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Questions tagged [speed-of-light]

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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39k views

How does gravity escape a black hole?

My understanding is that light can not escape from within a black hole (within the event horizon). I've also heard that information cannot propagate faster than the speed of light. It would seem to ...
41k views

Do we know why there is a speed limit in our universe?

This question is about why we have a universal speed limit (the speed of light in vacuum). Is there a more fundamental law that tells us why this is? I'm not asking why the speed limit is equal to $c$...
31k views

If all motion is relative, how does light have a finite speed?

I've often heard that Einstein shattered the notion of absolute motion (i.e. all things move relative to one another) and that he established the speed of light as being absolute. That sounds ...
13k views

Is it possible for information to be transmitted faster than light by using a rigid pole?

Is it possible for information (like 1 and 0s) to be transmitted faster than light? For instance, take a rigid pole of several AU in length. Now say you have a person on each end, and one of them ...
13k views

Why is light bent but not accelerated?

Light is bent near a mass (for example when passing close to the sun as demonstrated in the famous sun eclipse of 1919). I interpret this as an effect of gravity on the light. However, it seems (to ...
18k views

How did Einstein know the speed of light was constant?

I often hear the story of how Einstein came up to the conclusion that time would slow down the faster you move, because the speed of light has to remain the same. My question is, how did Einstein ...
10k views

What really causes light/photons to appear slower in media?

I know that if we solve the Maxwell equation, we will end up with the phase velocity of light being related to the permeability and the permittivity of the material. But this is not what I'm ...
11k views

Why is there no absolute maximum temperature?

If temperature makes particles vibrate faster, and movement is limited by the speed of light, then temperature must be limited as well I would assume. Why there is no limits?
16k views

If the speed of light is constant, why can't it escape a black hole?

When speed is the path traveled in a given time and the path is constant, as it is for $c$, why can't light escape a black hole? It may take a long time to happen but shouldn't there be some light ...
10k views

Could a human beat light in a footrace?

Is there anything preventing the following experiment from being done right now? Imagine that a human ran from point 'a' to point 'b' while light particles that reflected off a clock moved ...
11k views

If a mass moves close to the speed of light, does it turn into a black hole?

I'm a big fan of the podcast Astronomy Cast and a while back I was listening to a Q&A episode they did. A listener sent in a question that I found fascinating and have been wondering about ever ...
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If I run along the aisle of a bus traveling at (almost) the speed of light, can I travel faster than the speed of light?

Let's say I fire a bus through space at (almost) the speed of light in vacuum. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus toward the front, does that mean I'm ...
24k views

What is the mechanism behind the slowdown of light/photons in a transparent medium?

So light travels slower in glass (for example) than in a vacuum. What causes light to slow down? Or: How does it slow down? If light passes through the medium, is it not essentially traveling in the "...
10k views

Why can't the light from a candle light the whole of a dark room?

If you light a candle it starts emitting photons, right? And photons travel at the speed of light. But how come they can't light the whole of a dark room? It means that the photons are not reaching ...
13k views

Home experiments to derive the speed of light?

Are there any experiments I can do to derive the speed of light with only common household tools?
145k views

Would time freeze if you could travel at the speed of light?

I read with interest about Einstein's Theory of Relativity and his proposition about the speed of light being the universal speed limit. So, if I were to travel in a spacecraft at (practically) the ...
16k views

Why does the speed of light in vacuum have no uncertainty?

I could understand that the definition of a second wouldn't have an uncertainty when related to the transition of the Cs atom, so it doesn't have an error because it's an absolute reference and we ...
8k views

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, ...
37k views

Why and how is the speed of light in vacuum constant, i.e., independent of reference frame?

I was told that the Galilean relative velocity rule does not apply to the speed of light. No matter how fast two objects are moving, the speed of light will remain same for both of them. How and why ...
6k views

2002 research: speed of light slowing down?

Back in 2002 there was some research published hinting that $c$ may have been faster at some distant point. It was based on measurements of the fine-structure constant,  \alpha = \frac1{4\pi\...
9k views

If I am travelling on a car at around 60 km/h, and I shine a light, does that mean that the light is travelling faster than the speed of light?

