stupid physics questions

[gilesgoat]

"Message left there".

Is it possible to explain Hawking Radiation in simple terms ? ( i.e. is it possible to make someone believe it could be true on the basis of something without the need of a 20 years experience/course in theorectical physics to just try to figure out why this could be possibly, but not certainly/uncertainly yet, true ? ).

"A couple of particle/antiparticle thingies coming from nowhere that normally would destroy each other and you would never even know about them, happens that one of them falls into a black one while the other not creating some sort of universal unbalancing therefore the black hole looses mass ?".

One could not say "if there's a 50/50 probability that particle falls in or not could not also be a 50/50 probability that one time "A" falls in and the next time "B" falls in instead and so the balancing is restored ?".

??????

[gilesgoat]

Another one .. "what the fuck is a quantum entanglement ?" ..

"Ok something has state A, something else has state B, something else has state A and state B the same time" ..

.. "ok" .. "what's so great about it ?" and "why this quantum computers should be so awesome" could you not just simulate the same case with conventional logic/hw ?

Also what is this "state" we are talking about ?

[Dio]

Quantum compuing is about doing certain mathematical functions effectively instantly because they simultaneously compute every possible answer.

As for entanglement:
- Start with two (or more) identical things (electrons, but you can use photons, or larger things with more difficulty)
- You associate these things together and then they act as components of a single system, and that's entanglement.
- You can then operate on one of the things and it affects all the rest. (Almost, but not entirely, unlike a transistor, where the current at the base affects the current across the depletion region).

So the typical example is that you take two electrons (subject to the Exclusion Principle) and arrange for them to initially be in an indeterminate spin state (see Schrodinger's Cat for the thought experiement on indeterminacy) and then measure the spin of one of them to force the wavefunction to collapse. This measurement causes the state of the other to also be set, to the opposite of the first because the two have to have opposite spin. But there's a lot more you can do with it than that (and it all starts to get very hairy very quickly).

[pogo]

"A couple of particle/antiparticle thingies coming from nowhere that normally would destroy each other and you would never even know about them, happens that one of them falls into a black one while the other not creating some sort of universal unbalancing therefore the black hole looses mass ?".

One could not say "if there's a 50/50 probability that particle falls in or not could not also be a 50/50 probability that one time "A" falls in and the next time "B" falls in instead and so the balancing is restored ?".

The simple answer to this is whichever way it happens both the particle and antiparticle have mass. So whichever escapes it represents a loss of mass from the black hole. Other things, such as the charge lost, will balance out over time.

It's normally a minuscule effect. For any large black hole, such as formed from a star, it's far smaller than the gain in mass from cosmic ray particles randomly hitting it. Only in the case of the smallest black holes is the effect significant enough to evaporate them. Which is probably why there are no small black holes. Very many were probably formed when the universe was small and very dense after the big bang but if so they have all evaporated, and now they only form from large stars collapsing.

[Darkflame]

pogo - Actually that helped a bit. So a static object is drawing a "straight" line though time, which is also distorted by the mass....which then makes its "line in time" to us look like something accelerating ?
I cant visualize it, but is that ones being got at here?

Like most analogies, if taken too literally they become misleading.

No, a good and exact analogy does explain. It's poor analogies that don't. There are a lot of poor analogies about the place, generally made by people who don't understand what they are trying to explain, and some really good ones, generally made by people who do understand things, but sometimes just by raw luck.

A good analogy is like a balloon, if you over inflate it then it will burst.
A bad analogy, meanwhile, just use's an everyday object and then you force a parallel no mater if it helps enlighten or not.

[pogo]

pogo - Actually that helped a bit. So a static object is drawing a "straight" line though time, which is also distorted by the mass....which then makes its "line in time" to us look like something accelerating ?
I cant visualize it, but is that ones being got at here?

Yes. I can't visualise it either. The problem is the geometry of 4D spacetime is weird. Or more precisely it's a non-Eucidian geometry with a (+, +, +, -) signature. This is very useful as you can pretty much explain all of special relativity from the geometry, considered as a flat 4D space. But general relativity introduces a curvature to this space.

One thing might help clarify things. The equivalence principle in general relativity is a broad principle that covers many things but one I find especially useful is that:

The local effects of motion in a curved space (gravitation) are indistinguishable from those of an accelerated observer in flat space, without exception.

