First of all I am familiar with Einstein's Special Theory of Relativity, but I have noticed an inconsistency that is not addressed, so I figure that I must be missing something. Einstein's theory says that you move toward a clock at high speed, you not only see the hands of the clock speed up, but time for the clock relative to the observer really is speeding up, effectively sending the traveler into the future. So for example, if an clock is 1 light-hour away, and you are traveling at (.5)c towards it, it will take 2 hours to reach it, but the time elapsed off the clock will be much greater, and you would actually see the hands of the clock speed up as you moved toward it.\n\nHowever...\n\nWhat if the observer is stationary and the clock is moved toward him? To the observer, logic would tell you that the clock would still speed up, even though Einstein's theory would say that it slows down. How soes relativity know who is doing the moving?
124650
You've confused the statement of the theory. Let two observers be in the same inertial reference frame with two synchronized watches. One observer accelerates in a negligibly small amount of time to some fraction of the speed of light relative to the other observer. The latter observer will be called O1 and the moving observer O2. \nO2 will measure the time in 01's reference frame to be going *slower* than the time in his frame. Likewise, O1 will measure the passage of time in 02's frame to be going *slower* than in 01's frame (the half-life of particles "increases" depending on their speeds). In other words, given two events A and B that always occur t seconds apart in O, they will occur at some separation T > t when measured from O'. The same will occur if O' is switched with O in the sentence above.\nQuestions like this usually come from confusing the relativity of simultaneity. Remember that in relativity, there are no two events that are simultaneous with respect to all inertial frames. \nI recommend you check out the text "Spacetime Physics" by Wheeler et al.
625984
Here's the equation T = T' / sqrt[1- (v^2/c^2)]\n\nT is time measured by an observer at rest.  T' (read: T prime) is time measured by a moving observer.  v is the moving observer's velocity and c is the speed of light.\n\nThe speed of light is very fast.  So if v is 100 mph or 10 000 mph when you divide it by a number like c (186000 mps that's miles per second so a huge amount of miles per hour)  the v^2/c^2 value is zero and T = T'/sqrt (1) which is T = T'/1 so there is no measurable difference.  \n\nBut plug in 9/10 c for v.  That means an object going 9/10 the speed of light.  You get T = T' / sqrt[1-(9/10c)^2/c^2]\n\nc^2 cancels out and you get T = T'/[sqrt 1-(81/100)] T would be 2.2 times T'.  More time transpired for the non moving observer.\n\nHow did we get the equation?  Einstein was really smart.
453422
I think what you are discussing here has nothing to do with relativistic physics. It has everything to do with the time it takes for light to propagate through space. (That wasn't meant to sound paradoxical!) \n\nConsider the equivalent in the audible world. You are blind, and a train departs a station a "sound hour" away. (A sound hour is the distance sound travels in an hour) Every minute on the driver's watch, the train driver blows its whistle. The train remarkably travels at half the speed of sound from the instant it leaves the station to the instant it arrives at your doorstep. Now, when the train reaches halfway, it will have been travelling for an hour, and will therefore have blown its whistle 60 times, however you will not have heard the first toot yet. Once past the halfway point, you will hear the whistle blowing every 30 seconds, NOT every minute, since the train is catching up with the whistle toots as it travels along. For the remainder of the train's journey, you will hear the toots every 30 seconds, so you will still hear 120 toots, but they will occur once every 30 seconds according to your watch. So, from the train driver's point of view, he has travelled for two hours, tooting every minute. From your point of view, the train seemed to begin its journey an hour ago, but tooted its whistle every 30 seconds. The trick is that the doppler shift in the frequency of the whistle gives away the fact that the train is moving!! If you and the train driver agreed that the pitch of the whistle is 200Hz, and you hear the toot as a 400Hz tone, you know that the train is approaching at half the speed of sound! So you can incorporate that into your time calculations. On the other hand, if you agreed that the whistle will be blown every minute, you can also factor that into your calculations of time. \n\nSimilarly for the visible / spaceship analogy, the blue-shift in the light from the spaceship will tell you that the speed of the spaceship is tricking your mind!
