How fast is the earth moving.

[roger]

You can use the cosmic microwave background. The Cosmic Rest Frame is defined as that in which the CMB is isotropic.

You can, of course, as it is a rest frame, but there is nothing special about it.

 

[roger]

Fascinating, I've been aware of the balloon analogy for years yet never picked up only the surface of it is relevant to the demonstration. Thanks!

The problem is that the balloon is expanding in space; when we talk about space expanding, it really is space that is expanding, not just some stuff sitting in space moving away from each other.

 

[toddjh]

You can use the cosmic microwave background. The Cosmic Rest Frame is defined as that in which the CMB is isotropic.

Sure, but that's still completely arbitrary. There's nothing special about the frame in which the CMB is isotropic except for the fact that the CMB is isotropic in that frame.

In fact, I'd argue that, for human purposes, the cosmic rest frame is far more arbitrary than the frame of the sun or the Milky Way.

 

[Ziggurat]

Sure, but that's still completely arbitrary. There's nothing special about the frame in which the CMB is isotropic except for the fact that the CMB is isotropic in that frame.

This is incorrect. The CMB reference frame is the topologically unique reference frame for the universe.

Take a flat, infinite plane of paper: it's isotropic, and rotating a coordinate system within this plane does not change anything. This is exactly analogous to the reference frame invariance of special relativity. But now imagine that we roll that plane of paper into a tube. If we're confined to the surface of the paper, then geometry remains isotropic, but the topology no longer is. Draw a line in one direction and it'll come back to itself. Draw a line in any other direction and it won't. The same sort of topologically-induced unique reference frame exists in cosmology, and we can detect it experimentally with the CMB.

 

[lenny]

This is incorrect. The CMB reference frame is the topologically unique reference frame for the universe.

i think that's right.

(but only because i think we understand the CMB: claiming this as The frame is internally consistent with current theory, but most of our observations have been made from the same place in space-time.)

 

[Ceritus]

So our solar system isn't moving anywhere?

 

[ellindsey]

So our solar system isn't moving anywhere?

You still don't understand.

Our solar system is stationary relative to itself. Relative to other stars, the center of our galaxy, or other galaxies, it may be moving. But there is no single stationary reference frame which it can be said to be moving relative to.

You are still thinking of the issue in a Newtonian framework, in which some objects are moving, others are at rest, and it is possible to tell which objects are moving and how fast they are moving. The universe doesn't work that way. Away from some arbitrary fixed reference frame, it isn't possible to state that one object is moving and another is stationary. Not because of an inability to measure them, but because there simply isn't an objective stationary reference frame to measure movement against.

Imagine you have two objects in space, say object A and object B, that are moving away from each other at 100 MPH. An observer standing on object A says that object A is stationary and object B is moving at 100MPH. An observer standing on object B says that object B is stationary and object A is moving at 100MPH. Both of them are right, because each is measuring the motion in their own reference frame.

So, if you ask how fast the Earth is moving, and people respond "Relative to what?", it's not because they're being cute or trying to confuse you, and it's not because the Earth isn't moving. It's because there simply is no fixed reference frame that movement can be measured relative to. All movement is relative.

 

[MrQhuest]

Relative to earth's own axis, a person standing still on the equator is moving at about 1600 kph. (Circumference of the earth / 24 hours)

Relative to the Sun, the earth is moving at about 53000 kph. (circumference of earth orbit / 365 days / 24 hour)

I’m afraid I’m a little too lazy today to look up the distance to the galactic center, or the period of the sun's rotation around it.

MrQhuest

 

[Yllanes]

I’m afraid I’m a little too lazy today to look up the distance to the galactic center, or the period of the sun's rotation around it.

It was already given earlier in this thread. The speed of the solar system around the galaxy is around 230 km/s. Also, remember Monty Python's Galaxy Song

Just remember that you're standing on a planet that's evolving
And revolving at nine hundred miles an hour,
That's orbiting at nineteen miles a second, so it's reckoned,
A sun that is the source of all our power.
The sun and you and me and all the stars that we can see
Are moving at a million miles a day
In an outer spiral arm, at forty thousand miles an hour,
Of the galaxy we call the 'Milky Way'.
Our galaxy itself contains a hundred billion stars.
It's a hundred thousand light years side to side.
It bulges in the middle, sixteen thousand light years thick,
But out by us, it's just three thousand light years wide.
We're thirty thousand light years from galactic central point.
We go 'round every two hundred million years,
And our galaxy is only one of millions of billions
In this amazing and expanding universe.

 

[CurtC]

You can, of course, as it is a rest frame, but there is nothing special about it.

I think there is something special about it. When someone considers how fast we're moving, first you might consider the rotation of the surface of the Earth, and based on your latitude, you come up with something like 800 mph. Then you think about how the Earth revolves around the Sun, and that speed is like 60000 mph. Then you recall that the whole Solar System is rotating around the center of the galaxy, and that number is even bigger. The final thing that you can measure the speed relative to is the CMB. As we observe the CMB from here, it's red-shifted in one direction of the sky, and blue-shifted in the other direction, indicating that we're moving with respect to the big bang.

