# Time in relativity and the CMB

1. Oct 3, 2015

### palmer eldtrich

As understand relativity there is no "absolute time" however I have its possible to use the CMb as a way round this? Is this right and how does it work?

2. Oct 3, 2015

Staff Emeritus
What makes the CMB frame more special than the frame where Lincoln was shot?

3. Oct 3, 2015

### Orodruin

Staff Emeritus
Bad example, Lincoln being shot is an event which occured in all frames. He was not not shot in the rest frame of the galactic center.

4. Oct 3, 2015

### Chalnoth

The CMB does provide a valid choice of a reference frame. This choice of reference frame turns out to be very useful in that it simplifies the math for dealing with the expansion of the universe.

But it's still a choice: it's perfectly valid to select different reference frames. The math all still works in a different frame, and any physical calculation you do to predict an experimental result will come up with the exact same prediction.

5. Oct 4, 2015

Staff Emeritus
I don't understand this. An event doesn't occur in a frame. An event occurs in real life. A frame is how we assign coordinates to that event.

6. Oct 4, 2015

### Orodruin

Staff Emeritus
The point is that regardless of how you assign coordinates, Lincoln was shot. This is an event which, as you say, did not occur in a frame. You seemed to want to assign events to frames since you were talking about the frame in which Lincoln was shot.

7. Oct 4, 2015

### Staff: Mentor

So to make Vanadium50's point by rhetorical question, we need a different rhetorical question. How about "What makes the CMB frame more special than the frame in which the bullet that killed Lincoln was at rest?".

8. Oct 4, 2015

### Staff: Mentor

Not right.
There are some problems for which it is convenient to attach times to events using coordinates in which the CMB is isotropic, but that doesn't make those times any more "absolute" than any other. There are plenty of other problems for which those coordinates would be wildly inconvenient, and no problem in which the physics requires us to use them.

9. Oct 4, 2015