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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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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Two photons in a single universe [duplicate]

Since speed is relative. If the universe only ever had two photons in it that were heading towards each other. Relative to one of the photons, wouldn't the other be approaching it from twice light ...
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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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Can an object exceed the speed of light? [closed]

Consider an object travelling at speed $c/2 $. Consider another object travelling at speed $3c/2$ parallel to the first object but in opposite direction. If we are in the frame of the first object ...
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Alcubierre (warp) drive - a book for laymen

Recently I read article An Examination of Warp Theory and Technology to Determine the State of the Art and Feasibility by J.F.X. Agnew on state-of-art and feasibiity of warp drive. I started to be ...
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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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Calculate distance of galaxy 13 billion years ago [duplicate]

How can I calculate the distance of a point from Earth now vs 13 billion years ago? The excerpt below claims it is 8 times further now. Can someone provide relevant equations ?
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What would be the energy requirements for non FTL warp drive? [closed]

I have a question about Alcubierre Drive, or other Warp Drive concepts. I have read a lot about FTL warp drive and the massive energy requirements to perform a jump, varying from bigger than the ...
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If the method of FTL cannot be observed, does it still break causality [duplicate]

So, say we discover a method of FTL. However, anyone or anything that makes use of this FTL is unable to be observed by the outside world. For example. In the year 3000 a ship leaves Earth headed 6 ...
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Since gravitational waves travel at the speed of light how are Alcubierre warp drives possible?

There are clearly some very large technical challenges to overcome before actually making one but most writers seem to think Alcubierre warp drives are possible in principle. They work by propagating ...
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An electron in time

I've read many times now that when one measures the spin of an entangled electron (for example) the state of its partner is known instantaneously and this is true across any theoretical distance. ...
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Massive particles in de Sitter space

Can massive particles in de Sitter space move faster than light? For the radial coordinate (in static coordinates) I have got the hyperbolic expression $$r(\tau)\propto \sinh\left(\sqrt{\frac{\...
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Faster than light communication via entanglement? [closed]

Let's say site S and site S' have a very large number N of pairs of entangled particles: $(p_1 p_1') (p_2 p_2') \ldots (p_N p_N')$. so particles $p_1 p_2 \ldots p_N$ are located at site S and ...
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Information Propagation at the Speed of Light [duplicate]

Imagine a thought experiment: You have a rod 10 ft or 10 light nanoseconds long. On each side you have perfectly synchronized super high-speed cameras looking at the surface of the rod. You rotate ...
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Can an object achieve speed greater than speed of light? [duplicate]

An object of mass 1 kg at rest experiences a force of 1 N in space with no resistance force. What will be the speed of the object after 20 years? If I go with the high school physics. F=ma ; gives ...
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Is the speed of quantum entanglement interaction greather than $c$? [duplicate]

How much time should not elapse between measurements of e- and e+ spin computed at a distance ot 1km to not detect effects of the entanglement? There should be a finite speed? Maybe greater than $c$?...
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How is the speed of light constant in this situation?

Let's take an example, where A is a photon moving with the speed of light, and B is a car, that moves with half of the speed of light At t=0, a kid in B(thinking that the car is not moving) observes ...
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LHC on train tracks? [duplicate]

If the LHC can accelerate a proton to within 3 to 4 m/s of the speed of light, as a point of curiosity, what would happen if the entire LHC loop itself were constructed on train tracks, such that the ...
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Can the image of an object travel faster than the speed of light? [duplicate]

supposing an object moves from f to 2f of a convex lens at the speed of light, then will the image which moves from infinity to 2f of the lens be moving faster than the speed of light?
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Closed Timelike Curves and relatively stationary wormholes

Apologies in advance if this question is off topic, I wasn't sure if this was the most appropriate SE forum or not. Wormholes are one of the most often cited examples of a possibility for faster than ...
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Special Special Relativity Question [duplicate]

According to what I've learned from physics professors, no object can go faster than the speed of light. So if you have object A moving .8c one way and object B moving .8c the opposite way...relative ...
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Speed Of light spot [duplicate]

Let us suppose I have an laser.I'm swinging my hand so that the spot of light moves in screen. Screen is at arbitrary distance so when I swing my hands the spot go up and down on screen, Now It is ...
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Is it possible to effectively travel faster than the speed of light?

