Читать книгу Quantum Physics is NOT Weird - Paul J. van Leeuwen - Страница 19

Despite Maxwell's conclusion that the speed of light in a vacuum had to be constant in all circumstances, scientists kept trying to measure the speed at which the earth traveled through the ether by conducting sophisticated experiments with light. After all, it could still be possible that the ether would prove to be the basis of Maxwell's EM fields.

In 1887 Albert Michelson (1852-1931) and Edward Morley (1838-1923) devised and build an extremely accurate and quite advanced interferometer ^[12] to measure the movement of the earth in relation to this purported unmoving ether. They sought to determine the ether wind, an ether movement experienced due to the movement of the earth through this supposed ether, a bit like the wind you experience when driving a motorbike. They used roughly the same principle with which one could determine the speed, in relation to the stationary air, of a passing police car with its siren on, the so-called Doppler effect. Traffic police do something similar using radar. The radar waves reflected from your car are still traveling at the speed of light, however they have changed slightly in their frequency due to being reflected from your moving car. In that way, the police can measure your speed. So, beware.

Michelson and Morley’s experimental set-up made use of this expected Doppler effect of light waves sent out from an object moving through the ether, cleverly combined with the interference phenomenon of light waves. See figure 3.8. Using mirrors, they let monochromatic light beams bounce back and forth in two perpendicular directions and made the reflected light waves interfere with themselves. Rotating their arrangement would change the angle of the two perpendicular light beams relative to the direction of the presumed movement through the ether. This would affect the speeds of both light beams through the ether and their interference pattern had to shift visibly. The speed at which their interferometer would move through the purported ether could then be calculated by measuring the interferential shift.

Compare their set-up with a bike contest between exactly two equal fast engines. The start and finish are in city C. City N lies 50 miles north of C, city W lies 50 miles west of C. A strong wind blows exactly from the north with 25 miles/hour. One biker must reach N before returning to C, the other must reach W. Which race route would you choose if you were one of the bikers? If you do the calculations the answer is that the side wind path is the fastest. Biker C-W-C will be the first to finish.

Figure 3.8 shows the situation in which the interferometer moves to the right at a speed v relative to the ether. The dotted lines represent the path of the light relative to the assumed ether. The solid lines represent the path of the light relative to the interferometer. Half of the light is reflected upwards (grey vertical arrow) by a beamsplitter (a semi-transparent mirror); and then reflected down again by the upper full mirror.

Figure 3.8: Michelson-Morley experiment.

Source: Stigmata Aurantiaca on Wikimedia Commons.

On its way back half of it again passes through the beamsplitter (lighter gray downward arrow) reaching finally the interference detector. The horizontal grey arrow represents the other half of the light that passes first through the beamsplitter, reflects then from the full mirror on the right, returns to the beamsplitter where again half of it is reflected downwards to the detector. After having traveled different paths, both light waves (light and dark grey) will meet each other at the detector to interfere there. The more the travel time in vertical direction differs from the travel time in the horizontal one, the more their phases will differ and the more the interference pattern will shift.

Since the direction in which the earth would move through the ether was not known beforehand, Michelson and Morley’s interferometer could be rotated in a very controlled way. Their final and most precise experimental set-up was therefore mounted on a large heavy round granite plate that floated on a mercury bath. This prevented unwanted vibrations disturbing the interference. Once set in motion it would rotate by itself at a uniform speed, almost without any additional force.

For mathematics aficionados: in figure 3.8 you will also find the formulae for the duration T_t of the transversal light wave up and down (gray line) and for T_l that of the longitudinal wave to the right and back (black line). See if you can derive these plus a formula for ΔT = T_t - T_l.

The very small speed differences due to this movement of the earth through the ether would lead to time differences ΔT and therefore phase differences upon arrival at the detector. This would lead to a detectable shift in the interference pattern.

Michelson and Morley’s measuring instrument was extremely sensitive. It should be able to detect deviations in the speed of light due to the speed of the earth in its orbit, which is 30 km/s (18.6 mi/s), should that also be the speed of the earth relative to the ether. The reasoning was that even if the sun did not move with respect to the ether, the earth's orbit would show speed differences in opposite seasonal positions at twice that amount – so 60 km/s (37.2 mi/s). See figure 3.9.

Figure 3.9: The movement of the earth in summer and winter with respect to the ether wind.

If Michelson and Morley’s interferometer was supposed to move through the stationary ether at 30 km/s, their calculations pointed out that the light perpendicular to the direction of movement would lag 10^-15 seconds behind the light traveling in the direction of movement of their set-up. The resulting path difference of about 300 nanometer (nm) should have been detected by their very accurate device. This difference is of the same order as the wavelength of visible light, between 400 and 800 nm. A noticeable shift of the interference pattern should be observed.

However, to their disappointment, no shift was detected. So, exit the ether. Or one would have to reinstate the geocentric universe of Ptolemy, the earth residing immovably at the centre of the universe. Which was of course not a viable option anymore. Nonetheless, Edward Morley maintained his belief in the existence of the ether until his death. Their experiment, by the way, is still being repeated – every time with increased precision. So, in principle, the case is still undecided. But today's consensus is that the ether does not exist unless the opposite can be demonstrated convincingly.

Albert Michelson received the Nobel Prize in physics in 1907 for his performance in optics and his contribution to physics. A considerable prize for a "failed" experiment.