As I recall from years ago when I read Wrinkles in Time, we're going at about a million mph with respect to the CMB.

You wouldn't agree that measuring speed w/ respect to the big bang is the ultimate reference frame (not in a theoretical sense, but in a gut-feel satisfaction sense)?

 

[Ziggurat]

So, if you ask how fast the Earth is moving, and people respond "Relative to what?", it's not because they're being cute or trying to confuse you, and it's not because the Earth isn't moving. It's because there simply is no fixed reference frame that movement can be measured relative to. All movement is relative.

As I already said, this is incorrect. One of the ironies of physics is that while special relativity does not distinguish between any reference frames, general relativity does in fact reintroduce the notion of a prefered reference frame because of the topological constraints of cosmology. So there really is a unique reference frame for the universe, it's just that no local experiments can ever determine what it is. Measuring the CMB is, however, one of those nonlocal measurements we can do to accomplish precisely that.

 

[roger]

I think there is something special about it. [snip]
You wouldn't agree that measuring speed w/ respect to the big bang is the ultimate reference frame (not in a theoretical sense, but in a gut-feel satisfaction sense)?

I meant special in the physics sense. The laws of physics don't change, and you can't say something is "really" moving at X because of it's speed against the CMB. Motion is relative, there are no absolutes (which I know you agree with). In that sense the frame is not special.

It sure is special in the "gee whiz, neat!!!" way

 

[Ziggurat]

I meant special in the physics sense. The laws of physics don't change, and you can't say something is "really" moving at X because of it's speed against the CMB. Motion is relative, there are no absolutes (which I know you agree with). In that sense the frame is not special.

But it is special. It's special in the same way that the direction that raps around the surface of a cylinder is unique, even though there's no local difference between that direction and any other. It's a topological issue, and definitely non-local, but it's quite real nonetheless.

 

[JoeTheJuggler]

I am a little puzzled by this.

It is my layman's understanding that the Universe started a given "point" out of which it is (still) expanding. The analogy of a balloon being inflated is often used, but even this balloon as a center: why doesn't the Universe? If we were to turn around the galaxies and get them to go opposite their current directions, wouldn't they converge towards the actual "center of the Universe"?

The universe didn't start at a given point if the implication is that it's a point in space.

The Big Bang didn't happen somewhere in space--it was the origin of space/time. (The Big Bang happened everywhere in the universe.)

 

[Schneibster]

Personally I prefer the frame in which the Earth is NOT moving. It makes me feel more comfortable.

 

[Thabiguy]

Indeed, the CMB reference frame is as universal as one can ever get. It is basically the reference frame of the entire observable universe. When using it, you may "synchronize" your motion with someone in a distant galaxy billions of light years away, or indeed on the "opposite end" of the observable universe. That's universal enough for me. You may see it called the frame of the Hubble flow or the frame of the comoving observer.

While universal, it's not without interest that the frame is not global. It will actually be different for you and for someone in another galaxy. The difference is however predictable and you can tell it if you know the direction and distance of the other observer. That's why I prefer to call it the comoving frame; it moves along differently for each observer as the universe expands, but to each observer it offers a universal reference to measure against, one in which certain properties of space are shared across the universe. That's what makes it so important in cosmology.

What remains to be said is how fast the Earth is actually moving with respect to the Hubble flow. I've seen different values, but more or less they were around 370 km/s - that's the speed of the Sun. The speed of the Earth varies by about +/- 30 km/s as it orbits the Sun; it increases in December and decreases in June.

 

[Soapy Sam]

The Earth is Moving?
Why am I always the last to know?

 

[MrQhuest]

I have a question for the astrophysicists and the cosmologists here.

Assuming you could travel faster then light, what would the environment near, at, and beyond the edge of the universe look like?

As per my understanding, it is not just matter expanding into a void, it is space and time itself that is expanding at the speed of light. So would there be a barrier of some type, or would you just stop moving past the “boundary”?

MrQhuest

 

[wollery]

Since space time is curved you'd return, eventually, to where you began. The Universe has no boundary. Like the surface of a sphere, it is finite, but unbounded.*














*Probably

 

[Thabiguy]

Assuming you could travel faster then light, what would the environment near, at, and beyond the edge of the universe look like?

We don't know whether the universe has any edge. All we see is stars and galaxies that stretch as far as we can see, and beyond. The universe could be infinite; or it could have an actual physical edge, farther away than we can see; or it may be finite but without an edge (like the surface of a ball), and could wrap around on itself. In the part of the universe that we've been able to observe, we haven't seen any edge or wraparound or significant structure change. It's quite possible that the universe is much, much, MUCH larger than the part we've been able to observe.