# Tag Info

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No - the light is moving with the mirrors. To an observer in the mirror / clock frame, because they're in the same frame the light will appear to only move from one mirror to the other (up/down). However an observer not traveling with the clock, would still observe that inside the clock system (mirrors and the trapped light) the light still moves only from ...

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The point of that experiment is not that the light goes slower but that the light has a longer distance. This means that a single bounce of the light off the mirror takes longer for the observer's point of view. This is due to time dilation. So it is not because the light is slower, but it is because the light has to travel longer

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A force is defined as a change in momentum over time. In the Newtonian limit, this means a mass times an acceleration. But when dealing with things like photons, the formal definition is applied. Photons have no mass, only momentum. Therefore, if a force is applied to them, their momentum can be changed. This can happen in two important ways, a force can ...

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The observable universe radius is unique to any given point. By moving at all, we shift the center of our unique observable universes, causing the boundaries to shift.

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This question is a little naive but interesting. A common mistake is to confound speed of information and effective bandwidth. ie : my network documentation tells me that a flow of ( unzipped ) information travels at "speed of light" * k , with k between 0.3 and 0.6. After a new software release, using high compression , I see that I can send my ...

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Although the most common answer to questions like that is "relative to what?", there are possible side-effects. If I recall correctly, the earth's velocity relative to the cosmic background radiation is on the order of a mere 600km/s. If we were to be travelling arbitrarily close to the speed of light as compared to our current motion, part of this ...

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The cameras are at the same place, because we ignore the small distance between the cameras, right? When the cameras are at the same place, then the same information arrives at the same time at both cameras. The person in the train picture will look the same age as the person in the ground picture.

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You are basically correct. In the presence of any charged particles a light wave interacts with the particles to form an entangled system. Once this has happened we no longer have a photon and (for example) an electron, but instead we have a single system described by a single wavefunction that includes both particles. For weak interaction the system is ...

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In nature, there is a perfect (classical) vacuum (ignoring gravity, which I will get to). Think about atoms. In space, there are atoms, but if you "zoom in" there are gaps. In these gaps, couldn't light travel impeded? And about the gravity argument, gravity changes the frequency of light but not its velocity.

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It is not possible to find a frame of reference where a photon is at rest. I will argument in two different ways: 1. Maxwell equations and electromagnetic argument: From Maxwell it is expected that electromagnetic disturbances propagate in vacuum at a constant speed c~299792458 m/s which is the maximum speed for the propagation of electromagnetic ...

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The time dilation (to use the proper term - not distortion) occurs in the frame of reference of the moving object. So only the photon "feels" that time has slowed down. In our reference frame, we simply observe a photon whizzing alone at precisely c. So all our observations that depend on knowing the speed of light remain accurate. Actually, if you consider ...

