How did they decide "the speed of light"?

[Magnetic]

Who was the person who "clocked" the speed of light, and how was it accopmlished?

How CERTAIN are we of it's speed as we understand it as a constant?

[dragon]

Google is your friend

http://www.sigma-engineering.co.uk/ligh ... ndex.shtml

http://www.sasked.gov.sk.ca/docs/physics/u3a22phy.html

http://www.is.wayne.edu/mnissani/a&s/light.htm

[WyrdNyrd]

I don't have the definitive history at hand, but many, many people have "clocked" light over the centuries, each time getting a more accurate reading than the last.

These days, even undergraduate physics students can measure it to absurd degrees of accuracy, thanks to the miracle of interferometry.

These days, we're so certain that our measurements are accurate, we've redefined the "meter" in terms of the distance travelled by light, in a vacuum, in a certain time.

Seperately, the fact that it is constant in all reference frames is a consequence of the Special Theory of Relativity, which was in turn inspired by the Michelson-Morley (sp?) interferometry experiment.

This experiment didn't confirm that light-speed is constant in all reference frames, just in two different frames in which one would assume the speeds would be different. Alarm-bells rang when the measured speeds were, in fact, identical. Einstein then came up with the comprehensive theory explaining this.

And to answer the implied question: No honest person, with a working brain, and with the knowledge of all the facts, theories and direct experimental evidence, disputes either the currently-accepted speed, or the facts of Relativity, or believes that the speed has changed in any way since the big bang.

[AK_Jedi]

The speed of light is an outgrowth from Maxwell's equations on electric and magnetic fields. It can also be determined experimentally using a variety of techniques.

However, the speed of light does change going through different mediums. For example, light going through water would travel slower that light going through air. The constant speed: c=3.0X10^8 m/s is the speed of light in a vacuum.

[Surlethe]

Seperately, the fact that it is constant in all reference frames is a consequence of the Special Theory of Relativity, which was in turn inspired by the Michelson-Morley (sp?) interferometry experiment.

Actually, that the speed of light is a constant is a premise of relativity; all the weird effects on clocks and mass and length are consequences of a constant c. I don't know them well enough to guarantee this, but I'm fairly certain you can derive the value of c from Maxwell's equations for electromagnetism.

[Magnetic]

If YOU were a photon of light, would you recognize the fact that, as you travel through space, your "time of travel" takes a certain amount of your time? . . . . . .Someone stated (in another forum):

Consider the time for a photon. It goes from point A to point B in zero time from its point of view. This is likely because it has no rest mass.

When I didn't follow what he was saying, he replied:

Einstein's Special Relativity has demonstrated that time, and space, are observed differently as relative motions appraoch the speed of light. Newton, and Galileo, were wrong if you measure things that are traveling at fantastic speeds. SR shows that the time ticks of an object moving close to the speed of light relative to you will tick slower. For a photon, at the speed of light, there are no ticks for it, but our ticks stay the same. There are many good web sites that help explain it.

Without relativity equations, GPS locating would be very inaccurate, as the speed of the satelites and the speed of Earth are not inconsequential when measurements must be in centimeters. Also, even if something were to go faster than the speed of light, you would not observe it to do so. Think about a supersonic jet traveling overhead, the sound will never catch up to it.

I don't know enough about the Theory of Special Relativity to be able to reply.

[b00tleg]

If YOU were a photon of light, would you recognize the fact that, as you travel through space, your "time of travel" takes a certain amount of your time? . . . . . .Someone stated (in another forum):

Consider the time for a photon. It goes from point A to point B in zero time from its point of view. This is likely because it has no rest mass.

When I didn't follow what he was saying, he replied:

Einstein's Special Relativity has demonstrated that time, and space, are observed differently as relative motions appraoch the speed of light. Newton, and Galileo, were wrong if you measure things that are traveling at fantastic speeds. SR shows that the time ticks of an object moving close to the speed of light relative to you will tick slower. For a photon, at the speed of light, there are no ticks for it, but our ticks stay the same. There are many good web sites that help explain it.

Without relativity equations, GPS locating would be very inaccurate, as the speed of the satelites and the speed of Earth are not inconsequential when measurements must be in centimeters. Also, even if something were to go faster than the speed of light, you would not observe it to do so. Think about a supersonic jet traveling overhead, the sound will never catch up to it.

I don't know enough about the Theory of Special Relativity to be able to
reply.

LOL. I'm no scientist but yes, time applies even to photons travelling at the speed of light. Thats why they came up with the term "light-year" just to make it easier to comprehend how far light can travel in such a short amount of time. IIRC, light travels about 3 trillion miles in a year. But yes, even photons are subject to time.

