Why is everything rotating in the universe?

[Kerbin Dallas Multipass]

Galaxies, stars, planets, moons. They all rotate around a center and(or) are in a rotational motion themselves.

The universe itself is not rotating (or is it?). So what is causing this?

If I take a cloud of matter in space and all the laws of physics, why does it begin to rotate? Why isnt the stuff falling to dead center of gravity? What determines the angle of rotation? I just dont get it

[Themohawkninja]

This is because since virtually every single time that two objects gravitationally attract each other, they do so slightly offset, so they don't contact each other, but rather they orbit like so:

Note how both bodies are travelling (more or less) clockwise.

Now then, copy+rotate+paste this orbit many times, and you get an entire set of objects rotating.

They all tend to rotate the same direction, because there won't be an even 50/50 split of rotational directions, so in the end (after all counter rotating pairs of objects collide and fall to the center of gravity) there remains one sole direction of rotation. This is explained in this video:

P.S. As far as we know, the universe isn't rotating.

Edited December 23, 2013 by Themohawkninja

[Arran]

Short answer in one word : Gravity

I'm not a scientist, nor can i explain things correctly like and scientist, and someone here might correct me where i go wrong.

But i assume, if you look inwards to atoms and neutrons, they act in a simulair way as galaxies and solarsystems do, they all rotate around simply said a gravitational body.

All packed together they make up a whole thing. While the Atoms and neutrons are rotating and moving around themselfs and the rest, the object they consist of (IE your body) isnt rotating, but moving in a other direction.

Same is for the universe, wich makes an expanding motion, while everything in it is making rotational movement.

Now again, this is stated in a very simplistic way, and prolly not entirely correct in details, but you get the drift.

Again, i am a layman in this matter, so i explain it in terms "I" grasp it in, prolly enough smarter minds here that can correct me and explain it better as i can.

[Savage117]

I think this is a very good question and would like to see some theories on it. It may seem simple to explain at first, it's actually very complex, because we are talking about billions upon billions of bodies acting upon each other while flying out from a central point (The Big Bang).

My first gut answer to this is simple, when the stuff from the Big Bang exploded outward it all was given some spin like shrapnel from a grenade and that spin got amplified when other spinning objects started gravitating together, and that was all at a microscopic to near macroscopic scale. Then we get to planetary scale after a super long time, I have no idea how long in years, and this one is kinda easy. For example, Lets say this planet is absolutely unaffected by anything around it during its travel out from the big bang, its not spinning, nor is it rotating around another body, both near impossible but for examples sake we will use it, and don't concern yourself with how it was formed without spinning. This planet will sooner or later get hit by something or hit something itself, this is were its spin will come from, because no matter what, if something hits it, it would be impossible for that object to hit it exactly center on, it might come close but some very minor spin would still be passed to the example planet. After multiple hits more spin would be transferred, not to mention knocking it into orbit around other bodies sooner or later. This same thing is happening billions upon billions of times in other parts of space as well, a general flow will emerge and anything orbiting against the flow will get pulverized, or captured, by the larger number of objects going with the flow.

I'm sure there are lots of holes in that explanation, maybe someone can flesh it out a bit more, but I think I got the gist. There will also be many exceptions, for example: rogue planetary bodies that have been knocked out of their orbits by who knows what.

Anyway, hopefully that answers your curiosity.

Edit: got ninja'd by Themohawkninja appropriately, but this as well as impacts would contribute to the flow and spin of everything around it. And as he said the Universe probably isn't rotating, it has nothing to rotate around, it theoretically is the product of an explosion of some sort, so it would expand outward forever unless something were placed or formed where the explosion happened that had some sort of gravitational pull.

Edited December 23, 2013 by Savage117

[Themohawkninja]

Short answer in one word : Gravity

I'm not a scientist, nor can i explain things correctly like and scientist, and someone here might correct me where i go wrong.

But i assume, if you look inwards to atoms and neutrons, they act in a simulair way as galaxies and solarsystems do, they all rotate around simply said a gravitational body.

All packed together they make up a whole thing. While the Atoms and neutrons are rotating and moving around themselfs and the rest, the object they consist of (IE your body) isnt rotating, but moving in a other direction.

Same is for the universe, wich makes an expanding motion, while everything in it is making rotational movement.

Now again, this is stated in a very simplistic way, and prolly not entirely correct in details, but you get the drift.

Again, i am a layman in this matter, so i explain it in terms "I" grasp it in, prolly enough smarter minds here that can correct me and explain it better as i can.

Actually, electron orbitals are far more different than the cliche circular orbits that gravity dictates, not to mention the various quantum principles that limit what electrons can do (most notably the Pauli Exclusion Principle (I think), which limits the number of electrons per orbit to two). Such issues don't exist with gravity.

[Arran]

Actually, electron orbitals are far more different than the cliche circular orbits that gravity dictates, not to mention the various quantum principles that limit what electrons can do (most notably the Pauli Exclusion Principle (I think), which limits the number of electrons per orbit to two). Such issues don't exist with gravity.

