Speed of light Definition and 1000 Threads

Lets suppose we are in a space and there's no external force that affects our system,which our system is simply one object which we can think it is a box and it has 1 kg mass and there is us. The object ,lets call it A,Its initally rest relative to us so it can be our inertial referance...

I realize that unobserved light it going to be difficult to measure, but could the double slit experiment be set up to give us an average air time for the particles? I find it hard to believe that a particle acting as a wave would travel at exactly the same speed as an observed one.

Do laws of physics apply below the event horizon? It appears as if black holes had such gravity as to have an escapr velocity higher than c, which means that things are pulled inwards at higher speeds than the speed of light. Or am I overlooking something?

will there be any effect on the speed of light ,when it travels from higher dimensions to three dimensions of space ?

I'm still trying to understand the concept of the slower passage of time at or near the speed of light. Say for example we had the technology to travel this fast and wanted to send astronauts to visit the newly discovered planet Proxima Centauri b four light years away. My understanding is that...

I assume that, via scattering processes, the speed of light slows from that in a vacuum close to the centre of a black hole to zero at the event horizon. How is the gradient in its speed defined throughout this volume? Is an analogy with a sound wave reaching an interface appropriate?

E=mc^2 states that when you speed up matter to the speed of light, it becomes pure energy, of mc^2 joules. Now, if that is true, can you reverse the equation? Wouldn't energy speed up to the negative speed of light(-c^2), turn into matter? Or is that the wrong balance?

A spaceship traveling at speed of light close to speed of light (wrt inertial reference frame) sending some data every second on their clock to people who are stationary (wrt inertial reference frame). At what time these people would receive this data on their own clock? Let's say for a second...

In quantum field theory, we use the universal cover of the Lorentz group SL(2,C) instead of SO(3,1). (The reason for this is, of course, that representations of SO(3,1) aren't able to describe spin 1/2 particles.) How is the invariant speed of light enocded in SL(2,C)? This curious fact of...

Let's say a spaceship zooms around the Earth repeatedly at 99.99% of the speed of light for a period of time that is 50 years for observers on Earth. How much time will the passengers on board feel has elapsed after these 50 years? And, if someone on Earth was somehow able to remotely observe...

According to this video I was watching Maxwell was looking for the speed of EM waves and just divided coulomb's constant by the magnetic constant and then took the square root and that was the speed of light. √(Ke/μ0)=√(9e9/1e-7) = 3e8m/s So why is this a thing? I just don't understand why it...

I know there is a modern paradigm that states that nothing, including information, can travel faster than the speed of light. (With exception to quantum entanglement, but I'm not going to pretend like I know the exact details of that subject). Thought experiment. Imagine there are a series...

I learned in Analytical Mechanics: "Emmy Noether's theorem shows that every conserved quantity is due to a symmetry". The examples I learned where conservation of energy as symmetry in time and conservation of momentum as symmetries in space. Now I wonder, do universal constants are also due to...

Hey guys this question is a bit basic but I always get caught up on the idea, I'm not in school for physics so I don't really have a prof to ask and most of the internet always skips ahead of this. Say I am in an inertial frame traveling close to C speed, the only thing I see is a photon...

So there is lots of information on the Internet specifying that the speed of light from the headlights of a car traveling at 30mph is C and not C+30 because it is impossible to travel faster than light but what about the speed of light from the brake lights pointing backwards? Am I right in...

Hi I have a question about current start signals in a simple circuit such as explained in http://amasci.com/elect/poynt/poynt.html. Turning on a switch somewhere in the circuit, sphere of influences, i.e. motions of electrons in wire and generation of Poynting vector around the wire, start from...

Currently, I'm doing the CREST award for physics, and my topic is about different methods of finding out the refractive index of a medium. I have already tested out few ways, and now i am thinking about a method where a formula n=c/v will be used(n-refractive index, c-speed of light in vacuum...

Hi all. New to PFs. I deal in (exotic) human biology, which happens to have put me at the intersection of fields and, in this instance, conceptually more challenging physics. I'm presently involved in a comparative analysis of biological versus physical time contraction-dilation. I happen to...

Dear PF Forum, Before I get a NO answer, I'd like to ask a few question. https://en.wikipedia.org/wiki/Hubble%27s_law#Observed_values As of 13th July, 2016 Hubble flow is 67.6 km/s per mega parsec. Or 67km/s per 31 trillion * 1 million km or per 3.26 million light year. Say a galaxy 11.68 giga...

I understand that c is the "ultimate speed" and that it is a result of the fundamental constant, the permeability of free space. But if this is a constant only to a "stationary" frame of reference, how can we accurately measure it from Earth? Since Earth is moving relative to a supposed "fixed"...

Hello! I have a question that has been bothering me since I first started learning about Special Relativity: Given only the Minskowskian metric and/OR the spacetime interval, how can one reach the conclusion that the speed of light is invariant for every observer and how can one conclude that it...

Dear PF Forum, What is the limit of acceleration? I've been reading old threads, and I found this. G = 6.673 x 10-11 N m2/kg2 Solar mass = 1.989 * 1030kg And I tried to plug in some numbers... In a distance 30 km from a 10 solar mass object the acceleration is... ##a =...

