As the title suggests I would like to know how you measure the speed of light. I understand the distance over time thing but how does that get put into practise when it is to fast for the human eye to see and probably very hard for the average person to appreciate.
Also does all light travel at the same speed?(ultra violet, infra red, laser etc.)
Bounce a beam of light off of some object of known distance away, then measure the length of time it took that beam of light to make the round trip. For more accuracy, use a more distant object and/or faster equipment.
(I have a Bushnell Laser Rangefinder that I bought for $200 a few months ago that works on this principle, although it assumes the speed of light and estimates the distance of the bounced-off-of object. It can measure distances from as small as 21 yards to as large as 600 yards with an accuracy of one yard. Light takes approximately one nanosecond to travel one foot, so this puppy apparently can measure time intervals within three nanoseconds.)
I’ll let someone else handle this one. I think it does, but I really don’t know.
Okay thats pretty much what I thought, thanks.
For those of you that can answer the ’ does all light travel at the same speed?’ question, here’s another one for you.
When do you start to time light from the sun as it is not like a lightbulb you can turn on or off, it is constant so where is the defining point?
Yes, all electromagnetic radiation travels at the same velocity – whether X-rays, gamma rays, visible light, microwaves or radio waves.
Yes; all light travels at the same speed in a vacuum. However, light speed is slower through a substance (like air or water). I believe that the reduction in speed is linked to wavelength, so that all light does not travel through a medium at the same speed. The following URL has a little information, although it doesn’t go into great detail on the frequency dependence:
http://www.scimedia.com/chem-ed/light/em-rad.htm
Light comes in small particles, called photons. Photons move at the speed of light; so you can just follow a photon. Of course, light is both a wave and a particle, but that’s beyond my ability to clearly explain.
If I recall correctly, some of the most accurate measurements of the speed of light were done in the lab. A beam of light was bounced back and forth between a series of mirrors. The reason for doing this was to allow the beam of light to travel a great distance, without taking up a lot of space (the total length of this chamber was about 3 feet). This also allowed the test to be done in in a vacuum. From that point on, the test was pretty much similar to any other speed test–turn on the light and time the light beam. Of course the time-measuring instruments were incredibly accurate.
I can’t recall the name of the guy that first did this, but his experiment went something like this:
He had a gear of a known diameter and tooth pitch. He shined a light through a small hole and at the teeth on the gear at a known distance. He change the speed of the gear until the light stopped flickering.
You can calculate the linear velocity of the gear at the gap between the teeth knowing the angular velocity. The distance the light travels is known so bingo.
My dynamics is a little fuzzy from years of dormancy and I sold that text after I graduated so I can not post the equations, but you get the gist.
In 1728, James Bradley accurately (to about 1/2% error) measured the speed of light by measuring the difference in the angle of incidence from a star as it’s light struck the Earth at different positions in the Earth’s orbit around the Sun.
Arround 1850, Jean Foucault used a technique whereby light was reflected off of a rotating mirror and was allowed to strike a second mirror, reflecting back to the rotating mirror. Knowing the speed of rotation and measuring the angle of displacement, the speed of light can be calculated. Several years later Albert Michelson, refined Foucault’s method to achieve greater accuracy.
Until recently, this has been the standard way of measuring the speed of light. “Incredibly accurate time-measuring instruments” were not required.
Lately I’ve seen experiments that measure the speed of light using LEDs as photon emitters, fiber optics as a medium, photodetectors as receivers, and oscilloscopes as measurement devices.
I’m not aware of any experiments where light was reflected through a series of mirrors in a vacuum. I don’t see how this would work, actually, but would welcome a reference or a link to such an experiment.
horowitz wrote:
Armande Fizeau is the guy who did the toothed wheel experiment, however his results were not significantly more accurate than those of James Bradley, which is why I didn’t mention him.
If you spun a carousel fast enough, could
you get the edge of the carousel going nearly the speed of light so that time stood still for
people on the carousel?
Couple of quick notes…people are correct in saying the speed of light is measured by bouncing it off something far away (as I remember one experiment involved the moon) and measuring how long it takes for the light to return.
The speed of light is constant, period. IT does not slow down moving through air/water/etc…however as the light particles (photons) pass through material they may get bounced around, and it may appear as if light has slowed down, but it is merely taking a longer route…interestingly enough another kind of particle…neutrinos…pass through matter without stopping. Just why neutrinos do this and photons don’t (both have no mass) is not understood.
No in is SURE there are any particles involved in light, that is just a theory. The wave theory appears to have a bit more empirical support, though they are generally combined today.
