Speed of light Definition and 1000 Threads

so I was recently studying some ray optics and then suddenly a weird question came across my mind I just don't know if it's the correct thread to ask but let's continue anyway So the question is : suppose there is a spaceship traveling at light speed and someone fire a laser inside it what...

I've read a couple of other topics on the Physics Forums about the lumped circuit abstraction requiring that signal timescales (i.e. the period of the highest frequency signal component) be much longer than the propagation delay of the signals though the circuit and that the wavelength should be...

[Mentors' note: Split from https://www.physicsforums.com/threads/is-there-a-thought-experiment-to-show-that-the-speed-of-light-is-constant.986641/] If I could provide you with a device to measure the speed of light, and the accuracy can reach 0.01 m/s, what would you do?

I know the amazing thought experiment by Albert Einstein with the two light clocks. (The observer at the train station has a light clock and the person in the train.) It's amazing because you can even deduce the formula to calculate how fast the clock in the train goes. But this experiment...

The HR 8799 star is 129 light-years away from us, in this image we can see thermal emissions coming from these planets. Do these emissions move at the speed of light? If so does it mean that what we are seeing in this image is how the planets looked like in infrared 129 years in the past?

As much as i search Google, in an effort to find out how exactly the constancy of speed of light was historically deduced before 1905, from Maxwell equations or by any other means, i am not able to find such an explanation. In all of the search results that i could find, it is just stated that...

Hi, I read various threads in PF about the concept of invariant speed and the speed of light in vacuum that in our universe happens to be the same as the 'invariant speed'. My doubt is about the speed of the light in vacuum as measured from a non-inertial frame (basically in the context of SR...

Which one does it mean: "phase velocity" or "group velocity" or "speed of the wave front"? In the postulate of constant speed of light .

What is the equation/formula for the Speed Of Light? E=mc²? I thought it would be simple to find in a search.

Hello everyone! Let's say that you were to attempt to go as fast as possible on a spaceship with the mass of an average car in an absolute perfect vacuum. What I am wondering is, that if you were to reach a certain speed, and stop applying energy to this imagined spaceship, would the spaceship...

Isn't the meaning of speed, a variable of distance divided by a variable of time? Therefor isn't the meaning of a constant speed of light, a constant distance divided by a constant time? If there is any truth in this saying and there probably isn't, then what is the meaning of light constant...

If i am moving away from an object at a certain constant speed close to the speed of light, is that object also moving away from me at the same constant speed?

Would it be correct to say, that we are moving away from stars at the edge of the universe, at the same rate that these stars are moving away from us? I am relating to stars that are moving in relation to us, at a speed that is faster than the speed of light. Is the symmetry that maintains that...

To describe the movement of the planets, Newton assumed that there was such a thing as gravity. But he didn't know what gravity was. To derive the Lorentz transformation, Einstein assumed that the speed of light was absolute (not relative), but is it also known why the speed of light is absolute?

I understand that the meter is defined from the speed of light (distance light travels in 1/299792458 of a second). But how did man measure this exact distance to this level of precision? With any apparatus, isn't there an unknown amount of bottleneck somewhere?

Is there any reason to believe that the one way speed of light is constant? Not isotropic, but constant in the sense of being independent of position.

I take the following example to explain my question : when a « black-hole » is attracting a planet there is a Force which is proportional to the 2 masses and inversely proportional to the distance between the 2 masses. When the planet moves towards the « black-hole » this attraction force...

When we say speed of light = c m/s. Who is measuring meters and second. From SR perspective, that photon is moving towards you at speed of light so there should be time dilation, correct?

I am reading up on the special theory of relativity, and watched a video. In the video we have a train moving at ½ c towards a station, where an observer is waiting. The train's headlamp is on. How would the lamp's light appear to the observer? I was thinking that it¨'d appear to the observer...

In 2009, the same society together with the Tau Zero Foundation announced Project Icarus, a similar spacecraft that could achieve 15% the speed of light. That year, a physicist called Friedwardt Winterberg announced a fusion spacecraft that could be used as a capacitor to produce proton...

The wavelength of light from a moving source is red shifted which means that the wavelength has increased and the quantity of energy arriving per second at a relatively static destination is less than the quantity of energy emitted per second at the source. If so then the original quantity of...

Sorry if this is a stupid question but I couldn't find an answer anywhere. According to 2 scientific papers, the neutron star PSR J1748-2446ad has a rotation rate of 716Hz, which equates to a linear surface speed of 0.24c. What if this star was originally rotating, let's say, 5 times (or more)...

I know we end up with ##c=\sqrt{\frac{1}{μ _0.ε _0}}## The reason I would like a bit of help is that I understand that the value of c as deduced from Maxwell's equations is independent of any frame of reference. I can see that this is the case from the above equation involving the...

In relativity, the speed of light in vacuum is a universal constant. Also, it has wave-particle duality. So if the speed of light slows down in a different media other than vacuum, what exactly is slowing down? Macroscopically speaking, the speed of light does slow down. What about in the...

