Speed of light Definition and 1000 Threads

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...

If two objects are traveling at the speed of light in opposite directions, doesn't this mean that one object is traveling twice the speed of light relative to the other?

Daniel.Y.Gezari @ nasa.gov 's paper arxiv.org/vc/arxiv/papers/0912/0912.3934v1.pdf lunar ranging evidence of variable c fig 2 of this paper shows DLB as the distance between the observatory (Launch) and the retro-reflector at the moment of the Bounce, DBR as the distance between...

What If we accept starting point of the big bang as the central point and what If the equatorial expansion rate of a spinning universe was the speed of light, how would it impact universe? ( In this question, you may think yourself as an observer from out of universe. I try to mean with "the...

Hello everyone! Recently my work has been more involved with high energy physics. I have been looking into some things, and I would like help with an understanding of the speed of light. So far, the main ideas I have seen have been thought experiments, with minor (if any) direction to...

I know that a photon traveling from the Sun takes around 8 minutes to get to Earth, but, how long does the photon "think" {if it had a stopwatch} it took to get from the Sun to Earth ?

If two masses that are separated by a distance are created from pure energy using the equation E=mc^2, where did the gravitational potential energy between them come from? Does this mean the speed of light isn't really constant, and must be changed very slightly to accommodate it? Could it be...

Let's say a spaceship made of a transparent material moving past the Earth in a straight line at a constant speed of 0.99 c, and it's at its minimum distance from Earth. If a light is turned on in the middle of the ship, for an observer on Earth will the light move towards the front of the ship...

The Earth is moving around the Sun at around 30km/s, a change of 60km/s over the course of the year. Presumably, measurements of the speed of light would show a redshift up to ±30km/s? Are redshift measurements this accurate? Likewise, the speed of the Solar System around the Milky Way, is...

The speed of light is a parameter that attaches itself to what exactly, an inertial frame of reference or a massless particle moving therein?IH

I was thinking about the QED lagrangian and renormalization and I thought something like this: "There are three renormalizable infinites in QED so there should be three free parameters to adjust in order to obtain the low energy results. The three 'high energy' parameters are e, m and where is...

Is light's speed constant? The universe is expanding, i think the distant b/w the planets&stars are increasing is that true? is it true then is the time taken to reach sun light to Earth now from past change? if the time is constant then how the speed of light is constant?

Hello All, Light travels at 1,86,000 m/sec. i.e.2,99,338 km. The distance from Earth to Moon is 3,84,400 km. Is there any constellation or any physical object (to have an idea) which is not near to moon but falls within the range of 2,99,000 km? Just for curiosity. Thanks

I am a new user of this site, and have not studied physics in forty years. However, I was reading Janus' Special Relativity Primer at http://www.thephysicsforum.com/special-general-relativity/25-special-relativity-primer.html , with its excellent animated diagrams, but am having considerable...

Hi all,I originally posted this question as a reply to another post, but afterwards I thought it would be better to start a new thread. Unfortunately, I couldn't figure out how to delete the old post. With that out of the way... In the experiment, we send either light or sound from a source...

Hi! I know the mass of, for instance, an electron can be express in units of electronvolts over speed of light squared. However, I am not sure how to use it. When I have to calculate \frac{\sqrt{2mE}}{\hbar}, do I have to multiply m with the speed of light squared to get it in meters per second...

Hey there, I just got a theory I've been thinking about since a while and want to share it. So here it comes: imagine, we build a huge centrifuge in space, i mean really huge. It is made of super stable material which will not break in the following process. So we speed up that centrifuge until...

How do you calculate change in time at the speed of light?

Why is the speed of light 186,000 miles per second? Is that how fast the ether will allow it to travel? and if that is the case, if the edge of the universe; the edge to which the universe is speeding up, would the ether out there let light travel at higher or lower speeds? Which to me means...

I've read online that the fine-structure constant (alpha) is at a sort of "goldilocks value" for life generation; if it were 4% lower, stars would not produce carbon or oxygen in their fusion, and if it were greater than 0.1, fusion simply wouldn't occur. These are interesting facts, but can...

While trying to understand special relativity, i came across the following explanation several times: Imagine a train moving at c/4 from left to right. If a person A, inside the train standing near the back of the train, flashes a light, the light is supposed to travel in the direction of the...

For a lay person such as myself, it's very easy to get confused and stumble around a bit, trying to understand how galaxies can be receding greater than the speed of light when the speed of light is the cosmological speed limit. It's easy to make the mistake of saying 'Ha! There are whole...

