# Tagged Questions

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

4answers
582 views

### What if we could give photons some mass?

I was reading an article and these paragraphs got me wondering... Before I list the replies, here is some background. The Higgs mechanism describes an invisible field that, it is argued, split one ...
1answer
253 views

### Speed of Light, Photons or WaveSpeed?

The speed of light is almost 300 000 km/s. The photons have a speed along the wave, and the wave have a speed straight forwards. What is the speed of light? Is it the speed the photons have along ...
2answers
890 views

### why is mechanical waves faster in denser medium while EM waves slower?

Why is it that mechanical waves/longitudinal waves/sound travel faster in a denser/stiffer medium as in steel compared to say air, while EM waves/trasverse waves/light travels slower in a (optically) ...
1answer
174 views

### $E=mc^2$ why is it $c^2$ and not just $c$?

Why is constant for the conversion of mass to energy square of the ligths speed? is it bedside it's the fastest real matter?
2answers
149 views

### What are the implications of the speed of light broken? [duplicate]

Possible Duplicate: What would be the effects on theoretical physics if neutrinos go faster than light? I don't know if it's been asked before, but I couldn't find a thread about it. I ...
2answers
357 views

### Do gravitational waves slow down as they pass through matter?

I've heard that gravitational waves travel at the speed of light, and have some parallels to electromagnetic waves. EM waves slow down as they pass through matter (speed of light in glass is slower ...
2answers
210 views

### Does gravitational differences affect the distance light travels? (a thought experiment)

My question is: How is light passing through different gravitational densities affected? The value of "c" is constant in a vacuum. I'm curious about if various time frames have any effect. This is ...
2answers
221 views

### What happens to body chemistry at the speed of light?

Assume that I'm traveling at the speed of light in one direction. My brain is also traveling at the speed of light in that direction. Presumably there is at least one receptor site in my brain that is ...
3answers
889 views

### Does a photon have a rest frame?

Quite a few of the questions given on this site mention a photon having a rest frame such as it having a zero mass in its rest frame. I find this contradictory since photons must travel at the seed of ...
2answers
363 views

### At what fraction of the speed of light have people traveled?

I'm guessing that, this would be someone in a rocket or something... When they hit their top speed, at what fraction of $c$ are they traveling?
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215 views

### Does the Special Theory of Relativity “form” the foundation of Modern Physics?

My question is in reference to Geoff Brumfiel's Scientific American article "Particles Found to Travel Faster than Speed of Light", about which I have two questions. I have become engaged in ...
4answers
1k views

### Why is the speed-of-light “the upper limit” rather than the speed of “particle type X”?

Basically, I can't stop wondering why light (the photon) is so special, compared to all the other particles known (and unknown) to modern day physics. Could it be that there exists an upper limit on ...
1answer
194 views

### The relation between the speed of light and the Big Bang Theory

I would like to know how much of the Big Bang theory is dependent on the constancy of the speed of light. P.S.: It might be guessed that I am asking this because of the recent CERN news. Yes, of ...
1answer
266 views

### Do the particles that were found to break the speed of light really break Einstein's theory of relativity? [duplicate]

Possible Duplicate: What would be the effects on theoretical physics if neutrinos go faster than light? Update: Loose cable caused faulty results Apparently, researchers at CERN have found ...
0answers
472 views

### Can neutrinos travel faster than the speed of light? [duplicate]

Possible Duplicate: Superluminal neutrinos What would be the immediate effects if light does not go at the maximum speed possible? This is a hot topic right now, so I thought we should ...
3answers
91 views

### If neutrinos travel faster than light, how much lead time would we have over detecting supernovas?

In light of the recent story that neutrinos travel faster than photons, I realize the news about this is sensationalistic and many tests still remain, but let's ASSUME neutrinos are eventually proven ...
0answers
150 views

### What would the impact be on the physics world if neutrinos DO travel faster than light? [duplicate]

Possible Duplicates: What would be the immediate effects if light does not go at the maximum speed possible? Superluminal neutrinos I was reading this article about a group of scientist ...
2answers
502 views

### What do physicists mean when they say “speed of light”?

Does it make sense to say, "The speed of light varies?" Some may say right off the bat yes, it changes as a wave passes through a different medium. However, I'd like to say no, because when I hear ...
3answers
457 views

### High speed and low speed photons

Looking at the discovery of the neutron, and I came across this page: http://www-outreach.phy.cam.ac.uk/camphy/neutron/neutron3_1.htm The animation on the left, talks about low energy photons and ...
3answers
2k views

### Reflection At Speed of Light

I have looked online to no avail. There is two competing answers and I am curious to know which one is right. Someone asked me this question. If you are traveling at the speed of light can you see ...
2answers
267 views

### How to measure faster than light electric energy?

According to relativity,nothing can break light barrier.But a recent preprint shows energy transmission of commercial electric power (f=60Hz) is faster than light. (It is not the drift velocity of ...
1answer
363 views

### Spin up, spin down and superposition

I'm just starting to study quantum mechanics. Please explain the error in this thinking: You set up decay of two $\pi$ mesons and get $2\mathrm{e}^-$ on Mars and $2\mathrm{e}^+$ on Earth. On Earth ...
3answers
520 views

### At what speed does our universe expand?

Conceivably it expands with the speed of light. I do not know, but curious, if there is an answer. At what velocity, does our universe expand?
1answer
298 views

### Microsecond trading with neutrinos

The Spread Networks corporation recently laid down 825 miles of fiberoptic cable between New York and Chicago, stretching across Pennsylvania, for the sole purpose of reducing the latency of ...
2answers
301 views

### How can I explain the scientific basis of the constant speed of light to a C-Decay proponent?

This question asks why and how the speed of light is constant. I would like to ask a related, almost converse question: Given that the speed of light is constant, how could I explain to a ...
2answers
407 views

