# Speed of the Earth through the Universe

I am interested in knowing how fast the Earth is moving relatively compared to points outside the galaxy. I know that this may be impossible to mention totally accurately as the speed depends on a number of factors, including the rate of travel around the sun, the rate the solar system is orbiting the galaxy, and the way the Milky Way itself is moving relative to the other galaxies.

Is it possible for a theortetical person outside the galaxy in a fixed point in space with an assumed large enough telescope to isolate the earth to factor in all of these numbers and come up with a relative speed for the earth as it moves through the universe?

No, there’s no such thing as a fixed point in space.

I am not a physicist and I may be wrong about this but my understanding is that all points in space are moving relative to one another and there is no “fixed point in space” from which to observe. Therefore, the question has no answer.

I do understand that, and that everything is relative, and whatnot, but would not an observer was far enough away from most large sources of mass so as to be virtually unaffected by their movement not be close enough for the purpose if this thought experiment?

Let me get some coffee and rephrase that into English:
I do understand that, and that everything is relative, and whatnot, but would not an observer that was far enough away from most large sources of mass be able to remain relatively fixed with regards to the movement of the milky way, or galactic cluster so as to be able to deduce the relative speed?

No. Imagine an auto race where the cars, the finish line, and the press box in the grandstands were all moving at breakneck speed in different directions. That’s our universe.

I do get that, but if we know the movements in directions x, y and z, then a path through 3d space should be fairly constant (though changing slightly as the earth loops around the sun, and as the sun loops around the galaxy. I know I am missing something here though…

But we don’t know the movements, except in relation to other reference frames.

You are making the mistake of assuming that there is some “stationary” spot in the universe that you can find and go sit there. Doesn’t work that way. Things only move in relation to other objects. For instance, in the solar system it looks like the sun doesn’t move much, but everything else moves around it. Go to the next closest star, though, and the sun is flying thru the universe, too.

You could define motion relative to the Cosmic microwave background. This is the closest we have to a universal (heh) reference frame.

A bit of googling reveals that the Solar System orbits the center of the Milky Way at about 220 km/s, and the Earth orbits the Sun at about 30 km/s.

Now, if you were travelling alongside our galaxy (G), when the Solar System (SS) swept by you in its orbit, the Earth would appear to be moving at 220 km/s if the Earth were stationary relative to the sun. However, the Earth orbits at 30km/s, so if it were moving in the same direction as the orbit of the SS (on the “outside” portion of its orbit), its speed would appear to be 220 km/s + 30 km/s, or 250km/s. However, if it were on the far side of the sun (the “inside” half of its orbit), it would appear to be moving more slowly, as its motion would be retrograde relative to the orbit of the SS (220 km/s - 30 km/s = 190 km/s).

If you were to pick a reference point, with the info above plus the speed and direction of the reference point with regard to the Milky Way, you could get a back–of-the envelope calculation of how fast Earth would seem to be moving from there.

I anticipate being corrected by those who know (much) better.

There is a sense of “stationary” in the universe that has real and significant physical meaning. All the galaxies spin, but define the center of each galaxy to be its location. The expansion of the universe as explored by Hubble and scaled by the Hubble constant says that all these locations are getting further apart on average (though there is also an additional relative velocity between each of these locations and the average of all the others). So, there is an average framework which from our point of view has other galaxies retreating from us at velocities that are (at least for some considerable ways out) proportional to their distance from us, the Hubble constant being the constant in question. That average framework is measureable and has real significance. We are moving relative to that framework in a way that must have arisen due to gravitational interactions between our galaxy and others near it, and groups near our groups, and so forth.

The concept of Einsteinian relativity says that there is nothing about space itself that prefers any reference frame, but it doesn’t say that there may be physical reasons for scientists to prefer some reference frame for a particular purpose. In practice we do it very often for the reference frame of the Earth’s surface (which for the great majority of technical purposes is practically an inertial one).

There is no well-defined 3 dimensional “background” space. Firstly, space and time are essentially unified into 4 dimensional (well, 3+1 dimensional) “spacetime”. Secondly, there is no canonical 3 dimensional “slice” of spacetime to define space.

We’re moving at about 600 kps with respect to the CMB.

But how fast is the CMB moving?

But the OP was so vague as to preclude any preference. It explicitly invoked the notion of a “stationary point” with no relative clause. All measurements of motion in GR (and thus cosmology) must have a relative clause to be well-defined.

But for the question to be answered as I posed, the stationary point x could have been from anywhere not defined within the parameters of what I was asking to measure. Whether things were speeding towards the imaginatry point x, or away from the speed so to speak could be ascertained (or so I thought).

Assuming that the entire shebang was moving on a path not directly toward s or away from point x, a reasonable correction could be made perhaps.

In all this, I do understand that the speed of the earth orbiting the sin, and even the sun orbiting the milky way would be minor compared to the speed at which the galaxy itself was moving, but hey, I wanted to see them all added up anyway. The preference as outlined was for you to pick a point x from which you could answer the question outside of the area of that which is being measured.

So what you’re asking is that we pick any point outside, say, our local galactic cluster, and ascertain our speed relative to that point.

The problem is that there are a bazillion-bazillion-bazillion possible x’s, each one yielding a different answer, neither being more (or less) stationary than the others.

Simplify things a bit to see what is happening.

Picture a universe devoid of anything except Person-A and Person-B.

Person-A and Person-B are moving relative to each other. That is, from either of their persepctives they see the other person moving through the universe. Doesn’t matter if they are 10 feet or 10 million light years away. Doesn’t matter in what direction. Doesn’t matter how fast. Either of the two may consider themself at rest and decide it is the other person who is moving. Person-A can think they are sitting still and Person-B can think they are the one who is sitting still. Either of the two could also assume they have some motion while the other person has some motion that together combine to equal what they observe. All of those answers are correct despite intuition telling us that only “one” answer can be correct.

Slice and dice it any way you please. There is NO experiement either one can do to definitively determine which one is moving and which is standing still. Any combination you can come up with that matches the observations is correct. You see Person-B coming at you at 100,000 mph and assume you are sitting still. Or you are moving towards Person-B at 50,000 mph and he is moving to you at 50,000 mph. Both answers are correct…or at least you can never discern which is the “real” answer (or any of the infinite other possible combinations).

Expand that idea out to the current universe and it myriad stars and planets and you are no better off. All you can do is pick a frame of reference arbitrarily and go with it. This is ok since you can still get “correct” answers and do useful math (like how to aim your rocket to get form A to B).

Sure, you can arbitrarily choose your x, and make the whole question meaningless, or you can measure your velocity with respect to the largest, most distant structure available. The CMB surrounds us on all sides, and our movement towards one side, and away from the other produces a nice dipole in the evening sky.

Can’t you assign a fixed point where the Big Bang happened? I understand this would only work if the expansion outward was even in all directions. But it seems like you could use this origin as a reference point.

I apologize as I don’t claim to know much about astronomy, but I plan to do a lot more reading on the subject.