If you’re traveling at the speed of light TOWARDS a mirror, will you be able to see yourself?

So, pretty much I want to make an experiment in order to get the speed of light. What I plan to do is to have a lantern in the dark(initially off) perpendicular to a wall, two sensors(one closest to the lantern and the other closest to the wall), then turn on the light making sensor 1 go off as...

Why is speed of light in matter medium smaller than speed of light in vacuum?Are all photons must be absorbed by atoms and then the atoms re-emit radiations in matter?What is real picture of all photons in transparent medium?

I heard that the speed of light doesn't depend on the wavelength or frequency of it, how does it happen? I don't understand it because the ultimate equation is c = fλ (where c is the speed of velocity of light, f is the frequency and λ is the wavelength of it...).

Summary: Why is the speed of light limited? Why does light have a maximum speed?

Richard Feynman formulated quantum path integrals to show that a single photon can theoretically travel infinitely many different paths from one point to another. The shortest path, minimizing the Lagrangian, is the one most often traveled. But certainly other paths can be taken. Using single...

Problem Statement: I know that speed of light is 3000km per second Relevant Equations: Is it true? Can someone correct me if I 'm wrong,just interested thank-you

I'd like to know if anybody can tell me either; How large a wheel would have to be for the outer edge to travel faster than the speed of light if powered by a motor that's around now. We'll say the wheel is in outer space to eliminate drag and such

I am wondering if an object can fall into a black hole at faster that the speed of light. I have heard that the expansion of the universe can make distant galaxies appear to recede from one another at velocities faster than the speed of light.

 Intuitively, this makes sense to me. I am...

If my car can travel the speed of light will the head lights still work?

1. How did one came to the constant 299 792 458 m/s? 2. Why this particular number? Why not 521 334 992? 3. Why can't we travel faster than the speed of light?

Quantum entanglement does not imply that you can send information faster than the speed of light since you cannot manipulate what your sending. You don't know what you have until you have measured it. But you do know that you have the corresponding photon at the other location in space no matter...

In a vacuum the speed of light is a constant. What if its not in a vacuum? Okay, as always I'm confused. I'm very interested in physics but am a layman. Alright, so we know that time is relative to one's distance from a mass, so that as we go further from the Earth, let's say, time moves...

The second postulate says the speed of light is constant c independent of all inertial observers. Does it mean the speed of the wave front relative to the observer , that is, the relative speed between the wave front and the observer?

I'm studying history of electromagnetism. Here is my question regarding Maxwell's classical brilliant work. I understand these constants (for now) And I understand this: And I understand this: But I need to understand how this math below gets us to the speed of light. I don't understand...

I know that the speed of light is different in different mediums. The speed of light in Cesium as a medium is actually higher than the speed of light in vacuum. How is that possible? Shouldn't it be fastest in vacuum? [Mentors note: this post has been lightly edited as part of splitting it out...

I'm a bit confused as to why the speed of light changes in an accelerating ship relative to an onboard observer. In other words, on a ship with a clock at the nose and a clock and observer at the tail, in an accelerating ship, the clock at the nose will tic faster. The reason (according to a...

Is anyone expermently measured two way speed of light from moving source using time of flight?

Hello, I have a physics question that I am hoping the forum can answer. I have lots of them actually, but I would like to start with the one question and go from there. Ideally the answer to the question should be based upon current accepted physics theory. The Question: How do objects...

Romer measured speed of light using moon's of Jupiter but he got value with a error of 26%. Is anyone did same experiment in modern era? And what value they got?

It is commonly said that the speed of light when traveling inside materials is lower than that of light in vacuum, but I don't understand how this can be true. It is the same light traveling, so how can it act differently? Does light appear to be slower in materials because it is not following a...

Play from 3:00 It states that we will age slower if we were traveling at the speed of light compared to being on Earth. Would we not age the same but just be further away hypothetically? Next it states that if we travel away from a clock at the speed of light at 12:00, we would always see...

Homework Statement In the Fizeau's Experiment to determine the speed of light, let the gear have N teeth, the frequency of the rotating gear being f, the distance traveled by the light beam/ray L (distance b/w the gear and the mirror) and let there be n eclipses(blocking of the light beam)...

Hi, The speed of light is given c=1/√(ε0μ0) where ε0 is permittivity of vacuum and μ0 is vacuum permeability. The permittivity and permeability of a material is given as ε=εrε0 and μ=μr)μ0 respectively where ε is absolute permittivity of the material and εr is relative permittivity of the...

My question is about the speed of light. Our current understanding of light says that light is constant for all observers, and uses time dilation to explain this. Have we proven this? The speed of light emitted from a stationary object is equal to C. The speed of light emitted from a moving...

I know that special relativity is based on the postulate that an object that travels at the speed of light with respect to one inertial observer must travel with the speed of light with respect to all inertial observers. This postulate implies that no two inertial frames can differ by a relative...