Originally posted by FabianFnasIs there any proof that the velocity of light is a constant, or is it just a good conjecture?
In relativity, it's a postulate. Physics doesn't make proofs about anything, NEVER. It makes theories that might explain or predict reality in an certain way, nothing more.
Does general relativity work if the velocity of light changes with time?
velocity of light CAN change with time. General relativity would still work, at least locally. There are some theories that use non constant speed of light, but new problems arise with conservation of energy and other stuff. It's a field being explored right now. Investigation is very hard, because we are limited to a very small piece of space, and have only been observing things for a very small piece of time.
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Originally posted by ivan2908Any hole in my assumption ??[/b]
[b]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 the rope with speed of say 5 km/h , he sent a signal to moon that is faster than speed of light 😲
yes. The signal couldn't reach the moon that fast. The laws of physics for pulling small ropes is different from pulling long ropes with such high energy.
General relativity would have to be used.
Originally posted by wormwoodIt'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?
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...
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You are trying to apply special relativity reasoning to a general relativity problem. It can be solved with the Schwarzchild solution of the Einstein equations of general relativity for spherical symmetries.
The whole disk would be distorted, the disk would be indestructible the same way, you simply can't see "indestructible" as a synonym of "keeping it's apparent shape".
Originally posted by ark13You must apply general relativity. Your reasoning is correct for special relativity for non accelerated frames, but this is not one of those.
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 ...[text shortened]... te, as the moment of inertia approaches infinity the angular acceleration would approach zero.
Originally posted by wormwoodAhh but if that is so, my discussion in Sunday School concerning the age of the universe falls apart. I maintain that, even if Carbon Dating, evolution, AND the big bang are all bogus (which is a prevailing theory in my class), the fact that we know the speed of light and we know that the Doppler effect is true--thus those galaxies speeding away from us MUST be billions of years old, simply because it would take that long for their light to have reached us....right?
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.
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Originally posted by PinkFloydis intergalactic space filled with bose-einstein condensate? no. -what is it filled with then? practically nothing. what is the speed of light in practically nothing?
Ahh but if that is so, my discussion in Sunday School concerning the age of the universe falls apart. I maintain that, even if Carbon Dating, evolution, AND the big bang are all bogus (which is a prevailing theory in my class), the fact that we know the speed of light and we know that the Doppler effect is true--thus those galaxies speeding away from us M ...[text shortened]... f years old, simply because it would take that long for their light to have reached us....right?
bingo!
Originally posted by PinkFloydRight. Unless there us some unknown effect between, or God created some initial conditions to trick us to think Universe is older.
Ahh but if that is so, my discussion in Sunday School concerning the age of the universe falls apart. I maintain that, even if Carbon Dating, evolution, AND the big bang are all bogus (which is a prevailing theory in my class), the fact that we know the speed of light and we know that the Doppler effect is true--thus those galaxies speeding away from us M ...[text shortened]... f years old, simply because it would take that long for their light to have reached us....right?
Evolution and Big Bang are not bogus nor true. They are simply theories that fit reality we see, nothing more. They're just waiting to be improved.
Originally posted by wormwoodI would guess that the "speed of light in practically nothing" is the same as the speed of light in a pressure cooker, or in Timbuktu, or in my washing machine, or in my navel. c, is constant; it doesn't change. Therefore, the fact that there is practically nothing in the void of intergalactic space seems irrelevent.
is intergalactic space filled with bose-einstein condensate? no. -what is it filled with then? practically nothing. what is the speed of light in practically nothing?
bingo!
Originally posted by PinkFloydIn that case you have a lot of reading up to do.
I would guess that the "speed of light in practically nothing" is the same as the speed of light in a pressure cooker, or in Timbuktu, or in my washing machine, or in my navel. c, is constant; it doesn't change. Therefore, the fact that there is practically nothing in the void of intergalactic space seems irrelevent.
Originally posted by tomtom232I think the main problem with your grasp of this concept is your visualization of gravity: it's Newtonian. From what I gather, you picture gravity as a sort of rope, pulling on various celestial bodies to keep them close to larger celestial bodies. The generally accepted way today is not Newtonian, but Einsteinian, which pictures gravity as a warping in the continuum.
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
I'll use the age-old analogy: picture a rubber sheet extending infinitely in all directions, stretched taut. A plane, if you will. The sheet is flat, and anything rolled across it will continue rolling across it in a straight line by the law of inertia. However, if you place a bowling ball on the rubber sheet, it's no longer taut. It stretches beheath the weight of the ball, creating a warping. If you were to roll anything (a marble, for example) through this, its path would change as it hit the warp. This is gravity, where the sheet is the spacetime continuum, the bowling ball is a large celestial body, and whatever else you roll across is a smaller celestial body. (Technically, the continuum exists in four dimensions, not just two as described here. I kept it 2-D for the sake of clarity.)
Got it?
The light speed in glass is slower than in vacuum. But glass is only atomic nucleus surrounding in vaccum. Between the nuclei there is mostly vacuum. So why does the light go slower in glass when it is mostly vacuum?
Why are some materials opaque for light when it is mostly vaccum, and others are transparent (like glass) when the both have the same amount of vacuum between its atoms?
Originally posted by ivan2908The big hole in your conjecture is there is nothing and I mean exactly that, NOTHING, that would not get stretched out by your pulling on it, in other words, the pull would travel at the speed of sound, its called a longitudinal wave, like if you have a slinky and bunched several turns together and let them go, a wave of turns moving back and forth in the direction of the stretched out slinky would happen. That same kind of wave would proceed down the rope. Sorry, but that just doesn't happen. You can simulate that one earth, just get a strong wire and stretch it out and have some instruments like an electric guitar pickup or something, piezo pickup and hit one end with a hammer or something and start a very fast timer going and you will find the sound propagates down the wire a lot slower than the speed of light. A LOT slower.
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 ...[text shortened]... he sent a signal to moon that is faster than speed of light 😲
Any hole in my assumption ??
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Here is a nice problem for relativitists: I just heard about these cosmic rays coming in from across the universe, billions of LY away but have so much energy, they are going almost at C. They said if a light beam takes 300 million years to travel that many light years, this cosmic ray would take 300 million years, and ONE microsecond more.
So how much increase in mass would the particle get from going that close to C?
You know the formula, right?
Here is a start: 365 days per year, 86,400 seconds per day=31,536,000 seconds per year. So lets call it 3E7 seconds per year for grins, 30 million. So 3E7*1E6 =3E13 MICROSECONDS per year.
now times 300E6 (300 million years) or 3E8 * 3E13=9E21 microseconds in 300 million years. So the ratio is 9E21/(9E21+1)
So how close to C it gets. So using relativity, suppose the rest mass was a proton, what is the mass at that velocity? I don't even have enough digits on my calculator for it, it must take something like matlab to do properly. Anyone got one of those advance math programs?