Tagged Questions

"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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Klein-Gordon equation and wave velocity

It looks like solutions of the KG eqn travel faster than light, because if $$\omega^2 - k^2 = m^2$$ then $$\mid\ \omega\mid \ > \ \mid k\ \mid$$ and I thought the wave velocity was $\omega / k$. ...
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Speed of light and current dimensions of the universe [duplicate]

I've seen several documentaries explaining that the diameter of the universe is currently estimated at over 90 billion light-years. And which that - in the face of the age of the universe being about ...
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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 measurement affect multi-particle entangled particles?

It is my understanding that you can have two entangled particles, A and B, and measure their spins from two different angles, X and Y, such that: If you measure A using one angle (e.g. X), there's ...
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Pendulum moving faster than speed of light

In classical mechanics, the period $T$ of a pendulum is given by $$T = 2\pi\sqrt{\frac{l}{g}},$$ where $g$ is the gravitational field and $l$ the length of the rope attaching the bob to the pivot. ...
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Light travels in a medium

According to Snell's law : $${n_1 \over n_2} = {v_2 \over v_1}$$ $v_2 = v_1 n_1 / n_2$ Assuming that $n_1$ is vacuum , we will find the following equation: $$v = c / n$$ (We may find the same ...
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Relative Velocity Problem: Light? [duplicate]

Assume 2 rockets coming towards each other at each other at 99% of speed of light, and so as they pass each other the relative velocity between 2 rockets would be 198% of Speed of Light and so what ...
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Don't photons pass photons move away at speeds faster than light? [duplicate]

I understand there is a longstanding rule that nothing travels faster than the speed of light... but if you had two flashlight aimed at each other and turned them on inside a vacuum the photons from ...
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On the no-faster-than-light in special relativity

In the special relativity it is well established that, in the vacuum no one can ever travel faster than light, due to the relativistic velocity addition formula. Recently I saw some silly statement ...
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Nothing can travel faster than the speed of the light, but could this be an exception? [duplicate]

I often show off by asking people how fast electrons flow through wires. Then I tell them it's actually only a few millimeters per second. To clear the disbelief off their faces and boost my ego, I ...
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Is it possible to travel forward and backward along with time? [closed]

Is it possible to travel along through time if we found a thing that is more speeder than light?
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Was the expansion after the big bang faster than the speed of light? [duplicate]

I think it's called Planck time and it's the speed at which matter spread during the big bang. Was the big bang expansion faster than the speed of light?
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If I send two rockets from the Earth in opposite directions, at, say, 60% of the speed of light relative to the Earth, then relative to each other they are travelling at 120% of the speed of light. ...
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Does EPR permit information to travel faster than light? [duplicate]

This question always throws me: Discuss if the EPR paradox violates special relativity? The information of the state of one particle is instantaneously transmitted to the other particle, but ...
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Is it possible to apply force to a light particle?

I know, the universal speed limit is the speed of light $c$. So, is it possible to apply more force to a light particle? What would happen to it if we could?
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Travelling at a relative speed greater than the speed of light [duplicate]

I've wondered about this since I was a kid. I've been told the speed of light is a constant ($c$), which would mean that light always travels at the same velocity. Now, here's what interests me: if I'...
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Superluminal speed?

I came by the quotation below. I'm confused: does it mean that it's actually possible for light and even material objects to move faster than light? Gravitational fields are present the velocities ...
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Is a Super-luminal Laser spot possible? [duplicate]

Note:This is not the same as Is the "How to break the speed of light" minute physics video wrong? because a larger distance is used in a different direction so that the time of flight of the ...
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Can photons travel faster than $c$? (Feynman Lectures)

I apologise for the very non-technical nature of this question. I am new to QED and perhaps am interpreting things in the wrong way, but I'll ask anyway, and hopefully someone can provide a non-...
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Would one-way FTL travel be safe from a causality standpoint?

Any demonstration I've seen of using FTL signals to violate causality seemed to involve an exchange of FTL signals. Would FTL travel be consistent with causality if it always involved motion in one ...
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Is there serious research on superluminal travel? [closed]

I was wondering if there are any serious attempts, or any ideas for loopholes in the current theories, that would enable us humans to travel with a speed greater than the speed of light?
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Can we travel faster than the speed of light? [duplicate]

So my question is completely a theoritical question. For example i have a stick made of the strongest material and lightest material and its lenght is as long as the solar system's diameter. The end ...
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Tachyons and the light barrier

I read somewhere that if something breaks the light barrier, i.e. a tachyon, that it will take infinite energy to make it go slower than the speed of light. So, does that mean that the object will ...
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Exceeding the speed of light using gravity? [duplicate]

From what I have learned, objects must accelerate as they approach a heavy mass. But I also know that they cannot travel faster than the speed of light. What if you had a proton travelling at ....
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What is the speed of gravity? [duplicate]

Hypothetically, let`s say that one day our Sun is suddenly removed from the galaxy. For us, on earth, would take up to 8.3 minutes to notice that the sun was removed, since the distance from the earth ...
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Special Relativity - Distance Not Increased?

