Speed of light Definition and 1000 Threads

I had read about tachycons being faster than the speed of light and which are theoretical in that I think they have not been demonstrated empirically. Ok and also if they were real it brings up the paradox of essentially these particles traveling backwards in time. Such that and it sounds...

Really good blog post by Sean Carroll on a shockingly common misconception: http://www.preposterousuniverse.com/blog/2015/10/13/the-universe-never-expands-faster-than-the-speed-of-light/ Hopefully at least scientists will stop spreading this misconception soon.

I have read that very high precision attempts to confirm the constant speed of light in all directions have been successful. For example in 2009, Stephan Schiller's lab was able to achieve a precision level that was one hundred millions times more precise than the original Michelson Morley...

Hello, Is the speed of light constant at all speeds? So will the speed of light (3 x 10^8 m/s) be the same if a person was traveling at 1000 km/hr and if they were stationary. Thanks

Could the speed of light be accelerated by a huge gravitational field? For example, we know light doesn't escape a black hole and is strong enough to cause lensing, therefore could it accelerate light if it complimented the light's path?

I appreciate that this may be the 100th time this is asked but I am yet to see an answer which addresses the question head on. Given that reaching light speed creates both a length and time singularity; both length and time amount to zero for the propagating wave/photon. Why does that photon...

I have learned that light has a constant speed of 299 792 458 m / s or C and that this speed cannot be changed by anything, how can a black hole "trap" light if this speed cannot change? Is it because time is also trapped, so a second lasts infinitely long?

Hello, As we all know, speed of light of different wavelengths (let's say red and blue) in vacuum is same. However, in medium (like glass) it's different and depends on a wavelength and a refractive index. Let's say we send a short pulse of white light incident into a piece of glass (of length...

Homework Statement Using the special relativity formulae p = mv / [1 - (v/c)2] E2 = p2c2 + m2c4 derive linear relations between: (i) momentum and mass; (ii) energy and mass; (iii) energy and momentum, which involve only c, c2, β = v/c, and γ (= 1/sqrt(1 - β2)) The attempt at a solution I am...

The first postulate is perfect, the laws of physics are the same for all uniform inertial frames of reference. In fact the second postulate is perfect as well the speed of light is constant in all uniform inertial frames of reference. But here is my problem with it, the speed of any wave is...

How do we know the speed of light is constant? Is it possible that we only perceive it as constant?

If there is a person walking along the road at the speed of 5km/h and a car traveling in the same direction as the person at 60km/h relative to the person. How to calculate their the distance between them? Also the velocity of light is 186 000 miles per second and what if both object X and Y...

When the spin of an entanglement particle is established, it is said that the corresponding spin of its entangled twin is revealed immediately, (via wave collapse?), and that this interaction can occur across a substantial distance. It has also been said that this immediate interaction can occur...

Here is a question that might be somewhat more philosophical than this community cares for. If so, I apologize in advance. Are there any reputable theories as to why the speed of light is constant? I know that it is an empirical fact and therefore that it does not need to be proven. But on...

Hello Physics Forum. Given time runs slow as seen by a moving observer, why doesn't light travel further in the slowed time? Thereby negating the greater distance light has to travel, mirror to mirror in the light clock on the train thought experiment?

My understanding: Relative to stationary body, the speed of light is 3*10^8[m/s] In all intertial frame of reference the speed of light remains constant: menaing, if I travel in rocketship @ 0.5*c, the light coming out of my rocketship will also travel at 3*10^8[m/s]. The law of physics remains...

The way that I understand it is this. But correct me if I'm wrong. Because P=m*v, sense infinity * 0 in calculus is 1, and because the speed of light is the limit of velocity, we can express it as infinity, so p = 0 kg * infinity m/s. Is that at all right?

Hello PF. A stationary observer generates two pulses of light. The pulses are 2 seconds apart according to his watch. What is the distance between the two pulses? Assume he marks out 10 metres and places himself half way at the 5 metre mark. If the light pulses go past and align with the 10...

Without getting into a deep discussion about the vacuum, let's agree that c is the speed of light in a vacuum. If we direct a light beam in a vacuum to pass between the plates of a charged capacitor (perpendicular to the E field) will the speed of the light between the parallel plates of the...

Hello all, Suppose we can travel on a spacecraft at the speed of light, how long it would take for the person on the spacecraft to travel one light year, not to a person observing him/her from Earth, if there is any difference? Thanks

How is the "speed of light in glass", or any other non-vacuum classical medium, defined at the quantum level? Between atoms, the photons are traveling exactly at c, right? As always, thanks in advance.

In his book (Relativity) published in 1920 Einstein said that in General Relativity the speed of light is affected by gravity and therefore isn't a constant: In the second place our result shows that, according to the general theory of relativity, the law of the constancy of the velocity of...

The natural expression of speed in relativity (and thus the true meaning of speed) is through the concept of rapidity, which comes from incorporating the gamma factor. It turns out that the rapidity of light is infinite. So the question of whether there can be speeds greater than light becomes...

Hi all, After reading about Einstein's theory of relativity I have few questions as follows 1. Let's say I am in a space lab which is traveling at the speed of half of the speed of light. So when I try to measure the speed of light coming from space I record it as 'c'. Time is running slow...

