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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Special relativity: Two moving objects from different reference frames [duplicate]

Imagine a inertial frame of reference, in which two objects are moving with constant velocity in opposite directions. Object one is moving at $50$% the speed of light and object two has a velocity of $...
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Velocity of point of intersection of 2 rods [closed]

I'm not getting how are we getting u greater than c for part b by using formula for part a. Plz explain
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If there was a solid bar of diamond between here and Proxima Centauri [duplicate]

Or maybe not diamond, but imagine some substance that was crystalline, the hardest substance and the least ductile, which retained a perfect shape under enormous pressures. My question is, if you ...
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Speed of light (Special relativity) [duplicate]

If a light is fixed on the front of a spaceship, then for a person in spaceship, the speed of light is $3 × 10^8 ms^{-1}$. But when observed from earth, does speed of rocket add to the speed of light?
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What, if anything, is wrong with this proposal for faster-than-light communication using entanglement swapping? [closed]

What, if anything, is wrong with this proposed mechanism for faster-than-light communication? Here are pastes from the proposal: Spooky Communication at a Distance Faster-Than-Light Communication via ...
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Speed of a train plus speed of light [duplicate]

Einstein postulates that the light travels at the same speed $c$ in every inertial frame of reference. Now, if I'm within a train which goes right with velocity $V$ and throw a ball to the right with ...
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Matter moved faster than photons at big bang? [duplicate]

The observable universe is 90 billion light years across. But the age of universe is 13.8 billion years. Light would take 90 billion years to cross this distance, how could matter move that far in ...
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How can information be communicated with entanglement? [closed]

Forgive my overly simplistic view of this. But I have been wondering this for years. Any insight would be greatly appreciated. So imagine you have two entangled particles, and they are separated and ...
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Is High-Fidelity Quantum-Entanglement Data-Transfer Real?

I was discussing quantum entanglement with a friend and explaining that faster than the speed of light data transfer isn't a reality with our current understanding of physics. He brought up quantum ...
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Does faster than light lead to closed timelike curves?

If some signal can be sent to a spacelike interval then is it possible in some reference frame to have a close timelike curve? Novikov claims so in this paper. The argument lies below eq$7$ but ...
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How did we beat the light?

We observe that the universe is 13.8 billion years old, give or take a little. We can say this because we observe and measure the fact that distant galaxies and the CMB are that far away from us, ...
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Why is my contrived superluminal thought experiment wrong?

An astronaut is on an odd mission. Each day, the spacecraft is accelerated by 10% of the speed of light, and at night the astronaut sleeps and will loose all memories of the previous day. Each day ...
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Observed Behavior of Light When Relative Motion $> c$

I am trying to wrap my head around some of the implications of relativity. Hopefully somebody can help enlighten me with a specific example I am wondering about: We have 2 objects, A and B, that are ...
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Is there a force that a body experiences as it approaches light speed?

My question can be boiled down to a thought experiment. Assume you have a space ship in an empty universe. This space ship is capable of a constant $1 m/s^2$ acceleration. As it approaches light ...
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Does the effect of field travels faster than light? [duplicate]

As I was going through some work of my college level courses of fundamental physics, the formula caught my attention $F={G}\frac{Mm}{r^s}$. "r" seems can be any value; there would still be ...
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Nothing can exceed the speed of light but what if we accelerate a particle that contain electron that have a speed near to speed of light? [duplicate]

We all know that speed of light is $3\times 10^8$ $\text{ }\mathrm{ms^{-1}}$. The speed of electron inside the atom is $2.7\times 10^8$ $\text{ }\mathrm{ms^{-1}}$, which is close to speed of light. So ...
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Why consider spacetime trajectories that cut a $t=$constant at more than one points in the path integral of a relativistic particle?

Here is a picture from Padmanabhan's book on quantum field theory. Also described in the first part of this lecture within $17-25$ minutes. He says that a relativistic particle can follow a path like ...
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Exceeding the speed of light in view of special theory of relativity [duplicate]

Is there any rigorous way to show that it is impossible to have any inertial reference frame with the speed of light apart from the blow up of the Lorentz factor?
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Why can't we surpass the speed of light? [closed]

Can anyone provide me with a complete mathematical proof about why an observer frame cannot surpass the vacuum speed of light? I have looked for answers in Quora and FB groups but no one is really ...
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How, if at all, do Tachyons experience redshift?

I recently attempted to answer this question, to which I felt qualified because the possibility of Tachyons had been discussed when I took a course on Special Relativity. In that same course I learned ...
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Is Light our limit? [closed]

Suppose something existed faster than light will we be able to perceive it? And even if we encounter it wouldn't seem to travel with speed of light?
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A rod is attached to a star in a galaxy at the edge of the observable universe and it reaches earth. Will that rod start moving faster than light? [closed]

It is known that the galaxies at the edge of the observable universe are accelerating away from us and eventually they will start moving faster than light with respect to our frame. However, suppose ...
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Some areas of the universe are moving away from us FTL but surely as we get closer to that area, the speed of expansion would gradually slow?

If the universe is expanding uniformly, then wouldn't the rate of expansion of the universe change if you were to move closer to a point that is currently moving away from us faster than light? For ...
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A query regarding FTL Communication

This is a question that almost certainly has been asked before (but I am still struggling with it), thus at the risk of redundancy with due apologies here it goes: The Theory of Relativity asserts ...
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Theoretical question about waves and interference

Assume I’m given a standing wave function and I’m asked to find the phase velocity of the two waves that interfered and created the standing wave. Now, I have two questions: Is it possible to move ...
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Why can't light travel three days into the future then transmit that information back to us prior to us arriving at that position? [closed]

Example: If a football game between the Pittsburgh Steelers @ Tennessee Titans will happen in 3 days Sunday 10/25/2020 at 1pm, why can't light leave today 10/22/2020 and go to where the Earth will be ...
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Age versus size of the universe [duplicate]

QUESTION CONTEXT: The universe is roughly 14B years while its diameter 93B light years, making radius 46.5B ly. If I understand correctly if we froze everything, go on the one side, then go to the ...
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Wouldn't a negative mass be going faster than $c$ according to our current models of relativity?

I heard that there are some physicists trying to figure out, out least hypothetically, how things with positive and negative mass may interact with each other. I'm really confused about how this can ...
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If a black hole bends light what does to a beam that arrives at a zero degree angle directly towards the singularity? [duplicate]

We can only speculate what is going on but does the black hole singularity make the light arrive faster than it would in space not affected by gravity?
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Causality and processes in QFT

We have virtual particles in quantum field theory (QFT). In general, they don't have the need to obey causality. My question is: Do the processes in QFT (electron self-energy, photon self-energy, ...
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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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