stupid physics questions

[gilesgoat]

So inside the singularity of a rotating black hole ( probably all of them are rotating ) which is supposedly some sort of toroidal surface ( instead of a point ) space should be so warped that basically 'there's no escape from that torus because there are no other paths' altough already this is puzzling as there has to be at least one path 'from outside to it' because you have to cross the event horizon to actually start going in there so, gravitational forces apart, what would prevent you to go back the way you entered in ?

Some say "no this is impossible because once you crossed the event horizon there are no more paths in space actually going back" so there's no way back because no matter where you'd turn or try to go every direction in space you'd take would lead you into the singularity.

Hum.

Then what about time itself ? Is it CURVED as well into "closed paths" as well ? Would you be caught in an endless loop of past-present-future ? But some other says that in that situation you'd have a 'strange thing' where future events could change past events and so on and the laws of casuality are broken.

Or do you have time all squeezed/flattened into a thing where past-present-future are all simultaneously present in the same point cause it's all squeezed into the same thing ?

Then it makes you think "what is exactly space" ? .. Space unless there's matter in is "nothing" is just "empty" and probably if it would be not for gravity it would be still 'nothing' then again what gravity is ?

Hum.

[Dio]

Trying to reason about singularities is fraught with logical traps because so many of our physics concepts are not applicable.

[gilesgoat]

Trying to reason about singularities is fraught with logical traps because so many of our physics concepts are not applicable.

When Earth will be sucked into one, maybe there will be some vindaloo in it too. (*)










(*) will there be any Earth around when that will actually happen, probably not.

[Darkflame]

I think due to timey whimmy stuff if the earth started getting sucked in, the rest of the universe would seem to speed up by a crazy amount.

This is related to Einstein's Equivalence principle, that if something (a person, say) experiences a force they've no way to tell if it's due to gravity or due to acceleration as they are identical.

I was told on the space thread that all forms of acceleration need exponentially more energy the faster a object is going. That is, to go twice the speed you need 4 times the energy.

Yet gravity (near earths surface) is 9.81ish meters per second per second, the acceleration is constant untill you get to terminal velocity (which I always understood was due to frictional forces balancing gravity). If it normally takes more energy to increase speed by the same amount at higher speeds, surely gravity behaves quite differently? It seems to be apply constantly the same force, yet accelerate the same amount regardless of initial speed.

(for simplicity all this is ignoring speeds near C)

[Dio]

You've got a bit of a confusing mess of acceleration, velocity, force and energy there.

[zip accident]

The reason masses bend space but charges or magnets don't is because of the way gravity works. Gravity acts on everything, even things without mass, as e.g. even light has it's path changed by gravity.

Everything has mass. Energy is mass. Anything with energy has mass.

In extreme cases gravity can be too strong for light to escape, as in a black hole. So all things moving in space follow curved paths due to gravity, independent of their masses (providing they're not massive enough to curve space themselves).

Everything with any mass at all curves space, just less so if they're less massive. That's what gravity is. Under that paradigm, there is nothing else to gravity than the bending of spacetime by mass.

Another way to think of space curvature is as a convenient way of describing things.

Exactly, it's a paradigm.

It's because gravity works on everything that we can think of space as curved. But we can't go outside of space and measure the curvature. We can only measure it by placing masses or lighting lights and seeing how they're affected. So the curvature of spacetime and the universality of gravity are two ways to describe the same thing.

Because it applies to everything equally, gravity is the simplest case. There has to be a reason (and "because it's Friday the Fourteenth" is not a good Physical reason) to invalidate the application of the same paradigm to the other forces, or it does apply.

This is related to Einstein's Equivalence principle, that if something (a person, say) experiences a force they've no way to tell if it's due to gravity or due to acceleration as they are identical.

Except that you can accelerate under gravity without it being resisted by inertia that would squash you flat if it was due to say an explosion.

<edit>

After actually looking at the article, Einstein isn't saying anything there that I disagree with. I think that pogo was mistaken to think that there was anything there that disagrees with my suggestion. I could be wrong about the whole thing, but risking being wrong is scientific, not that there's enough maths in this yet to be more than a vaguely science related hypothesis.

</edit>

This comes from general relativity from where we also get spacetime curvature. It relates to curvature as the geodesic lines (straight lines) in spacetime correspond to a particle moving under gravity, in orbit around a mass for example. To move it out of orbit orbit you need to supply a force, which is a force to overcome gravity or a force to move it off the straight line (which is Newton's 1st law, although this is well outside the bounds of Newton's physics).