In his last book, Brief Answers to the Big Questions, Stephen Hawking states that: If one couldn't go faster than light the round trip from us (...) to the centre of the galaxy would take about 50,...
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Sky faster than light motion [duplicate]

Recently,I have found out, that if we consider the speeds at which stars or other space objects move from the point of view of the Earth, we can calculate that they would be faster than light, because ...
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Need help understanding the argument that “faster than light motion does not imply time travel” [closed]

The paper in question is https://arxiv.org/abs/1407.2528 . But it contradicts known theory. Could someone ELI5? Is it legit? Are there any gotchas or catches? Abstract Seeing the many examples ...
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Alcubierre metric without geometrized units

In the beginning of Alcubierre's paper, he defines his metric with the use of geometrized units ($G = c = 1$). While it makes the math simpler, it seems to hide the physical numbers away, leaving me ...
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Does quantum entanglement imply a universal “now”? [duplicate]

If a particle on earth is entangled with a partner particle in Andromeda, and the wave function collapses here on earth, the wave function would, as I understand it, collapse in Andromeda too. Does ...
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Can decoherence provide a loophole for FTL communication in a system of multiple entangled particles?

I am a layperson when it comes to the study of quantum mechanics and do not quite have the expertise to properly evaluate the questions I am going to post here. But here it goes. Suppose three ...
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Quantum entanglement and a possible loophole for FTL communication? [closed]

Does affecting particle {a} in such a way that it moves cause particle {b} to move? If so can we instead of measuring the particles, have them in a box that is made of sensors, such that if the ...
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Black holes, and faster than light communication

Lets say we have a black hole that has a diameter > 1 light year and we can somehow push and pull this black hole {though the push and pull itself can never exceed lightspeed of course}.Will the other ...
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What would be the perceived energy of two objects going near the speed of light in opposite directions?

Let's say there are two objects going nearly the speed of light, in opposite directions. Obviously from the vantage point of #1, #2 would be moving at about the speed of light, c. But assuming #1 ...
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Why is this $v > c$ example wrong? [duplicate]

Take an acceleration equal to g for 1 year. Isn't the speed of light reached? And exceeded after this time? In the Indian journal Pramana Soviet's scientists made this argument. They explained we ...
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Why is electricity not faster than light if all the electrons are just shifted in the wire?

https://www.youtube.com/watch?v=jbi7gJTPSXk Please check 1:33. Many people said that all the electrons are just shifted at the same time when a current flows in the wire. So electricity can move ...
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Why can't we calculate the absolute velocity of the Earth using the speed of light? [duplicate]

We know that the speed of light is always constant in space no matter the speed of the object emitting light, right? Now consider this. An object is moving at half the speed of light and shined 2 ...
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Can a black hole accelerate matter / light particles beyond the speed of light? [duplicate]

We know that light cannot escape the gravitational pull of the event horizon of a blackhole. This made me wonder if falling into the event horizon would accelerate the matter / light particles beyond ...
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How can we measure if something is travelling faster than speed of light?

The answer that brought this question in my mind: https://physics.stackexchange.com/a/298975/249090 Cant it be that the way we measure the speeds cant measure them faster than that? And for that ...
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Is relativistic statistical mechanics “foundationally” relativistic?

Does relativistic statistical mechanics average over particles that exceed the speed of light? This article seems to imply as such: It is interesting to note that the classical and quantum ...
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Is wave function collapse non-local or local?

Imagine a particle in a very large box which takes years to travel from one end to the other. Alice and Bob are outside the box, on opposing ends. Each can remove their side of the box to check if the ...
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Quantum entanglement and information transfer faster than light [closed]

According to quantum entanglement theory, if the spins of two particles are in superposition and then spin of one particle is observed, the spin of the other particle is determined immediately. While ...
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How would faster-than-light object traveling through space look like to us on Earth? [closed]

Inspired by this question on Worldbuilding, I'm intrigued by a question : How would an object, traveling faster than light through Solar System, appear to us on Earth? Lets assume that object is a ...
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Can the expansion of the Universe be faster than the speed of light? [duplicate]

I am reading the textbook "The Cosmic Perspective 8th ed.", in the chapter describing the earliest moments of the Big Bang, one of the sentences is [...] a sudden and dramatic expansion of the ...
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Escape a black hole by tunneling faster then the speed of light [closed]

I do understand that nothing can escape a BH (not particles, no information), and this question is not about why you cannot escape. This question is not about Hawking radiation, and I do understand ...
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If I throw a ball at a black hole, will the ball exceed the speed of light when it reaches the event horizon? [duplicate]

The escape velocity of a black hole at the event horizon is the speed of light, this means an object that is dropped from the top of the black hole's gravitational well will reach the speed of light (...
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What is a good way to visualize the movement of an Alcubierre drive? [closed]

So I know that an AD works by pinching space-time in front of the ship and expanding it behind the ship. A good way to describe gravity is having a blanket stretched out on all four corners and ...
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Is spacetime really not physical? [closed]

This question has nothing to do with the expansion of space or the speed of the expansion. I do understand that space expansion is not constrained by the speed of light limit, but I am not asking ...

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