What is time, and why does it only seem to move forward?

How do we know clocks really tick at different rates?

It has been measured many times. In October 1971, J. C. Hafele and Richard Keating flew four cesium-beam atomic clocks around the world on regular airline flights, once eastward and once westward. Compared with atomic time at the U.S. Naval Observatory, the flying clocks lost 59 billionths of a second going east and gained 273 billionths going west, in good agreement with relativity.

Speed slows a clock, and height speeds it up, because a clock deeper in Earth’s gravity ticks more slowly. GPS satellites feel both. In round numbers, speed alone would put their clocks about 7 millionths of a second a day behind ground clocks, and height about 45 ahead, for a net gain of about 38. Engineers correct for this, for example by setting the clocks to tick slightly slower before launch.

In 2010, NIST compared two clocks, each based on a single aluminum ion, and found that raising one by 33 centimeters (about a foot) made it tick slightly faster. Over a 79-year life, that adds up to about 90 billionths of a second, far too little to feel. In 2022, physicists at JILA, run by NIST and the University of Colorado Boulder, measured the difference across a single millimeter inside a cloud of atoms. This part of the answer is Established. See our answers on whether time travel is possible and why the speed of light is a limit.

Why does time run one way, and what is the strongest objection?

As far as we know, the basic laws of physics work almost the same run forward or backward. (A few particle interactions are exceptions, but they do not seem to cause everyday one-way behavior.) Yet heat never flows on its own from a cold iron bar into a hot one. In the Stanford Encyclopedia of Philosophy, Craig Callender calls this one-way pattern “one of the most salient and consequential features of the physical universe.”

Ludwig Boltzmann offered an explanation in the late 1800s. There are vastly more ways for the air molecules in a room to be spread evenly than bunched in one corner, so an uneven state almost always drifts toward an even one. Entropy is, roughly, a count of those possible arrangements, and the second law of thermodynamics says it tends to increase. That part is Established. But the same counting works backward too: on its own, it predicts that the past should also have had higher entropy, which is wrong. One widely discussed fix is a starting condition: the universe began with extremely low entropy. David Albert called this the Past Hypothesis, and Boltzmann, Einstein, Richard Feynman and Erwin Schrödinger all saw that something like it was needed. The encyclopedia calls it an elegant solution that still faces concerns: Supported, still debated.

The strongest objection is that it answers one mystery with another: a very special beginning. Roger Penrose estimated that, by the standard way of counting, such a start is astronomically unlikely. Some reply that it could be a law of nature, and some doubt that it needs explaining at all; others argue that it does. John Earman argued that it cannot even be stated in the language of general relativity, Einstein’s theory of gravity. So why the universe began with such low entropy is an Open question. For what may have come before, see whether time began at the Big Bang.

Many philosophers have tried to use entropy to explain other one-way patterns too, such as why we remember the past and not the future. The encyclopedia says such connections, if any, “are bound to be subtle.”

Does time really flow, or is it all already there?

Around 500 BC, Heraclitus wrote that everything flows; a generation or so later, Parmenides held that what truly exists simply is, whole and unchanging. The Stanford Encyclopedia says such questions have been debated for more than two millennia, “with no resolution in sight.”

The two main camps are the A-theory and the B-theory. A-theorists hold that our sense of time flowing matches something real, and that past, present and future differ in a deep way. B-theorists describe all change with “earlier” and “later” alone and treat time more like a dimension of space. A related view, eternalism, or the block universe, says past and future events are as real as present ones: “now” is like “here,” a mark of where you are. An analogy: a film reel holds every frame at once, yet the show seems to unfold. It breaks down because a reel still sits inside time, while the block includes all of time.

In relativity, whether two distant events happen at the same time depends on the observer’s frame of reference. If no frame is privileged, there is no single, universal “now.” The physicist Wim Rietdijk and the philosopher Hilary Putnam argued that this points to a block universe. Others disagree: the encyclopedia notes that believers in a special present can reject the no-privileged-frame assumption without any conflict with experiments. Einstein, Rudolf Carnap recalled, said the problem of the Now “worried him seriously,” since it seemed to lie outside physics.

In the 2020 PhilPapers Survey, which 1,785 philosophers completed, 38.2% of those who answered this question accepted or leaned toward the B-theory, 27.2% the A-theory, and 36.2% other answers. Some chose more than one, so the numbers add to more than 100%. We label this Beyond science, at least for now, because experiments do not decide it. If the future already exists, is it fixed? See whether free will is real.

Why does time seem to speed up as we get older?

Many people feel the years rush by as they age. Psychologists find the effect is smaller than the saying suggests. In a 2010 study of 1,865 adults aged 16 to 80, William Friedman and Steve Janssen found that people of all ages said time seems to pass quickly. Age differences were very small, except on one question: how fast the last 10 years had passed. Of seven theories tested, the findings supported one based on time pressure: how rushed people feel. A 2005 study of 499 people aged 14 to 94 found that the sense of speed does grow with age, but age explained at most 10% of the differences between people. A 2015 study that pinged young and older adults on their phones for five days found no age difference in how fast the present seemed to pass. Any age effect looks small, and in the biggest of these studies it appeared mainly for the past decade: Supported, still debated.

Does fear stretch time? In a 2007 experiment, volunteers fell backward 31 meters into a net at a Dallas amusement park. Afterward, they judged their own fall to have lasted 36% longer than a fall they had watched. But a flickering number on a wrist display, set just too fast to read, was no easier to read during the fall than on the ground. The authors suggest that the slow-motion feeling comes from memory, not perception: a frightening event may be recorded more richly. Only seven people gave time estimates, so the study is small. A 2023 lab study, though, found that emotionally arousing faces may sharpen people’s perception of very brief changes.

Lab results are mixed. A 2025 meta-analysis of studies with 437 stressed and 434 unstressed people found that acute stress tends to make time intervals seem longer. But in a 2020 set of experiments, the threat of an electric shock made intervals seem shorter, and in two further studies the same team could not replicate an earlier finding that fear makes time seem longer. And a 2010 review of 117 experiments found that busy people judge time as shorter if they know they will be asked, but longer if asked only afterward. Emotion and stress do change felt time, but the direction depends on the feeling and on how it is measured: Supported, still debated.

What do people often get wrong?

“Relativity means time is all in your head.” Relativity is about physical clocks, not feelings.

“Physics has shown that the flow of time is an illusion.” Some thinkers argue this, but it is still debated, and experiments do not settle it.

How can you check this yourself?

• Use your phone’s GPS. Every GPS fix relies on satellite clocks corrected for relativity, as Neil Ashby’s review explains.
• Watch entropy at home. Put a drop of food coloring in a glass of still water and watch it spread. It never gathers back into a drop on its own.
• Read both sides. The Stanford Encyclopedia entries “Time” and “Being and Becoming in Modern Physics” lay out the flow and block views. Our glossary explains the science words.

So physics measures how time behaves very well, its one-way direction seems to trace back to a special beginning nobody can yet explain, and what time ultimately is remains open.