■ Every experiment ever conducted showed that the speed of light is always the same. Ether, which humans again created, because we were desperate for a reference system, was not detectable. Also, if we moved relative to ether and measured the speed of light relative to ether, as we moved, we still got the same number for the measured speed of light. (In fact we were so desperate that we said ether was magical. If we moved with respect to it, “it” knew about our movement and adjusted itself. This is what Lorentz proposed and he precisely came up with all the equations Einstein did but his theory needed ether to be magical and almost have a brain of its own. This is why we call the length contraction and time dilation equations Lorentz transformations. His formulae were right. His theory seemed flawed for non believers in magical ether.

■ Although, measurement of speed is always relative and with respect to something. So if an object is moving, its speed can only be determined with respect to another object.So, measurement of speed is always relative BUT the measured speed of light with respect to anything else (moving or otherwise) always came out as a constant experimentally a contradiction!So, the absoluteness of time had to be questioned & that’s what Einstein did. Instead of freezing the manner in which time flows, he proposed freezing the ratio of distance covered by electromagnetic radiation in a given time period to that time period itself as a constant.This helped! Time flow could be adjusted to explain the constancy of the speed of light.When no one (such as Lorentz, Michelson-Morley) could let go off a reference system that explained everything before him/her, Einstein did. This is where he stands out.

**The Twin Paradox**

The key to understanding Special Relativity is trying to understand and think about what 'flow of time' actually is.

The paradox is usually described as a thought experiment involving twins. They synchronize their watches. One twin gets into a spaceship and makes a long trip through space. When he returns, the twins compare watches. According to the special theory of relativity, the traveller’s watch will show a slightly earlier time. In other words, time on the spaceship will have gone at a slower rate than time on the earth. So long as the space journey is confined to the solar system, and made at relatively low speeds, this time difference will be negligible. But over long distances, with velocities close to that of light, the "time dilation“ can be large. It is not inconceivable that someday a means will be found by which a spaceship can be slowly accelerated until it reaches a speed only a trifle below that of light. This would make possible visits to other stars in the galaxy, perhaps even trips to other galaxies. So, the twin paradox is more than just a parlour puzzle; someday it may become a common experience of space travellers. Suppose that the astronaut twin goes a distance of a thousand light-years and returns: a small distance compared with the diameter of our galaxy. Would not the astronaut surely die long before he completes the trip? Would not his trip require, as in so many science-fiction stories, an entire colony of men and women so that generations would live and die while the ship was making its long interstellar voyage? The answer depends on how fast the ship goes. If it travels just under the limiting speed of light, time within the ship will proceed at a much slower rate. Judged by earth-time, the trip will take more than two thousand years. Judged by the astronaut on the ship, if he travels fast enough, the trip may take only a few decades! An astronaut travels from the earth to the spiral nebula in Andromeda. Assume that the nebula is 1.5 million light-years from the earth (a conservative estimate; some astronomers believe it is closer to 2 million) and that the ship travels at such speed that the astronaut ages fifty-five years while making the trip there and back. When he returns, he finds that on the earth 3 million years have gone by!

You can see at once that this raises all sorts of fascinating possibilities. A scientist of forty and his teen-age laboratory assistant fall in love. They feel that their age difference makes a marriage out of the question. So off he goes on a long space voyage, traveling close to the speed of light. He returns, age forty-one. Meanwhile, on the earth his girlfriend has become a woman of thirty-three. Perhaps she could not wait fifteen years for her lover to return; she has married someone else. The scientist cannot bear this. Off he goes on another long trip. Moreover, he is curious to know if a certain theory he has published is going to be confirmed or discarded by later generations. He returns to earth, age forty-two. His former girlfriend is long since dead. What is worse, his pet theory has been demolished. Humiliated, he takes an even longer trip, returning at the age of forty-five to see what the world is like a few thousand years hence. This kind of time travel avoids all the logical traps that plague science fiction, such as dropping into the past to kill your parents before you are born, or whisking into the future and shooting yourself between the eyes. Consider, for example, there was a young lady named Bright, who traveled much faster than light. She started one day in the relative way, And returned on the previous night. If she returned on the previous night, then she must have encountered a duplicate of herself. Otherwise it would not have been truly the night before. But there could not have been two Miss Brights the night before because the time-traveling Miss Bright left with no memory of having met her duplicate yesterday. So, you see, there is a clear-cut contradiction. Time travel of that sort is not logically possible unless the existence of parallel worlds running along branching time tracks is assumed. Einstein's form of time travel does not confer upon the traveler any genuine immortality, or even longevity. As far as he is concerned, he always ages at the normal rate. It is only the earth's "proper time" that for the traveler seems to gallop along at breakneck speed.