The title says it all. If I was on a bus at 60 km/h, and I started walking on the bus at a steady pace of 5 km/h, then I'd technically be moving at 65 km/h, right? So my son posed me an interesting ...
44k views

How can a photon have no mass and still travel at the speed of light?

I've read a number of the helpful Q&As on photons that mention the mass/mass-less issue. Do I understand correctly that the idea of mass-less (a rest mass of 0) may be just a convention to make ...
9k views

Doesn't the speed of light limit imply the same electron can be annihilated twice?

If I understand correctly, there is a small probability the same electron to be found anywhere in the universe. Suppose that an anti-electron collides with an electron, annihilating it and producing ...
16k views

How does light speed up after coming out of a glass slab?

As I learned today in school, my teacher told me that when light enters a glass slab it slows down due to the change in density and it speeds up as it goes out of the glass slab. This causes a lateral ...
8k views

Without the Michelson-Morley experiment, is there any other reason to think speed of light is the universal speed limit?

If the Michelson-Morley experiment hadn't been conducted, are there any other reasons to think, from the experimental evidence available at that time, that Einstein could think of the Special Theory ...
17k views

Why doesn't light, which travels faster than sound, produce a sonic boom?

I know that when an object exceeds the speed of sound ($340$ m/s) a sonic boom is produced. Light which travels at $300,000,000$ m/s, much more than the speed of sound but doesn't produce a sonic ...
6k views

How does light re-accelerate after slowing down? [duplicate]

Light travels at speed x through a vacuum, and then it encounters a physical medium and slows down, only to leave the physical medium and re-enter vacuum. The speed of light immediately re-accelerates ...
8k views

How would night sky look like if the speed of light was infinite?

Would it be brighter? Different color? Gravitational lensing? Would black holes exist?
12k views

Does old light contain clues to its age?

Light from celestial objects is old. In the case of galaxies, it's millions of years old. It seems plausible to me that light might show signs of its age. I was surprised that a Google search only ...
13k views

Is light actually faster than what our present measurements tell us?

It is well established that the light speed in a perfect vacuum is roughly $3\times 10^8 \:\rm m/s$. But it is also known that outer space is not a perfect vacuum, but a hard vacuum. So, is the speed ...
25k views

Speed of light vs speed of electricity

If I arranged an experiment where light raced electricity what would be the results? Let's say a red laser is fired at the same time a switch is closed that applies 110 volts to a 12 gauge loop of ...
5k views

Why is a black hole black?

In general relativity (ignoring Hawking radiation), why is a black hole black? Why nothing, not even light, can escape from inside a black hole? To make the question simpler, say, why is a ...
6k views

What is the proof that the universal constants ($G$, $\hbar$, $\ldots$) are really constant in time and space?

Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain ...
4k views

Were the Michelson-Morley results a surprise?

How unexpected were the Michelson-Morley experiment results? Did physicists have theoretical reasons to predict that the speed of light would result to be invariant?
14k views

Why can't we see light travelling from point A to B?

Let's say we have a cloud of dust which is a lightyear across and someone shoots a beam of light from point A to B , why it is not possible for an observer far far away to see the light while it ...
8k views

What is so special about speed of light in vacuum?

I will try to be as explanatory as possible with my question. Please also note that I have done my share of googling and I am looking for simple language preferable with some example so that I can get ...
12k views

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 ...
12k views

Can you run away from your shadow?

Now this might be a silly question but it's actually bugging me, this one might be easier to understand if you have kids that watch (or used to watch) Peppa Pig. In one of the episodes, about shadows, ...
11k views

How can a black hole reduce the speed of light?

If the speed of light is always constant then light should escape from a black hole because if directed radially outwards it only needs to travel a finite distance to escape, and at a speed of $c$ it ...
167k views

Why does wavelength change as light enters a different medium?

When light waves enter a medium of higher refractive index than the previous, why is it that: Its wavelength decreases? The frequency of it has to stay the same?
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Does gravity slow the speed that light travels?

Does gravity slow the speed that light travels? Can we actual measure the time it takes light from the sun to reach us? Is that light delayed as it climbs out of the sun's gravity well?