# Tagged Questions

The speed of light is a fundamental universal constant that marks the maximum speed at which energy and information can propagate. Its value is $299792458\frac{\mathrm{m}}{\mathrm{s}}$.

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### Determining Ether Drag - Aberration of Starlight

In order to determine the relative motion between the Earth and the ether (the medium through which light supposedly propagated. It has zero density and complete transparency), scientists used the ...
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### Why does the speed of light $c$ have the value it does? [duplicate]

Why does light have the speed it does? why is it not considerably faster or slower than it is? I can't imagine science, being what it is, not pursuing a rational scientific explanation for the speed ...
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### Could light travel more slowly than the “universal speed limit”? Could this imply quantization of spacetime?

One description of relativistic effects that I've heard/read goes something like this: Everything moves through spacetime at a constant speed. An object's direction of travel through spacetime can ...
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### What starts the movement of a photon

Although a photon has no (rest) mass, it does have a measurable speed. Its movement can be altered by gravity. A photon "travels". If I turn on a flashlight, seen by someone at a distance, the photons ...
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### How the effect travel's? [duplicate]

Let us assume that we placed lot's of ball touching each other in a hollow cylindrical tube, now if we push one ball at the end the ball at the other end move's instantly. So how do the information ...
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### Angular velocity and the speed of light

So, accelerate a particle to just under the speed of light, then send it round a bend, technically it accelerates due to the change of direction, will its speed exceed or match $c$? Keep it simple, I'...
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### Why does light not refract when incidented perpendicularly?

I had read that light does slow down in glass because photons interact with atoms in glass. They are absorbed and re-emitted and during this phenomenon it's speed decreases. See also this and this ...
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### Can something travel faster than light if it has always been travelling faster than light?

I know there are zillions of questions about faster than light travel, but please hear me out. According to special relativity, it is impossible to accelerate something to the speed of light. However, ...
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### How would time duration and space distance change when we move in the speed of light? [closed]

It is possible to figure out how time duration and space distance change when we are close to the speed of light - but I am not sure what happens to them if we travel in the speed of light.
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### Where do photons get their energy from?

If energy required to accelerate a particle to the speed of light is infinite then where do they get it from? But first if photon's are massless, then why do they collide to some other thing and get ...
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### Why can't we accelerate to the speed of light? [duplicate]

Why can't we accelerate to the speed of light? It's just a speed and nothing else. Universe also crossed the speed of light at the time of big bang. Is this is just a interpretation or there is any ...
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### Light coming from a object travelling at speed of light?

Let us assume that an object is travelling at speed of light and a flash light is kept pointed in the opposite direction to the velocity of object. Then can we see the beam of light coming out from ...
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### Faster than the speed of light and future travel [closed]

I had read somewhere that if a person attains twice the speed of light, he can actually reach the future. But since the world belongs to one time and space, how will it change for one particular ...
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### Speed of Light in a Medium [duplicate]

For light travelling in a medium with refractive index greater than one: The "average" speed of light is slower than the speed of light in a vacuum. As far as I know, the instantaneous speed of light ...
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### Life of a photon [duplicate]

I am a student of class 12th and as far as i know when anything reaches about 99.99% of the speed of light it starts traveling in time or time for it slows down so that it don't breaks the speed ...
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### Can the speed of an electromagnetic wave be measured in the absence of neutrinos?

Let me explain better: from what I understand neutrinos are so pervasive they are literally everywhere. And since they have such a tiny electric charge they barely interact with anything and cannot be ...
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### How does a photon experience space and time?

To an an external observer it appears that time has stopped for photon. But this relation is reflexive, so for an observer travelling with the photon it appears the universe has stopped everywhere. ...
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### What is a photon's speed inside a dieletric?

We know that EM waves are slowed down in a dielectric. But at what speed does the photons that make up the wave travel? Do they always travel at the speed $c$, but colliding/being absorbed and re-...
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### Clarification on factors of $c$ in the Lorentz' force

I was told that Lorentz' force is given by $${\bf F}= q{\bf v} \times {\bf B}.$$ But I have read that it is given by $${\bf F}= \frac{q}{c}{\bf v} \times {\bf B}.$$ Why have these two relations ...
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### How accurate are our calculations about distant stars keeping in mind their enormous distances?

Since many stars are hundreds of light years away from the Earth and therefore, what we observe of them today is really their distant past, how can we say anything with certainty about their ...
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### If we are travelling with the speed of light, can we see what's behind us? [closed]

If we are travelling with the speed of light, can we see whats behind us(like if we are moving away from earth can we able see the earth)? And how we see the things that we are approaching with speed ...
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### Run with speed of light with a mirror in hand [duplicate]

Possible Duplicate: Reflection At Speed of Light Imagine you are able to run with the speed of light holding a mirror in your hand. Now will you be able to see yourself in the mirror?
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### What is the maximum time dilation between two objects, if one is standing still and the other is moving at $c$?

What is the maximum ratio in the rate of change in time in reference to object $A$ which is standing still and object $B$ which is moving at the speed of light?
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### Understanding that nothing can go beyond the speed of light [duplicate]

Possible Duplicate: Is the “How to break the speed of light” minute physics video wrong? Radius of the moon - 1737 km. Thus circumference of the side that can been seen from earth - pi * 1737 ...
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### Does light accelerate or slow down during reflection?

