagine (as is usual for these discussions) a spaceship travelling at some speed. There is no way for the occupants to know if they are moving fast by observing stuff - the ship could be stationary and objects fly by - but is the ship moving or the objects?
  
  So we look out the windows - and perhaps on one side of the ship we see more blue stars than the other. Aha! Thoise stars must be blue shifted, so we are generally headed in the direction of the bluer stars.
  
  But again, are the stars moving or the ship? Up to this point the question is empty - pick a frame of reference.
  
  But when, from your ship you observe the Cosmic Microwave background, if you go fast enough, the CMB itself will be blue shifted on one side and red shifted on the other.
  
  So you can define "at rest", relative to the CMB. Can't you?
  
  
  
• #1 by Redclaws on 05 Apr, 2021 18:04
• Yes, of course, but it’s not fundamental.  You can also define at rest relative to anything else - at rest relative to the earth, as I am (more or less) currently, for example.
  
  The CMB is sort of close to “everywhere”, but that doesn’t change physics itself.  It’s just another thing relative to which one can be at rest or in motion.
  
  The complex and more abstract implications of all this are a bit beyond me so I hope someone will come and explain them, but as I understand it, the fundamental thing is what you’re saying is more or less true, and it doesn’t matter.  Not in the sense of providing a universal absolute frame of reference that would undermine relativity as usually conceived.
• #2 by Proponent on 13 Apr, 2021 16:46
• Motion with respect to the CMB has been measured, by the way: the sun is moving about 368 km/s (see the "CMBR dipole anisotropy (ℓ = 1)" section of this Wikipedia article
• #3 by Hog on 16 Apr, 2021 17:27
• I am "at rest" even though I am hurtling through space/time at over 2,855,000mph/800miles/sec  or 4,495,000kph/1280kps/1,280,000 m/s.
• #4 by nicp on 16 Apr, 2021 21:18
• Quote from: Proponent on 13 Apr, 2021 16:46

  Motion with respect to the CMB has been measured, by the way: the sun is moving about 368 km/s (see the "CMBR dipole anisotropy (ℓ = 1)" section of this Wikipedia article
  

  Great reference, thanks!
  
  The point of my original post was that I had always considered any frame of reference arbitrary, and then realized one day that it isn't - due to the CMB.  I left physics (and wish I hadn't) 35 years ago, oops 37 years ago at the age of 18. I was going to (try) to deduce the relationship of temperature differences of the observed CMB to velocity, but someone a lot smarter than me obviously did that a long time ago.
  
  Thanks!