Absolute Velocity?

[Perpetual Student]

The earth is moving approximately at the rate of 627 km/s through the cosmic microwave background radiation. Since the CMB is a remnant of events after 300,000 years from t = 0, and as such would appear to define zero velocity, why does this not define absolute motion through space? How has this seeming contradiction to the concept of relativity of motion been addressed?

 

[NobbyNobbs]

I can't speak to the question of absolute velocity, but I can say I'm pretty sure your time frame is off. It's been quite a bit longer than 300,000 years since the Big Bang.

 

[LarianLeQuella]

The OP means that when we see CMB, it represents what it was like at t=300,000 years. So about 13.4 Billion years ago? As to the velocity question, I'll leave that to others. I got a headache this morning trying to explain that the universe's expansion is an expansion OF space...

 

[nathan]

The CMB could be used to provide a reference, but what does it give you? The laws of physics aren't special in that reference frame. What contradiction are you seeing?

 

[Reality Check]

It is not a contradiction. Things like Special Relativity just work without the assumption that there are preferred frames of reference (your "absolute velocity"). If we add in preferred frames of reference then SR is wrong and all of the experimental results verifying it are also wrong.
A question you should ask yourself is: What is the velocity of the CMB relative to?

 

[Perpetual Student]

It is not a contradiction. Things like Special Relativity just work without the assumption that there are preferred frames of reference (your "absolute velocity"). If we add in preferred frames of reference then SR is wrong and all of the experimental results verifying it are also wrong.
A question you should ask yourself is: What is the velocity of the CMB relative to?

Why? The universe we know has one big bang with the CMB as its remnant. What would the CMB be moving relative to? There certainly appears to be a preferred frame of reference, namely the CMB!

BTW, Nobby, I was referring to the event 300,000 years after the big bang that is the origin of the CMB we see today.

 

[Ziggurat]

The earth is moving approximately at the rate of 627 km/s through the cosmic microwave background radiation. Since the CMB is a remnant of events after 300,000 years from t = 0, and as such would appear to define zero velocity, why does this not define absolute motion through space? How has this seeming contradiction to the concept of relativity of motion been addressed?

Well, General Relativity is actually fine with having a unique reference frame for the universe (sometimes referred to as the "co-moving frame"). And that's actually exactly what the CMB will tell you. But it doesn't really get you anything. It's essentially a topological artifact of the big bang, but topology isn't synonymous with geometry.

One way to think about it is to consider the surface of a cylinder. If everything is confined to the surface of the cylinder, then the world looks flat (so, for example, the inside angles of triangles add up to 180 degrees - unlike on the surface of a sphere). Everything is the same in all directions, UNLESS you wrap all the way around the cylinder. So one direction is unique (the direction around the cylinder), but it's because of the topology of the cylinder (how the surface is connected), not the geometry (the intrinsic curvature, which is actually zero for a cylinder). Something very analogous is going on with the universe: there's a unique reference frame, but it won't make any difference for local experiments.

 

[Reality Check]

Why? The universe we know has one big bang with the CMB as its remnant. What would the CMB be moving relative to? There certainly appears to be a preferred frame of reference, namely the CMB!

BTW, Nobby, I was referring to the event 300,000 years after the big bang that is the origin of the CMB we see today.

Maybe the neutrino background?
After all you are picking out the CMB just because it was there when the universe was 300,000 years old. But there were neutrinos before then.

The basic fact is that we always measure velocities relative to something (typically the surface of the Earth). We could if we want pick an arbitary point in space, measure all velocities relative to it and call that an "absolute velocity". But that does not make much sense.

 

[Thabiguy]

The earth is moving approximately at the rate of 627 km/s through the cosmic microwave background radiation.

This may be nitpicking, but I feel it deserves correction: 627 km/s is not the speed at which the Earth is moving with respect to CMB rest frame, it is the inferred speed of the Local Group of galaxies wrt CMB.

Milky Way is moving with respect to the Local Group, and the Sun is of course moving with respect to our Galactic core, the result being that the speed of the Sun with respect to CMB is about 370 km/s (and this is also what we actually measured and derived the other speeds from).

As for the speed of the Earth wrt CMB, due to its orbit around the Sun, it periodically varies between about 340 and 400 km/s, being lower around June and higher around December.

Also, it is worth noting that the CMB rest frame is not global. Two observers in galaxies far, far away, even if both at rest with respect to CMB, will still be moving with respect to each other. This also means that there is no such thing as an inertial reference frame of CMB (in SR sense).