I just watched the movie Interstellar and the question was asked of why/how high-speed travel would slow time within an area. Curious as I am, I watched several YouTube videos to quickly 'educate' ...
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How can light travel at a speed greater than the speed of light

I was reading John Rennie's answer here, Can light travel faster? I now know that the sentence ''the speed of light is always constant in vacuum'' is just an oversimplification, for example light ...
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Is dark matter moving faster than light? [closed]

My way of thinking is as follows: Consider a balloon. Let this be blown into a nearly elliptical sphere. Now, the temperature inside the balloon is increased by some means; this leads to the ...
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Expanding universe and speed of light [duplicate]

Galaxies are moving away from us proportional to the distance between us and them , but nothing can travel faster than light, so even the farmost galaxies should be travelling away from us along with ...
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Acceleration due to Gravity: Free Fall [duplicate]

If we are in a free fall which implies we are accelerating at 9.8 m/sec every sec. And let's say that we are falling into a pit that has enormous depth. So isn't this be possible that we may ...
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Thought experiment on graviton breaking the speed of light [closed]

The effect of gravity travels at the speed of light. Suppose we can entangle a pair of gravitons (which are only theoretical, but who knows for sure?) and separate them over a vast distance. Hold on - ...
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If I move in a bus moving at the speed of light wouldn't I be disproving the laws of physics? [duplicate]

I'm just in 10 grade , so please bear with me. Just imagine If a bus was travelling at the speed of light , and I was was running inside it, wouldn't I be moving faster than light?
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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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Will an object always fall at an infinite speed in a black hole?

Most of you if not everybody will agree that the stronger the gravitational pull, the faster an object will fall. For example, on a planet with 50 times the gravity of Earth, any object will hit the ...
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Is FTL communication compatible with Lorentz ether theory?

I have come to understand, that faster-than-light communication is problematic with special relativity, because it leads to the possibility of sending signals into the past. I also read about the ...
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Phase velocity greater then the speed of light?

It is my understanding the the phase velocity of a wave can be greater then the speed of light. So imagine we had a wave packet consisting of a single sinusoidal wave; $$y=\sin(\omega t-kx)$$ Then ...
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Do tachyons move faster than light?

I am trying to understand whether or not tachyons travel faster than light. The linked Wikipedia page shows some seemingly contradictory statements, and they are confusing. For instance, the first ...
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A theoretical speed of light question [duplicate]

This is a hard question to explain so bear with me. Let's say that Earth is moving at 1/3 the speed of light to the 'right' (obviously I realize there is no constant left or right). If I were to ...
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What is the factor that controls the speed of electrons accelerating in vacuum?Won't they accelerate infinitely? [duplicate]

We all know that speed of light can not be exceeded. So the question is if electric field accelerates electron in a vacuum tube with enormous length won't the electron accelerate till a point where it ...
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Faster than light travel at half speed [duplicate]

Peter and Susan are facing each other in outer space. Both of them ignite their their incredibly powerful jet pack thrusters and accelerated away from each other to just over half the speed of light. ...
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Would an Alcubierre drive actually allow FTL travel? [duplicate]

If I do not phrase this question right, please forgive me in advance because I am a layman on the subject of physics, but a software engineer, nonetheless. I do understand many technical subjects. ...
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Did the Earth (amongst other things) travel faster than light?

It's often said that when you look far across space with a telescope you are looking back in time, as the light has only just reached our position in space. However, given that the Earth and many ...
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Does the induced Electric field develop instantly or lags by $\frac rc$?

Consider a loop of radius $r_0=3 \times 10^8$ cm. A thin bar magnet is passed through its center. This implies that the magnetic flux through the loop will change. Now according to Faraday's law a non ...
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Does a force propagate instantaneously to move the other side of the box of length one light-year? [duplicate]

This is a question of curiosity, and not for any academic purposes. Suppose I push a box along a surface that allows it to slide. When I initially start pushing it on one side, how long does it take ...
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Sending information faster than light

If I could ever send my friend any information faster than light it would violate causality. If he just guesses the information and acts on it before he could ever recieve it, everything is fine. What ...
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Does light have initial velocity? [duplicate]

You are standing still in a vacuum, and you throw a ball that moves 10 m/s away from you. Now you start moving in one direction at 5 m/s, and throw the ball in the other direction, away from the ...
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Why does moving near light speed keep you young? [duplicate]

As was shown in the movie Interstellar, when you go close to a black hole or move near the speed of light in space, then your clock runs slower than a clock on Earth. In the movie, Matthew ...
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Why can a particle have a nonzero amplitude outside its forward light-cone?

I'm having trouble grasping an idea that I think that is a very basic part of  quantum field theory. Many introductory QFT resources I have consulted often pose the following question: What is ...
Based on the classical light-cone approach it's easy to see you can't transmit information faster than $c$ but why does the speed of light (as far as I know) treat information transmission in this way ...