Science et Vie Junior dedicates this month an article about some strange phenomena due to relativity . A very good exercise pedagogy that we can prove without equations and with thought experiments to explain the funny idea of a theory yet complicated. I can resist not happy to tell the story to my sauce.
First step: landing or not? The equivalence between gravity and acceleration
Bob is an intergalactic astronaut sent on a mission to explore its mysterious planet Zorglub. To spare him the trouble of a long journey, he was asleep after taking off for several years. But following a breakdown somewhere, Bob wakes up before arriving. Or so he assumes everything is as black in the cockpit. The windows are closed, the measuring apparatus extinguished and there was a profound silence. What's going on? Has he arrived? Bob stands, poses one foot down and realize that instead of floating in the cabin can stand on the floor of the rocket. His brain begins to fuel up: "Would you ever Zorglub on which I feel the effect of gravity? Unless this is simply the effect of the acceleration of the rocket? Impossible to know because I don ' have no evidence outside: the engines are quiet and hide windows closed! "
Interlude1 : it is through this thought experiment that Einstein says he had a better idea of his life (with a lift because the rockets did not exist at the time). You have learned Second class of an object in a gravitational field g is subjected to a force equal to its mass times its acceleration a. Ie a = g, we can not differentiate an acceleration of a gravity field. It would not be the same if our object was subjected to another kind of strength, electrical, magnetic or other because the force of gravity is the only one that depends directly on the body. The equivalence between acceleration and gravity explains the confusion of our friend Bob without external index it is incapable of knowing if it is placed on the ground of a planet or if the vessel is in full acceleration. The genius of Einstein is to have extended this equivalence to all the laws of physics. He postulated that they all behaved the same way, whether you are subject to gravity or speeding up in the middle of space. But continuing the adventures of Bob ...
Second step: the light deflected by gravitation?
Groping, Bob tries to open the windows to hide and watch what happens outside. Damned is blocked! Bob tries to make a hole in it with his knife but he only succeeded in drilling a tiny notch. A thin ray of light enters the vessel. "Phew!" He said, looking closely the trajectory of the light I'll finally know whether or not I am arrested:
- if I'm lying on the floor of a planet, the ray of light inside my ship will draw a perfectly straight line.
- if for cons, I am trying to accelerate, the radius draw a (very) small satellite dish in my ship. "
The reasoning of Bob (who does not read Science et Vie Junior!)
Wrong, completely untrue! say Einstein equivalence principle which postulates that all laws of physics must be identical, which is subject to gravity or a acceleration. According to him, the path of a light beam to be deflected by gravity just as it would be if the rocket accelerates! We can tell Bob to stop trying to measure the curvature of the light in his rocket, it does not give him any information about his situation.
Interlude2: As crazy as it sounds, the more massive a planet is, the more it bends the path of light rays passing close to exactly as superheated air from the soil deflects the light and causes mirages water in the desert. There is a difference between the two situations. In air, the speed of light varies with temperature, it is understandable therefore that a ray of light, the fastest route between two points is curved. Conversely, the speed of light is always the same in a vacuum. Why in the conditions the shortest optical path between two points is curved?
The only possible explanation is that a field of gravity warps space itself, as if asking a ball in the middle of a sheet held by its four corners. In such curved spaces, the straight line is not necessarily the shortest path between two points and it is possible that the trajectory of light is curved. As described in the excellent blog Science amazing, the deflection of light in the vicinity of very massive galaxies may even cause some impressive astronomical mirage: the famous Einstein Cross and four stars ghosts or what would see if a black hole was between us and a galaxy (source: Wikipedia ) 
Bob is desperate. It's been several hours he tries to whether or not it is put on the damn planet. Communication with Earth is almost HS: Bob can only send signals in Morse spend Beep Beep Beep ... Not practical for cause, but perhaps is it enough? Bob is the following reasoning. Foo is almost stationary relative to Earth. If it is placed on it, time will flow at his watch at the same rate as on Earth: one minute at his watch exactly correspond to one minute in Houston. If cons, his ship is moving relative to Earth, one minute at his watch will be more than 60 seconds on Earth: it is the laws of relativity, which asserts. "So he told himself, just as I do transmit the ticking of my watch by radio to Earth as they know if I'm moving or resting on Zorglub. They will find out a way to send me the info ... "
Interlude 3: For those interested here is a simple explanation of the phenomenon of time dilation (borrowed from the blog of Alexandre Moatti ) . Readers eager to go directly to the next paragraph without touching 000F 20.
Instead of a clock, reason about the time it takes for a light beam to traverse the space ship in both directions. If the rocket is moving relative to Earth, Bob located inside of the rocket measure a shorter one observer that times this phenomenon from the Earth:
is one of the results the most paradoxical of relativity, a clock that moves seems slow for a stationary observer. Langevin drew a famous paradox: if you send someone flying through space at high speed, it will return to Earth by being younger than his twin brother remained on the spot!
Unfortunately for Bob, his ploy is still doomed for he comes again to forget the equivalence principle of Einstein. The laws of physics are the same in situations of gravity and acceleration situation. Bob's watch, as seen from Earth, appear to slow in all cases, it is placed on a planet or in full acceleration. In other words the gravity also slows the time! The only possibility left for Bob to know where he is in his space odyssey is to open a door and find a reference outside.
Conclusion: move, shake, wiggle! It will keep you young (a tad) longer. As we get older cons bizarrely in his head faster than her legs. Certainly not by much: after 79 years, our head is older than 83 billionths of a second that our feet. Not enough to become a sloth for so little!
source here
Sources: Science et Vie Junior, in January 2011 which are based computer graphics. Related posts:
The principle of least action, a jewel of physics
relativity light without light : how to find the equations of special relativity with no symmetry considerations and without postulating invariance of light.
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