Question for physics guru
- Thread starter salami1212
- Start date
Soldato
- Joined
- 12 Apr 2004
- Posts
- 11,788
- Location
- Somewhere
I'm not sure about the exact mechanics of gravitational waves, but there'd be some kind of wavefront generated by the change in mass that'd radiate outward towards the planets.
What exactly would happen to the planets would depend on the shape of the wavefront, but essentially they'd just fly off into space. If the wavefront is vertical then they'd just fly tangentially.
However, a mass suddenly popping out of existence is (as far as I know) incompatible with general relativity.
Last edited:
So if what u say is right, black holes shouldn't be black...
They appear black as no light reaches us from them. They could be electric pink for all we know.
They are black because light cannot escape them
How comes the light from stars orbiting the centre of a galaxy (a black hole) can reach us?
Soldato
- Joined
- 12 Apr 2004
- Posts
- 11,788
- Location
- Somewhere
Because the light didn't originate from within the Schwarzschild radius of the black hole.
Light can escape from the vicinity of a black hole as long as it's not coming from the other side of the event horizon, though the closer its origin is to the event horizon, the longer it'll take to reach a distant observer and the more red-shifted it'll be.
Incidentally, this is why it's impossible to see something falling through the event horizon of a black hole
Last edited:
Associate
- Joined
- 7 Jul 2007
- Posts
- 1,430
Only light that passes the inside of the black hole's event horizon cannot escape. Light passing outside this radius is simply refracted by the gravitational field. Also, photons are almost certainly massless, otherwise they could not travel at the speed of light, just near to it, and different frequency photons would travel at different speeds as shown by the de broglie relation, something that is not observed.
We'd freeze to death on account of there being no sun.
All forces are transmitted by massless particles which travel at the speed of light (at least according to our current best theories of physics). Electromagnetic forces are transmitted by photons (which we can detect with relative ease), whereas gravitational forces are transmitted by gravitons (which we cannot yet detect reliably).
It's certainly true that we do not have a fully unified theory for mavity, and this is where a large proportion of theoretical physics research is focussed. But, unless our theories require a massive revision, gravitons will not travel faster than the speed of light. People tend to think of the speed of light as being a "cosmological maximum speed", which is true in a way, but the speed limit applies to all information transfer - not just objects that have a mass.
Look into force carrier particles for more info.
Anyway, to answer the OP's questions:
1: The behaviour would depend on the material interface between the two boxes. If we take an idealised scenario with a perfectly isotropic surface (i.e. it "looks the same in every direction"; no alignment of microscopic surface irregularities etc), then the top box will not migrate, since the box below it is exerting a periodic force which has a net force of zero over the period of oscillation. Since the bottom box is moving so rapidly it's reasonable to assume that the force will be enough to overcome the static friction, so the top box would not "move with" the bottom box. There would, however, be a whole assload of friction created, which would heat up the surfaces of the two boxes very quickly.
In a less ideal scenario, tiny differences in the surfaces of the boxes would mean that the force exerted on the upper box, when the lower box is moving "left", would be a tiny fraction different that that exerted when the box is moving "right". The effect of this would be a non-zero net force on the top box, leading to a slight overall movement in one direction (in the same way that a vibrating phone may migrate across a desk). Eventually it will move off the top box and fall.
To answer question 2 you will need to get into string theory, or some other theory of quantum mavity. Since these are not yet complete, it's of little sense to get into specifics
It's interesting the way you chose to phrase the question though... There are many scientists who believe that the event horizon of a black hole represents a very real boundary to our Universe, and that inside of the event horizon the laws of physics as we define them cease to exist (Cosmic censorship hypothesis). In this way, you could argue that the original object does "cease to exist", but then you're getting into philosophy
Last edited:
Soldato
- Joined
- 12 Apr 2004
- Posts
- 11,788
- Location
- Somewhere
But surely once the lower box has moved out from beneath the upper box's centre of mass, the upper box will begin to tip to one side? This may be a very small effect for one oscillation, but since the lower box is oscillating so rapidly, it'll very quickly accumulate.
I was assuming that the period of oscillation for the lower box was smaller than the size of the boxes... But reading the OP again (the question being about "falling") I see that probably wasn't the question being asked...