[Kuroneko]

If YOU were a photon of light, would you recognize the fact that, as you travel through space, your "time of travel" takes a certain amount of your time? . . . . . .Someone stated (in another forum):

Consider the time for a photon. It goes from point A to point B in zero time from its point of view. This is likely because it has no rest mass.

No, "you" would not recognize it. Light follows what are called "null" paths in spacetime, along which the proper time interval is zero. This person is correct, but it it would be better to say that the notion of "time for a photon" is meaningless rather than stationary. But because one of the postulates of relativity is that light has the same velocity in all inertial reference frames, it follows that light has no inertial reference frame itself (otherwise, it would have zero velocity in some frame). Literally, light has no "point of view."

I don't know enough about the Theory of Special Relativity to be able to reply.

There isn't much to reply--he or she is correct, sans that one issue of interpretation. What exactly are you trying to do?

[Lord Zentei]

LOL. I'm no scientist but yes, time applies even to photons travelling at the speed of light. Thats why they came up with the term "light-year" just to make it easier to comprehend how far light can travel in such a short amount of time. IIRC, light travels about 3 trillion miles in a year. But yes, even photons are subject to time.

Light travels one light year per year from the POV of all observers, but not from its own POV, seeing as it has none. A light year is ca. 9½ trillion km or 6 trillion miles.

[kheegster]

Actually, that the speed of light is a constant is a premise of relativity; all the weird effects on clocks and mass and length are consequences of a constant c. I don't know them well enough to guarantee this, but I'm fairly certain you can derive the value of c from Maxwell's equations for electromagnetism.

The speed of light was first estimated in the 17th century by Roemer (sp?), who measured the time delay between a predicted eclipse on one of the Jovian (or Saturnian?) moons and the observed occurence of this phenomenon. Realising that this was due to the light travel time, he estimated a value for c which is within 30% of the actual value, which is reasonably impressive. Another, 19th century, method was to use a system of rotating spokes to 'chop up' a light signal into short pulses, and then observing the signal through another rotating spoke at a known distance away. With knowledge of the frequencies of the spokes and the distance between source and observer, it's possible to measure c to within 1% or so.

So c was reasonably well known without any prior assumptions about the nature of light. Then came Maxwell.

Combining the Maxwell equations into a 2nd order differential equation in time and space (two, rather; one for electric fields and another for mag fielsd) yields a wave equation which yields a wave speed equal to 1/sqrt(mu_0*epsilon_0), where mu_0 and epsilon_0 are the permeability and permittivity of free space, constants in SI units which determine how electric and mag fields 'spread'.

Plug in the values for these constants into the measured speed of this electromagnetic wave, and you get a value almost identical to the then known value of c, a remarkable discovery at the time.

[AK_Jedi]

If you want to insult someone, call them a photon, because photons have no "life" since time does not exist for them.


I know, I know, physics jokes are never funny.

[b00tleg]

LOL. I'm no scientist but yes, time applies even to photons travelling at the speed of light. Thats why they came up with the term "light-year" just to make it easier to comprehend how far light can travel in such a short amount of time. IIRC, light travels about 3 trillion miles in a year. But yes, even photons are subject to time.

Light travels one light year per year from the POV of all observers, but not from its own POV, seeing as it has none. A light year is ca. 9½ trillion km or 6 trillion miles.

Yeah thats true from the photon's frame of reference.

[Molyneux]

The way I understand it is this:

Any particle travelling at less than the speed of light in its medium must possess a finite mass. However, as that particle's speed increases, its mass increases as well, with infinite mass asymptotic to the actual speed of light. It cannot accelerate up to c, because that would require a literally infinite amount of energy.

For any particle travelling faster than the speed of light, it is actually travelling backwards in time (as compared to our viewpoint); as it DEcelerates towards the speed of light, its mass approaches infinity, and it similarly cannot reach the speed of light.

Any particle travelling AT the speed of light can neither accelerate nor decelerate; it must also be massless.

Is that about right?

[Lord Zentei]

LOL. I'm no scientist but yes, time applies even to photons travelling at the speed of light. Thats why they came up with the term "light-year" just to make it easier to comprehend how far light can travel in such a short amount of time. IIRC, light travels about 3 trillion miles in a year. But yes, even photons are subject to time.

Light travels one light year per year from the POV of all observers, but not from its own POV, seeing as it has none. A light year is ca. 9½ trillion km or 6 trillion miles.

Yeah thats true from the photon's frame of reference.

What? Did you miss what I was saying?