The idea is that if you're in a building experiencing gravity pressing you to the floor, or a starship undergoing acceleration pressing you to the floor, you can't tell the difference (assuming the starship's engine's are smooth and ignoring effects due to the curvature of the field near a mass such as the earth, which is only detectable with very fine instruments on a human scale).

According to general relativity these two forms of acceleration are the same: in each case you are accelerating by supplying a force to diverge you from your natural path - your worldline or geodesic. Remove the force and you follow that geodesic, falling towards the gravitational mass or drifting in space.

[Muttley]

No part of the rubber sheet analogy helps me know why objects move towards the "dense" space at the middle from standing still.

Nothing is ever "standing still" though.

[merman]

The Science of Doctor Who did a very good job of explaining black holes, the speed of light and why time travel might not be impossible...

[Darkflame]

No part of the rubber sheet analogy helps me know why objects move towards the "dense" space at the middle from standing still.

Nothing is ever "standing still" though.

ah, standing still relative to the body of mass that they are being attracted too? Would that do?
"still" also being defined as, say, not moving in any one direction and probably just vibrating slightly on very small levels. And the direction of movements I am looking at is purely spacial.
umm...any other caveats I have to put in

[pogo]

Or everything is standing still. Einstein's key insight in special relativity, the relativity postulate, is that there's no privileged frame of reference. So e.g. the speed of light is the same whoever measures it, even if they're moving (scientists already knew this from experiments: Michelson and Morley's measured the speed of light expecting to see variations due to the motion of the Earth through space but the result was the same within the limits of their experiments. But they couldn't explain it).

[zip accident]

I just said in another thread, that light is carried by the fields of atoms. However, that would mean that the Einstein thought experiment with the accelerating lift wouldn't work, because the light wouldn't be bent by the acceleration, because the atoms in the lift would be accelerating, so the light beam would be straight.

So, how is this wrong?

[pogo]

The lift thought experiment (that if riding in a lift/box you can't know whether the force you're experiencing is due to gravity, acceleration or a combination) would not be invalidated by light. If you're accelerating light will seem to curve due to the acceleration. If you're experiencing gravity that will also bend light. The effect in each case is tiny: even light going around and close to the sun is only deflected by a small amount, travelling more than a million km. It will be only detectable with the finest of instruments in a lift experiment, if at all. But it's a real effect.

Light isn't carried by atoms, or aether. It's carried by nothing, but travels of its own accord across the surface of spacetime.

[Absolute_Zero]

The lift thought experiment (that if riding in a lift/box you can't know whether the force you're experiencing is due to gravity, acceleration or a combination)

I've doubted that "can't" since the age of 14...

Under acceleration in deep space, drop two objects one meter apart and they should strike the "floor" of the accelerating lift/rocket exactly one meter apart.

Do the same in a static, earth-bound lift and the two objects should be ever so slightly closer at impact since the lines of force converge at the centre of the earth.

[zip accident]

The lift thought experiment (that if riding in a lift/box you can't know whether the force you're experiencing is due to gravity, acceleration or a combination) would not be invalidated by light. If you're accelerating light will seem to curve due to the acceleration.

Why? Its frame of reference is accerating. If its speed depends on its frame of reference, it shouldn't accelerate relative to that frame of reference.

If you're experiencing gravity that will also bend light. The effect in each case is tiny: even light going around and close to the sun is only deflected by a small amount, travelling more than a million km. It will be only detectable with the finest of instruments in a lift experiment, if at all. But it's a real effect.

Which experiment says that the bending of light in the lift is a real effect? I'm not at this point disputing the bending of light by gravity.

Light isn't carried by atoms, or aether. It's carried by nothing, but travels of its own accord across the surface of spacetime.

What is the surface of spacetime carried by then?

[llamaprotector]

I'll go off on a bit of tangent here but it makes much more sense once you realise everything is moving at the speed of light on geodesics. Everything. Light moves at the speed of light, obviously. But if you consider a mass it also moves, except in time.

Funny I just opened this thread now as there was a bit of glitch in the AVI on the program but I got the distinct impression that the ability to travel back in time is limited by the movement through space time which is limited to c.

So surely even the time component of the speed of time in space time is limited to c and hence the TARDIS has the black hole/collapsing star at its heart.

(not a huge Dr Who fan and that particular minute had a glitch at just the right time but that's what I presume he was talking about in The Science of Dr Who)