Questions tagged [faster-than-light]

"Faster-than-light", also known as superluminal velocities, refers to any sort of travel at a speed greater than the speed of light. Prohibited in mainstream physics due to the Special theory of relativity.

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Thought experiment using quantum entanglement in position and its effects

Consider we have two atoms $a$ and $b$. They are entangled with each other in position and momentum, with some wavefuction describing them in position space that is $\Psi(x_a, x_b)$. This ...
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Superluminal particles with causality

What kind of CLASSICAL theories would allow to true (non-apparent) superluminal particles (beyond speed of light, BSOL) agreeing with causality to exist? I mean, are causal superluminal classical ...
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Consequences for causality if superluminal neutrinos were explained by extra dimensions

One suggestion for explaining superluminal neutrinos (assuming for the sake of argument that the OPERA results should hold up) is that the neutrinos have taken a route through extra dimensions, with ...
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Feynman diagrams

Feynman diagrams are drawn on Space-time graphs. So is it possible for any particle path in that diagram to have a slope smaller than 45° or greater than 135°, i.e. can they travel faster than light? ...
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Exploring beyond event horizons

In the expanding universe the velocity of separation between galaxies depends upon how far they are. If they are much far away will they have relative velocity of separation greater than speed of ...
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Faster Than Light Communication Using Gravity?

Suppose two parties $A$ and $B$, some distance apart and at rest relative to each other both had control of one massive local object each. If party $A$ lifted their local object upward relative to $B$...
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Does the uncertainty principle imply the existence of particles that exceed the speed of light?

The uncertainty principle allows for the creation of virtual particles (with non-zero mass) that exist for very short durations. This allows empty space to have particle pairs that pop into existence ...
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Size of the Observable Universe [duplicate]

I wanted to know what the observable universe is so I was thinking and I thought, it must be age of the universe times 2. Well I was wrong. I found on one website that it is 46B LY across in each ...
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Superluminal expansion of the early universe how is this possible?

Is this a postulate? I get the expansion of the universe, the addition of discrete bits of space time between me and a distant galaxy, until very distant parts of the universe are moving relative to ...
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What prevents two objects from falling toward each other faster than the speed of light?

I was thinking about what happens when two objects fall toward each other in space. The farther apart they are when they begin falling, the faster they will be traveling when they hit each other due ...
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Chosing a reference frame in which the Earth is at rest and doesn't rotate

We may choose a non-rotating earth as our reference frame and ask ourselves: how about the planetary and stellar motions. A star at a distance of 10 million light years would turn around the earth in ...
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Faster than light signals and the price to be paid if we accept them : a very simple protocol

Some physicists currently understand entanglement as transferring information instantaneously, yet not violating causality. Is this really a satisfactory explanation, or should be look for something ...
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If the universe didn't expand faster than light, would our nights brighter like day?

There's a common QnA which has amused and inspired many kids: There are billions of Stars in the sky. If we combine lights received from all stars, wouldn't it beat Sun? Why is night dark really? ...
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How can I explain this fallacy of conveying information faster than Light?

First statement of this question: In superluminal phase velocities, what is it that is traveling faster than light? says that information cannot be transmitted at a velocity greater than speed of ...
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Faster than light in plasma

Some plasmas have a refraction index of less than 1. In these plasmas the phase velocity of light can be faster than light-speed. But the phase itself won't transfer information, so no paradox occurs ...
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Uncertainty principle and the speed of light

If an EM wave only gives us a probability of where a photon may be at a given moment, and the HUP tells us that we can't know the exact location of the photon. Then would it be correct to say that a ...
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Is General Relativity compatible with relative speeds bigger than $c$ between two inertial frames?

My question is motivated by a remark done by Tegmark in his book "Our Mathematical Universe". He says that GRT does not prove that relative speeds between material points are always smaller than c. It ...
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What would be the effect on gravity if space expanded at > $c$?

If space were to expand at > $c$ (as in inflation) would that mean gravity would no longer have any effect on the curvature of space, since gravity can only propagate at $c$?
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Faster-than-light communication using Alcubierre warp drive metric around a single qubit?

The Alcubierre warp drive metric has been criticized on the points of requiring a large amount of exotic matter with negative energy, and conditions deadly for human travellers inside the bubble. What ...
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Illusion that $v > c$?

I am currently studying length contraction in special relativity (and having the hardest time getting the harder exercises in my book right) and there is the following exercise in my book is driving ...
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What happens if an object that is rotating very fast also travels very fast?

For example, if you throw a frisbee and it travels away from you while spinning clockwise. At any single point in time, the left side of the frisbee will be traveling away from you faster than the ...
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Is it a coincidence that non-relativistic quantum mechanics prevents superluminal communication?

So non-relativistic quantum mechanics prevents superluminal communication. Given a bipartite state $\rho_{AB}$, the reduced density operators for systems $A$ and $B$ are given by the partial traces $$...
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Mutually inaccessible galaxies

It is said the farthest galaxy we can see in one direction is beyond the cosmic horizon of the farthest galaxy we can see in the opposite direction; they can't see each other. Doesn't this contradict ...
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Many times speed of light [duplicate]

http://www.huffingtonpost.com/2014/03/24/theory-of-everything-big-bang-discovery_n_5019126.html What does "many times speed of light" really mean in this context? For a layman it's easy to draw wrong ...
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Theoretically if you passed the speed of light in a medium, would there be a sonic boom equivalent?

