Why did Einstein use light to define simultaneity?
931618
Simultaneity in Special Relativity is based on 2 assumptions:\n\n- the laws of physics apply equally to all inertial reference frames\n- the speed of light in empty space is constant and independent of the relative motion of its source\n\nThis implies that there is no absolute simultaneity, meaning that two events that happens at the same time in one inertial frame may not happen at the same time in another inertial frame.\n\nSo Einstein is NOT defining Simultaneity based on the invariance of the speed of light in all inertial frames, but instead the failure of absolute Simultaneity is a consequence of the invariance of the speed of light in all inertial frames.\n\nSo why does the invariance of the speed of light in all inertial frames imply the lack of Simultaneity? The argument for this is exactly the same as why there can be no absolute reference frame if we assume that the speed of light is constant in all reference frames.\n\nThe only way for the speed of light to be constant in all reference frames is for distance and time to be relative and be a function of speed of the reference frame, i.e. speed = distance/time implies that distance must contract and time must dilate if the reference frame is moving with a certain speed.\n\nSo if distance and time is different in one reference frame than another, then there is no way that two observers, each in separate reference frames, can agreed on when some event has happened. Thus simultaneity is violated.
992432
Your question is similar to Einstein's question that led to a revolution in physics, so it's not a stupid question by far. Einstein asked, what would happen if someone could travel along a beam of light? What would the traveler see? \n\nThink of relative motion. When I'm on a train, watching the country side go by, it appears to me that everything else is moving and I, the tables, the chairs, and the people on the train are not moving. If I were standing on the ground watching the train go by, it would be the oppisote: I would appear, to myself, to be standing still, and the train, people on it, chairs and tables would all appear to be moving. Relative motion, hense the term "relativity."\n\nWhat Einstein discovered is that even if one could travel at the speed of light, that person would observe the light in your headlight traveling away from him AT THE SPEED OF LIGHT. Weird, huh? Think about it. If you're on a ski slope and an avalance is chasing you, what are you going to do? You are going to RUN. Why? It decreases the rate at which the avalance is approaching you. If the avalance is traveling 20  mph and you can run 15 mph, the avalance is only gaining on you at a rate of 20 mph - 15 mph = 5 mph. You slow it's approach to you. However, that does not work with light. If you ran away from a light beam, and managed to travel at the speed of light, the light beam behind you would STILL BE APPROACHING YOU AT THE SPEED OF LIGHT. Light speed is constant in every reference frame, no matter WHAT the speed of the observer. \n\nIn short, you would see the light in the headlight traveling away from you at the speed of light.
630475
Theory of Relativity- Light is the one speed that does remain constant.
801276
Light is a particle. Wave is its function.
176321
According to Wikipedia ( http://en.wikipedia.org/wiki/Theory_of_relativity ):\n"Albert Einstein's theory of relativity, or simply relativity, refers specifically to two theories: special relativity and general relativity. As a field of study, relativity also includes metric theories of gravitation in which special relativity applies locally.\nThe term "relativity" was coined by Max Planck in 1908 to emphasize how special relativity (which at that time was the only relativity theory)\n uses the principle of relativity.\nSpecial relativity\n\nMain article: Special relativity\nEinstein's 1905 paper "On the Electrodynamics of Moving Bodies" introduced the special theory of relativity. Special relativity considers that observers in inertial reference frames, which are in uniform motion relative to one another, cannot perform any experiment to determine which one of them is "stationary". This is known as the principle of relativity. While this principle was not new to Einstein's work, he found that including electromagnetism in this principle required a new formalism with many surprising consequences. In particular, it required the speed of light in a vacuum to be the same for all these observers, regardless of their motion or the motion of the source of the light.\nOne of the strengths of special relativity is that it can be derived from only two premises (although Einstein did not derive it this way):\nThe speed of light in a vacuum is constant (specifically, 299,792,458 meters per second).\nThe laws of physics are the same in any inertial frame of reference. This means that the laws of physics observed by a hypothetical observer traveling with a relativistic particle must be the same as those observed by an observer who is stationary in the laboratory.\nThe famous equation E=mc² (the energy contained in the rest mass of an object is equal to the product of its mass and the square of the speed of light) is derived from this theory.\n[edit]\nGeneral relativity\n\nMain article: General relativity\nGeneral relativity was developed by Einstein in the years 1911 - 1915. General relativity is a geometrical theory which postulates that the presence of mass and energy "curves" spacetime, and this curvature affects the path of free particles (and even the path of light). It uses the mathematics of differential geometry and tensors in order to describe gravitation without the use of the force of gravity. This theory considers all observers to be equivalent, not only those moving with uniform speed."
