# Tag Info

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As you note, spacelike-separated (what you call "FTL") events can't be ordered unambiguously in time. That means such events can't be said to 'cause' one another in any reasonable sense. One way around this is to abandon the idea of causation between events. If events don't really cause one another, but are actually just all equally caused by some sort of ...

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I am assuming you have 2 particles facing each other, and that they are approaching each other ? First, as mentioned elsewhere on this page, "..a particle moving at the speed of light does not experience time, and thus is unable to make any measurements." Instead, let's change the particle #1 that you are sitting on to having a specific velocity that is ...

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This is a perfectly good question; don't feel discouraged. Coming up with a notion of 'causality' that doesn't refer to any physical theory is pretty hard. I'll discuss it here in the context of general relativity (GR). The basic entities in GR are events; i.e. spacetime points $(t, x, y, z)$. Suppose we have two events $A$ and $B$ and that, according to ...

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Entanglement is not necessarily equivalent to measurement, if the entanglement is reversible. Measurement has to do with an irreversible event, like photon absorption or emission. Until an irreversible event occurs with X or Y, the entanglement of X with Y just results in the creation of an entangled, composite, XY wave function (and composite XY density ...

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The main issue is that one cannot use a non-relativistic dispersion formula $E=\frac{p^2}{2m}$ to deduce what happens at relativistic velocities. This Phys.SE post deals with the same theme.

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According to Einstein and many others, light speed c0 in vacuum is universal and measures about 299,792,458 m/s. So it is never possible to change the light speed in vacuum, which is the absolute upper limit for everything. Gravity does not affect a light ray, but the space and time through which the ray travels. In a strong gravitational field, the time ...

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I would suppose the short answer is no, but photons are affected by gravity. What is happening is naturally quite relativ to the observer. Suppose you are sitting on the photon, travelling past the gravity source with the speed of light. As has been argued above you would experience the force of the gravitational pull as an acceleration toward the source. ...

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You don't feel acceleration. When onboard the ISS, you are accelerating towards the earth (down) due to gravity: if you didn't, you would just fly away from the planet. Because you and the ISS are accelerating exactly the same way, you don't feel a thing. You don't feel a force if it's accelerating you: you feel pressure caused by opposing forces. Here ...

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To talk about acceleration in space is a little bit dangerous without exact definition. One has to separate free fall and acceleration from an impulse. Imagine, you are inside the ISS during an orbit correction. The impulse from the rocket engine you could feel, you get some weight, and and this is an acceleration. In all other time you are weightless and ...

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Photons are blue-shifted when attracted by gravity (I mean - moving towards a mass, not moving at right angles to the gravitational field like in an orbit). They can't go faster, but their energy goes up.

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Let's say we're sending a scouting mission to an Earth-like planet 100ly away to see if it's suitable for colonization. We could send the scouts out at near light speed, and due to time dilation they could easily survive the trip without dying of old age. If we send them out at .9c then the entire round trip will only take 20 years in their frame of ...

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Questions like this are complicated because you have to be clear what you mean by time. The simplest definition is that time is what is shown on a clock, so if I was holding some hypothetical clock that had been reset to zero at the Big Bang my clock would currently be showing 13.799 billion years i.e. the age of the universe. The question then becomes what ...

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This is true for galaxies beyond the cosmologic horizon. BTW they are not moving at that speed: their apparent speed seen from our place is such. Quite like the fact that the far galaxies we see close to the horizon seems both very young (which they aren't "in real life"), very red-shifted (while their emitted colors are indeed normal) and living very ...

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In Peskin's introduction to quantum field theory, he talks about this in chapter 2, section 4 in a subsection titled causality, which is on page 27 in my book. He explains that yes, there is some non-zero probability that a particle will move faster than the speed of light. However, he shows any two local operators that are separated by a spacelike ...

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The link about superluminal neutrions you cite is missing the fact that later on an error was discovered, and neutrinos do not, in fact, travel faster than light (see e.g. the Wikipedia article). To date, nothing that travels faster than light is known. The uncertainty principle does not "allow for the creation of virtual particles". The idea of such pairs ...

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Einstein told us that it would take infinite energy to accelerate a massive object to the speed of light, but that wasn't supposed to be the reason why massive objects can't move at the speed of light; it was supposed to be a consequense of them being unable to move at the speed of light. James Clerk Maxwell theorized that an electromagnetic wave could ...

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The rules of relativity theory basically tell you the following about velocity-related transformations: First, the vacuum speed of light is an invariant and second, the "inside" of the light cone (slower-than-light motion) and the "outside" of the light cone (faster-than-light) never get mixed up. So no matter how you manipulate your velocity, no matter what ...

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Can it be explained in a way that an 8 year old can understand? First demonstrate that there exist limits to velocities in various situations. Take a floating balloon and start adding hanging weights. For a fixed weight there is a terminal velocity, dependent of the size and weight of the balloon-weight system. This can be explained by the reaction ...

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I think a good way to approach this is as follows: An object moving at some finite speed needs to have a speed relative to something. You can't say you are moving at 3m/s without saying what you're moving relative to. This is not the case with the speed of light. When something moves at the speed of light it does so relative to everything. The resaon ...

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I would say there are 2 paths for explanations: for rockets-like problems you have to go into the relativity stuff, with the contraction of time and length which are just a fact. From that, it's explain everything, from paradoxs, differences from inside and outside, and extra cost of the last "pushes". (just saying alone that puffs get costlier and cause a ...

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