In the previous parts of this lesson we have used thought experiments and logic to argue the predictions of Special Relativity. This is not enough for science! We need actual experiments and data proving Special Relativity.
In antiquity, the speed of light was thought to be infinite. Galileo showed that the speed of light was finite, but he was unable to measure its speed. The first measurement of the speed of light was done in 1675 by a Danish astronomer Olaus Roemer. He used the delay in sunlight reflecting from Jupiter’s moons to calculate the speed of light (he measured c = 227,000 km/s) and proved that it is finite. In 1850, French physicist Fizeau and Focault measured the speed of light more accurately with a series of experiments using mirrors. But, the experiment that is credited for proving the absoluteness of the speed of light is the Michelson-Morley experiment.
At the time, space was thought to be filled with something called ‘ether’ through which light would propagate. The idea of vacuum/empty space was not yet around. In 1887, Michelson and Morley set out to prove that the speed of light must be different when Earth is traveling parallel to the ether versus when it is traveling perpendicular to the ether. The experiment failed! What this means is that they actually showed the opposite of what they set out to do, and instead proved that the speed of light is constant and is not affected by Earth’s motion around the Sun. Numerous experiments followed from measuring light from opposite sides of the Sun, or binary stars and all show the same result. The speed of light is absolute.
When cosmic rays (high-speed subatomic particles) from space hit Earth’s atmosphere, new elementary, extremely short-lived particles form and shower down to Earth’s surface. Muons typically only live for about one-millionth of a second before they decay into other types of matter. Muons that form in the upper atmosphere (10 km above ground) fall to the ground at speeds of 0.99c c They should decay in one-millionth of a second and we should never see them we do see them reach the ground is because of relativistic time dilation. Time is flowing more slowly in the reference frame of a muon when compared to time on Earth. Therefore, in the Earth-based reference frame, the muons can travel all the way to the ground before they decay.
Other tests proving time dilation are frequently done in particle
accelerators, where physicist accelerate subatomic particles to speeds close
to c
and study what happens when these particles collide. For
example, a particle called
Time dilation was even shown in 1975 in an experiment where time on an airplane flying in circles was compared to the time on the ground. After a 15 hour flight, the airplane clock lagged by little under 6 nanoseconds compared to the clock on the ground.
More recent experiments proving and accurately measuring time dilation were done using the Space Shuttle. Special relativity is actually one of the best-tested theories in physics.
Particle accelerators also confirm the mass increase as it is predicted by special relativity. The amount of energy released when high-speed particles collide depends on the mass of particles and their velocity. Particles created at high-speed collisions are measured to have a higher mass than the same ones created at lower speeds, as predicted by special relativity.
To this day, even in particle accelerators we have never been able to accelerate particles at exactly c . Getting particles to move at 99% of c is relatively easy, but the highest speed achieved in particle accelerators, to this day, is 0.00001% short of c .
Einstein’s famous E=mc^2 equation tells us that mass and energy are equivalent. What this means is that energy can be converted into mass and mass can be converted into energy. In Astro 1, you have seen that this is not an easy process but it does occur under certain conditions. One example is mass converting to energy in the core of the Sun where nuclear fusion reactions occur. In this case the mass of the four hydrogen nuclei is heavier than the mass of one helium nuclei that they make. So, this missing mass is converted into energy and is what provides us with light and energy from the Sun.
We also ran into energy converted into mass while studying the Big Bang. Very early on in the history of the universe, temperatures were so hot that energy was frequently converted into particles (mass) and mass into energy.
[1] The Cosmic Perspective by Bennett, Donahue, Schneider and Voit, 6th edition