Dear PF Forum, I'd like to ask a question regarding a post in Relativity sub forum. The formula for kinetic energy is ##E_k = 0.5mv^2## Which makes sense. Because if we push/accelerate 1 kg object for 1 m/s2 for 8 meters, we'll spend ##E = N.m = 8 joules## How much time do we need to do that...

[Mentor's note: This post has been moved to it's own thread.] Dear PF Forum, can I ask a random question here? 1. Is the speed of light defined by the speed of photon? 2. How do we measure EM wave speed? Just by putting transmitter and receiver and divide the distance by time? 3. If EM wave has...

One of the important predictions of relativity theory is that particles with mass can't reach speed of light in vacuum and will always be slightly slower. I wanted to know more about the maximum speed which can be reached by particles with mass and looked for relevant experiments. But to my...

Hello. Let's consider a beam of monochromatic light (just one frequency). 1. Light creates gravity field. 2. Gravity affects the speed of light. Q1: Are 1. and 2. true? Q2: Does the speed of light depend on the intensity and frequency of the light? Thank you for you time :) Greetings!

The speed of light is constant in all frame of reference... So the relative motion of the source of light and the frame of reference from which you are making measurement does not matter... Is it an Axiom in relativity ? I understand that they are experimentally proved concepts... But is it an...

As we known, the force applied to the plane that fly in the atmosphere is to keep plane moving forward. The force given is to against the pressure of the air. If a space aircraft flies in the vacuum, the force given to it will accelerate aircraft because there is no friction caused by air...

Homework Statement I am preparing a report on black holes and I recently learned about a phenomenon I was previously unaware of: the photon sphere of a black hole. While reading an article on said occurrence (I have now confirmed this on multiple sources) the photon sphere which is the minimum...

If the speed of light is constant regardless of the state of motion of the source, then doesn't this imply that it is possible to calculate the velocity of a reference frame by measuring the time it takes for light to traverse some known distance in that frame. For instance if our frame is...

I think about the following: For a black hole it seems quite natural to reflect about information loss. May be we have a firewall etc. We have an event horizont which does not allow any information exchange with the outer world. The difference between the universe and the singularity might be...

I know there is a problem with measuring the one way speed of light, but why can we not use just one clock. If somebody could explain the problems with this method, then it might help me to understand. Can we measure the one way speed of light the following way:- Have a long sealed tube with a...

Hi Folks-I am interested in knowing whether, in actual practice, people on Earth would see their 99% of c colleagues moving around in fast forward motion and if the reverse would be true from the vantage point of the relativistic astronauts who are moving away from earth. I suspect that since...

one thing every one says that our (any) galaxy is hundreds and thousands of lightyear long and also they rotate about their center then if they rotate with very low angular velocity then also the velocity of the extreme will cross the light speed? will it just think nd reply... (v=ω X r)

If I have two clocks in space at rest wrt each other and just a meter apart, I could synchronise them. If they were far enough away from any other mass so gravitational forces are nullified, then if I just let these clocks sit there for a few million years, expansion will separate them but...

We know that light's speed gets slowed down when traveling through a medium, and the more dense the medium the slower light can travel (of course c remains constant, but it takes longer to travel due to the continuous scatterings, absorbtions and re-emissions). Inside a black hole, just below...

Under Ideal conditions (a vacuum and no gravity) what is the limiting factor or factors for the speed of light ? Thank you, Pat Hagar

I saw the following question in the Physics-SE, and thought it is interesting. "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. However, it is legitimated to ask whether the...

I just wonder if there were any serious (and peer reviewed published, to be in line with guidelines) theory, which had proposed some link between speed of light in vacuum constant and between gravitational constant G. For example a calculation of speed of light based on G. Tried google search...

I hope to lay to rest two of the misconceptions about special relativity that are evident in the many questions asked here. 1) Why is the speed of light a constant ? Everybody believes Pythagoras's theorem that the length of the hypotenuse of a right angle triangle is ##\sqrt{s_1^2+s_2^2}##...

Dear PF Forum, There's one thing still bothering me. The speed of light. Supposed two observer, A and B A is from the west, B from the east. Separated by 100 lys. B sends a signal, say, B1 to A. So B1 will be received by A in 100 years, right? Now, supposed B travels at 0.8c and at the distance...

been answered numerous times i guess but i couldn't find it. i am an ignoramus. if spaceship doing 0.9c shines light forward we - from another inertial frame - see that light as proceeding forwards at 0.1c, do we not? But 'they' see it as proceeding forwards at c. fine. But if they shine a...

If you have the same liquid, water, but with different ions dissociated in it, changing it's colligative properties, does the index of refraction change? And/or does the speed of sound through it change?

black holes pulls light right, then when a black hole pulls photon will the speed that photon increase?

So someone is passing by me at ~the speed of light, which I believe would make them appear to be stationary from my point of reference. I decide to reach out and grab their hand. What happens?

I am no big expert on Relativity, though I know that objec cannot exceed the speed of light due to the restriction put by energy-mass equivalence. And since light and many waves have no intrinsic mass, they are able to move at such a speed, but why is the speed strictly the speed of light and...

If the speed of light is constant in any inertial frame then how do we measure a red shift or blue shift or why?

According to Maxwell's equation, the speed of light, ##C_0 = \frac{1}{\sqrt{\epsilon_0\mu_0}}##, is a constant regarding to some form of medium, called luminiferous aether. Shortly after the death of Maxwell, Michelson-Morley Experiment shows that the speed of light is constant regarding to the...