What do you mean by "the photon's frame of reference"?

[AK_Jedi]

The way I understand it is this:

Any particle travelling at less than the speed of light in its medium must possess a finite mass. However, as that particle's speed increases, its mass increases as well, with infinite mass asymptotic to the actual speed of light. It cannot accelerate up to c, because that would require a literally infinite amount of energy.

For any particle travelling faster than the speed of light, it is actually travelling backwards in time (as compared to our viewpoint); as it DEcelerates towards the speed of light, its mass approaches infinity, and it similarly cannot reach the speed of light.

Any particle travelling AT the speed of light can neither accelerate nor decelerate; it must also be massless.

Is that about right?

Thats more of less right.

Photons do accelerate though. When photons are reflected, they accelerate since they change direction.

The way I understand it, the actual mass of the particle approaching lightspeed does not increase, but the energy it takes to accelerate it does. It has the same end effect as increasing the mass.

[b00tleg]

Light travels one light year per year from the POV of all observers, but not from its own POV, seeing as it has none. A light year is ca. 9½ trillion km or 6 trillion miles.

Yeah thats true from the photon's frame of reference.

What? Did you miss what I was saying?

What do you mean by "the photon's frame of reference"?

No, I think I understand what your saying. From any outside frame of reference (or point of view), it takes a certain amount of time for even a photon to travel somewhere at the speed of light. From the point of view of a photon, time essentially stands still for it b/c its in a constant state of travelling at C. It'd be much the same way of throwing a person in a spaceship and safely accelerating them to say, a third of C. By doing so, the occupant of the ship wouldn't notice any real difference from his point of view. Time would pass at the same rate it always has. However for everyone else observing from the outside, decades, even hundreds of years could pass before the spaceship gets back.

Am I thinking of that right?

[Lord Zentei]

No, I think I understand what your saying. From any outside frame of reference (or point of view), it takes a certain amount of time for even a photon to travel somewhere at the speed of light. From the point of view of a photon, time essentially stands still for it b/c its in a constant state of travelling at C. It'd be much the same way of throwing a person in a spaceship and safely accelerating them to say, a third of C. By doing so, the occupant of the ship wouldn't notice any real difference from his point of view. Time would pass at the same rate it always has. However for everyone else observing from the outside, decades, even hundreds of years could pass before the spaceship gets back.

Am I thinking of that right?

Not quite. Two points:

* As Kuroneko pointed out, light has no "point of view", so measuring how long anything takes in relation to that point of view is meaningless.

* It is true that time moves more slowly for a moving observer in relation to a stationary one. However, light still moves at one light year per year from the point of view of the moving observer. From all observers, light in a vacuum always moves at light speed.

From the POV of a moving observer, it's the distance from A to B that gets less, not that light moves at a different rate.

[kheegster]

Thats more of less right.

Photons do accelerate though. When photons are reflected, they accelerate since they change direction.

Incorrect. The term 'acceleration' is meaningless for photons since their speed is invariant. However, it is possible to change their energy and momentum.

The way I understand it, the actual mass of the particle approaching lightspeed does not increase, but the energy it takes to accelerate it does. It has the same end effect as increasing the mass.

In the rest frame of the particle, its mass is still the same, but from another frame its mass would seem to increase.

[Braedley]

Another, 19th century, method was to use a system of rotating spokes to 'chop up' a light signal into short pulses, and then observing the signal through another rotating spoke at a known distance away. With knowledge of the frequencies of the spokes and the distance between source and observer, it's possible to measure c to within 1% or so.

Nitpick: It was one octagonal set of mirrors that was spinning, with the light source and observer at the same point. Downrange was a set of mirrors placed at a 90°. IIRC, the experiment was set up between two mountain ranges. The distance between the octagonal mirrors and the reflector mirrors could be measured to within a decent number of sig figs (maybe 3 or 4) at the time, and the rpm could be measured fairly accurately as well (again 3 or 4 sig figs), meaning you could get an answer with 3 or 4 sig figs. The biggest error is that it was conducted in air (although not at sea level) and not in a vacume, which would be practically impossible, although the %diff between c in space and c at sea level is less than 5%, and maybe less than 1% (don't remember the exact figure).

[Sriad]

If you want to insult someone, call them a photon, because photons have no "life" since time does not exist for them.


I know, I know, physics jokes are never funny.

I respectfully disagree!

One day, Heisenberg is traveling happily down the highway when a cop pulls him over.

"You maniac!" the cop yells, "do you have any idea how fast you were going back there?!"

"None at all," Heisenberg replies, "but I know exactly where I am."