Ask a physicist anything. (6)

[Wiccan_Child]

If you could rotate a magnet at near light speed would it do anything interesting?

I think that would be interesting in and of itself

I heard that there is no limit to how large a black hole can get, but is that true? I couldn't see how, I mean there has to be a hard limit. Can an infinitely large black hole exist? Would it do anything interesting?

Black holes are just objects with an extremely strong gravitational pull. There's no reason why an object that wandered near the black hole, like a planet or star, wouldn't get sucked in. There's no hard limit on their size. However, that said, while a black hole is properly a singularity of infinite (or near-infinite) density, its effects extend out to certain radii determined by its mass - heavier black holes have larger event horizons and photon spheres, for instance. So a black hole of infinite mass would have an infinitely large event horizon - but such a thing is impossible, as we don't have infinite mass in the universe, thankfully!

If a black hole's singularity is not infinitely dense, but had some mind-shattering object at its cold, dark centre, then that object would also be infinitely large if it was infinitely heavy. Crazy stuff.

What would happen if an infinite black hole tried to absorb another infinite black hole? Would it be doubly infinite, would the black holes be "full?"

If one black hole was had an infinity of matter, and its matter was infinitely compressed, then both holes would come together and form something with twice as much mass: infinity.

What would happen if something totally indestructable, like say the Juggernaut, were to be sucked into a black hole?

I doubt even the primordial magic that confers the Juggernaut his powers would be able to withstand the time-shearing abilities of a black hole.

 

[mzungu]

Hmmmmm!!!!!: BBC News - Darkest exoplanet spotted by astronomers

 

[mzungu]

Pluto has four moons! : BBC Solar System - Pluto is not a planet anymore

 

[Chalnoth]

If you could rotate a magnet at near light speed would it do anything interesting?

Well, when you rotate a magnet end-over-end (so that the north/south poles are continually switching), the magnet emits electromagnetic radiation at the frequency at which you are rotating it. The faster you rotate it, the higher frequency radiation is produced. There's nothing particularly special about getting it to rotate close to the speed of light, except for the fact that it is really really hard to do!

Besides the simple challenge in rotating a mass that fast anyway, rotating a magnet is even harder, because all of the electromagnetic radiation emitted tends to slow the magnet's rotation down.

I heard that there is no limit to how large a black hole can get, but is that true? I couldn't see how, I mean there has to be a hard limit. Can an infinitely large black hole exist? Would it do anything interesting?

The only limit is how much stuff falls into it. There isn't infinite amount of matter that can fall into a black hole, so you can't get a black hole that big.

 

[mzungu]

Well, when you rotate a magnet end-over-end (so that the north/south poles are continually switching), the magnet emits electromagnetic radiation at the frequency at which you are rotating it. The faster you rotate it, the higher frequency radiation is produced. There's nothing particularly special about getting it to rotate close to the speed of light, except for the fact that it is really really hard to do!

Besides the simple challenge in rotating a mass that fast anyway, rotating a magnet is even harder, because all of the electromagnetic radiation emitted tends to slow the magnet's rotation down.

Besides you will need a material with infinite tensile strength to withstand the centrifugal forces trying to tear the magnet apart.

The only limit is how much stuff falls into it. There isn't infinite amount of matter that can fall into a black hole, so you can't get a black hole that big.

How so? What would cause a black hole to be so saturated that it will no longer ingest more matter? Supermassive black holes still feed and there is no reason why this process should stop unless there is no matter to feed upon or any matter wandering close enough to fall into and beyond the event horizon.

Unless of course you mean that a black hole will ingest matter but not grow in mass after reaching an upper limit. This then begs the question of how can this be when more matter is added but the mass does not increase. Could it be possible that at this limit a "tear in the space fabric" is opened and the extra mass ends up being ejected into a parallel universe?

 

[impblack]

I don't know it was ever explained in this thread, i know it was talked about but i didn't found and explaination. Could someone explain or show me a post that explains quantum loop theory please? :b

 

[Chalnoth]

Besides you will need a material with infinite tensile strength to withstand the centrifugal forces trying to tear the magnet apart.

