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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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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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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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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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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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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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Faster-Than-Light Alcubierre Warp Drive Black and White Holes?

As you may know, the Alcubierre Warp Drive is a hypothetical system for space travel where the ship generates a space compression in the front of the ship and a dilation at the back,thus propelling ...
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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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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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Wheel that surpasses the speed of light? [duplicate]

I figure that the following scenario is impossible, but I figured I'd ask anyway. Assuming that a bike wheel has unending spokes, the rotational speed of the axle of the wheel should be greater than ...
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Ultimate Speed limit [duplicate]

I've been studying the chapter 'relativity' from this book 'Concepts of Modern Physics by Beiser' where I saw this enclosed article , which says that if u are traveling in a spaceship moving faster ...
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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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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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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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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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Can an absolutely unelastic object move faster than light? [duplicate]

When i emit a physical impulse on an object, the time it takes for this object to react to the impulse decreases the higher its elasticity is - but wouldnt that mean that if an object has an ...
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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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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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Is the speed of light constant regardless of the frame of reference? [duplicate]

I'm confused; I understand that the speed of light is constant for any observer, and that heading towards or away from a light source only causes it to blue or red shift (increase or decrease ...
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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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How can Tachyons fit in in an invariant speed theory like special relativity? [duplicate]

I mean, special relativity starts with the assumptions that the laws of mechanics are the same in every inertial frame of reference and that the speed of light is an invariant speed, constant for ...
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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 ...
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2answers
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Why doesn't Faster Than Sound information transfer imply time travel or violate causality? [closed]

I am struggling to understand the answer to What are some scenarios where FTL information transfer would violate causality? In particular, I have not seen a satisfactory rebuttal to the comment from ...
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Does the Einsteins theory of a clock tower observed from a tram really explain time dilation?

We were shown a video in our class about time dilation which explained a Einstein's theory - the one where he was travelling in a tram and realized that if he travels at light speed, time will flow ...
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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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How is time dilation associated with a ship with an Alcubierre drive?

Would a round trip cause the astronaut to age less or more or the same as an Earthbound twin? Normally, time dilation works as length contraction (an astronaut would see length contract as 1/gamma, ...
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What is a light cone? [closed]

What is a light cone? Why we can't escape the light cone? Why the speed of light being the limit for us to escape the cone the future and the past events of the light cone is that governs the future? ...
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Travelling at lights speed of light as universe expanding

I have read that the object, the light of which is reached us in 13.8 billion years is actually about 46 billion light years away from us now, due to the expanding Universe. Lets assume that we ...
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Why is the observable universe 90 billion light years in diametre? [duplicate]

The question is fairly simple, we know that the universe has been expanding for 13.8 billion years, so how did they determine the size of the universe that we can observe, and why can we not peek ...
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Speed of Light As a Maximum Speed Limit [duplicate]

I know the speed of light problem has been beaten do death by hundreds of questions and answers, but I wanted to explore a new possibility and perhaps show an indication that light-speed as a constant ...
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How can the Alcubierre drive itself move faster than light?

Assume that we have constructed an Alcubierre drive (we somehow managed to overcome all the difficulties). Now we want to want to move an object from galaxy A to galaxy B. We place it in the ...
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Can it be that apparent faster than light speed of shadow/laser dot is not actually faster than speed of light? [duplicate]

Many state that in fact laser dot or shadow can move faster than light but I beg to differ. My question, rather statement would be that nothing, not even apparent illusion of shadow or laser dot is ...
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Can you outrun a photon inside the event horizon of a black hole?

I was watching the YouToube video How Time Becomes Space Inside a Black Hole, by PBS Space Time, about how space and time flip within the event horizon. The way the narrator seems to explain it, the ...
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1answer
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Interpreting doubling of metric

Say you had coordinates $(x,y,z,t)$ and a beam of light traveling at speed=1. Now you make a change of coordinates $x'=\frac{1}{2}x, y'=\frac{1}{2}y, z'=\frac{1}{2}z, t'=t$ So now you have a metric ...
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How can the universe expand faster than the speed of light? [duplicate]

In Neil deGrasse Tyson's Astrophysics for People in a Hurry, he writes: By now, one second of time has passed. * The universe has grown to a few light-years across... If nothing can move ...
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How is traveling back in time different from traveling at a negative velocity? [closed]

A simplified explanation of why you can't travel through space faster than the speed of light is that you are already traveling at the speed of light, through spacetime. If you are stationary in ...
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1answer
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Seeing the Big Bang [duplicate]

The Big Bang happened almost 14 Billion years ago. Theoretically speaking , if we could teleport (Instant teleportation) to somewhere that’s 20 Billion light years away from the Earth , if we looked ...
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Why do the velocity components of molecules lose their independence when their velocity is more than speed of light?

In maxwell velocity distribution, maxwell assumed that the velocities of molecules are smaller than light, if the velocities are greater than light, then all the three components will no longer ...
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What's the fuss over quantum entanglement about? Is it because people have misunderstood? [closed]

The first video I've seen about quantum entanglement claimed that changing A's spin will immediately take effect on B's spin as well. The more I read into this topic the less it seems that this is ...
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Faster-than-light communication using Taylor's theorem? [closed]

I was thinking about Taylor's theorem and how if a function $f(x)$ is analytic at a point $a$ and one can measure all the derivatives at $a$, $f^{(n)}(a)$ then one knows the complete behaviour of the ...
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Questions on Bell's theorem and Disproving the hidden variable theory

I'm not a physicist (computer science background), but always was fascinated with the concept of entanglement. As many, the first intuition I had was - well, the two particles share some knowledge ...
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1answer
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Check where am I wrong. Here is how information can be sent faster than light [duplicate]

If I have a very long rigid rod which is of rectangular cross section and small mass. Let it's length be 1 light year and total mass be 1 kg now at one end if I rotate that rod, since the rod is rigid ...
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Is the common interpretation of quantum entanglement just a perversion? [duplicate]

The idea of spooky action at a distance seems to imply that the information was always present at each end, but the common interpretation is that information can be transmitted instantaneously. Is ...