How many people understand general relativity?

[nutball]

These objects as you refer to them (I presume you mean planets, stars, galaxies) tend to be born spinning, at least that's the way most of the models go. Shear turns into rotational motion rather easily.

Spinning up an Earth-sized mass using meteorite impact is incredibly difficult, not least because each meteorite carries very little momentum, and you need to arrange for the meteorites to strike in such a manner that there's a net torque.

 

[KimB]

Along the same axis...

I was always surprised that everything in the universe seems to rotate around itself.

Yup. This comes about because of the fact that if you collapse a large, diffuse cloud of matter into a small, compact body like a star, planet, or moon, the probability that there will be no net angular momentum is vanishingly small. It's basically a random walk problem: if I have a system where I randomly take either one step forward or one step back, each chosen by the flip of a coin, after N steps I will be, on average, sqrt(N) steps from where I started. Whether I end up sqrt(N) steps ahead or sqrt(N) steps behind is random.

It's the same with rotating bodies: you start with this huge, big, diffuse cloud. Each molecule in the cloud adds its own small angular momentum to the mix, in a random direction. Adding up the total angular momentum for the entire cloud, and on average you end up with some non-zero momentum. As the cloud collapses in on itself under its own weight, the small net angular momentum that is just randomly present translates into a larger and larger rotational speed.

Of course, this explains why a collapsing cloud would collapse into a single spinning object, but it requires quite a bit more work to explain why when a collapsing cloud collapses into a group of objects, such as our solar system, the individual objects within it tend to not only be rotating, but have correlated rotations. But at least it shows that for collapsed bodies, the expected result is rotation instead of no rotation.

 

[purpledog]

Yup. This comes about because of the fact that if you collapse a large, diffuse cloud of matter into a small, compact body like a star, planet, or moon, the probability that there will be no net angular momentum is vanishingly small. It's basically a random walk problem: if I have a system where I randomly take either one step forward or one step back, each chosen by the flip of a coin, after N steps I will be, on average, sqrt(N) steps from where I started. Whether I end up sqrt(N) steps ahead or sqrt(N) steps behind is random.

It's the same with rotating bodies: you start with this huge, big, diffuse cloud. Each molecule in the cloud adds its own small angular momentum to the mix, in a random direction. Adding up the total angular momentum for the entire cloud, and on average you end up with some non-zero momentum. As the cloud collapses in on itself under its own weight, the small net angular momentum that is just randomly present translates into a larger and larger rotational speed.

Looks like you just described a brownian motion and the associated diffusion process. As time goes by, this is still centered on zero but the variance increase with time.

Of course, this explains why a collapsing cloud would collapse into a single spinning object, but it requires quite a bit more work to explain why when a collapsing cloud collapses into a group of objects, such as our solar system, the individual objects within it tend to not only be rotating, but have correlated rotations. But at least it shows that for collapsed bodies, the expected result is rotation instead of no rotation.

What do you mean by "correlated rotations"?


Ok, here's my understanding of the flat things:

The flat stuff in the universe (some galaxy like ours, our solar system) seems to be made by something initially spherical getting smaller and smaller due to the effect of gravity. Being smaller means that it turns faster (like the second move of the girl here: http://www.youtube.com/watch?v=g2v5iadTh9o)

So fast that at some point the centrifuge force becomes bigger than the gravity and shlouf, that nice little sphere gets spread out all over the place, but within the place perpendicular to the rotation axis (more or less).

 

[Davros]

no because the cetrifugal force happens in all directions

 

[Bambers]

This seems to often be the case with even Newtonian mechanics: much of the time, it seems natural, intuitive. But throw up a situation that's a little bit out of the ordinary, and suddenly it ceases making sense. General Relativity is like that, except for the fact that we can never see the more out-of-the-ordinary situations that we can describe on paper, such as behavior near a neutron star or black hole.

Well I guess it depends on how able people are to accept new ideas and concepts. Yes, I guess many people are rather 'stuck in their ways' and no matter how many times they make a mistake, fail to learn from it. An example I can think of offhand is the tennis this weekend, centre line judges have a wonderful habit of consistently dodging the wrong way from spun serves, a few of them get it right but not many. However the players themselves, the good ones anyway, have little problem reading the spin.

But the lack of real practical examples isn't a problem either imo. e.g. the 4 body gravity problem mentioned above, or even just the videogame examples I mentioned. Some people are hopeless at adapting to a new system of rules, others will pick things up very quickly whether real or abstract and not just in physics. Some mathmaticians will end up with a gut feeling of what the right answer should be to a problem, some musicians like some of the great composers just 'know' what works with music, the best chefs just know what works with tastes. Providing they have some kind of reference of what the answer really is they can learn from it and relativity is no real exception to this.

Is all that understanding? True understanding, probably not. Working on that basis, nothing is truely understandable, but SR and GR are as understandable as any other abstract and not everyday system, to say its impossible to me strikes of painting everyone with the same brush.

no because the cetrifugal force happens in all directions

I didn't really do a great deal of astrophysics/cosmology in my degree, mostly because the lecturers for it were rather dull, however I'd imagine the same randomness which provides the intial spin also means the object is unlikely to be a perfect sphere and a bit squished in some directions. Over time, the resulting imbalance in gravity will spread the matter into a disc(ish) shape. That was invented btw so might be complete rubbish.