You are correct, but it was more ment as an simplistic example. most people with some electrotechnical background understand the movement of electrons and protons and to a degree they have simulairities. But there are some big differences like you mentioned.

On The Pauli Exclusion Principle you mention, from what i recal from school, doesnt mean a atom cant have more as 2 electrons, but no two electrons in an atom can have identical quantum numbers. This is an example of a general principle which applies not only to electrons but also to other particles of half-integer spin (fermions). It does not apply to particles of integer spin (bosons).

Now i can be wrong here, its been ages ago since i had to learn this stuff, and never really used it after schooldays, what is a half lifetime ago for me so i'm kinda rusty on the subject. Hope you can forgive me here.

[Kerbin Dallas Multipass]

snip

That video is pretty cool and sort or precisely explains my level of understanding of gravity or space-time.

I do understand that disks of matter rotating in the same direction HAVE to form once we have a spinning motion going. From then on I understand how matter can clump up in sub systems of orbits.

But where is the origin of this kinetic energy? The professor in the video explicitly states that he has to give those marbles a sideways motion. Thats the bit im puzzled about.

[Redjoker]

You are correct, but it was more ment as an simplistic example. most people with some electrotechnical background understand the movement of electrons and protons and to a degree they have simulairities. But there are some big differences like you mentioned.

On The Pauli Exclusion Principle you mention, from what i recal from school, doesnt mean a atom cant have more as 2 electrons, but no two electrons in an atom can have identical quantum numbers. This is an example of a general principle which applies not only to electrons but also to other particles of half-integer spin (fermions). It does not apply to particles of integer spin (bosons).

Now i can be wrong here, its been ages ago since i had to learn this stuff, and never really used it after schooldays, what is a half lifetime ago for me so i'm kinda rusty on the subject. Hope you can forgive me here.

You are both right in that 2 electrons can't have the same quantum number or be in the same orbital, a shell as opposed to a ring, since the portion of the quantum number that varies in electrons in the same orbital is the spin. Since it can only be clockwise or counterclockwise there is only two options and thus only two electrons per orbital.

[SFJackBauer]

Imagine a cloud of particles, a nebula, in space. It has an initial angular momentum, although very small. As the time passes, at the center of this nebula, the particles that get close to each other start to aggregate and collapse into a protostar. As more and more particles get attracted by this initial bundle, the gravity well gets bigger and bigger, like a sink hole increasing in size. However a small fraction of the nebula will never collapse into the center... rather, it will fall into its direction but its too far away to become part of the star, but too close to be ejected into the surrounding space. This will eventual form a rotating disk around the newborn star, and the local clumping of this disk will eventually give shape to planets. The planets spin and its rotation around the star all comes from the initial angular momentum of the nebula, and its subsequent collapse.

Now extrapolate this to bigger structures, like galaxies, and galaxy clusters. As for the universe itself, eh... that's complicated, I leave it to the real scientists to answer There are developing theories that point to a rotating universe though.

[K^2]

The fact that laws of physics are invariant under rotation leads to conservation of angular momentum. (Noether's Theorem.) And conservation of angular momentum pretty much guarantees that everything is going to be spinning.

Technically, the guy who answered "gravity" isn't completely wrong, either. Gravity is a consequence of a more general, Poincare symmetry obeyed locally. So it is all a part of the same thing. But in terms of classical physics, any central potential would have worked, and it is all about angular momentum.

Such issues don't exist with gravity.

They do. If you put an electron in orbit around a tiny black hole, you have to worry about all the same things.

Edited December 23, 2013 by K^2

[Themohawkninja]

That video is pretty cool and sort or precisely explains my level of understanding of gravity or space-time.

I do understand that disks of matter rotating in the same direction HAVE to form once we have a spinning motion going. From then on I understand how matter can clump up in sub systems of orbits.

But where is the origin of this kinetic energy? The professor in the video explicitly states that he has to give those marbles a sideways motion. Thats the bit im puzzled about.

If you mean to say why they aren't colliding head on all the time, then that is a simple matter of probability. It's much more likely that two objects will be offset than will be colliding head on. This probability is due to the fact that for two objects to be traveling directly at each other, they must be traveling 180 degrees apart. Any other number (excluding the radius of the bodies, which would allow for glancing blows) means that the two bodies won't touch.

Edited December 23, 2013 by Themohawkninja

[Themohawkninja]

They do. If you put an electron in orbit around a tiny black hole, you have to worry about all the same things.

Really? How so?

[Ralathon]

Really? How so?

Because the reason that electron shells exist is thanks to the particle-wave duality of very small things. Not thanks to electromagnetism. The equations would be exactly the same except you use gravity to determine potential energy instead of electromagnetism, and since both decrease by the square of the distance the outcome will be very similar.

[DaveofDefeat]

You spin me right round baby right round.

Dat angular momentum though, you should thank it.

[TechnicalK3rbal]

Because otherwise everything would be falling, and I don't think anyone would,like that.

[K^2]

The equations would be exactly the same except you use gravity to determine potential energy instead of electromagnetism, and since both decrease by the square of the distance the outcome will be very similar.

