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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What would it be like to go past the speed of light? [closed]

Question #1: I'm actually surprised that nobody has asked this question before, but theoretically speaking, what would it be like to go past the speed of light? This is actually only one of a few ...
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Dark energy as negative matter for Alcubierre drive

Can dark energy be used for the Alcubierre drive as a substitute for negative mass? After all, to make the Alcubierre drive work, it is necessary to expand the universe behind it, and that is what ...
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Collision of hypothetical Tachyon with normal particle?

If we are dealing in one dimension, what will happen if a hypothetical particle Tachyon (pure imaginary rest mass, $v>c$ and real momentum & total relativistic energy)? Will it interact with ...
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That the laser hits the same point on the opposite side of the box implies it is moving faster than $c$?

In Greene's video: https://www.youtube.com/watch?v=2pV20KuDucs In the video Greene describes a fairly well-known thought experiment. A person is in a rectangular box floating in space with a laser ...
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Is the speed of light not infinite? [closed]

The speed of light C is equal to 299 792 458 m/s at this speed it would take me 3.03 minutes to get to mars at the closest orbit but if I was going the speed of light would I not be at mars instantly ^...
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Light Clock and Length Contraction in General Relativity

I saw somewhere on here that you can't exactly talk about length contraction in General Relativity, you can however talk about time dilation. So I was wondering if I made a light clock, with a ...
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Why is the square of the neutrino mass negative?

Why is the square of the neutrino mass negative? In arXiv:hep-ph/0009291 this is explained by giving the example of: $$m^2_{\nu_e}= -2.5 \pm 3.3 \text{eV}^2 \tag{1}$$ "Thee negative value of the ...
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Feynman couldn't explain how the photon becomes instantaneously aware of the glass thickness. Do we have a better understanding now?

I remember reading in Richard Feynman's QED about this unknown physics mechanism which possibly involves information propagating instantly and it blew my mind: The probability of photon to reflect or ...
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In what sense does the “speed limit” $c$ prevent violations of causality? Could they be prevented some other way? [closed]

I have seen it said that if we want to avoid violations of causality (I can see why we would want to avoid that!), we need the information-transfer "speed limit" of c. I presume(?) this ...
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A question about Quantum entanglement, 45° Light Polarization, and faster than light communications

I recently read "Atomic Adventures" by James Mahaffey. In the book (Chapter 10) he mentioned a theoretical way of faster-than-light communication. My brain instantly searched for flaws in ...
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Why would FTL imply time travel? [duplicate]

Edit: Thank you all for the explanations, but I think until I fully grok special relativity, Lorentz transformations and relativity of simultaneity, the answers won't make any sense to me. Maybe the ...
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Can quantum entanglement be used to coordinate actions at “FTL speeds” without breaking causality or actual faster-than-light communication?

I know there are a lot of similar question but I don't believe this to be a copy. I understand that if two people lived far away they could not transfer information through quantum entangled particles ...
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What happens if an object spins faster than light? [closed]

Does a star disappears from space time, if it spins faster than light?
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If a photon takes all possible paths, does it travel faster than light?

Let's assume we are in a perfect vacuum and let a photon travel from a point $A$ to a point $B$. As far as my understanding of quantum physics goes, the photon takes all possible paths between $A$ and ...
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How do I interpret uncertainty in velocity greater than the speed of light?

I just studied Heisenberg's uncertainty principle in school and I came up with an interesting problem. Assume an electron which is moving very slowly and we observe it with a distance uncertainty of ...
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Does the classical relativity supports elastic particles going faster than $c$? [closed]

If we work in an experiment similar to that of Michelson-Morley and we assume that light is made up of small elastic particles, and we make light beams reflect multiple times from moving mirrors, ...
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Question about the circumstances of apparent causality violation due to objects with negative ADM mass

In The Generalized Second Law implies a Quantum Singularity Theorem, Aron C. Wall asserts that an object with a negative ADM mass would produce a "Shapiro advance" rather than the ...
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Does the notion of blurriness of lightcone appear in any theory of quantum gravity?

