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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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Aging while traveling faster than speed of light? [closed]

theoretically, if organic matter was able to travel faster than the speed of light, without becoming "pure energy", and since theoretically traveling faster then the speed of light would travel ...
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Why didn't Lorentz conclude that no object can go faster than light?

Based on Lorentz factor $\gamma = \frac{1}{\sqrt {1-\frac{v^2}{c^2}}}$ it is easy to see $v < c$ since otherwise $\gamma$ would be either undefined or a complex number, which is non-physical. Also, ...
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How Can the Universe Be Bigger Than It is Old? [duplicate]

My question is this: If we can look back to roughly 500m years after the big bang and have estimated the age of the universe at 13.7B years, why is the galaxy we are looking at not a part of the non-...
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Traveling to another galaxy impossible for ever?

If galaxies are moving apart from each other faster than the speed of light, does that mean that it's impossible to travel to another galaxy, regardless of what technology get discovered? Because you ...
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Can light in a medium travel faster than speed of light $c$ in vacuum?

Refractive index of medium $m$, $n_m$, is given by $$n_m = \sqrt{\mu_r\epsilon_r}$$where $\mu_r$ and $\epsilon_r$ are permittivity and permeability of the medium respectively. And, velocity of light ...
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What is Cherenkov radiation? [duplicate]

What is Cherenkov radiation? I came across this concept when I was learning SR. Why can't particles moving at speed of excess of speed of light in the medium. Why it does not violate the limitation ...
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So is quantum entanglement actually FTL “communication” or is it mundane pre-determination?

I have to say right off the bat, I'm a little frustrated that there seem to be very contradictory answers about this, at least to a layman like me. If two particles are entangled and you separate them ...
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Significance of the LIGO experiment with respect to length contraction

it might be best to first detail my (admittedly limited) understanding of the LIGO experiment. a beam of light is split along two 4Km evacuated pipes then reflected back where their interference ...
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Does gravity sometimes get transmitted faster than the speed of light?

Consider Earth moving around the Sun. Is the force of gravity exerted by Earth onto the Sun directed towards the point where Earth is "right now", or towards the point where Earth was 8 minutes ago (...
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Turning on headlights near light speed [duplicate]

If i turn headlights on in a car at v = 0.99*c, will the headlight speed be 0.01*c? I know speed of light is always the same but it really confuses me? My question is, does space contract so much at ...
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speed of freezing vs. moving [closed]

An idea came to my mind and like to discuss it. We know that out there in the space the absolute zero (aka. -273 C) is almost reached where all the particles and atoms would freeze up and stop giving ...
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Are tachyons time travelers?

If tachyons are faster than light, that means they have passed a certain point before anyone or anything can “see” them. Theories have mentioned that if anything can go faster than light, then it can ...
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Looking back in past [closed]

Can faster than light communication give us the ability to see in the past, that is to say, if we can imagine a ultra huge highly advance telescope 60 light minutes away from earth and we want to look ...
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1answer
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Observing supraluminally receding galaxies – is the “swimmer against variable current” analogy valid? [closed]

Davis and Lineweaver in Superluminal Recession Velocities state that “galaxies with distances greater than $D=c/H$ are receding from us with velocities greater than the speed of light and superluminal ...
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Delayed choice quantum eraser without randomization?

My understanding of delayed choice quantum eraser experiments roughly corresponds to the Wikipedia article about them, so please forgive if this question is stupid. In all delayed choice experiments ...
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Apparent FTL speeds seen by observer

MY guestion is if I was in a ship orbiting a black hole (not very close to the event horizon) I am orbiting pretty far and I am experiencing that my time is slowing down realitive to people on earth. ...
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Why do two moving inertial frames of reference atribute the same velocity to each other?

In both relativity and Newtonian mechanics, it is a self-evident axiom according to which two inertial observers who move relative to each other measure the same velocity of, say, $v$. I want to know ...
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Gear Theory — Entanglement — Spooky action at a distance explained? [duplicate]

Nothing can travel faster than the speed of light. Not even an electrical signal. Entanglement seems to violate this. But, does it? Imagine 100 gears lined up in sequence all the way across a ...
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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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How big is the universe 1 second after the Big Bang? [duplicate]

In his book Astrophysics for People in a Hurry, Neil de Grass Tyson starts of with the Big Bang and narrates on what (probably) happened within the first moments of our universe. He further states (p. ...
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How can I go and also not go faster than light?

Suppose I am in a spaceship. In this spaceship there is also my friend. So my friend and my relative velocity is zero. So no matter how fast my ship goes my friend's velocity relative to me is zero($0$...
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Is the speed of light greater if traveling at an initial velocity greater than zero? [duplicate]

I'm reading "a brief history of time" and he states "the speed of light should be the same whatever the speed of the source, and this has been confirmed," (24). Why is it not increased if it has an ...
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Relative velocity greater than speed of light

Is it possible a relative velocity between 2 objects to be greater than the speed of light? I know that it is impossible to reach the speed of light considering us to be immobile but is this ...
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1answer
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Trying to understand velocity addition and time dilation [duplicate]

I'm familiar with Einstein's formulae $V=\frac{u+v}{1+ \frac{uv}{c^2}}$ and $\Delta t'=\frac{\Delta t}{\sqrt{1-\frac{v^2}{c^2}}}$, the former being velocity addition and the latter being time dilation....
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Using entanglement for communication based on the double slit experiment [duplicate]

When we perform a double slit experiment we receive interference pattern. when we measure in a specific slit the exact location of the photon (or electron) the interference pattern disappears and we ...
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Exceeding the speed of light [closed]

I understand that the speed of light c is derived from the self-interaction between elections/photons, and is thus the maximum speed of anything composed of electrons/photons. Suppose that there is a ...
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No FTL information implies no FTL travel?

