How fast is the earth moving.

[joesixpack]

... 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...

Pardon me if this is a stoopid question, but don't some frames of reference create impossible circumstances? For example, if we were to assume that the earth were not spinning on its axis, but that the sun, planets, and stars, were instead orbiting us, then distant objects would have to move at light speed or greater in order to pass over a given meridian on the earth every 24 hours.
I guess my question is, even though there isn't one correct frame of reference, arent there many that should be incorrect?

 

[Ziggurat]

Pardon me if this is a stoopid question, but don't some frames of reference create impossible circumstances?

Not all reference frames are inertial reference frames. Rotating frames, which you refer to, are non-inertial reference frames. I think most of the discussion here was implicitly about inertial reference frames, though as you seem to have intuited, non-inertial frames exist and are distinguishable from inertial frames.

 

[TV's Frank]

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.

Except, of course, for the fact that there is no evidence (yet) from the CMB that the universe has non-trivial topology. Even if the universe did have non-trivial topology, as long as it's larger than the horizon (and inflation will always make the topological horizon larger than the physical horizon), we will never observe it! And since it doesn't affect any physics (GR responds only to geometry, but the topology can make some subtle changes to the vacuum energy via the Casimir effect, but that's it), we really can't tell the difference.

 

[Ceritus]

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.

If this were true would it be possible if you had a strong enough telescope to view the opposite end of the earth if there were no obstructions ofcourse by looking so deep into space you complete a slice of the sphere and come back on yourself/planet?

 

[wollery]

If this were true would it be possible if you had a strong enough telescope to view the opposite end of the earth if there were no obstructions ofcourse by looking so deep into space you complete a slice of the sphere and come back on yourself/planet?

No, because you'd be seeing the spot where you are as it would have been in the extreme distant past.


Light takes time to travel.

 

[Thabiguy]

If this were true would it be possible if you had a strong enough telescope to view the opposite end of the earth if there were no obstructions ofcourse by looking so deep into space you complete a slice of the sphere and come back on yourself/planet?

If universe is in fact compact and without edge, then yes, this would be theoretically possible. However, we have not seen such wraparound in the entire observable universe. If the universe wraps around on itself, it would take light more than 27 billion years to do the roundtrip - possibly many many million times more than that. If you could wait long enough to see the back of your head with a telescope, then the entire age of the universe so far was not nearly enough.

To make the prospect even more bleak, we do not know whether the universe actually wraps around on itself. It remains an open question. Recent research indicates that the observable universe is spatially flat. While this still does not exclude the possibility that the universe is "closed" in principle, it makes the consequences of that possibility rather irrelevant to us.

 

[Ziggurat]

Except, of course, for the fact that there is no evidence (yet) from the CMB that the universe has non-trivial topology.

My statements concerning a topological preference are for an isotropic universe, which is the simplest geometry/topology you can have. In fact, it's the other way around: if the universe is NOT isotropic, then the local CMB may not correctly indicate the prefered reference frame for the universe.

Even if the universe did have non-trivial topology, as long as it's larger than the horizon (and inflation will always make the topological horizon larger than the physical horizon), we will never observe it!

Not directly, that's true. But indirectly we can and have, through the CMB.

And since it doesn't affect any physics (GR responds only to geometry, but the topology can make some subtle changes to the vacuum energy via the Casimir effect, but that's it), we really can't tell the difference.

Actually, that's not quite true either. The CMB imposes an effective speed limit on charged particles originating outside our galaxy, because at speeds very close to the speed of light (and by that, I mean a lot closer than 99%) compared to the CMB, the extreme blue shift in the forward direction creates huge radiation pressures which will slow such particles down, and by quite a bit over intergallactic travel times.

 

[Cuddles]

Actually, that's not quite true either. The CMB imposes an effective speed limit on charged particles originating outside our galaxy, because at speeds very close to the speed of light (and by that, I mean a lot closer than 99%) compared to the CMB, the extreme blue shift in the forward direction creates huge radiation pressures which will slow such particles down, and by quite a bit over intergallactic travel times.

Of course, the main problem with that is that particles have been detected that break the speed limit. There's either a problem in the theory or a problem in the experiments (or both), but we're not sure which.

 

[Yllanes]

Of course, the main problem with that is that particles have been detected that break the speed limit. There's either a problem in the theory or a problem in the experiments (or both), but we're not sure which.

Well, the statistics for these ultra high energy cosmic rays are not yet very good, so we are not completely sure whether there is a problem. In a couple of years, with the results of the Pierre Auger observatory (and others), we will know.