 

[sol invictus]

The earth is moving approximately at the rate of 627 km/s through the cosmic microwave background radiation. Since the CMB is a remnant of events after 300,000 years from t = 0, and as such would appear to define zero velocity, why does this not define absolute motion through space? How has this seeming contradiction to the concept of relativity of motion been addressed?

The chair you're sitting on defines a rest frame too. If you stand up and walk away, is that a contradiction?

In anything other than a completely empty space, you can always tell whether you're moving with respect to the other stuff in the space. Relativity just tells you is that physics depends only on that relative motion.

The universe does have a preferred reference frame (at least the part of it that we can see does), that's correct. Our best explanation for where it came from is inflation, which made the CMB very regular (if it was wildly varying you wouldn't be able to say how fast we are moving with respect to it).

 

[nota]

I went thru this on a physics board
the nerds there keap up the same reference BS
I said it does not matter pick a point any point

example where was the earth one year ago
thats zero in X,Y, AND Z and time
now one year later how far away are we
and what direction are we going
adding in all the motions of the earth sun MWgalaxy local group ect

I never did get a answer

 

[wafonso]

I went thru this on a physics board
the nerds there keap up the same reference BS
I said it does not matter pick a point any point

example where was the earth one year ago
thats zero in X,Y, AND Z and time
now one year later how far away are we
and what direction are we going
adding in all the motions of the earth sun MWgalaxy local group ect

I never did get a answer

That may be because your question can't be answered. When you pick a fixed point as your origin... it has to be fixed in relation to something. What is that something?

Selecting an origin defines a frame of reference.

 

[jli]

A question you should ask yourself is: What is the velocity of the CMB relative to?

Isn´t it so that the speed of CMB relative to any observer is equal to the speed of light?. If I understand it correct, that property of light (electromagnetic waves) would make it impossible to use CMB as an absolute reference frame.

 

[Perpetual Student]

In anything other than a completely empty space, you can always tell whether you're moving with respect to the other stuff in the space. Relativity just tells you is that physics depends only on that relative motion.

The universe does have a preferred reference frame (at least the part of it that we can see does), that's correct. Our best explanation for where it came from is inflation, which made the CMB very regular (if it was wildly varying you wouldn't be able to say how fast we are moving with respect to it).

OK, that's the point. SR tells us that how time relates to the three space dimensions and how one frame of reference relates to another. I have not suggested that our movement through the CMB at about 370 k/s (thanks Thabiguy for the correction) changes that reality. The dogma that there is no preferred frame of reference appears to be contradicted by the CMB. The cosmological implications of this fact appear to be still undetermined and have not been explored yet -- as far as I know. Nor do I recall any major reaction to the discovery of the CMB anisotropy (other than the discovery of the small fluctuations that validated the big bang theory). I seems to me that finding a preferred frame of reference is a major discovery. For one thing it tells me we are moving through the universe as opposed to merely moving "relative" to something else!

 

[Thabiguy]

For one thing it tells me we are moving through the universe as opposed to merely moving "relative" to something else!

But it doesn't do that; it still merely tells us that we are moving relative to something else - the CMB, or the Hubble flow, or whichever you want to call it.

Specifically: how do you know that the CMB itself has zero "absolute" velocity? What if it is moving at constant speed of 42 km/s in some direction? Or 900 km/s? How can you tell? What the theory of relativity is telling us is that it doesn't matter what its "absolute" speed is, as it doesn't affect the outcome of experiments at all.

The CMB is just a thing that conveniently spreads throughout the universe so you can easily measure against it. But you don't actually need it; in principle, you could always measure against the rest of the universe.

Look at it this way... Let's assume that the universe is spatially unbounded and consists of a finite amount of mass, say, 10^100 kg. Then you can find the center of the mass and find your velocity (or anyone's velocity) with respect to it. But what does that really tell you about your absolute velocity? Why should it be true that all this mass, as a whole, is at rest?

Certainly, you can assume that it is at rest - and the theory of relativity ensures that such assumption is okay - but why would that contradict anything? The observable universe will always have some average velocity of unknown absolute value; the fact that you can assume it to be zero is unremarkable. The CMB that we observe has the same "average velocity" as the observable universe, but that's all.

Measuring against CMB just tells you your velocity with respect to a whole bunch of stars (rather than, say, the star next to you). It's more universal - arguably the most universal you can ever get - but still relative.

