Skulls in the Stars

Einstein’s special theory of relativity still is met with disbelief by a lot of non-physicists, and it is probably one of the most active areas of physics science denial out there. Write about relativity, and it is quite likely that you’ll get an angry commenter arguing that the entire theory is nonsense.

To be fair, it is extremely shocking and non-intuitive at first glance, as it forces us to reevaluate our notions of space, time, and simultaneity. In short: in response to a baffling inability to measure variations of the speed of light in vacuum with respect to relative motion, Einstein suggested that our laws of relative motion needed to be changed and he introduced two new postulates. The first postulate is that all the laws of physics are the same for every observer in acceleration-free motion. This was a big change from the Newtonian relativity picture, which said that the laws of kinematics (forces and motion) are the same for every observer, but not the laws of electromagnetism. The second postulate is that the speed of light is constant for every acceleration-free observer, regardless of the motion of the source or the observer.

This second postulate is the one that directly leads to some of the most bizarre consequences for physics. In order for observers to agree on the speed of light regardless of their relative motion, they must disagree on the length of moving objects (length contraction) and on the clock rate of moving objects (time dilation). Furthermore, the speed of light, c = 300,000,000 meters per second, ends up being an absolute speed limit that nothing can beat. Objects with mass always move slower than the speed of light, and light itself moves at c (though that is a bit of an oversimplified statement). Perhaps the strangest consequence of special relativity is the relativity of simultaneity: observers in relative motion will disagree about whether spatially separated events happen at the same time or not. Time and space cannot be considered separate independent quantities, and instead we must think about a unified spacetime.

These effects typically become significant only at speeds comparable to the speed of light, so in our day to day activities we don’t experience them. This is why relativistic effects are so perplexing at a first look: they conflict directly with our intuition.

Nevertheless, physicists are quite confident in special relativity, because it has been tested experimentally in many, many different ways since Einstein first introduced it in 1905. Some of these are so common that they hardly classify as “experiments” anymore! For example, high energy particle colliders like the Large Hadron Collider at CERN know how fast their particles are moving, and they’re always moving slower than c and in accordance with special relativity. Unstable muon particles that are created in the upper atmosphere can be detected at ground level because of time dilation: their average rate of decay is slower because of their high speed.

I’ve been looking into some of the actual laboratory experiments to test special relativity recently, and thought I would share a very elegant one that tests time dilation. Titled, “Measurement of the Red Shift in an Accelerated System Using the Mössbauer Effect in Fe⁵⁷“, it is work done in 1960 by researchers at the Atomic Energy Research Establishment in Harwell, England¹. It is a relatively simple and straightforward test, so let’s take a look!