After all, it does change direction when reflection occurs. So shouldn't it also accelerate? And since the acceleration cannot increase the speed of light, mustn't it slow down?
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### Speed of light in a given direction based on frame of reference [duplicate]

Possible Duplicate: How is the speed of light constant in all directions for all observers? Is the speed of light constant regardless of frame of reference? If not, why is it still measured as ...
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### Speed of light and lorentzian factors [duplicate]

Possible Duplicate: How can a photon have no mass and still travel at the speed of light? If light travels at the speed of light, and anything with rest mass will experience relativistic ...
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### What exactly are we doing when we set $c=1$?

I understand the idea of swapping from unit systems, say from ms$^{-1}$ to kms$^{-1}$, but why can we just delete the units altogether? My question is: what exactly are we doing when we say that $c=1$...
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### Time taken for gravity of a distant object to interact with a newly created particle? [duplicate]

Possible Duplicate: The speed of gravity Does gravity travel at the speed of light? Imagine there is a large mass $m_1$ (e.g. a star) 1 light-year away from us. It is stable, stationary ...
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### Time for Light to travel from most distant objects in universe

I understand that it is possible to detect the most distant objects in space over 13 billion light years away from us and that the universe is 13.75 billion years old. Does this imply that there were ...
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### How is the speed of light constant in all directions for all observers?

Please imagine the following thought-experiment: Order of Events: Pulse - A single pulse of light is emitted from the light towards the mirror Reflect - The pulse hits the mirror and is ...
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### Speed astronauts measure moving at the speed of light

Lets suppose a spaceship travels with v = 0.9c relative to the Earth. The time inside the spaceship would pass slower than on Earth. Would the astronauts measure a different speed (that means, a ...
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### What can $E=mc^2$ do? [closed]

In the famous equation $E=mc^2$, the variables stand for: $E$ is energy, $m$ is mass, and $c$ is the speed of light (in vacuum). And I understand the equation fairly but limited in knowing in ...
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### If airplanes' highest speed depends directly on the air it is moving through and not the ground,

If airplanes' highest speed depends directly on the air it is moving through and not the ground, does it mean we can build a time machine if only the air moves near the speed of light?
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### Extended Rigid Bodies in Special Relativity

I was reading Landau & Lifshitz's Classical Theory of Fields and I noticed that they mention that an extended rigid body isn't "relativistically correct". For example, if you consider a rigid ...
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### Is it really possible to break the speed of light by flicking your wrist with a laser pointer?

Minutephysics has a popular YouTube video called "How to break the speed of light". In the video it states that if you flick your wrist while pointing a laser that reaches the moon, that the spot of ...
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### How is quantum mechanics compatible with the speed of light limit?

Consider a free electron in space. Let us suppose we measure its position to be at point A with a high degree of accuracy at time 0. If I recall my QM correctly, as time passes the wave function ...
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### The definition of an inertial reference frame in Einstein's relativity

I'm reading Sean Carroll's book on general relativity, and I have a question about the definition of an inertial reference frame. In the first chapter that's dedicated to special relativity, the ...
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### Photons and Relativity

Consider a Photon from Sun and travels with a velocity $c$. Now think we are that photon. For us, it looks like Sun is moving away from us with a velocity $c$. So, why don't we get attracted back ...
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### Calculating the speed of an object moving relative to another moving Object (Frame of reference) [duplicate]

Possible Duplicate: How to deduce the theorem of addition of velocities? Let's say that you are in a rocket speeding at 90% the speed of light away from Earth. Now fire a bullet inside the ...
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### What does a sphere moving close to the speed of light look like?

What shape does the viewer in a reference frame with $v=0$ perceive? I suppose that since the sphere moves in one direction only (oX only, not oY) its section would change into an ellipse, where the ...
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### Question on “new CNGS/OPERA measurement of neutrino velocity” (hep-ex/1212.1276)

A "new measurement of the limit on muon neutrino velocity with respect to the speed of light " by the OPERA neutrino experiment in collaboration with CERN has become public recently, as preprint http:/...
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### Why doesn't pushing balls in a tube propagate the movement faster than the SoL? [duplicate]

Possible Duplicate: Is it possible for information to be transmitted faster than light by using a rigid pole? On one episode of QI they asked the question, "How fast do electrons move ...
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### Is there a way to formulate relativistic dynamics in a way that “hides” the finite speed of light?

I'm not referring to the dimensional choice that makes $c=1$; rather I'm imagining something more about replacing all references that apparently involve velocities with the appropriate $\gamma$ ...
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### Questions about the speed of light and the extent of the universe

What is the reason that speed of light is the maximum speed in the universe? Why there is no speed swifter than light? Or is it just we haven't found yet? A similar question is like this: why the ...
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### Photons arriving from the Sun

Given that the Sun is a bit less than 10 light minutes away from Earth, is it correct to assume in principle (I understand actual processes in the core of the Sun make the situation at a photon's ...
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### The Effects of Moving Matter Across Light-Year distances

If I were to stand at one end of a light-year long metal pole, and another person were to stand one light-year away at the other end, and then I were to push on my end of the pole. How long would it ...
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### Speed of light in a vacuum

I see many references to the speed of light in a vacuum implying that it is only truly a constant measurement in a vacuum. I can live with that, but what kind of vacuum? Are we still talking about ...