Yes, in your own reference frame you are stationary. So by definition all observers are at the same speed in their own reference frame, namely zero. What I was trying to say is that the velocity of the Earth relative to the Milky Way's SMBH is small compared to the speed of light.

Thanks for the clarification. This stuff can be clear and confusing at the same time.

I don't move relative to my reference point, but you move relative to my reference point and vice versa. So from my point of view it makes sense to say you are moving at some fraction of the speed of light.

That statement was very clearly talking about the difference between the reference frames of the Earth and the galactic core, and comparing that difference to the speed of light.

If we (sol / earth) were moving at a speed significantly closer to the (absolute) speed of light we would experience significant time dilation.

This is significant for observing external events and correlating phase shifts to figure out distances.

Eg. Things in the external universe may appear to happen extremely quickly.

It’s actually very important.

Regardless of how fast you appear to be moving in your own frame of reference, the speed you’re travelling in absolute terms affects what you observe externally (and specifically with regard to how fast you are moving compared to what you observing)

The thing we are at rest (ish) relative to is the cosmic microwave background. That's why we aren't moving that much relative to the rest of the universe, on average.

Time dilation is relative between you and some other reference frame. All the "stuff" in the universe nearby is (fairly) static, so we can say we are not moving fast, but it's still relative to the stuff around is.

Huh. So could one use that as a standard frame of reference?

Yes and no. The laws of physics do not pick out a preferred frame of reference, and they should be symmetrical under lorentz transformation, but the fact that we live in a cosmological universe starting with a big bang breaks that symmetry in practice.

It's similar to how the laws of physics are symmetrical under translation and rotation, but the presence of matter around us means the surface of the earth is not the same as the surface of the sun.

So, while we might use the CMB as a standard reference frame, the laws of physics should not.

Maybe, kinda. It's not obvious that it's more privileged than centering a reference frame on the Earth or the Sun. The CMB is maybe just an artifact of the Universe's history (like a Sun-centered one is an artifact of ours).

If we didn't have any CMB radiation floating around to measure, everything would probably work exactly the same. Maybe you could measure all the motion of all the matter in the visible universe and measure the motion of the "center" relative to us.

> the speed you’re travelling in absolute terms

There is no such thing. You always have to compare your speed to something else. Speed is calculated relative to something else. Relativity.

/shrug

My point was that if we (the solar system) were travelling as a considerable portion of the speed of light, then we would experience time dilation, which would significantly affect our observations of the external universe.

We're not.

So... it's fine.

I mean, you're basically right; speed isn't absolute, and it's wrong to refer to your speed relative to the CMBR as 'absolute'.

...but, well... I think it's fair to say 'The speed of light is constant in all frames of reference, so why [insert question here]...' is a very fair, and common question. 'Everything is relative' isn't an answer, it's just a platitude.