23952
That would be Einstein's Special Theory of Relativity.\n\nhttp://science.howstuffworks.com/relativity3.htm
424714
You misunderstand Einstein's Theory of Special Relativity. It doesn't say that you can't have something move faster than the speed of light. Only "information" can NOT move faster than the speed of light. By information, I mean any "signal", like electromagnetic waves (which includes light, radio waves, etc. etc.). Any material object within the Universe are also restricted by Special Relativity, since material objects also carry "information".\n\nFor the Universe or the fabric of spacetime to be expanding faster than the speed of light is NOT a violation of Special Relativity, and the Universe will not be moving backwards in time.\n\n--------------------------------------------------------\n\nEdit - Ok, here let me clarify why I said that Special Relativity (SR) does NOT apply to the expansion of the Universe:\n\nVelocity of something depends on which definition of distance we use. So, if we assume that the distance of an object at time t is the distance from our position at time t to the object's position at time t measured by a set of observers moving with the expansion of the Universe, and all making their observations when they see the Universe as having age t, then the velocity (change in D per change in t) can definitely be larger than the speed of light. This is not a contradiction of special relativity because this "distance" is not the same as the spatial distance used in SR, and the age of the Universe is not the same as the time used in SR.\n\nIn the special case of the "empty Universe", where one can show the model in both special relativistic and cosmological coordinates, the velocity defined by change in cosmological distance per unit cosmic time is given by v = c ln(1+z), where z is the redshift, which clearly goes to infinity as the redshift goes to infinity, and is larger than c for z > 1.718.\n\nFor the "critical density Universe", this velocity is given by v = 2c[1-(1/sqrt(1+z))], which is larger than c for z > 3. \n\nFor the "concordance model" based on CMB data and the acceleration of the expansion measured using supernovae, a flat Universe with Omega = 0.27 (this would be our Universe), the velocity is greater than c for z > 1.407\n\nAll this just means that for any expanding universe, there will be a value of z (i.e., redshift or cosmological distance) that will give the appearance of galaxies moving at velocities greater than c.
587704
I saw a lot of good answers out there that hit the answer pretty much on the head.  I only wanted to point out one minor mistake in what you wrote (which you probably won't run into in most freshman courses in relativity).  That the astronauts would actually be older than those who stayed on Earth.  The special relativity people were explaining was correct.  If you move => time slows down.  The thing is that gravity has a greater effect in slowing down clocks and the gravitational field you experience in orbit is weaker than on the surface of the Earth (obviously it won't matter though if you are flying around space at speeds near the speed of light, then the special relativity part becomes dominant over the general relativity i.e. gravity).
539547
Travelling back in time is not allowed by einstein's theory of relativity...however if you can instantly teleport youself to some faraway star,say a 101 light years away, you might be able to catch einstein receiving his nobel prize!! as stated earlier, there is no master grandfather clock that is regularly ticking away the seconds..rather that rate is adjustable...but one may zip into the future..consider an example where you are on a space-ship at a speed of 0.99990 c that is going away from the earth and coming back..in your time frame say,it takes about twenty years..using lorentz transformations, the time for your to and fro motion is close to 450 years!!
9361
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.
312536
*sighs* Why can't people do any research on their own? Anyway... you are traveling through time right now, how else did you push those keys in sequence to form your question?\n\nI think that what you are attempting to refer to (lol I'm never gonna get a best answer replying like this) Is that as you travel faster and faster, closer and closer to the speed of light, time flow for you slows down, and the misconception of the public that if you go faster than light you will reverse time. WRONG!\n\nThat's not specifically what you asked though, so I will focus on teh misconceptions regarding the speed of light. First of all, accelerating mass to the speed of light is impossible as it would require an infinite amount of energy, so your question is meaningless. Second, since your question is meaningless tehre is nothing more to say except maybe a supporting and enlightening example. Third, I do not feel creative tonight so that example is gonna have to pass tis time, unless you email me and I will explain it better.\n\ndavid_lapierre@sbcglobal.net\n\nUnless this is just a school project, which I doubt since they are going donw the drain is sciences these days... you might wanna ask me a question, I can be very helpful when I know my time is not going to be wasted on a person who won't bother to take a few minutes to learn something... :-)
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1. Time is a variable and not a constant. It ran backward only in Harry Potter's Vol 3 using Time Machine\n2.The perpendicular distance is the shortest distance\n3.Why not\n4.The variable nature of time can be felf only with objects moving with a velocity that approaches that of light. The theory of Relativity cannot be proved wrong because no one has understood it completely
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Wow.\n\nA few people here know what they're talking about.  A few.  Very few.\n\nMost of these responses are absolutely hilarious.  Most of them are from people trying to sound smart, in which case they do NOT.  Other cases are from kids with big imaginations, with "Anything is possible!"\n\nEinstein was brilliant, no question.  But all great scientists are capable of mistakes, and Einstein was no exception.\n\nHis theory on time travel was based on the speed of light and the "paradox" in which light travels at a fixed rate of speed.  This is true; light moves at 186,000 mps no matter what conditions are in place.\n\nHis theory's flaw is accounted to the fact that light has NO mass.  Light, in it's fixed movement, cannot be compared to any living organism (which is made up of mass) or any other object.  Any object made up of mass (for example, a human being, or any kind of lightspeed-capable rocket ship) does NOT have a fixed speed; therefore the velocity equation paradox that Einstein based his theory on does not apply to them.\n\nThis is all I want to say tonight.  Anyone who understands his theories will understand what I've said here, so think about my statements.  Let me know what you think (again, only those of you who understand what Einstein's theories were all about), as I'm no expert on relativity.  Like everyone else posting on a Yahoo answers forum about Albert Einstein and his theories on time travel, I'm just a fan.  Don't act like any of us are any more.\nEveryone else:  Please don't worry about it.\n\nAlthough I am skeptical by nature, like any smart person, it is NOT my goal to bash the idea of time travel.  After I spotted the flaw in Einstein's theory I presented it to many professionals in the field (or the few that I know, anyway) and have so far received very positive feedback.  I searched online as well and have discovered that others seem to have come up with the same idea, or similar.  Let me know what you think about the whole deal.\n\nBeing proven wrong only makes you smarter.
