The One-Way Speed of Light

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Speed of light experiments measure the average speed to a destination and back, leaving open the possibility that the speed may differ over each leg. Now Canadian physicists have measured the one-way speed of light to test whether Einstein really was rightOne-Way Speed of LightOn any list of the most important discoveries in 20th century physics, the constancy of the speed of light must come near the top. The idea of a universal, constant ‘c’ has profoundly influenced our ideas about the universe ever since.

When Einstein put forward this idea in the special theory of relativity, it immediately solved an important conundrum. Many physicists believed that light, like other waves, must travel through a medium which they called the luminiferous ether. Since the Earth moves around the Sun, it must move relative to this ether. Therefore, the speed of light in a given direction ought to change during the course of the year and even throughout the course of a day.

The now famous Michelson-Morley experiment knocked this idea on the head when it failed to find any variation in the speed of light relative to the Earth’s motion through space. Physicists have repeated this experiment with increasing accuracy many times since with the same result. The speed of light really is constant.

But there’s an interesting loophole in these experiments. They all rely on measuring the round trip speed of light between two points, for example, by bouncing light back and forth between a pair of mirrors. In other words, these experiments measure the average speed of light over two legs, there and back again.

That leaves open the possibility that the speed of light over each leg could be different. So various physicists have attempted to close this loophole by measuring the one-way speed of light.

This is no easy task. The history of these tests is filled with controversy, disputed results and more than one sensational claim that, yes, the one-way speed of light is different from the conventional two-way result.

Today, Farid Ahmed at York University in Toronto and a few pals reveal the results of their own measurement of the one way speed of light. And their results offer some comfort to fans of the status quo.

First, how do you measure the one way speed of light? There is no shortage of experiments which claim to have done this but have been later shown to have measured the two-way speed because of some overlooked factor.

So Ahmed and co’s method is important. These guys create two identical pulses of light and send them in opposite directions along the same length. If there is any difference in the speed of these pulses, that ought to be detectable by photodiodes at each end of the experiment.

Of course, the devil is in the detail. To measure any difference, the experiment has to be run over at least 12 hours, to allow the Earth’s rotation to reorient the experiment. Ahmed and co performed their experiment on 14-15 November and 28-29 November 2009.

And the effects of any environmental changes need to be carefully accounted for. The most significant of these is temperature which can play havoc with experimental equipment and therefore the results. But there are also other factors such as humidity and so on.

Ahmed and co say they’ve carefully accounted for all these factors and that their results are conclusive. “Our results do not report any significant diurnal variations,” they say. “This is consistent with Einstein’s Special Relativity.”

Ahmed and co’s results will need to be carefully scrutinised by the community. Nevertheless, the result will be the one that most physicists expect.

But the result will also be a disappointment for the small band of theorists who say that string theory predicts small variations in the speed of light in these kinds of experiments.

Of course, Ahmed and co don’t rule out the possibility that these variations might exist on a scale too small for this experiment to pick up.

But a test of that will have to wait for another day. Which in some ways is the beauty of science—there is always new physics to be found somewhere over the horizon (we hope).

Ref: Results of a One-Way Experiment to Test the Isotropy of the Speed of Light


Written by physicsgg

October 14, 2013 at 11:53 am


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  1. I think you can synchronize the clocks to measure one way speed of light:
    Let’s have 2 points A and B and point C exactly in the middle. We can indirectly synchronize the clocks at A and B by sending the signals to C and calculating back the time on A and B clocks so the signals from A and B would arrive at C at the same time. Let’s assume temporarily that uni- directional speed of light is the same in both directions and equals c.
    We can now synchronize the times at A and B with C knowing that at the moment signals were send towards C (assuming the signals arrived at C at the same time) the time at A and B should be same as time at C minus travel time of the photon from A (or B) to C.
    Now we can send the signal from A and B towards C at the same time as indicated on clocks A and B. If the speed of light is indeed same in both directions, both signals will arrive at C at exactly the same time. If there were any differences, we could easily calculate speed of light in either direction


    May 15, 2014 at 2:08 pm

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