283968
Because its the formula that summarizes his relativity theory... meaning energy is mass times the speed of light elevated to the second potence.\n\nThe formula was said the first time when ladies in NYC went to receive Dr. Einstein at the port and one of them made a stupid question.\n\n"Dr. How can you summarize your theory?"\nEinstein said "E=mc^2, have a nice day"
979220
Albert Einstein answered this question a long time ago.  He theorized (and every experiment since then has borne him out) that the speed of light is constant from ANY inertial (non-accelerating) frame of reference.  This includes the speed of light relative to your headlights.  \n\nLight speed is constant.  What changes is time and length.  Weird but true.
61320
Michaelson and Morely did some experiments to try to see if the speed of light incipient on the earth was different for light coming from different directions.  Th idea was that if light travels in waves (as it seems to in many regards), then these waves needed to travel through a medium (like when sound waves travel through the air).  This medium was termed "the ethereal ether" and was thought to pervade space, including places with a vacuum; light was thought to be a wave travelling through it.  So Michaelson and Morely decided to try to see our speed relative to the ether by using somthing called an interfereometer.  This compared the speed of lightcoming from different directions.  The idea was that a star we are headed towards should reveal a higher speed of light, while ones we're rushing away from should have a lower speed, due to the Doppler effect.  What they found was that the speed was the same in all directions, meaning that if there was an ether, then we we perfectly at rest relative to the ether.\n\nThen mathematicians and physicists looked at the theories of Scottish physicist James Clerk Maxwell.  Maxwell had taken all existing theories on electricity and megnetism and unified them into a single, cohesive set of equations.  These equations showed how electricity could create a magnetic field, and a magnetic field could induce an electric current.  This could happen in such a manner that an electric field would induce a magentic field, which in turn induced an electric field, which in turn induced a magnetic field, and so on.  The equations made it clear that this wave would travel at a certain specific speed.  Incidentally, that speed was similar to the speed of light measured by French scientist Fizeau in 1849 (several decades earlier) and confirmed by Foucault.\n\nIt was all of this that led Einstein to formulate his special theory of relativity.  Since light moved as an electromagnetic wave, and since those waves had a speed that was predicted directly by Maxwells equations and was independent of the velocity of the observer, Einstein came to recognize that simultaneity was not a universal concept, and that time changed with velocity.  The idea of the ether was discarded as needless baggage.
830585
Y'know, I've been trying to get a handle on that for a long time, but so far I understand it to mean that post-Einstein, once you  include time as a fourth axis to the 3 spatial ones, the universe makes sense in terms of our physical laws working everywhere and everytime.  That is a neccessity of a good physical law - it must always work no matter where in the universe you test it or how fast you're travelling.  In other words, translations and rotations of objects or fields or whatever must have no effect on the validity of the law.  If you do not include time as an axis perpendicular to the 3 spatial ones (mathematically, of couse), what we predict won't agree with what we see, especially in such cases as high relative velocities (as in approacing the speed of light), and strong gravitational fields.  Amoung the oddest result of this view is that, in truth, all time actually exists simultaneously, and there is some not-quite-understood mechanism that only lets us see one instant at a time.\n\nAnyway, to answer your question, "How does spacetime work?", the answer is, much better than space or time alone and separate.  They are, in fact, as inseparable as the pair of poles of a magnet.\n\nThis all might be not quite accurate, especially since I'm so far just an "armchair physicist" with no formal education in the area, (tho I'm looking forward to taking physics, including the higher levels, if I can ever get thru these friggin' calculus classes).  Anybody else have something to contribute or set me straight on??