 

[Fredrik]

The OP means that when we see CMB, it represents what it was like at t=300,000 years. So about 13.4 Billion years ago?

You may want to look up the word "billion".

 

[Ziggurat]

The dogma that there is no preferred frame of reference appears to be contradicted by the CMB.

It goes against what is typically taught to students of special relativity (in the same way that special relativity contradicts Newton's 2nd law, as typically taught to Freshmen), but it doesn't go against what's been well-known to students (and teachers) of general relativity for a long time now.

The cosmological implications of this fact appear to be still undetermined and have not been explored yet -- as far as I know.

There are few consequences of this fact. It matters if you want to try to find ghost images, for example, but I don't think it matters for much else. Mostly, I'd consider it a consequence of other facts.

I seems to me that finding a preferred frame of reference is a major discovery.

Except that it's an expected discovery. It would be quite something if that were not the case.

For one thing it tells me we are moving through the universe as opposed to merely moving "relative" to something else!

Well, not quite. We're moving relative to the average mass distribution of the universe. That's what determines this unique reference frame.

 

[nathan]

OK, that's the point. SR tells us that how time relates to the three space dimensions and how one frame of reference relates to another. I have not suggested that our movement through the CMB at about 370 k/s (thanks Thabiguy for the correction) changes that reality. The dogma that there is no preferred frame of reference appears to be contradicted by the CMB. The cosmological implications of this fact appear to be still undetermined and have not been explored yet -- as far as I know. Nor do I recall any major reaction to the discovery of the CMB anisotropy (other than the discovery of the small fluctuations that validated the big bang theory). I seems to me that finding a preferred frame of reference is a major discovery. For one thing it tells me we are moving through the universe as opposed to merely moving "relative" to something else!

You're equivocating two different interpretations of 'preferred frame of reference '.

Physical laws do not have a preferred frame -- they are invariant in all inertial frames.

Physical measurements are taken in a reference frame, and you may prefer one frame over another.

The reference frame in which the CMB appears isotropic is similar to using, say, the north pole as a reference point for latitudes on Earth. But unlike the north pole, it's not a very useful reference, unless you happen to be measuring velocities of galaxies relative to CMB.

 

[69dodge]

Isn´t it so that the speed of CMB relative to any observer is equal to the speed of light?. If I understand it correct, that property of light (electromagnetic waves) would make it impossible to use CMB as an absolute reference frame.

Yes, the CMB is electromagnetic radiation, so it moves with the speed of light relative to you, regardless of how you're moving. But it can change color (i.e., frequency), even though it can't change speed. It comes from all directions. If you see the same color in all directions, we say that you're stationary relative to the CMB. If you then start moving, you'll see higher frequencies coming from the direction that you're moving towards, and lower frequencies coming from behind you. So you can tell which way you're moving relative to the CMB.

 

[sol invictus]

SR tells us that how time relates to the three space dimensions and how one frame of reference relates to another.

Right.

I have not suggested that our movement through the CMB at about 370 k/s (thanks Thabiguy for the correction) changes that reality.

Hmm, OK. Go on...

The dogma that there is no preferred frame of reference appears to be contradicted by the CMB.

Again - the earth's atmosphere defines a reference frame, one that's "preferred" by those living inside it. So what?
There is not and never has been any dogma that says otherwise. The facts are simply that the physics only depends on relative motion - that doesn't mean you can't have a bunch of stuff at rest with respect to itself.

The cosmological implications of this fact appear to be still undetermined and have not been explored yet -- as far as I know.

The entire discipline of cosmology is devoted to exploring the implications of that and very closely related facts. The CMB is "at rest" (remember, it's composed of photons which can never be at rest in any reference frame - the true statement is whether or not it has a dipole in any particular frame) in the frame defined by time=constant using the standard coordinates of the Robertson-Walker metric.

In other words, when we say the universe is homogeneous and isotropic, that means there exists a preferred reference frame in which the CMB, galaxies, etc. are on average at fixed spatial coordinate. That's the fundamental basis of cosmology, and it was discovered 80 years ago.

Nor do I recall any major reaction to the discovery of the CMB anisotropy (other than the discovery of the small fluctuations that validated the big bang theory). I seems to me that finding a preferred frame of reference is a major discovery. For one thing it tells me we are moving through the universe as opposed to merely moving "relative" to something else!

See above. The presence of a dipole was expected - it would be an enormous coincidence if the earth happened to be at rest wrt the CMB, and anyway it could be true at most at two times of the year.