That too, but I suspect that the energy required to spin it up will be the limiting factor.

How so? What would cause a black hole to be so saturated that it will no longer ingest more matter? Supermassive black holes still feed and there is no reason why this process should stop unless there is no matter to feed upon or any matter wandering close enough to fall into and beyond the event horizon.

Perhaps you misunderstood me. I was trying to say the same thing that Wiccan_Child said, that there just isn't infinite matter around in the first place, and a black hole can only grow as massive as the matter that falls into it.

In reality, though, it turns out that black holes do tend to stop growing because as they absorb matter, the gravitational potential energy of the infalling matter gets released as a stream of particles and radiation. When this outgoing blast of energetic particles and radiation gets strong enough, it blows away the gas and dust that was feeding the black hole. Because of this effect, it is a bit challenging to explain how to get black holes that are many billions of solar masses in size.

 

[Chalnoth]

I don't know it was ever explained in this thread, i know it was talked about but i didn't found and explaination. Could someone explain or show me a post that explains quantum loop theory please? :b

Loop quantum gravity? Unfortunately I don't know that much about it, except that it is one proposed theory of quantum gravity, and that nobody yet knows how to get our current theory of gravity out of it. That is, it's a quantum theory of gravity, but nobody knows whether or not it actually describes the gravity we observe. Because of this failure, I've not been interested enough to investigate it in detail.

I do know that this theory of gravity stems out of attempts to take our current theory of gravity, and make it into a quantum theory. This is to be contrasted with string theory, which proposes a fundamental way that the universe behaves (it proposes that everything is made of vibrating strings), and then from that proposal a theory of quantum gravity is derived. Note that unlike in loop quantum gravity, the theory of gravity that you derive from string theory has been proven to act like the gravity we observe.

If you want to read more about Loop Quantum Gravity, I'd recommend the Wikipedia article:
Loop quantum gravity - Wikipedia, the free encyclopedia

 

[mzungu]

In reality, though, it turns out that black holes do tend to stop growing because as they absorb matter, the gravitational potential energy of the infalling matter gets released as a stream of particles and radiation. When this outgoing blast of energetic particles and radiation gets strong enough, it blows away the gas and dust that was feeding the black hole. Because of this effect, it is a bit challenging to explain how to get black holes that are many billions of solar masses in size.

So basically black holes achieve a size limit after which anything ingested will be blown out as supercharged energy jets

But does this not happen anyway when a black hole is feeding? Some matter is expelled in opposing jets of energy from both poles?

Strange and wondrous things these beasts! They are after all the Dr4agons of the universe

Muse - Supermassive Black Hole - YouTube

http://www.youtube.com/watch?v=cW7BvabYnn8&feature=relmfu

 

[Chalnoth]

So basically black holes achieve a size limit after which anything ingested will be blown out as supercharged energy jets

Um, not quite. Whenever anything falls into a rotating black hole, the gravitational potential energy of that matter is blown out in energy jets (most of it exiting the poles of the black hole). At some point, this outgoing radiation becomes strong enough to blow away the matter that is trying to fall into the black hole.

 

[chris4243]

Um, not quite. Whenever anything falls into a rotating black hole, the gravitational potential energy of that matter is blown out in energy jets (most of it exiting the poles of the black hole). At some point, this outgoing radiation becomes strong enough to blow away the matter that is trying to fall into the black hole.

And tying back to the other question of a rapidly rotating magnet, black holes rotate like most every other astronomical body only more so, generating some very interesting electromagnetic effects. Also, for a material that won't fall apart when rotating, you probably can't do any better than a black hole.

 

[pgp_protector]

And tying back to the other question of a rapidly rotating magnet, black holes rotate like most every other astronomical body only more so, generating some very interesting electromagnetic effects. Also, for a material that won't fall apart when rotating, you probably can't do any better than a black hole.

Scrith, it might be easier to get.

 

[mzungu]

And tying back to the other question of a rapidly rotating magnet, black holes rotate like most every other astronomical body only more so, generating some very interesting electromagnetic effects. Also, for a material that won't fall apart when rotating, you probably can't do any better than a black hole.

You will have to have a magnet with the gravitational field of a neutron star in order to withstand the centrifugal forces.