 

[KimB]

Looks like you just described a brownian motion and the associated diffusion process. As time goes by, this is still centered on zero but the variance increase with time.

Yes, any random process where you're accumulating the effects of a large number of random variables is the same in this regard. It's worth noting, however, that it is only the collapse of the cloud that makes the net angular momentum visible: without that collapse, the net angular momentum would be so small compared to the size of the cloud that it'd be completely imperceptible. Sure, the variance increases with the size of the cloud, but the size increases faster.

What do you mean by "correlated rotations"?

Well, I'll give a couple of examples. First, nearly all of the planets orbiting the Sun rotate in the same direction. I believe this was likely caused by shear in the protoplanetary disk that our solar system formed from.

Another example is with certain moons and Mercury: they rotate at just the right speed so that the same face always points towards the body they rotate. Our own moon is like this, as is Io. I'm sure there are others as well, but I don't know of any. This stems from the tidal forces: bodies in the solar system aren't perfectly spherical, and aren't perfectly solid. The slight deformations make it so that relative rotation of orbiting bodies is opposed. So, for example, as time goes on the Earth will slow down its rotation, as its kinetic energy is transferred to the Moon, which will achieve a higher orbit. This will continue until the Earth and Moon always show the same face to one another.

Or at least it would continue if our Sun didn't go red giant before that can happen, which it will.

Ok, here's my understanding of the flat things:

The flat stuff in the universe (some galaxy like ours, our solar system) seems to be made by something initially spherical getting smaller and smaller due to the effect of gravity. Being smaller means that it turns faster (like the second move of the girl here: http://www.youtube.com/watch?v=g2v5iadTh9o)

So fast that at some point the centrifuge force becomes bigger than the gravity and shlouf, that nice little sphere gets spread out all over the place, but within the place perpendicular to the rotation axis (more or less).

Well, no. As Davros correctly points out, the centrifugal force is in all directions. It cannot cause objects that were orbiting off-axis to start rotating within a plane. And furthermore, the gas cloud doesn't start off as a sphere: it starts as an amorphous blob.

But as it collapses and gets more dense, it experiences friction. As it collapses one direction of rotation starts to dominate. Friction brings the rest of the galaxy in line, if the conditions are right (astrophysicists are still working on what those conditions are...what makes for a spiral galaxy versus an elliptical galaxy is still poorly understood).

 

[purpledog]

no because the cetrifugal force happens in all directions

Hum... I don't understand your answer.

As far as I can tell, if you consider an object rotating around a constant axis, the centrifuge forces will be perpendicular to this axis (and yes, in all the directions whithin this plane).

If you put sand on top of that "tourniquet" (don't know the english name):

... and start to spin it, the sand won't go up or down, it will spread all over the spinning plane (forget about the gravity here).

I read "somewhere" that some galaxy were flat because they went throught a similar process in their life (getting denser and denser and then spreading in the rotating plane).

Does it make sense to you?

 

[KimB]

They can't spread out in the rotating plane. That would be a violation of energy conservation. They can only collapse as friction dissipates energy from the system.

 

[Danalys]

hmm about that whole time thing. you could measure time as a distance of light since light is invarient in a vacumm. so then time wouldn't be an issue, and the mass of something which slows light down would just be compressed space. and that's sort of a good way to think about some aspects of relativity really.

 

[KimB]

hmm about that whole time thing. you could measure time as a distance of light since light is invarient in a vacumm. so then time wouldn't be an issue, and the mass of something which slows light down would just be compressed space. and that's sort of a good way to think about some aspects of relativity really.

Well, this only recasts time in terms of distance. A better definition is to look at a physical system which oscillates and count the oscillations. This is actually the way the second is now defined:
"the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom."

 

[Danalys]

yes but to recast time in terms of distance give you a different vantage point of looking at things much like heliocentricism did for looking at the solar system. i think that vantage points and different ways of thinking about things can have a strong impact on your intuitive grasp of a concept.

i wouldn't say the present system is better empirically. it's certainly more usefull for building atomic clocks tho.

 

[KimB]

Well, the idea here is that to use distance as a measure of time, you first need a standard distance measure. The problem is that that's not a very easy thing to do. It's much easier to set up a simple oscillating system and just count the number of oscillations than to deal with a macroscopic distance measurement, which, because of the complexity, tends to be rather less reliable.

This is why the meter is now defined in terms of time. It used to be defined in terms of the length of a bar of platinum housed Sevres, France. But that turned out to change with time. Basing the length of a meter on the second, and basing the second on an oscillating system (in this case the radiation emitted by a cesium-133 atom in the transition between the hyperfine levels of the ground state) allows us to define our units in terms of the behavior of fundamental physics, which is invariant with time, unlike the previous bar of platinum.

 

[ninelven]

Yes, from that perspective all black holes would appear to have the same mass...

 

[KimB]

Yes, from that perspective all black holes would appear to have the same mass...

Huh? Why?