I am a student, and a few days ago in physics class we were discussing about the 'bullet fired from a train scenario', where e.g. if a train is heading towards a certain direction at 200 km/h and a bullet is fired at 800 km/h, the bullet would travel at 1000 km/h relative to the ground. However...

Homework Statement [/B] A circle of radius R travels in the x direction with velocity c. The center of the circle is at -R at t = 0. a vertical line rests at x = 0. Find dr/dt, where r is the position of the intersection between the circle and the vertical line, and explain if this violates 2nd...

I need to teach special and general relativity to IB Physics students in high school, and this is a question I've been wondering for a while now and want to understand before teaching it. If the speed of light is constant no matter what the speed of the source or observer, does that mean that...

If we can motion at speed of light,can we pass a small hole like a keyhole? Of course in suitable conditions...

Had a thought the other day. If a spool of a measuring tape was on a vehicle traveling close to the speed of light away from the Earth, with the spool anchored to the Earth, due to time dilation, would an observer on the vehicle see the spool unraveling faster than the speed of light?

Hi All, Recently I mentioned about a new method to measure the one-way speed of light. Quickly I was pointed to the existing roadblocks to such experiments, from members here and veteran professors in email. I have analyzed those roadblocks and putting here my understanding of the problems. To...

a question has made my mind busy...it is told that there is no frictions at space...frictions of air...friction of gravity and etc...none of these does exist in the space (outside the earth)...so we can launch a spacecraft with a primary speed(orbit speed of earth) and equip it with a...

Hi All, This is my fist post here! Recently, I came across the elusive problem of one-way speed of light which has not yet been measured in any agreeable way. So far, all the speed of light experiments have involved clock synchronization or cyclical path problem and hence provided only...

I know that in a vacuum, speed of light is constant. My question is more about the speed of light in a material like air. Dispersion of light in a prism tells us that the speed of light or the material index depends on the wavelength ( or frequency which is constant ) so I thought that air...

At the beginning of his 1905 paper 'On the Electrodynamics of Moving Bodies' Einstein defined the synchronization between two clocks using light signals, and on this basis formulates the following theorems 1. If the clock at B synchronizes with the clock at A, the clock at A synchronizes with...

I just found this interesting game by MIT game lab. It illustrates a world where the speed of light can become comparable to the walking speed of a child by collecting some orbs. So this way you can see relativistic effects directly. I post this thread so we can discuss the game and the effects...

If a object with mass and density just below what is required to form a black hole is traveling near the speed of light relative to an observer, its mass should appear to increase, and its length contract, to the observer. The objects mass and density will increase pass the threshold necessary...

how fast does an electron spin? if an electron has no volume and is a point, then surly speed would have no meaning. So could it spin at what ever speed it likes? would it just be pure spin (think of a square, it has 4 corners and 4 sides, if the length of the sides were zero, then it would be...

I tried searching the web for this topic but got an answer like "formulae used in classical mechanics are approximations or simplifications of more accurate formulae such as the ones in quantum mechanics and special relativity". My question is that why do the laws of Sir Isaac Newton no longer...

I was having a discussion with a colleague earlier today regarding exoplanet exploration. Being that the biggest challenge for mankind to travel to distant planets is that time dilation would make such an extreme effect that by the time we reached our destination thousands of years may have...

if we look on a star that it something like 10 light years from us with a telescope we see the star like he look before 10 years because the time the light take to come to us, so my question is what happen if we go to this star in the light speed or more, we will see all the 10 years in high...

Are all physics theories based on "the equivalence of energy E and mass m always reliant on the speed of light c? Are there any alternative theories related to the equivalence of energy and mass that are not reliant on "the speed of light"? I would appreciate your attention, ttvlr

If velocity is relative and dependent on an observer then how does an isolated object "approach the speed of light"? Approaching the speed of light relative to what? Does the ubiquitous constant velocity/closed compartment analogy break down at relativistic speeds? If one were traveling "near...

Hi! If you started a 50 light-year journey at the age of 20, traveling at 99.9% of the speed of light, would you look like 70+ years old when you arrived at the destiantion? How much would you have aged? And how would you experience the time on the spaceship? Would it feel like 50 slow years...

The speed of light changes according to the medium that its in and I don't understand why. Is it because each particles in the medium has a bit of gravitational forces so that the light doesn't travel in a straight line, or perhaps because of something else?

Hi guys, So I'm a bit confused about why strings can't move at the speed of light. I understand that the end points do if the string is open, but the rest of it doesnt. To paraphrase, really I want to know why you must have timelike and spacelike tangent vectors to a point on the world sheet...

okay, so I know that light is faster than sound, and we can prove it with many visuals, but can y'all give examples of speeds through the same medium. For example, I know that the speed of sound is 343 in room at 200 C