The Careful Art of Watching Time Slow Down

The classic explanation of Einstein's theory of special relativity involves simply a clock moving really fast. Because the speed of light always remains constant, the closer we come to reaching that constant speed, the more time has to slow down (or "dilate") to compensate. If it didn't, then from the perspective of an observer that wasn't moving as fast as us, light might be going faster than its universal speed limit. Instead, time itself slows because nothing in existence fucks with the speed of light. So: that fast moving clock would record more time than a clock that wasn't moving fast. Special relativity.

Testing this notion is a popular activity for scientists still, despite piles of experimental evidence indicating that, yes, time slows down. These tests often occur in labs and rely on interferometry, in which beams of light get split up and then recombined, with the resulting interference patterns giving up some information on light behavior under varying conditions. For testing special relativity (and time dilation), a beam is split such that it travels along pathways of different lengths. This results in a specific interference pattern that, if time dilation did not occur and special relativity was bunk, should shift as the velocity of the experimental device shifts. Experiments have found no shift.

Nonetheless, science being science, researchers are always testing and probing, trying to prove themselves wrong. One recent experiment conducted at Johannes Gutenberg-University used particles sped up (in a particle accelerator) to nearly 40 percent of the speed of light, which is far, far faster than anything that could be imagined in a laboratory interferometry experiment. While strapping a clock onto a speeding lithium ion isn't exactly feasible, it's possible to test out time dilation by measuring the frequency shifts found in front of and behind the particles, e.g. the Doppler effects.

These shifts should be symmetrical in a world of constant light speed and warping time; meaning, if you were to multiply the shift in front by the shift behind the particle, you should find that the product is equal to the absorption frequency of the particle at rest.

"In theory, this simply means firing ions down a tube as fast as you can," physicist Chris Lee writes at Ars Technica. "You then shine a laser into the oncoming ions, and you shine a second laser on the ions from behind. After measuring two absorption spectra, you are done and can go home." Basically, light shouldn't care about the velocity of the particle, or really any measurement device. It's only time that can change.

Imagine riding along on the top of a fast-moving train, armed with a flashlight, a few mirrors, and observational superpowers. With the mirrors you split the flashlight beam into two, one facing the direction of travel and one facing behind. Each one will experience the Doppler effect, a change in frequency in proportion to the train's speed. If the speed of light is to remain constant, those shifts should only be perfect opposites of each other. So far, they are.

"The experiment effectively measures the shift in the laser frequencies relative to what these transition frequencies are for ions at rest," Michael Schirber explains in a synopsis of the Gutenberg team's research, which is published in the current edition of Physical Review Letters. "The combination of two frequency shifts eliminates uncertain parameters and allows the team to validate the time dilation prediction to a few parts per billion, improving on previous limits."

String theory predicts a small variance in time dilation, as do some other theories of quantum gravity, the murky principle that hopes to bridge the very large-scale force of gravity with the quantum scale forces of electromagnetism, the strong nuclear force, and the weak force. These tests only get more precise with increased speeds, which leaves physicists with still a lot of ground to cover. So Einstein is still right—for now.