590909
GOOD QUESTION: well done! There is not really any such thing as time, it is merely our way of describing a particular abstract concept. If it was a solid physical reality it surely couldn't be relative. Lets all go and read Einstein and try to figure it out!
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As the others have said, all light travels at exactly the same speed.  This is even true if the observer and/or the  light source are moving very very fast.  Once this was proved by experiment, everyone was very upset until Einstein figured it out and showed that time and space can distort. If you want a real bad headache, you can read all about this.  If it doesn't give you a headache, become a physicist!
175508
If the clock is constantly moving at constant velocity, then you're right, the people on earth will say the plane's clock is going slower, whereas the people on the plane will say the earth's clocks are slow.\n\nHowever, when the plane starts and stops in these experiments, it undergoes an acceleration, which differentiates the two frames. That's why the plane's clock is behind when it's compared to the earth clock. Basically it's because the plane changes which inertial frames it is a part of when it accelerates. Your point is similar to the twin paradox, I suggest you check out my source, that should clear things up, you sound like you have a solid enough background to understand it.
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I think what you're looking for is\n\nt' = t * sqrt (1 - (v/c)^2)  where c is speed of light, v is the velocity of the observer, t is time external to the observer and t' is the observer's experienced time.\n\nThe problem with this equation is that when v>c, t' goes negative.  This is thought to believe that one can go backwards in time if the speed of light is exceeded.  However, there can also be the interpretation that the speed of light is an asymptotic limit.  In this case, backward time travel is not possible as one could not exceed the speed of light.\n\nIn terms of traveling to the future to go to the past, I'm not sure what exactly you mean.  An early theory posited a cyclic universe so that by traveling far enough in time, you would indeed end up in the past but it would be the past of the universe after the next cycle.
98466
What you are missing is about the reference frame.\n\nYou and the other observer are still in a same reference frame, of course a lengthy frame of reference. The relative speed between you and the other is zero.\n\nBoth of you are having instruments to measure the time and distance.\n\nYou will measure the speed of the rod as 1 inch / sec and he will also measure the speed of the rod as 1 inch/ s.\n\nAfter the rod has moved 1 inch (That is at that instant) both want to inform the calculated speed to the other. You have to rely only on some E. M wave (light). You will receive that information after one year from that instant. Similarly the other will also receive the information from you after one year. \n\n The theory of relativity deals with reference frames moving relative to each other.\n The theory of relativity cannot be applied to a single frame of reference.\n\nI will give another example to clear your doubt.\n\nImagine two railway stations 1000 km apart. The engine is at one station followed by no of carriages with the lost one at the other station. If the engine starts moving both station masters will know this information simultaneously.  But to transmit this information they have to rely upon some radiation which is limited by the speed of light. (note the difference between knowing the information and transmitting the information)\n\n Another thing is that theory of relativity has not contradicted cause and effect. Cause comes first and effect comes after,
437498
It is exactly that question that inspired Einstein to formulate his theory that nothing can ever travel faster than light. I am not saying the Einstein had a goal to make a theory saying nothing can travel faster than light. I am saying that the question made him think about how to explain what would happen in such a case and then the math lead him to a theory of motion in general which says nothing can travel faster than light.