- 25 Feb '08 11:44What is general relativity?

Einstein's earlier theory of time and space, special relativity, proposed that distance and time are not absolute. The ticking rate of a clock depends on the motion of the observer of that clock; likewise for the length of a "yardstick." Published in 1915, general relativity proposed that gravity, as well as motion, can affect the intervals of time and of space. The key idea of general relativity, called the equivalence principle, is that gravity pulling in one direction is completely equivalent to an acceleration in the opposite direction. A car accelerating forwards feels just like sideways gravity pushing you back against your seat. An elevator accelerating upwards feels just like gravity pushing you into the floor.

If gravity is equivalent to acceleration, and if motion affects measurements of time and space (as shown in special relativity), then it follows that gravity does so as well. In particular, the gravity of any mass, such as our sun, has the effect of warping the space and time around it. For example, the angles of a triangle no longer add up to 180 degrees, and clocks tick more slowly the closer they are to a gravitational mass like the sun.

Many of the predictions of general relativity, such as the bending of starlight by gravity and a tiny shift in the orbit of the planet Mercury, have been quantitatively confirmed by experiment. Two of the strangest predictions, impossible ever to completely confirm, are the existence of black holes and the effect of gravity on the universe as a whole - 25 Feb '08 12:21 / 1 editIn one of the books (I do not exactly in which) there was interesting analogy about space distortion caused by gravity. Imagine soft bed and a small but pretty heavy ball thrown in the middle of it. (representing our Sun which is pretty small star)The surface of the bed wont be flat anymore, it would become slightly distorted... and now black hole is analogy of the very small imaginary ball on bed which would be as heavy as bus or something, then the bed (if is stretchy enough and wouldn't be ripped apart, but it is theoretical bed anyway ) would bend to such degree that the central hole would be very deep and the hole surface would be distorted...

Now in universal meaning, you add one more dimension (something human mind can not comprehend but according the books it can be mathematically proven) and that is the staff that stars, neutron stars or black holes are doing with space....

very interesting stuff indeed. I am sad that I red all that books which were translated in Croatian language so I can not join discussion well since I lack English terminology... - 25 Feb '08 15:09

general relativity is simply the laws of physics when you assume that there's no preferred inertial reference frame and the velocity of light is constant.*Originally posted by tomtom232***What is general relativity?**

Einstein's earlier theory of time and space, special relativity, proposed that distance and time are not absolute. The ticking rate of a clock depends on the motion of the observer of that clock; likewise for the length of a "yardstick." Published in 1915, general relativity proposed that gravity, as well as motion, can af ...[text shortened]... onfirm, are the existence of black holes and the effect of gravity on the universe as a whole

Special relativity is just when you assume there's no acceleration and all thing get simpler. - 25 Feb '08 16:22

Is there any proof that the velocity of light is a constant, or is it just a good conjecture?*Originally posted by serigado***general relativity is simply the laws of physics when you assume that there's no preferred inertial reference frame and the velocity of light is constant.**

Special relativity is just when you assume there's no acceleration and all thing get simpler.

Does general relativity work if the velocity of light changes with time? - 25 Feb '08 16:52

the*Originally posted by FabianFnas***Is there any proof that the velocity of light is a constant, or is it just a good conjecture?**

Does general relativity work if the velocity of light changes with time?*speed*of light is not constant, it depends on the medium, and can be as slow as only a few meters per second.

but general relativity is based on the notion that there*exists*a top speed*which is constant*, and in practice that's assumed to be the speed of light in a vacuum. there's nothing to suggest that speed of light in vacuum wasn't the top constant speed, but it's totally possible that there are greater speeds. the theory makes no assumptions of what the actual top constant speed is, only that there*is*one. - 25 Feb '08 17:04

I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (And we now we are about 384000 km far away from the moon) and astronaut is holding one end of the rope, Earthling the second... So lets say that man from earth pull the rope (ignore practical problems like how to place the rope between moon and Earth), assuming that rope isn't flexible at all ,signal between two people which are 384 000 kilometers far away could result in less then a second. So although Earthling pulled the rope with speed of say 5 km/h , he sent a signal to moon that is faster than speed of light*Originally posted by wormwood***the***speed*of light is not constant, it depends on the medium, and can be as slow as only a few meters per second.

but general relativity is based on the notion that there*exists*a top speed*which is constant*, and in practice that's assumed to be the speed of light in a vacuum. there's nothing to suggest that speed of light in vacuum ...[text shortened]... akes no assumptions of what the actual top constant speed is, only that there*is*one.

Any hole in my assumption ?? - 25 Feb '08 17:25 / 1 edit

disregarding*Originally posted by ivan2908***I was always wondered something which is pretty stupid but interesting. Theoretically if you connect Earth and moon with some indestructible rope (And we now we are about 384000 km far away from the moon) and astronaut is holding one end of the rope, Earthling the second... So lets say that man from earth pull the rope (ignore practical problems like how to he sent a signal to moon that is faster than speed of light**

Any hole in my assumption ??*all*practical problems, I think it might even work just like that? it's really easy to make mistakes with these kind of 'practical' analogues though, because your mind refuses to fully detach from the practical problems no matter how you try...

it's just like I once asked my physics professor what would happen with an indestructible disc, accelerated so that its edge would travel near light speed. because as the edge starts getting really fast, the time and length of it changes, while it's still attached to the slow moving center of the disc. I mean, what the hell is going on in there? does the angular speed of the outer rim change? and if so, how's that indestructible disc still attached to the center? what would an observer sitting at the edge observe as he looks towards the center?