Hi everyone, Recently I randomly thought of a thought experiment of something going faster than light, along the lines of a shadow or some other "non-information carrying object". In this case it would be an empty seat. The idea is to take 800 million and one chairs of half a meter breadth...

I know that no object can travel faster than the speed of light, but if two objects travel in the opposite direction, both at almost the speed of light, then would one object be traveling faster than the speed of light relative to the other?

How close to the speed of light would you have to travel to be able to traverse the entire span of the known universe(94 billion light years i think?) in a persons 80 year lifetime?

If a guy gets on a rocket that travels at the speed of light for a finite period of time (in relation to other observers not moving at C) the guy in the rocket experiences no passage of time. In this scenario his leaving and arrival of the rocket happens at the same time in his frame, correct...

I am confused about Einstein's thinking. I understand when he formulated his general theory of relativity, he wanted to incorporate 3 foundations for his theory: The relativity principle, the equivalence principle, and Mach's Principle. He believed that inertia and weight were essentially the...

Are objects beyond the observable universe in some sense moving faster the the speed of light. Given that objects at the edge of the observable universe are "maximally" red shifted, objects beyond that theoretically must be more red shifted, which is impossible. Is it therefore a nonsense to...

According to this video, , if a black hole is large enough you could actually travel for some time within the event horizon without dying because the event horizon is so far from the actual singularity. So, assuming that's true, what would you see while you were inside the black hole? Here's...

Light travels more slowly through light-transmitting substances such as air or glass, otherwise no lens would refract light. so does this mean that the speed of light is not a universal constant?

Assuming there is a multiverse, has the speed of light (or casuality) to be the same for all universes?

I do not have a problem with the concept of the constant speed of light as it has no mass and therefore no inertia and therefore no relationship to any IFR. However it seems to be expressed as constant in all IFR's which I do not understand. This seems to say that if I am traveling at 1/2c and I...

Dear PF Forum, I've been wondering about how on Earth (and I do mean it, from earth :smile:) that we know there's a galaxy 20 billions light away. Considering that https://en.wikipedia.org/wiki/Age_of_the_universe is 13.8 billions years old. But before I'm asking about Supernova Ia and Hubble...

If we could imagine a medium that could slow down light quite significantly, if a sound wave and a light wave were both passing through this medium, would the sound wave see the light wave passing by at the speed that light passes through that medium or would it see it passing by at the speed...

I believe that the speed of sound is constant in the same medium as is the speed of light. I would like to understand why we need the the theory of relativity to explain the speed of light being constant but I believe it is not used to explain why the speed of sound is constant within the same...

I know that if an object moves at the speed of light, from its persepctive time will stop to a halt. However the smallest unit of time should be the plank time. So my question is, will the object (say a photon) experience throughout its life a single still "frame", or will it experience...

Is it true that the speed of light is only dependent on the size of the universe, if the universe were much smaller would the speed of light be faster?

I was researching variable speed of light theories as alternatives to inflation theory; from Wikipedia I cam across this: "The idea from Moffat and the team Albrecht–Magueijo is that light propagated as much as 60 orders of magnitude faster in the early universe, thus distant regions of the...

It's said that, speed of light is same in every frame is reference. Consider an ideal situation, if I'm also moving at the speed of light, will I feel light to be at rest or still at the speed of light itself according to my frame of reference?

If I were of speed 299 792 457 m/s relative to an observer and throw a ball, what would the observer see?

Is there a simple way to prove to someone that light travels at c in a vacuum? I was having a debate with a friend and he said the speed of light isn't real.

Einstein said that the speed of light is constant for all observers. When Maxwell derived that the speed of light in the ether was 299,792,458 m/s could he or did he surmise that the speed of light is constant for all observers (regardless of the motion of the source)?

why can't speed of light be relative to it's source?

If things such as quantum entanglement and the expansion of space can travel faster than light, then why can't gravitational waves, which are vibrations of spacetime? I thought that only matter cannot move through space faster than light. Also, has it been 100 percent proven that gravity waves...

Alright, so I'm by no means a trained physicist and most of what I know comes from sporadic readings on the internet, but I had a strange theory the other day. I'm more the type to think in visualizations or analogies rather than cold hard math, so I'll explain it the way the idea came to me...

So I've heard from multiple sources that one explanation for why light slows down whilst traveling through mediums other than a vacuum is that the light "takes every possible path at the same time" through the medium. Below I've drawn my two possible interpretations of what that means. Can...

I'm having a difficult time researching the answer to my question about the speed of light. Now obviously it is a speed not only reserved for light but also all other massless particles/waves. It's obviously a constant property of our Spacetime since we can manipulate th speeds of different...

I'm not sure how the speed of light squared makes any sense? What is MASS X C2?

...were infinite? In other words: is capacitance a reflex of the finitude of the speed of light? Please elaborate. Thanks a lot in advance.

I was listening to a podcast about the solar neutrino problem, and they discussed how we have deduced that neutrinos are not massless due to the fact that they interact with other particles (even if this interaction occurs rarely). I paraphrase: "a particle traveling at the speed of light is...