the real speed of light

[bwanaaa]

measuring the speed of anything requires detecting it at 2 different locations a known distance apart and accurately knowing the time that it is detected at each of those locations. Help me with a few simple problems:

How can you detect a photon without destroying it? so you really cannot measure the speed of a photon.

let's put that problem aside and consider a collections of photons on the starship enterprise. As the Enterprise accelerates towards lightspeed, the things and people on the starship are approaching the speed of the light from the lightbuulbs on the starship. Someone standing in front of the captain and looking backwards would see the view get redder and slower and larger because more time would elapse as succesive waves of photons reached the eye. At lightspeed, the scene would fade to black as the last wave of photons hiits the retina. the next wave never gets to the retina because the eye is traveling away from the light as fast as the light is being reflected off the captain. So how do you measure the speed of light if you cant see the clock?

I guess the non-smart ass way to ask the question is: how was the velocity of light first measured?

 

[Geniere]

I?ll only sta b at the first and third.

#1 Yep we can only make inferences. Does a photon actually move at all?

#3 If memory serves I think Galileo was the first to record his experiments which used lanterns and shutters. But the most precise, the most exacting method is as follows:

Buy a bunch of chocolate bars. Take the rotary tray out of your microwave oven. Line the entire bottom of the microwave oven with the chocolate bars (no wrappers). Turn oven on and observe the chocolate bars. When small melted depressions appear on the chocolate, quickly turn the oven off. Measure the distance between any two of the melted areas which provides the wavelength of the microwaves. Remove the magnetron tube from the microwave and read the small tag wherein the manufacturer will have inscribed the operating frequency. You now know the frequency and the wavelength, that?s all you need to calculate the speed.

 

[bwanaaa]

 

[bwanaaa]

Indeed, speed=wavelength x frequency, but then you have to measure each of those properties first. I am looking for a fundamental way to measure the speed of light involvng nothing more than a detector and a stopwatch.

 

[sgtroyer]

I'm not sure if it was the first, but the first accurate way of measuring c involved rotating mirrors. Rotate a mirror very fast. Shine a focused light on it (laser is best, but didn't exist when Michelson and Morley did this). Light will reflect off mirror, travel a certain distance, reflect off a fixed mirror, and return to the rotating mirror. While the light was taking this trip, the mirror has rotated by a small amount, so when the light hits the mirror again, the beam will be shifted slightly. If you know the distance between the two mirrors, the rotational speed of the mirror, and the angular shift, you can deduce the speed of light.

EDIT: As far as Star Trek goes, first of all, Star Trek doesn't exist. But if you want to wonder about what's possible, read "The Physics of Star Trek"

 

[superHARD]

Would the light be moving? I mean once you reach light speed would it be a solid room of light. Like what you saw once light speed was reached you would continue to see until you slowed down.?

I am shooting in the dark btw.

 

[VictorLazlo]

Originally posted by: bwanaaa
Indeed, speed=wavelength x frequency, but then you have to measure each of those properties first. I am looking for a fundamental way to measure the speed of light involvng nothing more than a detector and a stopwatch.

There is a reflector on the moon (left there by astronauts). Scientists routinely bounce laser beams off this reflector to keep track of the moons distance and to measure the speed of light. The distance is great enough that the travel time can be measured with a very accurate "stopwatch" in the method you are proposing.

 

[sgtroyer]

You can't measure the distance to the moon and the speed of light. Since what you're actually measuring is a time, speed will depend on distance, and distance on speed. Presumably they use the known speed of light, and use the mirror to measure distance.

 

[vtqanh]

What he just said is right, they don't measure the speed of light using the reflector in the moon. It's only for distance.
Galileo was the first to measure the speed of light using lattern. Obviously it didn't work out very well
Then somebody else measured it by using the moon of Jupiter (Io)
Then somebody measured it by using Rotating mirror.
After 1980s, they define meter as 1/299,xxx,xxx ( I don't have these x's on top of my head) speed of light. So measuring speed of light is no longer necessary, since you fix speed of light as a constant and define meter as 1/299,xxx,xxx of that constant.
Scientists have stopped measuring the speed of light for a while since they have more important things to measure

 

[uart]

Hi sgtroyer, the first somewhat accurate terrestrial based measurement actually involved shining light through a "toothed wheel" (to chop it into pulses) and then reflecting it back (from a distant mirror) to a second synchronized toothed wheel. The speed of the wheel was then adjusted to find the first minimum in the reflected light passing back through the second wheel. This occurrs when the light transit time is equal to the time for the wheel to rotate one half of one tooth pitch. The ?rotating mirror? method you describe was a later refinement.

However, nearly two centuries before these first terrestrial based measurements a Danish astronomer by the name of Roemer inferred the speed of light from observations of one of the moons of Jupiter. (based on apparent variations in the predicted times of ?moon set? and moon rise? as the distance between Jupiter and Earth varied due to their respective orbits).

While not ?accurate? by today?s standards, Roemers figure of about 2 x 10^8 m/sec was quite an achievement considering that there was no prior knowledge at all, other than Galileo?s figure of ?either very fast or instantaneous?.

 

[rjain]

the distance a photon travels from source to detector divided by the time it takes to move that distance is the speed of the photon. So what if the photon is destroyed at the end?

 

[rneff50]

I did this experiment back in High School. Used a frequency modulated laser and beam splitter and two photodiodes. One half traveled a known long distance ~20 meters to the diode and the other half traveled about a tenth of that to it's own diode. I used an oscillscope to look measure the phase shift between the waves. This is all you need to calculate the speed of light. It was really quite simple and very accurate. I seem to remember only couple percent error.

