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Compressed Matter

chracatoa said:
Yes, but if the big bang is right, the universe would have to expand faster than the speed of light if there are parts of it that are invisible for us. Or am I completely off base here?
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The Big Bang is compatible with an infinite universe. As Schneibster said, only the observable universe was the size of a pea, the whole would have always been infinite. Even if the universe is finite, with all of it the size of a pea a long time ago, its radius is bigger than its age times the speed of light. This is not in contradiction with SR. You can read this tutorial or start with these two FAQ:

How can the Universe be infinite if it was all concentrated into a point at the Big Bang?

If the Universe is only 10 billion years old, how can we see objects that are now 30 billion light years away?
 
Just dropping off a few more book ideas....

"Einstein's Theory of Relativity" by Max Born is my favorite. After reading a stack of 'popularizations' that left me unsatisfied, this book had enough detail to make me happy.

Also, the relativity primer in Brian Greene's "Elegant Universe" presents some of the concepts in a brilliantly clear manner.
 
chracatoa said:
Yes, but if the big bang is right, the universe would have to expand faster than the speed of light if there are parts of it that are invisible for us. Or am I completely off base here?
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The matter from the Big Bang didn't expand at that rate. Space itself was expanding faster than light. The indirect result is that the matter was moving faster than light. But since it was not moving faster than light relative to space itself, no laws are broken.
 
Schneibster said:
Hee hee, actually, regarding instantaneous forces, there's something really interesting to talk about. I've talked about it here before; I'll give a link in a moment. It turns out that the fact that charged particles obey SR, and that the transmission of the electromagnetic force is not instantaneous but propagates at the speed of light, is responsible for the existence of magnetism; [snip].
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I may be repeating a point since this conversation spans threads.

There should also be an force analogous to magnetism for gravity also. Since gravity propagates at the speed of light a spinning massive body should exert a torque on another nearby spinning massive body.
 
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Hmmm, might that be the origin of "gravity waves?"

Hey, I wonder what's going on at LIGO? Haven't checked that out recently.
 
No, I was wrong- that's at least partly frame dragging. IIRC. Yllanes will no doubt step in and correct me if I don't.

Frame dragging means that the actual spacetime surrounding a spinning massive object is twisted by the spin, as opposed to just being curved by the mass. Or anyway that's a good way to understand it from a layman's POV, which I try not to get too far beyond.
 
I don't think you're wrong. I think frame dragging, gravity waves, and the magnetic analog (gravitomagnetism???) I referred to are all closely related.

The magnetic analogy goes like this: In a circular loop of wire with with non moving charges a charged particle at a distance is not affected by the wire because all charges within the wire are balanced out. An equal number of protons and electrons. But start a current moving in the wire and the situation changes. At first glance the charges are still balanced but the electrons moving away from the isolated charge are now exchanging virtual photons with the isolated charge that are red shifted while the ones from the side of the circle with the current moving toward it are blue shifted. A similar thing happens with mass and gravity. If a mass is rotating, the mass on one side is seen as receding leading to "red shifted" gravity waves and the other side is seen as approaching and "blue shifted" gravity waves. That leads directly to frame dragging and magnetic analog. And then given that there is a magnetic analog that leads to wave propagation. So I think they are all closely related.


BTW Is LIGO still in operation? Has LISA made any progress? Has Gravity B produced results yet?
 
I did a post on those two experiments here. There'll be some more interesting stuff over there shortly, too, I think. Feel free to join in.
 
Yllanes said:
You can read this tutorial
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Very nice tutorial. I will spend some time reading it. I thought that the Big Bang included the entire universe, not only the observable universe.

Now, let us assume there is something beyond our observable universe. Will we be able to see something in the distant future or it will remain invisible for us forever? Perhaps we cannot even speculate about it...
 
Schneibster said:
Hee hee, actually, regarding instantaneous forces, there's something really interesting to talk about. I've talked about it here before; I'll give a link in a moment. It turns out that the fact that charged particles obey SR, and that the transmission of the electromagnetic force is not instantaneous but propagates at the speed of light, is responsible for the existence of magnetism; and it further turns out that the reason that the color (strong) force acts the way it does is that the magnetic component is dominant, which implies some rather curious things about how the color force manifests.

Here is the thread. Cecil had a cool experience here, and I was happy to share in it, along with epepke and some others.
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So magnetism is a type of electro doppler effect?
 
chracatoa said:
Very nice tutorial. I will spend some time reading it. I thought that the Big Bang included the entire universe, not only the observable universe.
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It depends on your definition of "universe" and "big bang". Popular thinking currently is that the universe is quite possibly infinite, or at least close enough that it can be seen on a clear day, but that the observable universe is, obviously, finite. In this case we can either refer to what is observable as "the universe" or to the entire infinite space as "the universe". Equally, the big bang can either refer to the start of the whole thing, the end of the inflationary period or just the origin of the observable universe (in most theories incorporating inflation the last two are the same, and in most without inflation the first and third are the same). It is easy to get confused when different people are using different definitions.

Now, let us assume there is something beyond our observable universe. Will we be able to see something in the distant future or it will remain invisible for us forever? Perhaps we cannot even speculate about it...
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Maybe. We can speculate about it, but without knowing the the shape, extent and actual amount of energy and mass in the universe it is impossible to say for sure. If the universe is closed and heavier than the critical mass it will eventually stop expanding and collapse back, which would mean that things outside will become visible in the future (unless light slows down as well, which is also possible). The most likely outcome at the moment is that the universe will expand forever, and in fact its expansion will accelerate. This will mean that while the observable space becomes larger, the actual matter will become more widely spaced and things visible now will pass over the horizon, eventually leading to every particle effectively living in its own isolated universe, since it will not be possible for any of them to interact with any others.
 
Doesn't an explanation based on considering the Doppler effect on the exchanged virtual particles lead to a consistent explanation also?
 
69dodge said:
In Newtonian theory, gravity is instantaneous.
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Isn't it more like Newtonian theory didn't even address the speed that gravity might propagate? I think most of the situations considered involved bodies whose gravitational field hadn't changed in millions of years. So the question of how fast a gravitational change might propagate didn't even arise. All gravitational fields considered were pretty much static.
 
Crazycowbob said:
The same for gravity (though we have not as of yet found a "carrier" for gravity), an object's gravity is felt instantly by everything within it's influence, otherwise Pluto would orbit a spot far behind the suns motion in the galaxy would it not?
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One simple point about this seems to have gone unmentioned. Pluto is as much in orbit around the galaxy as the Sun is. Just as the Moon is in orbit around the Sun as the Earth is. Regardless of the more complicated concept of what speed gravity might propagate at, Pluto is in no danger of the Sun leaving it behind.

For most practical purposes, including the orbits of the planets about stars and the stars about the galaxy, the propagation speed of gravity is a moot point. The stars and the galaxies are homogeneous and static enough that the gravity field propagated everywhere a long time ago. And the field that gets here tomorrow or next year isn't going to be much different from the one that is here today.
 
RecoveringYuppy said:
Isn't it more like Newtonian theory didn't even address the speed that gravity might propagate? I think most of the situations considered involved bodies whose gravitational field hadn't changed in millions of years. So the question of how fast a gravitational change might propagate didn't even arise. All gravitational fields considered were pretty much static.
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Actually, I think 69dodge had that one correct.

That Newtonian gravity propogated at the speed of light was a central problem with it in Einstein's view, once he had created Special Relativity.

And clearly, static isn't the case either. The planets move about and have gravity of their own that effects each other and other smaller bodies for certain.

There was no allowance for any propogation time due to the constant movement of the planets, thus instantaneous.
 
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