27 Oct 07
OK, this is neither a poser nor a puzzle. But I know all the smart people hang out here so I thought I'd ask.
I'm trying to understand relativity a little better, and have been reading some books.
One of the foundations of relativity is that the speed of light (in a vacuum) is a constant. From this it can be then shown that the speed of time is variable (i.e. time slows as speed approaches c). I understand all that (mostly), and the math behind it.
This all seems counter-intuitive to most people, who instead want to suppose that a) time is constant and b) the speed of light varies.
Can anyone propose an informal proof by contradiction to show where this breaks down. That is, assume:
a) Time is constant and cannot vary
b) "c" can vary
and demonstrate these lead to invalid conclusions?
27 Oct 07
Originally posted by RookRAKThe first one, time is constant and cannot vary has already been proven to be wrong, the variability of time is programmed into every GPS receiver because not accounting for variable time, the GPS would be useless for navigation, and could be off more than 3 Km, due to the fact that the distance and angle calculations depend on C being absolute on earth.
OK, this is neither a poser nor a puzzle. But I know all the smart people hang out here so I thought I'd ask.
I'm trying to understand relativity a little better, and have been reading some books.
One of the foundations of relativity is that the speed of light (in a vacuum) is a constant. From this it can be then shown that the speed of time is variab ...[text shortened]... stant and cannot vary
b) "c" can vary
and demonstrate these lead to invalid conclusions?
Using the assumption that C is invariable and time is not fixed allows corrections to the timing of the signals to the GPS receivers. If it were the other way round, there could be no accurate measurements of distances.
Any variation in C (there are some cosmological data suggesting a tiny change in C from 10 billion years ago, unverified) would be so small as to make no difference in anything we can calculate. If C was variable, radar would be inaccurate, but for instance, the corner reflector left on the moon allows lasers pointing at it to establish the distance to the moon at any given point by measuring the time of flight of the photons, since they are the same photons that left the laser and a few of them (very dam few, but enough) reflect off the mirrors and come straight back at earth and therefore the distance to the moon is known within ten cm or so and work is underway right now to get that down to about 1 cm to further test relativity. All that impossible if C varies. That is a direct measurement that shows the invariability of C to many decimal places.
When you maybe respond 'well show me it doesn't vary in any decimal place', we get into philosophy not science, there is no way to show C doesn't change by say 1 angstrom per second per billion years, even theory can't peg it down that fine since you run into the uncertainty principle which says you can peg the velocity of an object or its momentum or mass but not both at the same time and time indeed is the variable in relativistic calculations, even at normal velocities. We can calculate how much time flow changes if a beetle walks slow vs standing still.
27 Oct 07
Originally posted by sonhouseNone of these show that c is a constant and time varies!
The first one, time is constant and cannot vary has already been proven to be wrong, the variability of time is programmed into every GPS receiver because not accounting for variable time, the GPS would be useless for navigation, and could be off more than 3 Km, due to the fact that the distance and angle calculations depend on C being absolute on earth.
...[text shortened]... ties. We can calculate how much time flow changes if a beetle walks slow vs standing still.
27 Oct 07
Originally posted by wolfgang59I beg to differ. We can measure C with exquisite accuracy and all the tests show there to be no difference in any test. But it has been conclusively shown time can vary by simply having satellites orbiting the earth with an atomic clock on board and comparing the ticks to one that had been synced with it on earth. The two go out of sync rapidly. That is a direct measurement having nothing to do with a change in the speed of light.
None of these show that c is a constant and time varies!
It can be shown that C is constant at the same time a large change in the rate of time simultaneously and it is always the constancy of time that loses out to the constancy of the speed of light. We can not conduct any experiment that shows a variation in the speed of light but we can for absolutely sure conduct experiments that directly show a change in the rate of time flow, for instance, not even having to leave the earth, just put an atomic clock on a hikers back and have it be in sync with a central station and then climb a mountain and lo and behold, the lesser gravity on the mountaintop speeds up the clock. The distance between the equipment on the ground and the equipment on the hiker can be kept exactly the same simply by having a roll of very thin optical fiber that both ends are connected but still can unroll like the garden hose affair and that keeps the time of flight of the signal exactly the same no matter how far the hiker goes assuming there is enough left on the roll, so it is easy to show time itself speeds up and slows down depending on how steep the gravity field is. That is an undisputable finding.
28 Oct 07
1 edit
Originally posted by RookRAKI don't think it's so much a logic argument as an experimental one. Einstein's equations explain things that Newton's do not.
OK, this is neither a poser nor a puzzle. But I know all the smart people hang out here so I thought I'd ask.
I'm trying to understand relativity a little better, and have been reading some books.
One of the foundations of relativity is that the speed of light (in a vacuum) is a constant. From this it can be then shown that the speed of time is variab ...[text shortened]... stant and cannot vary
b) "c" can vary
and demonstrate these lead to invalid conclusions?
Maxwell's equations were not consistent with the laws of mechanics. In relativity they are.
28 Oct 07
Originally posted by AThousandYoungI understand the difference between Einstein and Newton.
I don't think it's so much a logic argument as an experimental one. Einstein's equations explain things that Newton's do not.
