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Gravity Probe B

Testing Einstein's Universe

Special & General Relativity Questions and Answers

Is there a simple way to measure the speed of light?

No. Galileo tried to do it with opening and closing lanterns real quick, but failed. The Danish astronomer Roemer used the predicted eclipses of Jupiter's moons at different places in Jupiter's orbit. This was very simple, but you need to know the earth-jupiter distance from an ephemeris for two different times, and then predict when the eclipses should occur. You then need a telescope to see when they actually occur. It is elegant but requires an ephemeris and a telescope.

In the lab, you need a pulsed laser, an oscilloscope and a long fiber optic cable to serve as a delay line. You send a pulse down the line which triggers the oscilloscope trace, then you detect the pulse when it gets to the other end of the cable to trigger the oscilloscope again. You measure on the oscilloscope the time interval between triggers and divide this into the length of the fiber optic cable.

Another possibility that was pointed out to me by Mr. Dejan Vucinic on 1-dec-1995 is to use the propagation of radio signals in a long distance relay. Here's how I think one might do it. You need two amateur radio 'ham' operators on preferably a trans-Atlantic radio link, an oscilloscope, and an independent link to the WWV time signal. The operator with the oscilloscope would set it up to trigger on a pre-arranged WWV time signal, and on a received signal from the second ham operator. At a pre-arranged time, operator A sends a short ( few millisecond), single pulse of energy at a pre-arranged frequency. Operator B's oscilloscope triggers its sweep on the same WWV time signal and on the received pulse. For a 10,000 km path between Operator A, the ionosphere and operator B, the delay should be about 33 milliseconds which is why the pulse has to be short. Once a delay is detected, you now have to note the geographic locations of the two operators on a globe, and for an ionosphere height of 20 miles, find the minimum propagation distance between the two sites. With the distance and the time delay, dividing the two should give you the effective propagation speed which ought to be close to light-speed. Doing the experiment many times and averaging should eliminate some random errors, but there could be several systematic errors that have to be identified and compensated for.

Can anyone, perhaps as a science fair project, think of another way to do this?

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All answers are provided by Dr. Sten Odenwald (Raytheon STX) for the NASA Astronomy Cafe, part of the NASA Education and Public Outreach program.