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The observer sees you travel from A to B - a distance that, in his frame of reference, is greater than one light year. He sees that you take more than a year. He concludes you are traveling at less than the speed of light. You, traveling so fast, "see" a much shorter distance (this is the concept of length contraction) $L' = L_0/\gamma$ where $\gamma = ... 1 The Lorentz transformations are used to transform between different inertial frames. For example if you and I are in relative motion then the Lorentz transformations convert the positions of spacetime points in my rest frame to the positions of spacetime points in your rest frame. However anything travelling at the speed of light has no rest frame, so the ... 3 Light moves at about a foot per nanosecond, or a meter every three nanoseconds. In order to capture it propagating across a room over a few frames, you would need to gather something like a billion frames per second. No consumer camera -- indeed no camera on Earth -- is capable of this. Now there have been people playing with "fempto-photography," but they ... -1 If the speed of light is constant no matter the speed of the source Actually, the speed of light isn't constant. It varies with gravitational potential, see Einstein talking about that here. However in gravity-free space the speed of light is constant. In addition you measure it to be constant regardless of your motion through space. That's because of the ... 0 It does not matter what your velocity is relative to a photon - the photon will always move at a fixed speed from your point of view. This is the 2nd postulate of special relativity - the speed of light in a vacuum has the same value (it's invariant) for all non-accelerating observers (in a medium of uniform density, the speed would be different from its ... 0 Einstein's postulate is that the relative speed between the observer and the source does not affect the speed the observer sees the photon moving, as long as the observer isn't accelerating, that is. This basically means there is no such thing as absolute 0 speed. Speed of light is your new and only constant friend. 1 The speed of light in a vacuum, or c, is 299,792,458 meters per second. Any other medium will slow light down. For example light takes about 40% more time to go through glass than vacuum. But media that would increase the velocity of light would violate fundamental laws of physics and can therefore not exist. 0 You're right that there are numerous stars that are so far away and were created so "recently" that their light has still not arrived at Earth. However, most processes in the Universe happen on extremely long timescales (for human standards). In the Milky Way, on average approximately one star is born every year (see e.g. here). But the timescales from the ... 0 Tme dilation refers to time in the accelerated frame (the rocket). So a clock in the rocket will run slowly compared to a clock on earth. It is us who would measure the rocket as moving at 0.5c. The speedometer in the rocket could actually show a speed greater than that. From the POV of the crew in the rocket, their clock is running normally. It is the ... 0 [...] neither country's delegate wants to sign the treaty before the other delegate and thus, a simple system is devised to ensure that both delegates sign the peace treaty simultaneously. The solution involves setting a light bulb at the center of a table in such a way that the light bulb is exactly between the [two] delegate[s ...] the light bulbs ... 1 You're right that there would be a disagreement over who signed the treaty first, but it would not be between the diplomats on the train; it would be between the people on the train and the people not on the train. The setup initially is that the two diplomats are sitting in the dining car with the curtains drawn, for security. Let's say the light that the ... 0 For instance, take a rigid pole of several AU in length. [...] The person on the opposite end should receive the pushes and pulls instantaneously as no particle is making the full journey. As other answers have pointed out, you can't have perfect rigidity, and the signal would propagate at the speed of sound in the material. If tap a steel rod with a ... 1 Your "live stream" would be affected by the Doppler effect. Since the speed you mentioned is far from relativistic you don't have to consider the relativistic Doppler effect. If$t_0$is the time when the camera was launched,$t$is the time at which you watch a certain frame,$t_f$is the time the events on that frame took place,$v$is the speed of the ... 0 I don't know the specifics, but I do know that: a) the feed would be red-shifted due to the Doppler effect, comparative to a firetruck's wail decreasing in pitch as it drives away, and b) the feed would slow down incrementally due to time dilation, moving at high speed. This would become more severe as it entered gravitational fields also. 0 The key point here is the "weird" change in the values you measure depends on what frame you observe them in relative to the object. There is no experiment you can conduct to determine an absolute value for your speed. Speed only makes sense relative to an object. So if a near light speed object relative to us was heading towards us, from its perspective ... 2 The basic point is that you can't shine light away from the centre of a black hole once you are inside the event horizon. Far away from the black hole, light cones are oriented so that propogating light from an event can travel anywhere inside a cone bounded by two lines at 45 degrees in a standard (Schwarzschild coordinates) space-time diagram. Nearing ... 0 My understanding (which may be dated) is that inside the event horizon the reference frame (or space-time itself?) is falling faster than light toward the singularity. So light DOES continue to travel at C, but it is doing it in a reference frame that is falling faster than light. Actually the reference frame is falling at exactly the speed of light at ... 0 It depends on how you define mass. I like to think of it as mass is just rest mass. I mean the mass you weight on a scale when nothing is moving. On different media light moves slower not because it gains mass but because of its interaction with the atoms in the media. The photons get absorbed and reemited in such a manner that when you sum the waves for ... 1 First, we will look at the energy of a free relativistic particle of (rest) mass$m$moving with velocity$v$: $$E = \frac{mc^2}{\sqrt{1-\frac{v^2}{c^2}}}$$ where$E=mc^2$when$v=0$. We now consider a few cases:$m\ne0$: In this case,$E\rightarrow\infty$as$v\rightarrow c$. Therefore, a massive particle that at any point of time is moving at less than ... 2 Which is more fundamental is probably a meaningless question, but you can think of the geometric notion of a manifold (the mathematical abstraction of spacetime) as being more general than Lorentzian manifolds, i.e. ones whose metric is locally like that given in John Rennie's Answer. So one could in principle conceive of universes that were described by ... 1 Every particle needs to have energy to be a particle (if it had none it wouldn't even exist). Since energy is equivalent to mass and therefore gravitates I would say YES, all particles that have a speed less than the speed of light must also have mass. Because the speed of the particle is less than the speed of light an observer could travel with the same ... 0 fundamental is a choice dependent term, its what one considers something to be more basic. spacetime is a consequence of both constancy of c + invariance of physical laws, which describes how the general nature of laws should be, which is called Lorentz invariance. spacetime is one consequence of Lorentz invariance, there are others like energy and momentum ... 2 It is an artificial distinction to say one is more fundamental than the other. The geometry of flat spacetime is given by the Minkowski metric: $$ds^2 = -c^2dt^2 + dx^2 + dy^2 + dz^2$$ and this is fundamental in the sense that all of special relativity is described by this equation. But it is also fundamental that the parameter$c\$ in the equation (which ...

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