Almost surprisingly so. In general, physics of a black hole is very different from that of a charged particle. However, to get equivalent of a Hydrogen atom by using a tiny black hole instead of a proton for nucleus, you need a black hole that has mass of approximately 3.79x10¹²kg. Such a black hole would have a Schwarzschild radius of roughly 5.6x10^-15 meters. In contrast, Bohr radius for Hydrogen atom, which is where you are most likely to find an electron, is 5.29x10^-11 meters. That is almost 10,000 times larger.

In other words, the region of space where black hole has all of its funky physics is very small compared to the size of the atom. So the electron would behave exactly the same way as it would in Hydrogen.

The ground state orbital does have non-zero probability to be found at the center, however. So I'd guess that the electron would eventually get pulled in, but I don't know how long it will take. (It'd be a lot of work to estimate this even from classical QM, and I suspect this is a QED problem.)

[christok]

The ground state orbital does have non-zero probability to be found at the center, however. So I'd guess that the electron would eventually get pulled in, but I don't know how long it will take. (It'd be a lot of work to estimate this even from classical QM, and I suspect this is a QED problem.)

QED models electromagnetism and can hardly tell us much about a gravitational problem but I agree that it plays a role. The scenario probably requires a working theory of quantum gravity for accurate modeling.

(I'm going to have to add a disclaimer here: I'm still slowly trying to learn QM and GR so take what I say with a pinch of salt.) I agree that the electron should fall in sooner or later. We need to figure out whether the electron will be able to lose energy as photons--I have a hunch it is so--in which case it will rapidly end up in the black hole. Problem is, QED must tell us whether photons are emitted but doesn't play nice with gravity.

Back to the original question, I'm surprised no-one has mentioned Mach's Principle. The question of whether the universe as a whole is rotating is in fact very important in General Relativity: http://en.wikipedia.org/wiki/Mach%27s_principle. The question of preferred reference frames is often interesting.

[ZetaX]

What exactly is the reasoning for it to have a nonzero probability to be at the center, while it (seemingly¿) having a zero (or very very close to zero) one in an actual hydrogen atom¿

[christok]

What exactly is the reasoning for it to have a nonzero probability to be at the center, while it (seemingly¿) having a zero (or very very close to zero) one in an actual hydrogen atom¿

Electrons in atoms pass through the nucleus quite often. It's at those times when electron capture (one of several forms of beta decay) can occur.

Edit: Just to be pedantic, I should say "often" refers to times when it has a definite, known location. What it's doing at other times can of course not be known.

Edited December 26, 2013 by christok

[Beduino]

Funny OP question. I believe there are many correct answers.

If you take your time to study the Eistein´s relativity math, specially the steps showing the transformation from eistein formulas to newton physics (F=ma) in cases of smaller velocities and masses. You will see that somehow everything is related with spheres and the sphere formula.

[K^2]

QED models electromagnetism and can hardly tell us much about a gravitational problem but I agree that it plays a role. The scenario probably requires a working theory of quantum gravity for accurate modeling.

Actually, you don't need Quantum Gravity here. The scales we are dealing with here are many orders of magnitude above Plank scales. So there are no inherent QG effects. Next, we look at whether we have to consider QM of the black hole itself. Since its mass, again, is many orders of magnitude above that of the electron, we can treat it as a fixed object at the center. Any uncertainty in its position is going to be much smaller than event horizon size. So we're clear there as well.

The only place where gravity and QM interact in this picture is behavior of electron near event horizon. And we know how to deal with this. Because we assume BH to be a static object, we can simply do QED in Scwarzschild geometry. Basically, any place you see a partial derivative in QED Lagrangian, you replace it with covariant derivative. And anywhere you see a dot product, you'll need to insert the appropriate metric tensor. From there on, you just do normal Quantum Field Theory. I'm oversimplifying a bit, but this is the general way in which you'd compute interaction between tiny black hole and an electron. In a nutshell, that's also how Hawking Radiation is derived, except that you consider the vacuum of relevant QFT there.

I don't know how complicated of a problem this is going to be as I haven't tried to work it out. I suspect, proper treatment is very difficult, but there might be nifty shortcuts for a good estimate that I can't think of without writing this all out. At any rate, you'd be able to estimate the probability of such an "atom" eating its own electron without getting into Quantum Gravity.

And like I said earlier, this is only relevant if we want to know the life time of such "atom". The electron's distribution, on average, is far enough from the center to be considered as a classical QM problem, giving you solution identical to Hydrogen atom. So we basically know all the energy levels right off the bat.

What exactly is the reasoning for it to have a nonzero probability to be at the center, while it (seemingly¿) having a zero (or very very close to zero) one in an actual hydrogen atom¿

Just to clarify a bit on what christok said. Hydrogen atom is exactly the case where the probability is non-zero. In fact, radial component of probability amplitude has a maximum at zero. (I don't recall if it actually diverges, but that doesn't really matter in this discussion.) So at any given time, there is probability that electron is actually inside the proton. But because electron doesn't interact strongly, it just passes through like it's not even there. The only interaction is electromagnetic, and that's the very thing that gives you an orbital in the first place.