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Time is defined as the change we experience. Which is things being different in two different places in time. Which is annoyingly circular. Why can't we freely move back and forth in time like spatial dimensions? Because then it wouldn't conform to our idea of time. It's perfectly plausible though, to have another kind of being who perceives one of our ...


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Time such a pesky pest To leave it alone will be the best Cause its sure not to let you rest Until your thoughts explode in chest. What you say cannot be refuted. But speed of light alone cannot define it But you are accepting the notion of space , then of course space and motion will together be able to define time. Similarly c can be thought of as a ...


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First of all I would like to thank you for letting me participate of this discussion. I've been trying to get an answer for this question myself for at least six years now. Ever since I've realized that the experiment hasn't been done. Then one day in the same year (2009) I found a video on YouTube made by a German Blacksmith named Martin Grusenick. We've ...


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Newtonian mechanics, with no upper limit on velocities, is perfectly consistent and has no problem talking about time.


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Why isn't time just a function of the speed of light being finite In a way it is. If the speed of light was infinite, everything would happen at once. And it doesn't. But more generally I think it's better to say time is a function of motion. The mechanism of a clock is called a movement. A clock doesn't literally measure the flow of time like it's some ...


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Time is a physical quantity which cannot be defined, just like mass and length. They happen to be the fundamentals of our knowledge regarding understanding of nature. Nobody in this world can define time. Moreover, c ,that is, the speed of light in vacuum is used to define the unit of time second. This is because speed of light in vacuum has been observed ...


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Simple answer: no. Reason: Let's say you are a photon. A mirror is placed in from of you. If you do not smash in to it then it is traveling as fast as you. there for, the photon will never reach it to be redirected backwards.


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There are (at least) a couple problems with your proposals: In experiment A, you start with "a laser ... which can be controlled to emit individual photons." However, such a laser is, as far as I know, impossible to make, because the stimulated emission process that generates the output in the laser is a random process, emitting photons at random times. ...


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If I can expand a little bit on Sofia's answer the polarization of the medium opposes time variations in the electric field thus slowing down the phase velocity of the wave. This can be seen from Ampere's circuit law (the 4th Maxwell equation) which is central as you stated in arriving at the wave equation describing light. It can be written in vacuum as ...


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Although the answers here are good, in my opinion, they're too mathematical for the OP to get it. Before explaining what Brain Greene intended to convey in his article, I'd like to clear up your misunderstanding. There's no such thing as "time speed" vs "space speed", speed is the magnitude of one's velocity, and velocity is the rate of change of one's ...


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The speed of light in a medium depends on the frequency of the electromagnetic radiation $$v(\nu) = \frac{c}{n(\nu)}$$ where $n(\nu)$ is the refractive index. In a general case, $n(\nu)$ is a complex number, and its imaginary part accounts for the absorption of the medium (i.e. if a material is not transparent at frequency $\nu$, then $\textrm{Im } n(\nu) ...


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What you and I view as transparent or opaque are only that way in relation to photons in the visible spectrum (electromagnetic wavelengths between 390 and 700 nm) - this is because electrons in the material interact with those energies. In the case of opaque materials it's a safe bet that if you use a high enough energy photon (ultra violet and beyond) you ...


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Although Chris' answer contains all you need to know to answer your question, I thought I add a few lines to make things more clear. Your question was: Now, when we calculate the path difference between the two light paths, why do we multiply this time difference with the velocity of light $c$? As Michelson didn't know that the speed of light is ...


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How do we know the speed of light is constant and spacetime dilates rather than vice versa? We know that the speed of light is not constant. I'm afraid it's a popscience myth that the speed of light is constant. See Irwin Shapiro talking about it here: Some conspiracy nut was telling me that Einstein was BS and there's a giant conspiracy that he's ...


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You might be confusing some issues. In special relativity, space and time do not stretch or compress. It really comes down to measurements with clocks and rulers made by people that are moving uniformly with respect to each other. One option that is consistent with observations for SR is that there is one family whose clocks and rulers are right and ...


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The point is that it's a test of the spatial consistency (jargon word: "isotropy") of the speed of light. If light takes a different time to go down the two paths then there should be some phase difference between the light rays when they hit the half-silvered mirror; you tune the lengths of the "arms" for some test light (sodium) so that this phas ...


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The key to understanding this somewhat surprising result is that the relativistic velocity addition formula is not applicable to this calculation. As an example of when to apply the velocity addition formula, sssume there is an object with (1D) velocity $\mathbf u$ in some inertial coordinate system. Now, what is the velocity $\mathbf u'$ of that object in ...