In case anyone is wondering about this digression, cosmic rays have been detected with energies of 10²⁰ eV (the typical comparison is a single proton with the energy of a tennis ball or a baseball fastball). These particles cannot go farther than a few tens of Mpc, mainly because of their interaction with the CMB (photoproduction of pions for protons, photodisintegration in the giant dipole resonance for nuclei, etc.) This is known as the Greisen-Zatsepin-Kuzmin or GZK cutoff. So if we detect a particle with these energies, it has to come from a source less than some 20 Mpc away. What's more, these UHE cosmic rays are not deflected by magnetic fields, so if we detect one we just have to point a telescope in its incoming direction and see its source, at least in principle. This identification of the source should be relatively easy, because there are very few objects capable of such accelerations. But this is not working so well.

As an example, let us consider the most energetic event yet, 320 EeV (3.2·10²⁰ eV), detected by the Fly's Eye observatory in 1991. The only feasible sources at <20 Mpc were Cen A, M85 and Virgo A. But all of them implied a very high magnetic deflection, much too high to be reasonable for a particle of that energy.

So the origin of these UHE cosmic rays is still a puzzle. Maybe the only reason is that we have poor statistics (have detected few events) at these energies. But maybe there is some new physics involved. Bear in mind that these energies are 100 000 000 higher than what the LHC is going to be capable of, so cosmic rays are a potential laboratory for physics beyond the standard model. Many ideas have been developed to account for the flux of UHECR. Some of them imply minute Lorentz violations, new interactions for neutrinos, exotic primaries (such as supersymmetric particles), or even topological deffects (cosmic strings, magnetic monopoles) or superheavy relics (particles with 10¹² times the mass of a proton).

Several experiments are now in progress aimed at this area of the cosmic ray spectrum, to decide whether these new ideas are necessary or not. Most probably not, I'm afraid.

 

[robinson]

Of course, the main problem with that is that particles have been detected that break the speed limit. There's either a problem in the theory or a problem in the experiments (or both), but we're not sure which.

What particles are these you speak of? Is this data online?

 

[Yllanes]

What particles are these you speak of? Is this data online?

Well, I explained the situation in my previous post. If you want more information, you can start, for example, with the pages of the Pierre Auger collaboration.

 

[robinson]

Well, I explained the situation in my previous post. If you want more information, you can start, for example, with the pages of the Pierre Auger collaboration.

OK thanks. I think.

 

[robinson]

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.

I used to think that was what the Theory of Relativity was about. I have been assured that this is not the case. In which case, what is that concept you just explained called?

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.

Well, it does seem to when it comes to practical matters.

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.

I hear statements all the time about how fast something is moving. The reference frame seems to be where the person reporting the movement is standing. In the case of "how fast is the Earth moving", the answer changes depending on where we imagine the "fictional observer" is reporting from. While this is an imaginary point, it is still interesting to hear the answers.

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.

Now, again, what is that called? The statement, "All movement is relative". And, if that is true, then doesn't that mean the Earth isn't going around the sun? From our point of view, here on the Earth? This was mentioned upthread. In fact, from my point of view, the Universe revolves around me.



And, isn't the speed of light, or of all electromagnetic radiation, isn't that speed not dependent on a reference frame? Unlike all other motion, light is a constant, that doesn't depend on relative motion?

I know, I know, this is probably already been discussed to death. But I want to know, from the point of view of, say, another nearby galaxy, how fast is the Earth moving?

Or from the center of our Galaxy?

 

[wollery]

http://www.earthmove.info/

Just for fun!

 

[robinson]

That is pretty cool, and helpful. Now, somebody please explain how it can be correct. I underlined the part that doesn't seem possible.

From the page http://www.earthmove.info/

The Earth is moving at 187,500 miles per second??? Isn't the speed of light 186,282.397 miles per second?

 

[wollery]

You're right, I'll check that and let them know.

 

[wollery]

Ah, got it.

The edge of the visible Universe is receding at the speed of light, and that's a rounded off figure for that value. Not accurate, but close enough.

 

[robinson]

Good point. Now, how fast is the Earth (in the Galactic cluster), moving towards the constellation Hydra? That would be a reasonable relative velocity to calculate.

 

[politas]

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

I don't believe the unboundedness of the universe is due to curvature, but rather constant expansion. If space is expanding at the speed of light, it is impossible to ever reach the edge from anywhere within the universe, even though the volume of the universe is always finite.

So you wouldn't return to where you'd started, you'd just keep going forever, chasing an edge which is always receding.

 

[tkingdoll]

The speed of light question for the www.earthmoves.info site has come up before, this is a comment I've received in defence:

"Due to rounding we are super-Einstein by one part in 186000 or so
- about 6 parts in a million 0.0006%. In any case special relativity does not apply to the stretching of the universe "

Now, if someone just wants to tell me what that means, I'll be delighted to change the website