There is a theory based on Einstein’s relativity that the quicker you travel (particularly relative to the speed of light) the slower you age. Thus if you were at the outside of a carousel travelling at the speed of light, you would theoretically age much slower…you wouldn’t be able to go BACK into time per se or have it stop altogether, but it would slow down. This is just a THEORY however and there is no empirical proof of it…neither is there any proof that mass increases the faster you travel. This is based only on mathematics, which historically tend to be faulty, and some of the logic for this theory is actually quite poor. Physicists have myopia like all scientists and have trouble realizing when they are spouting fantasy. I wouldn’t count on a time travelling carousel.
IIRC, Michelson (of the Michelson-Morley experiment) was obsessed with improving the measurement of the speed of light. He spent his last years in California measuring light bounced back and forth through a fairly long underground tunnel. As his measurements grew more precise, he noticed a “drift” in his values … he had built his tunnel across the San Andreas fault, and needed a correction for the change in length of the tunnel!
Formally, since 1983 it has been “impossible” to measure the speed of light. In that year, the meter (the fundamental length unit for everyone in the world) was defined as “the length of the path travelled by light in vacuum during a time interval of 1/299 792 458 of a second.”. That is, the speed of light in a vacuum is a fundamental constant equal to exactly 299,792,458 meters per second. It’s “impossible” to measure it because your measuring stick is defined in terms of the speed of light in a vacuum. See Speed of Light in Vacuum and Unit of Length (meter).
The original question as well as some of the other subjects brought up are addressed in The Usenet Relativity FAQ, which covers, among other things, the constancy of the speed of light, how it is measured, and (this is addressed to avalongod) references concerning empirical proof of relativistic time dilation, etc.
Uhh, avalongod, I think you need to have a discussion with someone who’s familiar with physics. Virtually everything you said in your message there was completely wrong. 
Just for the record, Time dilation has been proven and shown to be a real physical effect. A famous experiment involving flying atomic clocks verified that time does indeed slow down at higher speeds.
BTW, avalongod, you are misusing the word “theory.” Just because it is called the “Theory of Relativity” does not mean that the truth of it is in the slightest bit of doubt.
I think avalongod deserves some credit here for at least sounding authoritative.
Yeah, there are a couple of howlers in avalongod’s message.
There are many experimental measurements of time running slower at high velocity and faster in low gravitational fields. Every particle accelerated in a particle acelerator exhibits the first effect. Similarly for many particles produced high in the atmosphere by cosmic ray collisions; we wouldn’t see them at the surface of the Earth unless their clocks run slower. The clock on each GPS satellite is adjusted for both velocity and position-in-the-gravitational-field; GPS wouldn’t work if relativistic time effects didn’t exist.
Similarly, all particles in particle accelerators increase in mass by the exact amount predicted by relativity.
A couple of other methods of measuring the speed of light (or alternately, measuring the length of the meter, since the definitions got changed): First of all, you can just use an ordinary clock, if your distances are large enough. The first scientific measurement of the speed of light used this method. In 1676, by careful measurement of the timing of the eclipses of Jupiter’s moons, Ole Christensen Roemer calculated the speed of light to within about 50% accuracy-- not so hot by today’s standards, but pretty good for a first try.
Another method relies on the fact that according to Maxwell’s equations of electrodynamics, the speed of light should be directly related to the values of two constants, epsilon-naught and mu-naught, which determine the strengths of the electric and magnetic forces, and these constants can be measured relatively easily in the lab. Specifically, c=1/sqrt(e[sub]0[/sub]*m[sub]0[/sub]). Of the reasonably accurate methods for measuring the speed of light, this is probably the easiest to carry out-- I recall making these measurements in a freshman level lab.
A couple more notes to prevent mislearning from avalongod…neutrinos do have mass. Or rather, they almost certainly have mass, and the uncertainty on the lower bound is small enough to make it a pretty sure thing. That should be tested to better uncertainty within the next few years (the good evidence for neutrino mass is less than 2 years old, IIRC).
Another thing: The photon and the neutrino are not related enough to make the kind of “oh wow!” comparison done here. Neutrinos are matter particles (leptons, I think, like electrons). Photons are force carriers (they mediate the electricmagnetic force). Comparing the strength of interaction of one to the other is meaningless. If you want to know why photons interact the way they do, take a couple courses in QED.
That sounds distinctly 18th century. The modern answer, of course, (not that it helps confusion of people unfamiliar with physics, but I haven’t heard a better way yet!) is neither or both, depending on how you want to look at it. Of course, wave/particle duality is true of all particles, not just photons.
Others have taken care of the “just a THEORY” part, but one real nit: If you were on a carousel at the speed of light, you would not just age much slower; time would stop entirely (infinite time dilation; relative, of course, to an observer in another frame). Of course, you could never reach the speed of light, since you would then (being composed of matter and thus having mass) have infinite kinetic energy, which Nature doesn’t like too much…
Come back in a few years when I have a PhD and I’ll be really insufferable.
avalongod,
How can you believe that light is comprised of waves AND believe that the speed of light is a constant through all mediums?