Pulsars & Neutron Stars - YouTube

 

[Chalnoth]

And tying back to the other question of a rapidly rotating magnet, black holes rotate like most every other astronomical body only more so, generating some very interesting electromagnetic effects. Also, for a material that won't fall apart when rotating, you probably can't do any better than a black hole.

As I mentioned earlier, for a magnet to emit radiation, it has to be rotated end over end. This is a critical point, because it is only if it is rotating end over end that the magnetic field changes with time.

But a black hole can't do that: a black hole can't have any intrinsic magnetic field. Its magnetic field is only created by its charge and rotation. Because the magnetic field is generated by the rotation of the black hole, the magnetic field will always be oriented in the same direction as its rotation, and so no electromagnetic radiation will be emitted.

 

[Mr. Pedantic]

Surely you wouldn't be able to detect any intrinsic black hole radiation, given that it's inside the event horizon? All you'll be able to detect is Hawking radiation and radiation from infalling matter?

 

[Chalnoth]

Surely you wouldn't be able to detect any intrinsic black hole radiation, given that it's inside the event horizon? All you'll be able to detect is Hawking radiation and radiation from infalling matter?

That's sort of the point. Hence it can't have a magnetic dipole moment oriented differently from its rotation axis.

 

[Cactus Jack]

Explain the Time/Space Continuum theory (I don't understand it myself).

 

[oriel36]

Explain the Time/Space Continuum theory (I don't understand it myself).

It is junk,the idea that space and time are combined could be found in any fiction section of a bookstore in the late 19th century but how science fiction turned into the monstrosity it became in the early 20th century takes quite an effort to untangle .

Anyway,here is the original science fiction explanation for 'time travel',a wonderful fictional novel that should have been left that way -

"‘Now, it is very remarkable that this is so extensively overlooked,’ continued the Time Traveller, with a slight accession of cheerfulness. ‘Really this is what is meant by the Fourth Dimension, though some people who talk about the Fourth Dimension do not know they mean it. It is only another way of looking at Time. There is no difference between time and any of the three dimensions of space except that our consciousness moves along it. But some foolish people have got hold of the wrong side of that idea. You have all heard what they have to say about this Fourth Dimension?’" The Time Machine,H.G Wells 1898



The existence of so much empirical junk is merely a symptom of Christianity abandoning its astronomical heritage or not having a spokesman good enough to deal with the issues.

 

[mzungu]

It is junk,the idea that space and time are combined could be found in any fiction section of a bookstore in the late 19th century but how science fiction turned into the monstrosity it became in the early 20th century takes quite an effort to untangle .

Anyway,here is the original science fiction explanation for 'time travel',a wonderful fictional novel that should have been left that way -

"‘Now, it is very remarkable that this is so extensively overlooked,’ continued the Time Traveller, with a slight accession of cheerfulness. ‘Really this is what is meant by the Fourth Dimension, though some people who talk about the Fourth Dimension do not know they mean it. It is only another way of looking at Time. There is no difference between time and any of the three dimensions of space except that our consciousness moves along it. But some foolish people have got hold of the wrong side of that idea. You have all heard what they have to say about this Fourth Dimension?’" The Time Machine,H.G Wells 1898



The existence of so much empirical junk is merely a symptom of Christianity abandoning its astronomical heritage or not having a spokesman good enough to deal with the issues.

 

[Chalnoth]

Explain the Time/Space Continuum theory (I don't understand it myself).

The word "continuum" is usually not used in the context of space-time, except in Star Trek. Continuum is more used in the context of understanding the behavior of fluids (air, water) and in understanding the effects of stresses and strains on solid objects. Continuum mechanics ignores the fact that matter is made of atoms, and assumes it is just smooth, continuous stuff (hence the word continuum).

That said, within relativity, we talk about space and time as one construct, space-time, because the two get mixed up in various situations. When you have, say, a train moving at very high speeds, as we watch the train pass we see it as being both compressed length-wise and we also see clocks on the train slowed down. An observer on the train, by contrast, sees its clocks moving at a normal rate, but the rail on which the train is traveling is now shorter.