In the above diagram, black lines are $x=\pm t$ and the highlighted space is what I meant by the blurriness of the lightcone. Personally I have 2 arguments and both seems to suggest there shouldn't be ...
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Can we think of relativity as a measure to determine the finiteness of universe? [closed]

Let’s consider a hypothetical situation wherein an observer is in the standpoint of a light ray. The observer faces infinite time dilation. Thus time stops for the observer and it'd have transversed ...
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Can an object move faster than the speed of light in a medium? [duplicate]

In water the speed of light changes to $2.25 \cdot 10^8m/s$. Can a electron travel faster/travel at the same speed in water. I don't see any violation of SR. So shouldn't it be possible for an ...
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How can a shadow move faster than light? [duplicate]

So I understand that similar questions has been asked before. There is a specific aspect I don't understand which I don't think has been explained. If I shine a light on a surface and then I add a ...
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Can a particle have a speed exceeding that of a photon in vacuum? [duplicate]

Actually I have studied that the smaller the mass of a particle, the fastest it can travel in space. (But i don't know the mass of a single proton) and my question is: can any particle travel faster ...
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Does light travel faster when it is in moving spacetime? [closed]

I was wondering if light can travel faster when it is within a region of moving spacetime. I know in general, that when you move a medium such as water, like in a river, water waves travel slower when ...
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Quantum Entanglement and transfer of time information, is it possible? [duplicate]

If two particles are quantum entangled...let’s call them particle A and particle B. You measure the state of particle A. At this point, can you know the exact time at which the particle B goes from ...
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Tachyons and Speed of light [duplicate]

Hi guys I am so sorry if the question maybe odd or not right but I am not an expert in this area I do apologise in advance if the questions is actually stupid. I was a reading a bit of relativity and ...
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Exceeding the speed of light by means of a shadow, will we ever be able to transmit information faster than light? [closed]

I bumped into this: Can a shadow move faster than the speed of light? while trying to see if I could find out anything about what I'd been thinking for 2 years now. Bear with me here, cause I'm only ...
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A free particle moving faster than light? [duplicate]

Say I observe a free particle at the origin at time $t=0$, for $t>0$ the particle is free and hence its wavefunction evolves in space as time passes. Now if it's wavefunction has non zero amplitude ...
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Why can’t an elementary particle be forced to have a particular outcome in an entangled pair?

I read in a blog Quantum Entanglement: Slower Than Light that one can not force a particle from an EPR pair to have a not statistical outcome for the entanglement parameter. I can not understand why? ...
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Which Doppler formula if light were to travel at non-relativistic speed?

I was going through this article about Doppler formulas and what it says is, that we only really have 1 Doppler formula, not 2. I only want if someone can confirm if I am understanding what it implies ...
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Is the speed of light constant relative to the observer? [duplicate]

Is the speed of light constant relative to the observer? Hypothetically, I am standing at the front of a train. The train is traveling 100 mph. I shine a flashlight pointing forward in the direction ...
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If I'm moving at half the speed of light $c/2$, and another object with $c/2$ in the opposite direction, will I move with speed of light? [duplicate]

... c/2-(-c/2)=c relative to the other object? (title character limit) Alternatively If my speed is c/n to the object behind me, which also has c/n to the one behind it and so on I should have a ...
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Are superluminal Lorentz contraction velocities real velocities?

Starting with the equation for Lorentz contraction $L = \sqrt{1-(v/c)^2}L'$ and making the substitution $v=at$ (be sure to see a better substitution two paragraphs down) then taking the derivative of ...
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Why are the most distant galaxies “only” around 13 billion light years away?