Well lets see: As the lower box moves out from the centre of mass for the upper box, the upper box will begin to tip, then fall (very slightly). As the lower box heads back, its leading edge will encounter the bottom surface of the upper box at a slight angle. Say the lower box moves to the right and back again; the upper box will have dropped very slightly and rotated slightly anti-clockwise. When the lower box comes back, the leading edge will impact the right-hand half of the upper box's base. This will create another anti-clockwise rotation around the centre of mavity, and so will work to further increase the anti-clockwise rotation caused by the tipping.
I suppose that if it oscillates fast enough you could get into a scenario where the amount dropped / rotated initially is smaller than the lengthscale of the surface irregularities, in which case we revert back to the "stationary" scenario where an assload of friction is generated. But in the real world with non-ideal boxes, I can't imagine any other scenario than the box tipping over very quickly (gets rammed by the bottom box during the first oscillation).
Last edited:
First of all in order to offer a precise answer these cubes need a mass. Galileo. Secondly on the properties of the speed of light, is relative to the observer and as such we the observer would not see 3 different cubes we would in fact observe a contracted cube, it’s length would be squeezed depending on the direction it was travelling. Einstein.
Galileo again and a little bit of Newton. Given the gravitational pull of the earth on the upper cube. The upper cube would need mass in order to fall otherwise it will remain stationary. However given a mass the cube will fall in accordance with the related theory.
Newton’s law of universal gravitation. Forces will be exerted by and upon the falling object and the lower moving object. Gravitational radiation (wave displacements) will theoretically extend along the ‘x’ axis in both directions (left and right) from the lower cube and along the ‘y’ axis from the upper cube(although possibly much smaller displacements).
Come on you people this is a simple clear case of classical mechanics. As observers it would appear to the eye that there is no movement. That the upper cube is not displaced. However it can be measured that the upper cube does in fact wobble however the displacement would be in the sub microscopic realm. Of course this is merely theory since the gravitational effect of the earth exerts a stronger force than these theoretical cubes. However it does not follow that this may be the case at all since we have no value of mass for either cube and that the results could be much different in a vacuum with inertial frame of reference.
------
Galileo again and a little bit of Newton. Given the gravitational pull of the earth on the upper cube. The upper cube would need mass in order to fall otherwise it will remain stationary. However given a mass the cube will fall in accordance with the related theory.
Newton’s law of universal gravitation. Forces will be exerted by and upon the falling object and the lower moving object. Gravitational radiation (wave displacements) will theoretically extend along the ‘x’ axis in both directions (left and right) from the lower cube and along the ‘y’ axis from the upper cube(although possibly much smaller displacements).
Come on you people this is a simple clear case of classical mechanics. As observers it would appear to the eye that there is no movement. That the upper cube is not displaced. However it can be measured that the upper cube does in fact wobble however the displacement would be in the sub microscopic realm. Of course this is merely theory since the gravitational effect of the earth exerts a stronger force than these theoretical cubes. However it does not follow that this may be the case at all since we have no value of mass for either cube and that the results could be much different in a vacuum with inertial frame of reference.
------
Soldato
- Joined
- 12 Apr 2004
- Posts
- 11,788
- Location
- Somewhere
Needs more name-dropping.
Lol I see the humour in your post.
Was meant more as a quick bibliography citation.
------
Associate
- Joined
- 3 Feb 2009
- Posts
- 895
- Location
- Sydney
Photons are massless
Surely they can't be as if they were they would be unaffected by mavity. Doesn't an object have to have mass for it to be affected by mavity?
Last edited:
Always thought that due to way that mavity in a black hole affected space time that it "bent" it so much that the photons just ended up travelling in a loop. Like a race car going round a very large track, from their perspective its a straightline but really they are just going round and round and thus never reaching an observers eyes / detection. This of course may involve some form of rubbish.
Last edited:
Not entirely accurate
http://en.wikipedia.org/wiki/Nuclear_force
http://en.wikipedia.org/wiki/Mesons
No. Think of it more as mavity effecting space itself that the photons have to travel in. As a 3D example, stretch out a large sheet of elastic material. Place a large weight in the centre, creating a dip (this is our mavity well). If you were to shoot a lot of tiny balls across this sheet, as the balls come within decreasing radii of the weight they be increasingly effected, initially as a change in direction (gravitational lensing) but within a certain radius they will not be able to escape (they have crossed our even horizon).ThePirateHulk said:
This is slightly crude, but I hope it helps.