how is the speed of light figured out?
364109
In 1676, by a Danish astronomer, Ole Römer, working at the Paris Observatory. He had made a systematic study of Io, one of the moons of Jupiter, which was eclipsed by Jupiter at regular intervals, as Io went around Jupiter in a circular orbit at a steady rate. Actually, Römer found, for several months the eclipses lagged more and more behind the expected time, but then they began to pick up again. In September 1676,he correctly predicted that an eclipse on November 9 would be 10 minutes behind schedule. This was indeed the case, to the surprise of his skeptical colleagues at the Royal Observatory in Paris. Two weeks later, he told them what was happening: as the Earth and Jupiter moved in their orbits, the distance between them varied. The light from Io (actually reflected sunlight, of course) took time to reach the earth, and took the longest time when the earth was furthest away. When the Earth was furthest from Jupiter, there was an extra distance for light to travel equal to the diameter of the Earth's orbit compared with the point of closest approach. The observed eclipses were furthest behind the predicted times when the earth was furthest from Jupiter.\n\nFrom his observations, Römer concluded that light took about twenty-two minutes to cross the earth's orbit. This was something of an overestimate, and a few years later Newton wrote in the  Principia (Book I, section XIV): "For it is now certain from the phenomena of Jupiter's satellites, confirmed by the observations of different astronomers, that light is propagated in succession (NOTE: I think this means at finite speed) and requires about seven or eight minutes to travel from the sun to the earth." This is essentially the correct value.\n\nOf course, to find the speed of light it was also necessary to know the distance from the earth to the sun. During the 1670's, attempts were made to measure the parallax of Mars, that is, how far it shifted against the background of distant stars when viewed simultaneously from two different places on earth at the same time. This (very slight) shift could be used to find the distance of Mars from earth, and hence the distance to the sun, since all relative distances in the solar system had been established by observation and geometrical analysis. According to Crowe (Modern Theories of the Universe, Dover, 1994, page 30), they concluded that the distance to the sun was between 40 and 90 million miles. Measurements presumably converged on the correct value of about 93 million miles soon after that, because it appears Römer (or perhaps Huygens, using Römer's data a short time later) used the correct value for the distance, since the speed of light was calculated to be 125,000 miles per second, about three-quarters of the correct value of 186,300 miles per second. This error is fully accounted for by taking the time light needs to cross the earth's orbit to be twenty-two minutes (as Römer did) instead of the correct value of sixteen minutes.
932886
the speed of light = 299 792 458 m / s or 1079252848800 M/H\ncircumference = 2*Pi*R \nso 2* 3.14159*R= 107925848800M/H\n1079252848800/(2*3.14159)=R\nRadius= 1.71768571 × 1011(<--- times 10 to the 11th power) meters
68058
Two of these answers were correct (the speed of light in a vacuum is exactly 299792458 meters per second), although the appropriate reference is not wikipedia but the Bureau International des Poids et Mesures (BPIM).  This is the internationally recognized "official" source.\nhttp://www.bipm.fr/en/si/base_units/\n\nThe speed of light in a vacuum is exactly known because the meter is defined (by the BPIM) as being the distance that light in a vacuum travels in 1/299792458 of a second\n\nThe answer from Trsmd is not quite correct in stating that the speed of anything can be measured by determining the distance in travels in a given amount of time.  Since our units of distance are defined in terms of the speed of light, the correct statement would be that distance can be determined by measuring the amount of time that it takes light to travel that distance.\n\nAlso notice the qualifier "in a vacuum".  The speed of light travels more slowly when it travels through something like air, water, or glass.  How much more slowly depends on the properties of the material (density, composition, temperature, etc.)
104857
The speed of light varies depending on the medium it is passing through. The optical density of the material it is passing through is used to find the speed of light through the material. \n\nWhere c is a constant is in a vacuum. If that is what you are referring to, I have no idea.
