What is time? | Live Science

Time is the apparent progression of events from the past to the future. Although it is impossible to fully define the nature of time, we all share many common experiences related to time: causes naturally lead to effects, we remember the past but not the future, and the evolution of time seems to be continuous and irreversible.

Is time relative?

Einstein’s special theory relativity revealed that the experience of the passage of time is relative to the observer and his situation. Previously, the work of Isaac Newton had assumed the existence of a “master clock” that kept time synchronized throughout the universe. This clock was not really thought to exist, but the concept allowed Newton’s equations to work. The key idea was that all observers could agree on the exact same moment, according to the Internet Encyclopedia of Philosophy (opens in a new tab).

However, building on the work that preceded it, Einstein discovered that the passage of time is relative. In special relativity, moving clocks run slowly; the faster you move through space, the slower you progress through time. The closer you get to the speed of light, the greater this effect becomes.

Einstein's special theory of relativity illustrated with trains.  Mark Garlick/Science Photo Library via Getty Images

Einstein showed in his special theory of relativity that two observers cannot agree on simultaneous events. This can be understood by this diagram. On the left, a train carriage is depicted with a person, Alice, inside. Alice turns on a light in the middle of the cart and observes light beams arriving at both ends of the cart at the same time, T2. On the right, we see the scenario from Bob’s perspective on the platform as the train passes at speed v. He sees both light beams emitted at the same time, just like Alice. However, as the train is moving to the right, the rear of the train intercepts the fire to the left first, at time T1 T1. So, from Bob’s point of view, the events that Alice saw to be simultaneous occur one after the other. (Image credit: Mark Garlick/Science Photo Library via Getty Images)

In the decades since Einstein first proposed this concept, physicists have made multiple measurements that demonstrate this effect. An atomic clock on board a jet plane will run at a slower rate than a clock on the ground. A subatomic particle called a muon does not exist long enough to travel from the atmosphere, where it is generated when cosmic rays hitting the air molecules, to the ground. But because muons travel at nearly the speed of light, they appear to exist longer from our perspective, allowing them to complete their journey.

When Einstein developed his theory of general relativity, he extended this concept, known as “time dilation“, to situations involving gravity. The presence of strong gravity also slows the passage of time, so a clock in a strong gravitational well (for example, on the Earth’s surface or near a black hole) will run at a slower rate than a clock in the middle of space, according to physicist Christopher S. Baird (opens in a new tab).

Related: 9 Ways to See Einstein’s Theory of Relativity in Real Life

Is time travel possible?

Time travel into the future is not only allowed, it is required. Indeed, with each passing second, we are all moving towards our own future. The future is inevitable and there is no escaping it. But the reality of relativity clearly shows that “jumping” in time is perfectly acceptable.

If a twin goes off in a rocket and spends a few years traveling near the speed of light, when it returns to Earth it will have aged less than its Earthbound twin. Although only a few years passed on the spacecraft, decades or even centuries could have passed on Earth, depending on how quickly the rocket traveled, according to Cosmos magazine (opens in a new tab). In a real-life example, NASA astronaut Scott Kelly lived a few milliseconds faster than his twin Mark (Scott also has six minutes faster), thanks to a longer time in space, traveling at speeds of approximately 17,500 mph (28,100 km/h), according to Live Science sister site Space.com (opens in a new tab).

photograph of an older woman's hand next to a younger woman's hand.  Peter Finch via Getty Images

Time passes differently in space: the faster you move through space, the slower you progress through time. (Image credit: Peter Finch via Getty Images)

But time travel to the past seems to be forbidden – at least in all experiments and observations ever made. On the one hand, the possibility raises all sorts of uncomfortable problems, such as the famous grandfather paradox who asks what would happen if you went back in time and killed your own grandfather: you wouldn’t exist, so you couldn’t travel back in time to commit the deed.

Second, there is no known mechanism in physics that allows us to go back in time. While some time travel situations can be constructed in general relativity, these situations require entities that don’t seem to exist in our universe (like matter with negative mass or infinitely long cylinders).

However, physicists currently do not have a full understanding of why time travel to the past is prohibited.

Can time be reversed?

Almost all of the laws and equations that physicists use to understand the natural world are time-symmetric. This means that they can be reversed without changing the results. For example, if you were to watch a video of a ball rising through the air and falling, without any other context, you wouldn’t be able to tell if the video was playing forward or backward.

However, there is one aspect of physics that seems to respect a flow of time: the concept of entropy, which is a measure of disorder in a system. According to second law of thermodynamicsentropy always increases in a closed system, and this evolution cannot be reversed.

Physicists aren’t sure if growing entropy gives rise to the “arrow” of time or if it’s just a coincidence, according to the Stanford Encyclopedia of Philosophy (opens in a new tab).

Expanding Universe.  A conceptual illustration of the

A conceptual illustration of the “arrow of time”. (Image credit: NASA/GSFC)

Is time discrete or continuous?

Almost all physical theories treat time as a continuum, this is also how we perceive the flow of time. There is not the smallest “unit” of the passage of time. All events flow without interruptions or hitches in the next one.

However, a theory of quantum gravity, called loop quantum gravity, hypothesizes the existence of a smallest possible unit of space-time. This unit would represent the smallest possible extension of space and duration. In this theory, what we perceive as regular, continuous time is actually a jerky, stop-motion progression from past to future. But because it happens for such an incredibly short duration, it appears to be continuous, like frames from a movie blending together, according to a 1998 paper by physicist Carlo Rovelli in the journal Living Journals of Relativity (opens in a new tab).

Is the time real?

Scientists, philosophers and others have studied the nature of time. And while we’ve learned a lot about time, like the reality of time dilation and the possible connection between time and entropy, we haven’t been able to come up with a complete description of what time dilation is. time.

Some philosophers and physicists have argued that what we experience as time is just an illusion, an artifact of our consciousness. From this point of view, the passage of time is not real; the past and the future already exist in their full extent, just as the entirety of space already exists. What we perceive as the flow of time is a by-product of how our brain works when we process sensory information from our environment, according to physicist Sean Carroll (opens in a new tab).

Additional Resources


Baird, CS (2013, June 24). Does time pass faster at the top of a building than at the bottom? Scientific questions with surprising answers. https://www.wtamu.edu/~cbaird/sq/2013/06/24/does-time-go-faster-at-the-top-of-a-building-compared-to-the-bottom/ (opens in a new tab)

Callender, C. (2021, June 8). Thermodynamic asymmetry over time. Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/time-thermo/ (opens in a new tab)

Carroll, S. (2013, October 18). Is the time real? https://www.preposterousuniverse.com/blog/2013/10/18/is-time-real/ (opens in a new tab)

Hunter, J. (nd). Time travel. Internet Encyclopedia of Philosophy. Retrieved April 5, 2022, from https://iep.utm.edu/timetrav/ (opens in a new tab)

O’Connell, C. (2021, August 3). Time travel: five ways to do it. Cosmos. https://cosmosmagazine.com/science/physics/five-ways-to-travel-through-time/ (opens in a new tab)

OnlineRovelli, C. (1998). Looping quantum gravity. Living Journals in Relativity, 1(1). https://link.springer.com/article/10.12942/lrr-2008-5 (opens in a new tab)