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Let me quote the relativistic velocity addition formula for easy reference: $$v_{AB} = \frac{v_A - v_B}{1 + \frac{v_Av_B}{c^2}}\tag{SR}$$ I'm guessing you interpreted these quantities as follows: $v_A$ is the speed of the light beam relative to George $v_B$ is the speed of Gracie relative to George $v_{AB}$ is the speed of the light beam relative to Gracie ...


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The clocks are all in sync in the ground frame, but they are not in sync in the train's frame. An observer on the train would think that the clocks in the front of the train are ahead, while those to the rear are behind. Measuring the forward traveling beam against the nearby clocks will show a long time difference, while measuring the rearward traveling ...


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According to the principle of relativity, motion is relative. You can think about it this way: There could be no physical experiment which would tell you if your reference frame is moving or not. Who told you that we can attach a reference frame to the light ray? This is completely untrue. Yes, both times "slow down" in comparison to each other! And ...


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Why is the speed of light limited to (only) 299.792.458 m/s? Because space is the way it is. The speed of light is related to the permittivity and permeability of space via the expression: $$c_0={1\over\sqrt{\mu_0\varepsilon_0}}$$ See Wikipedia. There's a somewhat similar expression for shear wave velocity in mechanics: $$v_s = \sqrt{\frac {G} {\rho} ...


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Scale is not symmetrical in physics. Say, if you zoom in stuffs, you will eventually see atoms, which is totally different to the "continuous" things you see in your scale everyday. Therefore, there is nothing special to find light move "not impressively" in a cosmic scale. Not to mention that being "fast" or "slow" is a matter of comparison. Besides, ...


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Yes it is possible, but the difference would need to be miniscule. Effectively your question reduces to one of the following: "does light have a truly nonzero rest mass?" and / or "is there a highly diffuse optical medium all around us which modern repetitions of the Michelson-Morley experiment have not yet detected?". Look up "Experimental Checks on Photon ...


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Does the accelerated rate of expansion of the Universe have any effect on the velocity of light? This is the subject of some debate. Check out the Wikipedia Variable Speed of Light article: "The idea from Moffat and the team Albrecht–Magueijo is that light propagated as much as 60 orders of magnitude faster in the early universe, thus distant regions ...


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The expansion of the Universe has no effect on the local speed of light. Any local measurement of $c$ will yield $c$, and $c$ won't change. There is one thing that often causes confusion about the speed of light or faster-than-light travel. A photon moving in an expanding space-time appears to move at an average speed faster than $c$. Consider a ...


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The Michelson-Morley experiment indicated (contrarily to it's original intent), that the speed of light perceived by an observer is not dependent on their speed relative to anything. Along these lines, our movement in space with respect to any arbitrary point has no impact on the speed of light measured in the earth system. Disclaimer: did not hear a ...


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It's a good question, and illustrates an important principle in relativity. The proper time of an observer is equal to the length of their world line. For any observer the time shown by the clock they carry is called the proper time, and the proper time is an invariant i.e. all observers in all frames of reference will measure the proper time to have the ...


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If the photon you described is going to move from A to B, there's no reason you can see it at position C. Because if you were able to see the photon, it had moved from A to C, where you are, not B. Therefore, you are not able to see the path light travels through, unless, as others said, something like an atom deflects it. In all the answers above, something ...


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Under the conditions stipulated, the question is a false statement. We would, in fact, be able to "see" the light beam between points A & B. As the photons travel from point A to B, some photons will be deflected (by colliding with the dust particles) in our direction, allowing us to "see" the beam. Only in the absence of dust particles (or any other ...


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Possibly in the slightly negative energy region of a Casimir mirror pair However, the effect is really small


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There may be scope in this question to mention the faster than light (FTL) capabilities of Tachyons (should they exist). If (in the language of Speacial Relativity) we apply the first postulate of SR to tachyons and there is an instantaneous transmission of information in one inertial frame then it would be true to say this occurs in all inertial frames. I ...


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You are right, in the frame where $O$ is at rest, the time $P1$ and $P2$ take to travel across $O$ will be the same, call these times $t_1=t_2$ But it is also true that in $S$, $P1$ will take an longer time to cross $O$ than $P2$. How is this possible? There are various ways to begin understanding how. Here is one. Go back to the rest frame of $O$, and ...


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Yash, Imagine S is the Sun sending two photons, P1 and P2, and the object O is represented by two asteroids, O1 and O2, equidistant from each other all the time and from S at $t_0$ - they are moving in the same direction at the same velocity (c/2). So for some time one will be moving towards the Sun and one away from it. So you are right that the speed of ...



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