1120142
The question is too philisophical. We don't know WHY light travels so quickly, we just observe that it does. We can derive such a speed and we can derive the fact that it always travels at this speed no matter what your reference speed is. We have failed to find a situation where the speed of a single photon has not been the accepted speed of light.\n\nAlso, contrary to a previous post, light travels at a constant speed, independant of its frequency. This opposed to mechanical waves such as sound and water waves..
13401
It's true that the speed of light is constant no matter what speed at which you or the source is moving.  This is a fundamental postulate of relativity, and there is no explination.  It's just accepted because our experiments indicate that it is true.  You used the formula "lights speed + X = actual speed"  this type of velocity addition is called "Galilean".  It turns out that speeds don't actually add that way.  Far out.\n\nWhere is the rest of the energy? Good question.  The frequency of the light you observe from the sun DOES depend on how fast you and it are moving.  The energy of each photon is given by E = hf, where f is the frequency of light.
24020
Neglecting the lengthy derivation of why light speed travel is impossible, the first postulate of relativity is that light has the same velocity regardless of how fast you're moving.  So, at ANY speed you can achieve in your car, when you turn the lights on you'll see the waves moving away from you very very quickly.
90556
Always exactly the same speed, c = 299 792 458 meters/second, no acceleration or deceleration or anything.\n\nThe *apparent* speed of light through glass is less than c, but that is because the photons (the light) bounce off all the atoms and that slows down the speed through the glass. The speed of light from one atom to the next is exactly c.\n\nAlso, sometimes you will read about experiments where the popular press version says some bunch of light has been made to faster than c, but this is just the phase speed and so isn't quite the same thing. You can't send any information faster than c with this method.\n\nThere is an apparent something that goes faster than light in what is called the EPR-effect, a wierd property of quantum mechanics often misleadingly called "quantum teleportation," but again, no information travels faster than c.\n\nThere are hypothetical particles called tachyons which *always* go faster than light - they can't go slower. But no evidence for these have been found and most physicists think they can't exist.\n\nAs for warp drives and other faster-than-light science fiction type things, that, in my opinion isn't possible. Certainly everything we know about physics and the universe now says it is impossible, and it is unlikely that any future theory will show we are so wrong now that we missed this one. Also if FTL is possible then you can have a sort of time travel, and then cause and effect get ruines and, well, its a mess.\n\nBe glad that c is absolutely the same to all observers in every situation - the universe wouldn't work without it.
495659
The accurate speed of light is 299,792,458 m/s.
166913
Actually, photons in vaccum are found to travel sometimes faster and sometimes slower than speed of light, but during very brief instants of time. If you are measuring velocities in a distance thats long enough (always in terms of atomic distances) the avarage speed turns to be the speed of light. But if you are measuring shorter distances (I don't know how short it has to be) you have to take into account that maybe you will measure a higher or lower speed than the light speed.\n\nWe can say that photons have a very high probability of travelling at the speed of light, and very small probabilities of travelling a bit faster or a bit slower. If the distance is long enough, the faster velocities will cancel with the slower ones, and the avarage will be light speed.
990328
The speed of light is always constant in any reference frame.\n\nNo matter the reference frame, an observer will always measure the speed of light in a vacuum to be exactly,\n299,792,458 m/s.
600967
No there is no 'summation' of speeds involved with light.  Light is Electro-Magnetic Energy, although light travels as particles as well as waves.  In this Universe the speed of light is a constant, roughly 186,000 miles-per-second, or 300,000 km/sec or 300 Mega-Metres / sec.  Unless some POWERFUL external force acts upon light, it cannot change speed.  This entails the gravity of black-holes and such. (Not even light can escape from them.)\n\nA vehicle might weigh somewhere in the neighbor-hood of 3,300 lbs. (Approximately 1,500 Kg, for those of you who have not learned to count properly, just yet ...)\nCheck out da' math ...\n\n60mph = 100 kmph = 27.77r mps. \n(1,500 Kg x 27.77r mps) / 9.81 mps = ~ 4,246 N\n\nI doubt, very seriously, that a little over 4,000 Newtons is enough force to even put a dent in the factor of light-speed.  For more info, look-up Einstein's Theory of Realtivity' (Although I believe 90% of it to be short-sighted ancient crap ...)  This subject is covered in it ...\n(umm covered in Einstein's Theory, not copvered in crap .. which err .. is the same thing at times ...
