As we approach the vernal equinox, the days start to stretch out and time seems to follow. It is a consequence of the Earth's orbit that the lengthening of each day reaches a maximum at both equinoxes and at a minimum at the solstices.
Certainly, our perception of time changes with the longer days and the return of long hours of daylight. But time itself does not change. Each day is composed of 86,400 seconds that pass by like clockwork.
Or, at least, that is the way that physicists once viewed time. To quote Sir Isaac Newton, the universe is based on "absolute, true, and mathematical time, which of itself, and its own nature, flows equitably without relation to anything external."
Time passes in a stately procession without concern for anything else. To Newton, it was the one solid fact in the universe.
Unfortunately, in the late 1800s, scientists began to question this idea and discovered that it was wrong. Actually, "wrong" is probably too harsh a word. Rather, it might be better to say that the Newtonian view of time is inadequate as an explanation for all physical phenomena.
In order for an absolute, true, and mathematical time to exist, everything in the universe would need to be set against an eternal backdrop. This backdrop was labeled as the "aether" by scientists and provided the frame of reference for Newtonian calculations. But when technology finally caught up to imagination and scientists were finally able to look for the aether, no one could find it.
Perhaps the most famous attempt to measure the presence of a firmament was the Michelson-Morley experiment which was intended to measure the difference in the speed of light travelling with or against the aether as opposed to perpendicular to it.
The idea was to set up two beams of light at right angles to one another and look for a difference in the time it takes for the beams to travel their paths. But no difference was found. And by extension, no aether could exist.
An explanation for this experiment was offered by two other scientists, Lorentz and FitzGerald. They postulated that the results could be explained if length and mass were affected by velocity. That is, there was a contraction in the distance that the light beam had to travel that exactly offset the delay that should have been there.
They worked out a fairly simple equation to explain this.
It was another decade, though, before the final death knell for absolute, true, and mathematical time was struck. Albert Einstein argued that if mass and length are variables, then time should also be variable. That time depends on velocity. This is the basis for the Special Theory of Relativity.
Is time a variable?
It took some time before anyone could truly answer that question. But one answer was provided by the development of atomic clocks in the post-war period.
In 1971, two scientists, J.C. Hafele and R. Keating, set out to test Einstein's equations and predictions with the help of four atomic clocks and commercial airplanes. They loaded up the clocks and flew around the world heading east.
When they got back to the U.S. Naval Observatory, the four clocks were on average 59 nanoseconds or 0.000000059 seconds slow relative to clocks that had remained fixed at the Observatory. They then headed west and flew around the world. The clocks were 273 nanoseconds fast when they got back.
The reason that the time differences are not the same is that the rotation of the Earth produces a time dilation effect, as predicted by Einstein. But when everything was accounted for, the time dilation due to the speed of the airplane was exactly as Einstein predicted. Time's variability had been experimentally confirmed.
Of course, there are other experiments that have demonstrated that time dilation occurs. For example, due to their high velocity, some cosmic ray particles have much longer life times than would be anticipated. This caused some confusion in sub-atomic physics until time dilation was taken into account.
Similarly, in high energy accelerators, such as the Large Hadron Collider, the relativistic velocity of the particles and the by-products of collisions can result in significant time dilation with particles that should only last for nanoseconds have lifetimes that stretch out into the millisecond range.
All of the experiments that have been done over the past century lead to the same conclusion. Time is a variable. Immutable time does not exist.
But that is something that most of us have always suspected. After all, our internal sense of time is quite variable. Time flies when you are having fun and it can drag when things are boring.
Or, as the physicist John Wheeler once said: "Time is nature's way to keep everything from happening at once!"