### Does gravity spread instantly? [duplicate]

Possible Duplicate: The speed of gravity I am real noob in physics, so sorry if this question is really stupid. Today in a casual conversation I claimed that if the sun were to instantly ...
3answers
880 views

### What really cause 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 is related to the permeablity and the permitivity of the material. But this is not what I'm interested in, ...
4answers
751 views

### Double light speed

Let's say we have 2 participles facing each other and traveling at speed of light Let's say I'm sitting on #1 participle so in my point of view #2 participle's speed is c+c=2c, double light speed? ...
4answers
214 views

### Massive particles and speed of their propagation

Can one show that in quantum field theory at least some example massive particles propagate with speed less than speed of light, while massless travel at speed of light? Well, motion is a different ...
2answers
497 views

### Tachyon and Photons

Is there a particle called "tachyons" that can travel faster than light? If so, would Einstein's relativity be wrong? According to Einstein no particle can travel faster than light.
2answers
164 views

### Thought experiment that seems to involve something growing at twice the speed of light. Is anything wrong?

Let foo be some unit of distance and bar be some unit of time which have been chosen so that the speed of light c = 1 foo/bar. Position several observers along a line each separated by one foo, and ...
1answer
183 views

### What happens when light moves perpendicular to a moving object?

Imagine the folllowing situation: A coherent light source is attached to a car such that the emitted light beam path is "being crossed over" by the car i.e. the long parallel light beams are struck by ...
2answers
853 views

### Michelson rotating mirror experiment

Could someone explain the calculation required to answer this question. It is from a text book and the answer is recorded as 585Hz but I cannot replicate the answer. In 1931 Michelson used a ...
2answers
410 views

### Why is travelling around the speed of light a problem?

I don't fully understand what would happen if we could travel at the speed of light. But I saw somewhere here that it would mean events happen out of order. But why is this a problem. It is said that ...
3answers
548 views

### Can the speed of light become complex inside a metamaterial?

The speed of light in a material is defined as $c = \frac{1}{\sqrt{\epsilon \mu}}$. There are metamaterials with negative permittivity $\epsilon < 0$ and permeability $\mu < 0$ at the same time. ...
2answers
648 views

### Can you use a laser to measure the speed of light with a rotating mirror?

I have learned that the classical measurement of the speed of light with a rotating mirror does not work with a laser (as opposed to, say, a mercury-vapor lamp). Can you tell me if and how coherency ...
3answers
419 views

### Max rocket speed in interstellar space?

Interstellar space propulsion...if a spaceship were to get beyond our Sun's gravitational pull and since there is no atmosphere/wind/friction in space...does that mean, if an engine was constantly ...
5answers
1k views

### Maximum speed of a rocket with a potential of relativistic speeds

Ultimately, the factor limiting the maximum speed of a rocket is: the amount of fuel it carries the speed of ejection of the gases the mass of the rocket the length of the rocket ...
3answers
3k views

### Why is the speed of light defined as 299792458 m/s?

Why is the speed of light defined as 299792458 m/s? Why did they choose that number and no other number? Or phrased differently: Why is a metre 1/299792458 of the distance light travels in a sec in ...
3answers
243 views

### Seeing light travelling at the speed of light

Imagine there are two cars travelling "straight" at the speed of light*, $A$, and $B$. $B$ is following directly behind $A$. Suddenly, $B$ switches on its headlights. Will $A$ be able to see this ...
3answers
698 views

### Alcubierre Drive - Clarification on relativistic effects

On the Wikipedia article on the Alcubierre drive, it says: Since the ship is not moving within this bubble, but carried along as the region itself moves, conventional relativistic effects such as ...
5answers
2k views

### Why can't light escape from a black hole?

Photons do not have mass (that's why they can move at speed of "light"). So, my question is how the gravity of black hole can stop light from escaping?
1answer
986 views

### Can you see yourself in a mirror when you are riding on top of a light stream?

What happens if you would ride on top of a light stream and you would look into a mirror that is in front of you, could you actually see your own face? I am asking this because I heard that nothing ...
4answers
1k views

### Travelling faster than the speed of light

Let's say I fire a bus through space at the speed of light. If I'm inside the bus (sitting on the back seat) and I run up the aisle of the bus will I in fact be traveling faster than the speed of ...
3answers
857 views

### “Speed” of Gravity and Speed of Light

Some threads here touching speed of gravity made me think about that. This lead to some questions. The speed of gravity was not measured until today (at least there are no undebated papers to that ...
6answers
1k views

### What does it mean to say that mass “approaches infinity”?

What does it mean to say that mass "approaches infinity"? I have read that mass of a body increases with the speed and when the body reaches the speed of light, the mass becomes infinity. What ...
4answers
676 views

### In superluminal phase velocities, what is it that is traveling faster than light?

I understand that information cannot be transmitted at a velocity greater than speed of light. I think of this in terms of the radio broadcast: the station sends out carrier frequencies $\omega_c$ but ...
3answers
212 views

### The speed of light also applies for 'distance' materials?

The question is hard for me to put into one sentence so please try to completely read the example: If I had a stick that is 1000KM long and I would push it forward with 1 millimeter in lets say... ...
5answers
680 views

### How long would it take for electricity to flow from one terminal to other, via a 1 LY long wire?

Basically, how long does it take for electricity to determine there is a closed circuit and how does it know that the circuit exists? I'm curious to know how it knows there is a closed circuit at any ...
1answer
376 views

### What's a better phrase than “speed of light” for the universal spacetime speed constant? [closed]

The phrase "speed of light" is commonly used for the constant c =3E8 m/s, a feature that's "hardcoded" into the structure of spacetime. All massless waves and particles move at this speed, and it's a ...
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