7206
To be precise, what we usually call the "speed of light" is really the speed of light in a vacuum (the absence of matter). In reality, the speed of light depends on the material that light moves through. Thus, for example, light moves slower in glass than in air, and in both cases the speed is less than in a vacuum. However, the density of matter between the stars is sufficiently low that the actual speed of light through most of interstellar space is essentially the speed it would have through a vacuum, so we don't make much error by ignoring the difference.\n\nTo make it easier for you to convert, here is the speed in alternative units :\n\n(1) 983,571,056.4304461 foot/second\n(2) 11,802,852,677.165352 inch/second\n(3) 1,079,252,848.8 kilometer/hour\n(4) 299,792.458 kilometer/second\n(5) 582,749,918.3585312 knot\n(6) 880,991.0899527 mach\n(7) 670,616,629.3843952 mile/hour (mph)
265157
It takes an infinite amount of energy to reach the speed of light, so the quick answer is "You can't".\n\nIf you are traveling at CLOSE TO the speed of light and you turn on your lights they will appear (to you) to create cones of light that travel at light speed relative to you.  If you measure the speed of the light, you will get c (approx 300,000 k/sec).  To you, the light will appear to be traveling 300,000 k/sec faster than you are.\n\nAn observer who is not moving (one who see's you as traveing at nearly light speed) will also see the light as traveling at speed c, but he will see it as moving only a little faster than you are.\n\nHow can this be?  It happens because of something called "time dialation/compression" (see the Theory of Special Relativity).  The "seconds" measured by you (in the car) are MUCH longer than the seconds measured by the motionless observer.  That's the magic of special relativity.\n\nThis is confusing stuff.  If you want to really understand it, look up special relativity and read carefully.
872993
E=m c^2\nEnergy=Mass &#92;times (speed of light)^2
791590
E = m x c^2,\nso \n\nmass = Energy / c^2 (speed of light squared)
972241
Firstly, please allow me to substitute your term "speed" for the more accurate term "velocity".\n\nAll particles and waves have an intrinsic velocity  in a vacuum.  The subatomic particle which makes up light is the photon.  The photon travels at an average maximum velocity of 3x10^8 m/sec in a vacuum.  For the exact numerical speed, here is a good source\n\nhttp://en.wikipedia.org/wiki/Speed_of_light\n\nbut for the purposes of almost all calculations, 3x10^8m/sec is used.  There are physical forces which can slow down the speed of light, and the chief force is gravity.\n\nDarkness is the absence of light, and therefore, the absence of any photons. Therefore, the concepts of speed and velocity are not applicable.\n\nP.S.  There is only one particle theorized to be faster than the speed of a photon, or more aptly put, faster than the speed of light.  That subatomic particle is the "tachyon".  It is further theorized that , because it is travelling faster than the speed of light, it is also travelling backward in time.\n\nP.P.S.  Many of the theories in physics are derived from solutions to various "so-called" unsolvable problems derived from math theory.  In this sense , physics and math go hand in hand.  Therefore, many physics theories are simply supported by math equations until a real life proof can be generated, characterized properly, qualified, and quantified.  The problem here is that just because a math equation may have a solution , it alone BY NO MEANS is proof in and of itself of the existence of subatomic particles and physical phenomenae.  It can only be used as a measuring stick,of sorts,  in order to look for the existence, and therefore, FORMAL PROOF of such phenomenae.