I suspect that all of that applies to acceleration due to magnets or electro-static charges. If a non-magnetic object was placed inside a magnetic object that was accelerated by a magnet, it would experience inertial resistance to that motion, but a magnetic object wouldn't. Thinking of magnetic or electric "black holes" you would have to be thinking of pulsar strength fields, and even those are probably not absolute black holes for magnetic objects, but they'd kill us if we got close none the less.

Gravity doesn't bend space. Mass does.

Fair enough "gravity is the bending of spacetime by mass". I'm suggesting that all field forces are the bending of spacetime by their relevant factors. This is testable, if it doesn't work, that's in itself interesting, I would suggest very interesting. If it does work, it's just a new paradigm that doesn't invalidate any results that were found before, though it may provide a different way of looking at them that gives them a different aspect.

There was a fascinating article in New Scientist a couple of months back that postulated that what we currently view as the bending of spacetime is simply the second law of thermodynamics at work (since gravitational clumping causes a lower entropy state).

I think I saw that while browsing in a shop, but didn't read it all. Since I believe entropy is the cause of natural selection, I don't have a problem with it being the cause of most things in the universe, but then again, if there is a causative factor for entropy that would be interesting too.

... three macroscopic forces? The strong and weak forces are not macroscopic - a macroscopic force needs an extremely light (i.e. probably massless) force boson.

I was counting electricity and magnetism as different. I am aware that they're connected, maybe there are only two macroscopic forces, which would eliminate the coincidence.

[lazerwalker]

Lots of stuff

These sorts of comments are difficult to answer because, at the risk of coming across as a dick, you know very little actual physics but are familiar with the analogies we use to try to understand & explain what's going on. It's then easy to try to extend the analogies in ways that don't make sense, from the point of view of the original theory which produced the analogy[1]. An important part of physics is to marry together the ability to work with the maths & also to develop the physical intuition that comes with the analogies, so that you can understand what the mathematical models are telling you.

To take your original point though, no, we don't and can't describe electromagnetic effects in terms of space-time[2], since there is only one space-time, and if charge bent space-time then non-charged matter would also be affected by charged bodies & EM fields. Each point in space-time will have an EM field associated with it, so the EM field can be bent and distorted by gravity just as matter is. More fundamentally, the motion of charged particles through EM fields just isn't the same as motion of particles through gravitational fields.

[1] This is a simplification, as sometimes the analogies can come first, and the maths is used to flesh out the analogy. Ultimately though, the maths, not the analogy, describes what happens.

[2] Obviously, EM fields have an energy, and thus will cause space-time curving, but this is generally a negligible effect.

[Darkflame]

You've got a bit of a confusing mess of acceleration, velocity, force and energy there.

I'll try to break it down then:

a) Is the effect of gravity to produce a consistently increasing velocity ? (again, ignoring the distance fall-off effect, and ignoring any atmospheric effects ).
Maybe I was just taught a over simplification, but to my knowledge gravity accelerates everything the same.
So gravity would apply the same change in speed over the same time period regardless of the objects original speed.

b) To consistently increase velocity any other way do you need to expend increasing amounts of energy to do so?
So, in other words, does it take more energy to accelerate something from 2 to 3 kph, then it would to go from 1 to 2 kph.

If it does take a increasingly larger amounts of energy to cause the same (relative) change in speed, surely that implies that a gravitational field accelerates objects differently then anything else that could give a object a impulse.
Gravity applies the same change in speed over time, everything else gives a progressively reduced speed change over time given the same energy expenditure.

[Absolute_Zero]

This is related to Einstein's Equivalence principle, that if something (a person, say) experiences a force they've no way to tell if it's due to gravity or due to acceleration as they are identical.

Except in practise they are distinguishable per the non-existence of requisite perfectly homogeneous gravitational fields.

e.g. if you drop two horizontally separated objects in an accelerating lift-like cabin, then they will fall in precisely parallel paths. Whereas [for example] under a planet's gravitational influence, the paths will converge slightly as they fall toward the gravitational/mass centre.

/pedant

[pogo]

The reason masses bend space but charges or magnets don't is because of the way gravity works. Gravity acts on everything, even things without mass, as e.g. even light has it's path changed by gravity.

Everything has mass. Energy is mass. Anything with energy has mass.

No, wrong. We have particles with mass and without. Photons, that is light, don't have mass. We know this as they don't of themselves generate gravity. And they can travel at the speed of light, which is impossible for any particle with mass (as it would take an infinite amount of energy). And of course we're pretty good at measuring the mass of things.