- 25 Feb '08 17:58

Well... that is a good question. What did he reply ?*Originally posted by wormwood***disregarding***all*practical problems, I think it might even work just like that? it's really easy to make mistakes with these kind of 'practical' analogues though, because your mind refuses to fully detach from the practical problems no matter how you try...

it's just like I once asked my physics professor what would happen with an indestructible d ...[text shortened]... ? what would an observer sitting at the edge observe as he looks towards the center?

- 25 Feb '08 21:00

Not excatly true. On relativity we assume that light speed is constant in vacuum. And it isn't stated nowhere, besides bad books, that light speed is an limit speed. What one can get is that by the relativistic formula for adding velocities is: if v1 and v2 are both minor than c to start with than their composition will always be less than c.*Originally posted by wormwood***the***speed*of light is not constant, it depends on the medium, and can be as slow as only a few meters per second.

but general relativity is based on the notion that there*exists*a top speed*which is constant*, and in practice that's assumed to be the speed of light in a vacuum. there's nothing to suggest that speed of light in vacuum ...[text shortened]... akes no assumptions of what the actual top constant speed is, only that there*is*one.

Other than that you were spot on. - 25 Feb '08 21:01

This a textbook problem more or less. If you look it up you can find approximate answers to yiur question.*Originally posted by wormwood***disregarding***all*practical problems, I think it might even work just like that? it's really easy to make mistakes with these kind of 'practical' analogues though, because your mind refuses to fully detach from the practical problems no matter how you try...

it's just like I once asked my physics professor what would happen with an indestructible d ...[text shortened]... ? what would an observer sitting at the edge observe as he looks towards the center?

- 25 Feb '08 21:11

Only assume? There is no theoretical proof?*Originally posted by adam warlock***... On relativity we assume that light speed is constant in vacuum. ...**

When universe was young, might the speed of light in vacuum possibly be different from what it is nowadays, or in far future?

Is the possibility of a non-constant speed of light not completely ruled out? - 25 Feb '08 21:44
*Originally posted by FabianFnas***Only assume? There is no theoretical proof?**

When universe was young, might the speed of light in vacuum possibly be different from what it is nowadays, or in far future?

Is the possibility of a non-constant speed of light not completely ruled out?**Only assume? There is no theoretical proof?**

In physis theoretical proofs hasn't got the same bearing that they have on math. The physical axioms are dictated by experiment (thermodynamics is an excelent example) and as time passes we experiment moreand learn more. So the truth from the old days may not exist today. Physics advances by approximations. And this is only natural since we are humans and finite in space, time and knowledge. What I'm trying to say is that theoretical proofs on physics are always based on some metaphysical assumption made by the physicist/physicists and on the context of the given theory evrything that is logically*proved*is proved. Now if you start to probe deeper intonature you will always find out that your previous metaphysical assumption is incomplete.

**When universe was young, might the speed of light in vacuum possibly be different from what it is nowadays, or in far future?**

A nonchallant ansewr is yes. But the full answer is everytime you are saying something new in physics you have to give very good reasons for doing so. We have a building that for the most part works very well so if you are giving new assumptions you have to explain why the old assumption worked in their given limits and why your new assumptions are more global and should be used. João Magueijos works in VSL theories and a lot of other people work too and do this. So just stating something won't really cut it.

**Is the possibility of a non-constant speed of light not completely ruled out?**

No, but care to give us a goor reason for doing so. In the context of a lot of theories the speed of light is taken to be constant during some derivation. So now you say that c changes. But unless you give us a law of how to express derivatives of c and other such things a lot of formulas that we know have can't be achieved. And one other thing is that in the the current theories the formulas that are achieved assuming c is constant are remarkbly acurate so if c indeed changes it has to be change very slowly in order for both fact to be able to coexist.

I think you are hinting into João Magueijo and his VSL were light speed varies in cosmic scales but you have to know that cosmic scales are very big indeed. So in the everyday laboratory things will pretty much flow as we know them. Of course if he is right the metaphysics behind everyday physics is invalid**but**to the accuracy needed in evryday physics they are just what we need. It's more or less General Relativity and Newtonian gravity. Everybody knows that newtonian gravity isn't the last word but every space ship that is flying in the solar system is flying due to the solution of newtonian equations. The thing is that Einstein equation, nonlinear partial differential equations are very hard to solve, and newtonian equations are easy as pie the trajectories in both cases may differ in the nanometer or so but what we are interested in are movements in the Km scale so as far we know both theories are equally good in practice. - 25 Feb '08 23:04

I'd say it would be impossible to accelerate. Take the fact that as something approaches light speed, its mass approaches infinity. While the mass of the inner disk wouldn't change much, the outer would approach infinity. And as the mass of the object increases, especially the outer portions of it, its moment of inertia would approach infinity with its mass. Since Torque = moment of inertia * angular acceleration, assuming torque is constant and finite, as the moment of inertia approaches infinity the angular acceleration would approach zero.*Originally posted by wormwood***he smiled and replied:**

"well... that is a good question."

damn teachers and their pedagogic ways...