In the 1970s, Herbert Dingle, an English physicist, refused to believe the paradox. For years he wrote witty articles about it, accusing other relativity experts of being either obtuse or evasive. The superficial analysis to be given here will not clear up this controversy, which quickly plunges into complicated equations, but it will explain in a general way why there is almost universal agreement among experts that the twin paradox will really carry through in just the manner Einstein described. Dingle's objection, the strongest that can be made against the paradox, is stated this way. According to the general theory of relativity, there is no absolute motion of any sort, no "preferred" frame of reference. It is always possible to choose a moving object as a fixed frame of reference without doing violence to any natural law. When the earth is chosen as a frame, the astronaut makes the long journey, returns, finds himself younger than his stay at-home brother. All well and good. But what happens when the spaceship is taken as the frame of reference? Now it must be assumed that the earth makes a long journey away from the ship and back again. In this case it is the twin on the ship who is the stay-at-home. When the earth gets back to the spaceship, will not the earth rider be the younger? If so, the situation is more than a paradoxical affront to common sense; it is a flat logical contradiction. Clearly, each twin cannot be younger than the other. Dingle likes to state it this way: Either the assumption must be made that after the trip the twins will be exactly the same age or relativity must be discarded. Without going into any of the actual computations, it is not hard to understand why the alternatives are not so drastic as Dingle would have us believe. It is true that all motion is relative, but in this case, there is one all-important difference between the relative motion of the astronaut and the relative motion of the stay-at-home. The stay-at-home does not move relative to the universe. How does this affect the paradox? Assume that the astronaut is off to visit Planet X, somewhere in the galaxy. He travels at a constant speed. The stay-at-home's watch is attached to the inertial frame of the earth, on which there is agreement among clocks because they are all relatively motionless with respect to each other. The astronaut's watch is attached to a different inertial frame, the frame of the ship. If the ship just kept on going forever, there would be no paradox because there would be no way to compare the two watches. But the ship has to stop and turn around at Planet X. When it does so, there is a change from an inertial frame moving away from the earth to a new inertial frame moving toward the earth. This shift is accompanied by enormous inertial forces as the ship accelerates during the turnaround. In fact, if the acceleration during the turnaround were too great, the astronaut (and not his twin on the earth) would be killed. These inertial forces arise, of course, because the astronaut is accelerating with respect to the universe. They do not arise on the earth, because the earth is not undergoing similar acceleration. From one point of view it can be said that the inertial forces produced by this acceleration "cause" a slowing down of the astronaut's watch; from another point of view the acceleration merely indicates a shift of inertial frames. Because of this shift, the world line of the spaceship - its path when plotted on Minkowski's 4-D graph of space time - becomes a path on which the total "proper time" of the round trip is less than the total proper time along the world line of the stay-at-home twin. Although acceleration is involved in the shifting of inertial frames, the actual computation involves nothing more than the equations of the special theory. Dingle's objection still remains, however, because exactly the same calculations can be made by supposing that the spaceship instead of the earth is the fixed frame of reference. Now it is the earth that moves away, shifts inertial frames, comes back again. Why wouldn't the same calculations, with the same equations, show that earth-time slowed down the same way? They would indeed if it were not for one gigantic fact: when the earth moves away, the entire universe moves with it. When the earth executes its turnaround, the universe does also. This accelerating universe generates a powerful gravitational field. As explained earlier, gravity has a slowing effect on clocks. A clock on the sun, for instance, would tick more slowly than the same clock on earth, more slowly on the earth than on the moon. Now, it turns out, when all the proper calculations are made, that the gravitational field generated by the accelerating