1018446
I know Einstein came up with it and it deals with energy.\n\nEnergy(E) = Mass(m) * Speed of Light(c2)
30733
If you were traveling at the speed of light, whether or not your lights work would probably be the least of your concerns.\n\nIn reality, the laws of physics prevent any object from traveling faster than the speed of light.\n\nSomething that is perhaps just as interesting, however, is what happens as your vehicle APPROACHES the speed of light.  Take a look at some of the work done by Einstein to get a general idea on that.
25302
There isn't an easy explanation, though it isn't that hard either.  \n\nThe whole idea of special relativity is that all motion is relative to the speed of light and that nothing with mass can travel that speed.  So, in answer to your first two questions, the speed of the light in both directions is still the same as it would be if you were standining still.  \n\nWhat Einstein proved was that there are no fixed points in space, but that space and time are constantly streching and contracting to accomodate the fixed speed of light.  The important thing to understand is that distances of space and lengths of time are different depending upon your relative motion.  I hope that begins to answer your question.
202455
That's nearly a philisophical question . . .\n\nIt's like asking "what is magnetism? I know what it does and how radios work and how to generate electricity, but what is it?" \n\nor \nasking "what is blue? I can see it and know it when I see it, and I know the wavelength of light that is associated with it, but what is it?"\n\nHaving said that, I still think that it is a very relevant question, but I don't have an answer beyond what the standard physics book says . . . which is actually a bit of a tautology (circular reasoning). \n\nPhysics book: Gravity is the tendency of an object with mass to attract another. Mass a measure of the inertia of an object when a force (such as gravity) is acting on it. Gravity begets mass which begets gravity which begets mass ad infinitum.\n\nThe warping of spacetime very simply implies that time is another dimension. I know that statement sounds very generic becuase it's been used in every sci-fi show in the past 50 years to explain alternate universe and the like, but all it means is that we move through time just like we move through space. Recall from basic right triangle trigonometry, that if something is moving in the X direction and the Y direction at the same time, then we can solve for the total motion and the direction of the motion using basic trigonometry. Now the same can be done for motion in three dimensions. It takes more triangles and more math but the net speed and direction can be calculated. And the same can be done for four dimensions. We can't draw it, but we can do the math for it even if we can't visualize it.\n\nNow let's make some assumptions:\n1. We always move at the speed of light. Always.\n2. That fourth dimension is time.\n\nSo now, if we always move at the speed of light (call it 'c'), then what happens when we are not moving in any of our three normal directions? We must be moving through the dimension of time at speed 'c.' What happens if we start moving in the three other dimensions? Then we must slow down a bit in the dimension of time, because the total speed in all dimensions in always c. What happens if we move through the three other dimensions at speed c (theoretically impossible, but an important thought problem), then we stop moving in the fourth dimension, the time dimension . . . hence the notion that time will stop for someone moving at the speed of light. (Now you know something about Special Relativity).\n\nGravity "warps" the dimensions. It changes their shape . . . So all the math we used to describe or speed of travel through the other dimensions no longer applies. In particular, the time dimension. This is what some of the other answer mean when they say gravity warps spacetime. It changes the shape of the dimensions, they are no longer linear, so the math we just did does not apply without some special corrections to account for gravity.\n\nI know that's probably not the explanation that you were looking for but it tries to describe how gravity affects time and space.\n\nAnd don't be fooled by explanations of an expanding universe that say hold something infront of your face and let it go . . . that makes absolutely no sense. If the universe is expanding (most scientists believe that it is), then when the universe expands, and you hold something in front of your face, the thing expands, you expand, the space between you and the thing expands, the yardstick or meterstick expands, etc. Since everything is exapnding at the same rate invluding you, there is no way to visually measure the expansion. Relative to you, everything always appears to be the same size. In fact since the whole universe is expanding, relative to the whole universe, everything appears to be the same size.