I know that it is technically impossible to reach the speed of light in vacuum since the mass of the object travelling would reach infinity. However in a medium, would there be some sort of ...
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Speed of light $> c$? [duplicate]

lets say we have two person A and B. Person A is holding a light source (lets call this light source 'S1') which is pointed towards north. Now person A starts moving at speed 0.86c WRT (with respect ...
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Why can an object not reach the speed of light by falling in a gravitational field with constant acceleration?

I recently learned in school that the gravitational potential energy of an object is equal to $mgh$. I also learned that the kinetic energy of an object is $\frac12 mv^2$. If an object falls in a ...
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If space can expand faster than light why can't gravitational wave?

I heard that gravitational wave is the measure of stretchiness of space time, so since there is no limit to how fast space can stretch what about gravitational wave?
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Is the GW150914 signal consistent with a superluminal gravitational wave burst?

I've been following the news about the detection of gravitational waves with interest and had a question for those with a physics background. I've gathered that the preferred explanation for GW150914 (...
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Speed of light and warp drives in general relativity

Velocities can be a tricky thing in general relativity. A cool concept seemly consistent with the Einstein field equations) is an Alcubierre drive, described by the Alcubierre metric. However, I ...
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How does gravitation propagate along curved spacetime?

In this wikipedia article it is described how a beam of light, with its locally constant speed, can travel "faster than light". That is to say it travels a distance, which, from a special relativistic ...
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Is the maximum possible measurable velocity = 2c

What is the maximum velocity that can be measured between two objects? Is $2c$ the correct answer? Two photons (A & B) are emitted simultaneously from my position; photon A going north and ...
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Thought experiment about no-cloning theorem and FTL information

The quantum no-cloning theorem states that one cannot "build" a perfect cloning device for arbitrary quantum systems. There also exists a famous thought experiment where Alice transmits information ...
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Relative Speed vs speed of light [duplicate]

Possible Duplicate: Travelling faster than the speed of light Someting almost faster than light traveling on something else almost faster than light I've got two questions which are related and ...
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A telegram from Mars

Say, people fly to Mars, get in trouble, and want to send us a SOS signal. The average distance is 225 Gm, so they must wait 25 minutes to hear our response at the speed of light. This may just be too ...
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Black Hole rotation and acceleration

I have been reading some about the black holes and the rotation about it, and have also heard about the fastest spinning black hole (NGC 1365), which as I understand, its rotation is 86% the speed of ...
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Are some gravitational wavelengths forbidden by causality?

Consider a gravitational wave in linearized gravity $d_{\mu \nu}(X_{\eta}) = D_{\mu \nu} e^{i X_{\eta} K^{\eta}}$ with $K^{\eta} = (-\omega t, \textbf{k})$. Let $d=| \textbf{D}|$ the scalar maximum ...
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About the nonlocality of QM and faster-than-light/backward-in-time machines

The fact the quantum mechanics is nonlocal is known already for a long time, since the Bell works (1966 and later) and the Aspect's group experiments confirming the Bell-type CHSH inequality (1980 ...
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Why does FTL not leave behind a motion blur? [closed]

I once thought anything traveling faster than light would leave behind a motion blur that on one end recedes at the speed of light and at the other increases at the velocity of the object. If an ...
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Relative speed when getting close to the speed of light

I was thinking about the relative speed of an observation reference frame and an object which has been accelerated to a speed close to the speed of light. I'm by no mean an expert and the last physics ...
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Is it possible that the universe is not expanding faster than the speed of light? [closed]

What if the speed of light increases proportionately to the expansion of space? Is it possible that light traveling in the medium of space gets to travel faster if the medium is expanding? An analogy ...
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Faster than light speed transfer of certain event

I am CS/Math person I am not quite sure what it means in physics information cannot be transferred faster than light (FTL). I tried to understand the proof of no transfer theorem but I lacked the ...
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Superluminal speed at cosmic horizon [duplicate]

The observable universe is limited by a cosmic horizon. Galaxies beyond the horizon move away from us faster than light, so we cannot see them. Imagine a spaceship in the vicinity of our horizon. If ...
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Twin Paradox: Why does't the traveling twin measure faster-than-light photons on Earth?

Sorry if this question has been asked, but it was difficult to search through given the volume of Twin Paradox questions. Anyway. My question isn't with the twin paradox per se; it's with what the ...
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Can space move? [closed]

I have some questions; I hope you don't mind: $\bullet$ If the space between two distance galaxies is increasing, then is the volumes of space in which the galaxies find themselves also moving apart? ...
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Observable Universe from two different perspectives

My layman understanding is that the universe is much bigger than that we can observe however due to the cosmological constant the observable universe is the matter etc. that is not moving away from us ...
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What're some criticisms of the paper “Six observations consistent with the electron neutrino being a tachyon with mass: mνe² = −0.11±0.016 eV²”?

Today I read about this paper making the case that neutrinos have imaginary mass and travel faster than light. I want to get excited, but I have no way to know how likely it is that this paper will be ...
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A problem with $E = mc^2$

(Rest) mass of proton = (rest) mass of neutron (approximately) = $1.67 \times 10^{-27}$ kg. let $m$ be the mass of nitrogen atom then $m = 2.32 \times 10^{-26}$ kg. $c = 3 \times 10^8$ m/s (approx). ...
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If it turns out that neutrinos do travel at faster than lightspeed, how will the success of special relativity be explained?

As per in the title. If it turns out that neutrinos do travel faster than the speed of light, how will the success of special relativity be explained? My apologies if this has been asked before; I've ...
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What happens if we exert more force on an object which would be travelling at near maximum light speed? [duplicate]

If we exert force on an object which would be travelling at near light speed, to an extent to make it move greater than speed of light in vacuum, what would happen? If object doesn't move faster than ...