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The passage of time often seems like the most malleable thing in the world. A minute may feel much longer when you’re asked to hold a plank position in a workout class, and an hour may fly by when you’re chatting with a friend. In these cases, your perception of time is altered. But real, physical time dilation also happens. According to Albert Einstein’s special theory of relativity, clocks that are closer to Earth’s centre of gravity, say at sea level, tick more slowly than those at the top of a mountain. A similar slowing happens when a clock is physically travelling extremely fast.
Undeniably, this sounds bizarre, but decades’ worth of experiments haven’t managed to disprove it. Yet in what exact sense time dilation is real – and what it means for a physical effect to be real at all – has inspired philosophical debates ever since Einstein first formulated his theory in 1905.
In fact, in June, at the Foundations of Physics conference in Irvine, California, I heard Marco Giovanelli discuss how time dilation is both real and apparent at once. It sounds paradoxical, but when I called Giovanelli, who is a historian of philosophy and science at the University of Turin in Italy, to talk through it, I started to understand that it reflects the facets of our physical world that Einstein’s work first uncovered.
For Einstein, it was all about rods and clocks, explains Giovanelli. Suppose that you and a friend each had an identical clock and a metre-long rod. You stand still, while your friend gets into an incredibly fast spaceship and sets off on a journey. Compared with yours, their clock will tick less quickly, and their rod will contract in size. If you want to know by how much, Einstein’s got your back: he worked out those calculations more than a century ago. If you happen to have a second friend who has a slightly slower spaceship and their own clock-and-rod set, the same trick will work again, except that their clock will slow and their rod contract a little less than your speedier friend. Finally, if you also acquire a spaceship and catch up with either of your friends, your clocks will start to tick at the same rate again, and your metre-long rods will again look the same length.
To discuss these effects, physicists use the language of “inertial frames”, similar to comparing a room at rest, or a rest frame, with a spaceship travelling at a constant speed, or a moving frame. In your individual frames, you and your friends can each establish a coordinate system, the familiar grid of width, depth and height. Your coordinate systems will match when you are all at rest or when your spaceships move side by side at the same speed.
When one of you moves more quickly, though, special relativity shows that their grid will deform, contracting in the direction of motion. It is this detail that bothered Einstein’s contemporaries. It seemed that the change in the rod length was uncomfortably intertwined with changes in the coordinate system used to situate it. But an effect that is a function of a coordinate system cannot possibly be real, critics argued. Ultimately, coordinate systems themselves are of dubious reality. They seem to simply be tools that we humans made up to help us organise our theories and observations of the physical world.
In 1911, physicist Vladimir Varićak argued that the rod’s length contraction “is only a psychological and not a physical fact, i.e., the body has not really undergone any change”. Giovanelli recounts another anecdote where, in the 1970s, physicist John Stewart Bell surveyed his colleagues at the CERN particle physics laboratory about whether length contraction could break a thread suspended between two spaceships accelerating at exactly the same rate. Most physicists said the thread wouldn’t break, endorsing the idea that nothing physically changes about the atoms in the thread. Could they have possibly been wrong? This made my head spin.
I will admit to having always been a rather terrible student of relativity, only learning it just well enough to pass my preliminary exams in graduate school. However, I’m certain I was previously taught that relativistic time dilation and length contraction are unquestionably real. We know they are, from experience in everyday life. The precise clocks on GPS satellites tick at different rates than those on Earth. This is partly because they move quickly and partly because of the gravitational effects explained by Einstein in his general theory of relativity. For GPS to be accurate, the difference in ticks must be corrected for. Failing to do so would have very real consequences, like ending up in someone’s backyard when I try to use my phone to visit a new café or bookstore. But here’s the crucial thing: the clocks on GPS satellites derive their precision from leveraging quantum control over atoms rather than some macroscopic clock gears, so it feels especially egregious to suggest that time dilation does, well, nothing to an atom.
Einstein himself was thinking about this, too, Giovanelli tells me. In fact, an atomic effect was at the centre of a 1906 debate he had with Johannes Stark, a physicist who was awarded the Nobel prize for discovering how electromagnetic fields can affect the energy states of atoms. Stark was studying fast-moving charged atoms, and Einstein recognised that these experiments may display the transverse Doppler effect – in the same way that a siren changes pitch as it approaches you due to a shift in the frequency of its sound waves, a similar effect can happen due to time dilation. It is a distinct prediction of special relativity, an effect that doesn’t exist in other theories, says Giovanelli.
For the standard Doppler effect, you know that you could cancel it out by driving after the police car and that what is happening isn’t due to something changing inside the car’s speaker. The same question reared its head for the transverse Doppler effect. “Stark really thought, kind of naively but naturally, that something is going on in the atom,” says Giovanelli – in our analogy, the speaker in the car would be physically changing. Einstein rejected this. He posited that all the atoms remain the same and that their inner workings are unaffected by motion. Time dilation was the only explanation. Seemingly apparent, here it was confronted with a real, empirical test.
Ironically, when the transverse Doppler effect was experimentally verified in 1938, one of the two physicists involved was Herbert Ives, who never accepted special relativity. Ives thought that he had found signatures of the “luminiferous aether”, a hypothetical substance that some researchers thought permeated the universe and provided one preferred, true reference frame. A preponderance of other experimental evidence, much of it obtained even prior to Ives’s work, completely ruled out this option, and special relativity became the only game in town. For all the clamouring about the oddity of an effect related to an observer’s perspective, one that many physicists wanted to dismiss, time dilation withstood the test of time. Over the ensuing decades, ever-increasingly precise atomic clocks hav
e never uncovered any evidence that something more “real” could be happening.As far as Giovanelli is concerned, what happened was not so much a dispute over physics, but rather a run-in between physics and philosophy. Underneath the sticky question of whether time dilation is real hides another, even stickier philosophical one: what does it mean to be real?
“At a certain point, you cannot avoid it,” says Giovanelli. “And so, Einstein was forced to address the question philosophically. OK, really, let’s define: ‘apparent’ means disappears for the co-moving observer, ‘real’ means empirically testable.” In some sense, the situation ought to never have become that complicated, he says. Scientific theories provide testable hypotheses – it is what they are designed for – and special relativity has passed every test we’ve put it to. What more could you ask from a theory?
Well, it’s hard to understand from an intuitive point of view, says Giovanelli. Einstein even had his own questions, primarily about how certain he could really be that two clocks, or two atoms being used as clocks, could be reliably identical. The advent of quantum physics offered some assurances, but Einstein had his qualms with quantum theory and spent time worrying about it way beyond how it applies to rods and clocks. But he never questioned the empirical reality of time dilation and length contraction, says Giovanelli.
As we talked, I wondered whether for me, like Einstein’s critics, it was my notable lack of relativistic intuition that made time dilation so hard to grasp. It may have been easier to accept that when atoms within a clock or a rod move at a certain speed, some as-yet-unknown mechanism physically changes them, leading to time dilation and length contraction. Instead, I have to grapple with there really being no such thing as absolute time.
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