880820
Einstein has 2 theories of realivity.  The Special theory of relativey and the General theory of realtivity.  \n  The first simply means this in layman's terms.  All laws of physics laws are valid given that the state of object is in Uniform motion.  (uniform motion means a constant state of acceleration)\n  The General theory of relativity is all about gravity.  Basically it is a theory to explain the force of gravity.  \n   Although Neither theorys specifically mention time travel, many people have used the specifics of these theorys to try to show whether time travel is indeed possible.  \n    One of the principals of the general theory is that no object can travel faster than the speed of light.  Although this seems like qquite a simple statment it really isn't.  If you think about light and the things you are seeing "lets say your looking at the stars"  The light coming from those stars has traveled billions of miles to get to earth.  You are in fact seeing the light that a certain star has emmited in the past.  Now if you were to travel faster than light you could in effect travel through time and reach light events that people have yet to see.  \n   Now at face value you would say well since the theory of relativly states you can't travel faster than light this is impossible.  Here lies the catch.  The speed of light is always the speed of light (meaning if you were in car traveling at the speed of light down the road and you turned your headlights on the speed of the light from the headlights would still travel away from the car at the speed of light).\n   Now let's say your a person standing on some point and were watching the car travel at the speed of light.  The light that is coming from the cars headlights would to you seem to be traveling at 2times the speed of light. Get it?  \n   So when speaking relativisticly the frame of refrence is the most important factor.  Now one thing to note that time is NOT a constant.  It has been proven that the faster an object moves the slower time is for that object.  This indeed does work for everyday activities such as your car driving down the road.  Time for you is accually an extremely tiny bit slower than it is for the tree that you past on the road. \n    SO if we "could" get an object to move the speed of light or greater time could either stop or go in reverse for that object.  We cannot get matter to the speed of light at this time.  We have gotten close, but right now the laws of physics seem to prevent that.
127218
Relativity, as known to Galileo and Newton, refers to the idea that motion is relative to the observer.  Giving a simple example, on a calm day standing outside my house I think the air is not moving, while my friend riding in the back of a pickup truck feels the air blowing in his face.  However, both Newton and Galileo still believed there existed an "absolute" motion, as measured by the unchanging framework of the universe.\n\nWith the theory of Special Relativity in 1905, Einstein changed all that and revolutionized physics.  The main idea of special relativity is that the laws of physics should be the same for all observers moving at a constant speed. (Acceleration is not addressed in Special Relativity, but both acceleration and gravity are included in General Relativity, which Einstein formulated 10 years later.)  According to Special Relativity, there are no such things as absolute rest or absolute motion - speed and motion can only be measured against an arbitrary frame of reference, and one reference frame is as valid as any other.  Special Relativity also includes the concept that light travels at an absolute speed which is the same for all observers, regardless of the motion of the observer or the light source. Building on this concept, Special Relativity shows us that not only is motion relative, but so are many other familiar concepts like distance, time, and even energy.  In other words, different observers moving at different speeds will measure quantities such as length, time, and energy differently.  But each observer's measurement is equally valid.  It is the relativity of time that gives us the famous "twins paradox", where an astronaut travelling at very high speeds will age less rapidly than his twin back home on Earth.  Finally, Special Relativity gives us  the famous equation E=mc2, which establishes the equivalence of mass and energy.  All of these ideas were revolutionary and marked the end of Newtonian physics, which had reigned unchallenged for nearly 300 years.
242501
Relative motion between objects was not new to Newton and his contemporary scientists.\n\nEvery experimental result proved and confirmed again and again Newton’s laws of motion even in the frames of reference moving relative to each other.\n\nLatter with the improvement in measuring technology and particles moving nearly equal to the velocity of light, their behavior contradicted Newton’s laws.\n\nEinstein with a firm belief that any physical law (including Newton's law) must be the same in any reference frame moving relative to each other postulated that speed of light is independent of the source of light.\n\nThis explained every experimental result well and obeyed Newton’s laws.\n\nThe only difference is that  the basic quantities mass , length and time which during Newton’s time was thought of absolute in the sense that  they will not change with the velocity of the frame in which they are , is now considered to vary with the speed of the reference frame, and the velocity of light is independent of the motion of the source.
1024005
Einstein did not believe in quantum mechanics, he developed relativity, which says nothing can go faster than light.\n\nThis is not a simple subject, but you might try the source below.