cosmos slows down the spaceship clocks until they differ from earth clocks by precisely the same amount as before. This gravity field has, of course, no effect on earth clocks. The earth does not move relative to the cosmos; therefore, there is no gravitational field with respect to the earth. It is instructive to imagine a situation in which the same time difference results, even though no accelerations are involved. Spaceship A passes the earth with uniform speed, on its way to Planet X. As the ship passes the earth it sets its clock at zero time. Ship A continues with uniform velocity to Planet X, where it passes spaceship B, moving with uniform speed in the opposite direction. As the ships pass, A radios to B the amount of time that has elapsed since it passed the earth. Ship B notes this information and continues with uniform speed to the earth. As it passes the earth it radios to the earth the length of time A took to make the trip from the earth to Planet X, together with the length of time it took B (measured by its own clock) to make the trip from Planet X to earth. The total of these two periods of time will be less than the time (measured by earth clocks) that has elapsed between the moment that ship A passed the earth and the moment that ship B passed the earth. This difference in time can be calculated by the equations of the special theory. No accelerations of any sort are involved. Of course, now there is no twin paradox because there is no astronaut who goes out and comes back. It can be supposed that the traveling twin rides out on ship A, then transfers to ship B and rides back, but there is no way he can do this without transferring from one inertial frame to another. To make the transfer he must undergo incredibly strong inertial forces. These forces indicate his shift of inertial frames. If we wish, we can say that the inertial forces slow down his clock. However, if the whole episode is viewed from the standpoint of the traveling twin, taking him as the fixed frame of reference, then a shifting cosmos that sets up gravitational fields enters the picture. Regardless of the point of view adopted, the equations of relativity give the same time difference. This difference can be accounted for by the special theory alone. It is only to counter the objection raised by Dingle that the general theory must be brought into the picture. It cannot be stated too often that it is not correct to ask which situation is "right": Does the traveling twin move out and back or do the stay-at-home and the cosmos move out and back? There is only one situation: a relative motion of the twins. There are, however, two different ways of talking about it. In one language, a change of inertial frames on the part of the astronaut, with its resulting inertial forces, accounts for the difference in aging. In the other language, gravitational forces overbalance the effect of a change of inertial frames on the part of the earth. From either point of view, the stay-at-home and the cosmos do not move relative to one another. Thus, the situation is entirely different for each man, even though the relativity of motion is strictly preserved. The paradoxical difference in aging is accounted for, regardless of which twin is taken to be at rest. There is no need to discard the theory of relativity. An interesting question can now be asked: What if the cosmos contained nothing except two spaceships, A and B? Ship A turns on its rocket engines, makes a long trip, comes back. Would the previously synchronized clocks on the two ships be the same?

The answer depends on whether you adopt Eddington's view of inertia or the Machian view of Dennis Sciama. In Eddington's view the answer is yes. Ship A accelerates with respect to the metric space time structure of the cosmos; ship B does not. The situation remains unsymmetrical and the usual difference in aging results. From Sciama's point of view the answer is no. Acceleration is meaningless except with respect to other material bodies. In this case, the only material bodies are the two spaceships. The situation is perfectly symmetrical. In fact, there are no inertial frames to speak of because there is no inertia (except an extremely feeble, negligible inertia resulting from the presence of the two ships). In a cosmos without inertia it is hard to predict what would happen if a ship turned on its rocket motors! As Sciama says, with British understatement, "Life would be quite different in such a universe". Because the slowing of the traveling twin's time can be viewed as a gravitational effect, any experiment that shows a slowing of time by gravity provides a kind of indirect confirmation of the twin paradox.

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