 

[PowerMacG5]

Well, about your Enteprise analogy, it doesn't work like that in Roddenberry's Star Trek. In Start Trek, the Warp Theory (Miguel Alcubierre's theory) is used. What happens is the ship is encapsulated in an energy bubble creating two spaces, inside and outside space. The ship is moving at 0 m/s relative to the inside space, therefor there are no time dilation or light bending occurences happeneing relative to the inside space. Alcubierre's theory is terchnically possible because there is no speed limit on the speed that space can travel within space (Universe is expanding greater than c). Although, in theory, Alcubierre's warp theory requires almost all energy present in the known universe to warp create the gravitons and exotic matter needed to move the inside space through the outside space.

 

[rjain]

I don't know which Universe is expanding faster than c, but it's not the one we live in. I don't know how someone can build a theory of how to engineer a machine based on theories of particles that we haven't even discovered or characterized yet.

 

[RossGr]

let's put that problem aside and consider a collections of photons on the starship enterprise. As the Enterprise accelerates toward light speed, the things and people on the starship are approaching the speed of the light from the lightbuulbs on the starship. Someone standing in front of the captain and looking backwards would see the view get redder and slower and larger because more time would elapse as succesive waves of photons reached the eye. At lightspeed, the scene would fade to black as the last wave of photons hiits the retina. the next wave never gets to the retina because the eye is traveling away from the light as fast as the light is being reflected off the captain.

This is a whole different topic then the measurement of the speed of light.

Have you ever heard of a fellow named Albert Einstein?

His paper ON THE ELECTRODYNAMICS OF MOVING BODIES is based on works of Clerk Maxwell, who in 1867 showed that the speed of light is a constant. It's value does not change with the observer. In other words no matter how fast you are moving you will still measure the same speed of light. Thus is it impossible to move at light speed. Simply because no matter how much you speed up you will still measure the speed of light as c.

So in the example above, all will appear normal to the inhabitants of the Enterprise, no matter how fast it goes.

 

[PowerEngineer]

Michelson-Morley Experiment

 

[Shalmanese]

The problem is that now we don't measure the speed of light, we measure the length of a metre. The speed of light is EXACTLY 299something hundred km/s. From this, we can deduce exactly how long a metre SHOULD be.

 

[rjain]

No. The length of a meter is defined as a specific number of cycles of light emitted from a particular kind of laser.

 

[chicsdiggit]

Are you asking for the type of experiment required for accurate measurement of light? If so, then I don't know the exact experiment done, but if you are merely asking how they calculate it, they simply use a distance = rate x time formula, or rather, rate = distance/time.

I wonder, is the universe really expanding faster than c? (c is the speed constant of light) Einsteins General Theory of Relativity says no. A free body cannot accelerate to the speed of light, and his famous equation E = MC^2 shows that. Plainly speaking, in order to accelerate, you need to apply more and more power (E). Since E must continually become greater, the c^2 becomes insignificant, and therefore goes 0, using limits. So the more you thrust, the greater mass (M) becomes. Mass goes to infinity, therefore it would take an infinite amount of energy to accelerate to the speed of light.

 

[ZeroNine8]

Originally posted by: Shalmanese
The problem is that now we don't measure the speed of light, we measure the length of a metre. The speed of light is EXACTLY 299something hundred km/s. From this, we can deduce exactly how long a metre SHOULD be.

Originally posted by: rjain
No. The length of a meter is defined as a specific number of cycles of light emitted from a particular kind of laser.

The 1889 definition of the meter, based upon the artifact international prototype of platinum-iridium, was replaced by the CGPM in 1960 using a definition based upon a wavelength of krypton-86 radiation. This definition was adopted in order to reduce the uncertainty with which the meter may be realized. In turn, to further reduce the uncertainty, in 1983 the CGPM replaced this latter definition by the following definition:

The meter is the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.

http://physics.nist.gov/cuu/Units/meter.html

 

[rjain]

hmm, I thought it was based on some kind of cesium laser.

The size of the universe can't increase faster than the speed of light unless you define the universe in a way that makes part of it inaccessible from the rest of the universe forever. (of course, black holes complicate that issue further)

Edit: oops, the cesium one is the definition of a second. From that, we can measure the distance travelled by light in a second and compute the length of a meter.

 

[f95toli]

Originally posted by: rjain
The size of the universe can't increase faster than the speed of light unless you define the universe in a way that makes part of it inaccessible from the rest of the universe forever.

Which is, according to some astronomers, actually what is happening. It is entirely possible to have two regions in space separated in such a way that they never have been (and never will) in "contact" with each other, in principle even the laws of nature could be different in the two regions and you would never be able to tell.

 

[maluckey]

Makes sense. Otherwise we would see a night time sky as bright as daylight from the countless stars radiating light for billions of years. The universe would be saturated with light.

I understand that gravity affects the path of light, but wouldn't grossly irregular shapes also affect wavelength and reflected particles enough to cause the darkess that we see?

 

[silverpig]

Originally posted by: rjain
I don't know which Universe is expanding faster than c, but it's not the one we live in. I don't know how someone can build a theory of how to engineer a machine based on theories of particles that we haven't even discovered or characterized yet.

Simple. You invent the particles to have whatever properties you want.

 

[rjain]

Originally posted by: f95toli

Originally posted by: rjain
The size of the universe can't increase faster than the speed of light unless you define the universe in a way that makes part of it inaccessible from the rest of the universe forever.

Which is, according to some astronomers, actually what is happening. It is entirely possible to have two regions in space separated in such a way that they never have been (and never will) in "contact" with each other, in principle even the laws of nature could be different in the two regions and you would never be able to tell.

Right, but all those are speculations and imaginations, not theories with any kind of scientific utility or relevance.