You're getting confused with mass-energy equivalence, which is a real thing. If you obtain energy from a body by e.g. chemical or nuclear means it loses mass. The mass change can be very small but it is measurable. In theory you could convert 100% of a mass to energy but the only way we can do that is by annihilating it with anti-matter. But anti-matter doesn't exist in the universe, unless we make it first, currently a very difficult process. We might do it though, as a way of storing energy for interstellar travel.

Except in practise they are distinguishable per the non-existence of requisite perfectly homogeneous gravitational fields.

e.g. if you drop two horizontally separated objects in an accelerating lift-like cabin, then they will fall in precisely parallel paths. Whereas [for example] under a planet's gravitational influence, the paths will converge slightly as they fall toward the gravitational/mass centre.

That's two bodies, not one. One one body you could in theory measure the internal stresses generated by the non-parallel gravitational forces but they would be minute. If you actually wanted to do this experiment it would be an interesting one to do on the ISS, which is in orbit so effectively in free-fall and has zero net acceleration. Except the forces on the extremities will be not quite parallel so will exert stresses, varying as it rotates around the Earth. But even for that I think the effect will be minimal and swamped by e.g. thermal expansion and the drag forces that slowly degrade its orbit.

[zip accident]

These sorts of comments are difficult to answer because, at the risk of coming across as a dick, you know very little actual physics but are familiar with the analogies we use to try to understand & explain what's going on. It's then easy to try to extend the analogies in ways that don't make sense, from the point of view of the original theory which produced the analogy[1]. An important part of physics is to marry together the ability to work with the maths & also to develop the physical intuition that comes with the analogies, so that you can understand what the mathematical models are telling you.

I attended a tech college, there were a class of about ten or twelve taking A level physics, I was one of two who passed, I believe the other pass was also a grade E. I failed A level Maths at that time, retook it decades later and only scraped through, and now have trouble remembering any maths.

To take your original point though, no, we don't and can't describe electromagnetic effects in terms of space-time[2], since there is only one space-time, and if charge bent space-time then non-charged matter would also be affected by charged bodies & EM fields.

I'm suggesting charge bends spacetime for charge only, non-charged matter doesn't experience the curvature. It would mean spacetime is different for different particle types, if we know that isn't the case, how do we know that isn't the case.

Each point in space-time will have an EM field associated with it, so the EM field can be bent and distorted by gravity just as matter is. More fundamentally, the motion of charged particles through EM fields just isn't the same as motion of particles through gravitational fields.

There would be differences due to the different ranges at which the effects of the fields occur. If there is (or are) more difference(s) than that, that's relevant.

[1] This is a simplification, as sometimes the analogies can come first, and the maths is used to flesh out the analogy. Ultimately though, the maths, not the analogy, describes what happens.

However, maths is also an analogy. Maths is very good, but it's a description of the universe, not itself the universe.

[zip accident]

Photons, that is light, don't have mass.

They don't have rest mass. However, they don't have a rest existence. I recall Einstein having said that he couldn't imagine being at rest relative to a photon.

We know this as they don't of themselves generate gravity.

Eh? how would you attempt to measure the gravitational effects of a mass as small as an electron, which is hugely more massive than a photon?

And they can travel at the speed of light, which is impossible for any particle with mass (as it would take an infinite amount of energy). And of course we're pretty good at measuring the mass of things.

Any particle with a rest mass, yes. However, it's when they are at the speed of light that they have, or appear to have, mass.

[Absolute_Zero]

However, maths is also an analogy. Maths is very good, but it's a description of the universe, not itself the universe.

Ye olde 'maths invented or discovered'? I'm with Pythagoras, it's all maths... and Riemann shaped.

http://seedmagazine.com/content/article ... t_hitched/
http://www.maths.bris.ac.uk/research/hi ... /random-m/

[zip accident]

However, maths is also an analogy. Maths is very good, but it's a description of the universe, not itself the universe.

Ye olde 'maths invented or discovered'? I'm with Pythagoras, it's all maths... and Riemann shaped.

http://seedmagazine.com/content/article ... t_hitched/
http://www.maths.bris.ac.uk/research/hi ... /random-m/

I'm not saying it's invented, I'm saying the maths we have discovered are an approximation to the maths of the universe, and always will be.

[pogo]

Any particle with a rest mass, yes. However, it's when they are at the speed of light that they have, or appear to have, mass.

No that's just wrong. The energy required to accelerate a mass to lightspeed is infinite, so particles that go at the speed of light must have no mass. This is fundamental special relativity, and only requires high school maths. Look in particular at Lorentz factor.