Maxwell's equations were not consistent with the laws of mechanics. In relativity they are.
I am simply looking for an experiment, or equation, that would fail given my wrong assumptions.
Again, I'm not *disputing* the fact that c is a constant, and that time is not. I just want to know where mathematically, or experimentally reversing them falls. I'm sure there are many, many examples, but I need someone better at physics than I am.
28 Oct 07
unless i'm missing something . . .like a whole lot of classes, the "speed of time" doesn't make a lot of sense . .i mean, like in the physical reality sense. . .the perceptual sense . of course, like time in a dentist chair compared to time shopping or kissing good night, but the speed of time? what's that about?
28 Oct 07
Originally posted by coquetteYou're very right! The language that both posters are using till now is wrong. Talking about the speed of time is a little bit nonsense. It's better to talk about if time flows to a constant rate than use the term speed, and the difference is not only on semantics, but it is a very real difference.
unless i'm missing something . . .like a whole lot of classes, the "speed of time" doesn't make a lot of sense . .i mean, like in the physical reality sense. . .the perceptual sense . of course, like time in a dentist chair compared to time shopping or kissing good night, but the speed of time? what's that about?
28 Oct 07
Originally posted by sonhouse"conclusively shown time can vary by simply having satellites orbiting the earth with an atomic clock on board and comparing the ticks to one that had been synced with it on earth. The two go out of sync rapidly. That is a direct measurement having nothing to do with a change in the speed of light. "
I beg to differ. We can measure C with exquisite accuracy and all the tests show there to be no difference in any test. But it has been conclusively shown time can vary by simply having satellites orbiting the earth with an atomic clock on board and comparing the ticks to one that had been synced with it on earth. The two go out of sync rapidly. That is a d ...[text shortened]... up and slows down depending on how steep the gravity field is. That is an undisputable finding.
This is the proof. 😀
I think the original poster was asking how you can measure ANY speed accurately if the flow of time changes. ie. If I measure something to be half-light speed how do I know its not really quarter light-speed but time is running slowly?
A simple experiment with the atomic clocks does tell us that time is variable.
28 Oct 07
Originally posted by RookRAKA very simple experiment is the decay of muons in our atmosphere. This fact is one of the most strong proofs that we have for special relativity and if you ask me its strength come for the fact that it is a pure natural experiment. I'll check out some links for you and I advice you to go to youtube and check out some of the lessons on special relativity that they got there.
I understand the difference between Einstein and Newton.
I am simply looking for an experiment, or equation, that would fail given my wrong assumptions.
Again, I'm not *disputing* the fact that c is a constant, and that time is not. I just want to know where mathematically, or experimentally reversing them falls. I'm sure there are many, many examples, but I need someone better at physics than I am.
http://www.astro.ucla.edu/~wright/relatvty.htm This is one of my favorite aproach to relativity. Using space-time diagrams. If you ask me that's the way it should be done and it would avoid a lot of common misconceptios that the general public has on this subject. Another thing that I think should have more emphahis is the the metirc matrix. On special relativity the metirc is non-euclidian and that's why a lot of fun stuff happens.
I don't have the general scorn that people have to wiki so I'll give you two wiki links too http://en.wikipedia.org/wiki/Special_relativity http://en.wikipedia.org/wiki/Introduction_to_special_relativity
And if you have the time check this link too: http://nobelprize.org/educational_games/physics/relativity/
The reason I'm giving you a lot of links instead of staright answers is that I believe that if people do their own research things can be learned much better. In case you have anymore doubts just bring them on and we all can discuss it here. My only catch is that I see alot of people using a wrong language on this matters while we are on the foruns.
31 Oct 07
The idea of "speed of light" and "time" are confusing.
Just how confusing can be summed if you can answer the following question correctly. Not many people can.
Postulate - The Universe is 13.7 Billion Years Old and the speed of light is SOME value and photons travel at that speed where speed is distance divided by time.
How long does it take a photon to cross the entire universe?
31 Oct 07
Originally posted by StarValleyWyFrom edge to edge:the age of the universe.
The idea of "speed of light" and "time" are confusing.
Just how confusing can be summed if you can answer the following question correctly. Not many people can.
Postulate - The Universe is 13.7 Billion Years Old and the speed of light is SOME value and photons travel at that speed where speed is distance divided by time.
How long does it take a photon to cross the entire universe?
I'm assuming universe means observable universe.
31 Oct 07
2 edits
Yes. We could estimate that the distance across the universe is now about 47 billion light years.
Edit: The reason it is not "27.4" light years across is because of time dilation. We dont need that to understand the "photon" crossing question though.
I have to leave, so I thought I better leave an answer to my poser.
The correct answer to how long it takes a photon to cross the universe is "Who Knows?" Literally. Since time does not exist AT THE SPEED OF LIGHT, what is anything divided by zero? It is undefined.
But it isn't that simple. IF the photon is "measured or observed" then it becomes what we call "real". It drops into "our universe"... whatever that is. Technically, we "brake its waveform". If it is never observed, it never exists. Period. Philosophers have asked "If a tree drops in the forest and noone observes it, is it real?" In our universe, the answer is a resounding NO. I'll be back in a week or so. Mike