According to Wikipedia's List of the most distant astronomical objects, the most distant galaxy (GN-z11) is estimated to be around 13.39 billion light years away from earth. However, the observable ...
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FTL communication and paradoxes with local causality

I have Yet Another Question about FTL communication, causality and sending a message into your past. I've read this article and this excellent answer and I understand why it's possible to find a ...
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If the ER = EPR hypothesis is correct, could the Casimir Effect be used to allow for information transfer?

ER = EPR HYPOTHESIS If two entangled particles containing Einstein-Rosenberg Bridges were extremely carefully "probed" through a strong example of the Casimir Effect, could the negative density cause ...
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Alcubierre drive: what is the nature of exotic matter? [closed]

In connection with Alcubierre drive we often hear about necessity of an exotic matter in order to produce a negative energy density. I suspect that such matter has no place in standard particle model. ...
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Self-acceleration of a light beam? Airy beam?

Can someone provide me with a quick explanation of an accelerating (or self-accelerating) beam of light? Something called an Airy beam is an example? I get the impression that an accelerating Airy ...
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Does effect of force travel faster than light? [duplicate]

Suppose force (sufficient to move a block & imagine it(the block of course) to be extremely long and rigid ) is applied at one end of the block and the Block starts moving. Now force must be ...
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Using faster than light signals to synchronize clocks

What would happen if one could communicate by non-luminous signals whose velocity of propagation differed from that of light? If, after having adjusted the watches by the optical procedure, we wished ...
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Why is gravity fictitious?

I understand from a non-rotating falling reference frame that that gravity is fictitious since there is no apparent force acting on the falling observer. However, from a rotating frame of reference ...
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Partial reflection for fast communication

I've been rewatching the Feynman Auckland lectures and have a question about using partial reflection as a communication medium. Say you had two glass lenses in a vacuum, positioned several miles ...
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What would a Natario Warp Drive look like, outside & inside the warp bubble?

Within a Natario Warp Metric, all mass in front of the ship is pushed outwards by it's negative energy density, this means no light would interact with the front of the ship at all. And since the ...
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Does phase velocity or group velocity change when light enters a new material? Contradictory sources

I'm so confused. I've read in this book (page 28) that group velocity of light can exceed $c$ in certain gases. However a lot of people online and in the forums say that phase velocity can exceed $c$. ...
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Would Doubly Special Relativity or Mach's Principle allow faster-than-light travel to occur without violating causality?

Doubly Special Relativity posits that the Planck length is the same in all reference frames. Mach's Principle posits inertia is produced by the combined mass of the universe. From my limited ...
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Why do matter waves have superluminal phase velocites?

Specifically why do they have $V = \frac{c}{β}$ as stated in this paper written by De Broglie: http://aflb.ensmp.fr/LDB-oeuvres/De_Broglie_Kracklauer.pdf ? I follow the proof of “phase harmony” ...
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Breaking the light speed barrier [duplicate]

Assuing that our current understanding that the speed of light can't be matched or breached is correct, if you were travelling at the fastest possible speed in a space ship, say 99.99% of light speed, ...
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In regards to the hypothetical Alcubierre Drive (hear me out), are energy costs consumed immediately, or during transit?

I am aware the Alcubierre Drive is highly hypothetical and likely cannot exist. However, there are significant quantities of research material into Warp Drives of many types, and presumably the answer ...
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Is the speed of light universal for all mediums? [duplicate]

We know that the speed of light is constant in a vacuum and has the value c= 3× 10^8 m/s. But in a medium such as glass(refractive index=1.5) the speed of light is 2× 10^8 m/s. Now considering the ...
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Approach for calculating Alcubierre energy function?

NOTE: I know that the Alcubierre drive is on shaky ground and I'm not asking about its practicality or physical feasibility. I'm also not looking for answers like, "It's sci-fi so you can just make it ...
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Thought experiment: wired pull long distance communication

Let's imagine that doing research for the space elevator someone manages to build a very strong wire. But instead of using it for the space elevator a wire several millions of kilometers long is ...

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