The general consensus in the scientific community is that it is impossible to transmit information faster than light. There is also speculation that it might be possible to open wormholes or travel ...
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Why does my thought experiment involving entanglement appear to violate the speed of light?

I have a thought experiment that "seems" to allow messages passed at faster than the speed of light. Since that is impossible I'd like to learn what is wrong with my experiment/reasoning. Start with ...
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1answer
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What if a spaceship going half the speed of light turned on its headlights in both directions?

If the spaceship, travelling at c/2 were to turn on only its forward facing headlights, then after one second (relative to us, the stationary external observers) we would see that the ship has moved ...
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2answers
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How do we know that light cannot travel faster than it does?

We assume the speed of light in vacuum is its maximum speed but can we not assume that it could be faster, or slower?
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2answers
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Does basic QM allow for superluminal “particle movement” during wavefunction collapse?

Can particles move superluminally away from their "expected values" using basic quantum theory? Here's an example: The eigenstates of a harmonic oscillator are defined from $(-\infty, \infty)$. This ...
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6answers
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Thought experiment - faster than speed of light travel [closed]

I came up with a thought experiment and it has been on my mind for quite some time now. Would like to hear the thoughts of others as my wife has no clue what I'm talking about. Given two points of ...
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Can two electron moving in opposite direction move faster than light? [duplicate]

Suppose we are moving two electrons in opposite direction both having a speed of 1.6e8 meter per sec.When they will cross each other then their relative velocity will be 3.2e8 ms_1.Isn't it faster ...
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Can Information created by entangled particles be received faster than light by an observer?

I am the captain of a spaceship at the moon of Tellus, and my colleague Trillian is the captain of another spaceship at Proxima Centauri. I need to deliver a package to Trillian, who is going to meet ...
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Is this news article about quantum radar legit? [duplicate]

From https://www.digitaltrends.com/cool-tech/quantum-radar-stealth-aircraft/ In principle, a quantum radar functions like a regular radar — only that instead of sending out a single beam of ...
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3answers
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Velocity of massless particles

Due to the equation $E = \frac12{mv}^2$, can a massless particle travel at an infinite speed in a vacuum (as its mass would be $0$ so its energy would also be $0$)?
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How does measurement-affected spin of an entangled particle not communicate information ftl?

Am I not understanding this video correctly: https://www.youtube.com/watch?v=tafGL02EUOA&t=5m20s Since up/down or left/right orientation of the particle depends on the physicist's choice at ...
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How do we know quantum entanglement works no matter the distance?

It is said quantum entanglement works regardless of distance. 2 particles can be entangled and information is shared instantaneously, even if they are lightyears away from each other. But how do we ...
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5answers
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Is there a medium less dense than vacuum, in which light can travel faster than $c$? [closed]

Is there a medium less dense than vacuum, in which light can travel faster than $c$? If not, can we make it?
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2answers
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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 ...
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1answer
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Super luminal speed paradox? [duplicate]

Chosing a reference frame in which the Earth is at rest and doesn't rotate is related, but different. Here I'm asking if there is a paradox and if my attempt at resolution has merit, and if not ...
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3answers
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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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3answers
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What is the fastest possible acceleration?

Context & Question In physics, the fastest possible speed is accepted as the speed of light. No other mass has been recorded to travel as fast as the photon. What is the acceleration of the ...
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1answer
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What will happen if the refractive index of lens is less than unity? [duplicate]

Generally the refractive index of any lens is greater than air. My question is what will happen if the refractive index of lens is less than unity?
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1answer
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CNOT and FTL communication

Let's consider two qubits $A$ and $B$. They are related by a CNOT gate, I call $U_{NOT}$ the matrix of the CNOT. So, I have $|A^{out}\rangle |B^{out}\rangle = U_{NOT} |A^{in}\rangle |B^{in}\rangle$ ...
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Black Hole Speed of Information Paradox?

Suppose you have a non-charged, non-spinning, spherically symmetric super massive black hole. Suppose also you could manipulate a sizable mass distribution within 10% of the Schwarzchild radius ...
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5answers
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Does time dilation mean that faster than light travel is backwards time travel? [duplicate]

Ok. So my question is, I've always heard it that Faster Than Light travel is supposedly backwards time travel. However, the time dilation formula is $$T=\frac{T_0}{\sqrt{1-v^2/c^2}}$$ And while it ...
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1answer
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Speed of light, can it be not constant? [duplicate]

What if we switch on a flashlight while travelling at the speed of light (we are travelling in vacuum, in a straight path)? What will happen and what will be the speed of light from the flashlight? ...
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1answer
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Faster than light in quantum gravity?

Imagine there's two objects a light second apart in a space with a certain metric. So no signal can reach the other under a second. But in quantum gravity where we sum over metrics, there may be a ...