Time is the apparent progression of events from past to future. Although it is impossible to completely define the nature of time, we all share many common experiences bound by 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 flow of time is relative to the observer and his situation. Previously, Isaac Newton’s work had assumed the existence of a “master clock” that kept synchronized time throughout the universe. This watch 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 exactly the same time, according to the Internet Encyclopedia of Philosophy (opens in new tab).
But by building on the work ahead of him, Einstein discovered that the passage of time is relative. In the particular theory of relativity, moving clocks run slowly; the faster you move in space, the slower you go through time. The closer you get to the speed of light, the greater this effect.
In the decades since Einstein first proposed this concept, physicists have made several measurements showing this effect. An atomic clock on board a jet will tick slower than one on Earth. A subatomic particle called a muon does not exist long enough to travel from the atmosphere where it is formed when cosmic rays beat air molecules to the ground. But because muons travel close to the speed of light, they appear to exist farther from our perspective, allowing them to complete their journey.
As Einstein developed his general theory of relativity, he extended this concept, known as “time extension“for 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 tick slower than a clock in the middle of space, according to physicist Christopher S. Baird (opens in new tab).
Related: 9 ways you can see Einstein’s theory of relativity in real life
Is time travel possible?
Time travel into the future is not only allowed – it is mandatory. With each passing second, we all move into our own future. The future is inevitable and it is impossible to escape. But the reality of relativity makes it clear that it is perfectly acceptable to “jump” forward in time.
If a twin sets off in a rocket ship and spends a few years traveling close to the speed of light when they return to Earth, they will have become less older than their terrestrial twin. Although only a few years have passed on the spacecraft, decades or even centuries could have passed on Earth, depending on how fast the rocket traveled, according to Cosmos magazine (opens in new tab). In a real-life example, NASA astronaut Scott Kelly has experienced a few milliseconds less time than his twin Mark (Scott is also six minutes younger), thanks to spending more time in space and traveling at speeds of around 17,500 mph (28,100). km) / h), according to Live Science sister site Space.com (opens in new tab).
But time travel into the past seems to be forbidden – at least in all the experiments and observations that have ever been performed. First, the opportunity raises all sorts of unpleasant problems, like the famous ones grandfather’s paradox who asks what would happen if you went back in time and killed your own grandfather: You would not exist, so you would not be able to travel back in time to commit the act.
Second, there is no known mechanism in physics that allows one to travel back in time. While certain time-traveling situations can be constructed in general relativity theory, these situations require entities that do not appear to exist in our universe (as matter with negative mass or infinitely long cylinders).
But physicists currently do not have a full understanding of why time travel into the past is forbidden.
Can time be reversed?
Almost all laws and equations that physicists use to understand the natural world are symmetrical in time. This means that they can be reversed without changing any results. For example, if you were to watch a video of a ball rising in the air and falling again, without any other context, you would not be able to see if the video was playing forward or backward.
However, there is one aspect of physics that seems to respect a stream of time: the concept of entropy, which is a measure of disorder in a system. According to the second law of thermodynamicsentropy always rises in a closed system, and this development can not be reversed.
Physicists do not know whether the growth of entropy gives rise to the “arrow” of time, or whether it is just a coincidence, according to Stanford Encyclopedia of Philosophy (opens in new tab).
Is time discrete or continuous?
Almost all physical theories treat time as a continuum, which is also how we perceive the flow of time. There is not the slightest “unity” of the passage of time. All events flow smoothly without interruption or hiccups into the next.
However, a theory of quantum gravity, called loop quantum gravity, assumes that the smallest possible unit of space time. This unit would represent the smallest possible extension of space and the duration of time. In this theory, what we perceive as smooth, continuous time is really a stuttering, stop-motion progression from past to future. But because this happens for such an incredibly short duration, it appears to be continuous, as does the setting of a film that mixes together, according to a 1998 article by physicist Carlo Rovelli in the journal Live reviews of relativity (opens in new tab).
Is the time real?
Scientists, philosophers and others have considered the nature of time. And although we have learned a lot about time, such as the reality of time expansion and the possible connection between time and entropy, we have not been able to come up with a complete description of what time is.
Some philosophers and physicists have argued that what we experience as time is merely an illusion, an artifact of our consciousness. In this view the passage of time is not real; the past and the future already exist to their full extent, in the same way that the whole space already exists. What we perceive as the flow of time is a by-product of the way our brains function when we process sensory information from our environment, according to physicist Sean Carroll (opens in new tab).
Baird, CS (2013, June 24). Does time go faster at the top of a building compared to the bottom? Science 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 new tab)
Callender, C. (2021, June 8). Thermodynamic asymmetry in time. Stanford Encyclopedia of Philosophy. https://plato.stanford.edu/entries/time-thermo/ (opens in new tab)
Carroll, S. (2013, October 18). Is the time real? https://www.preposterousuniverse.com/blog/2013/10/18/is-time-real/ (opens in new tab)
Hunter, J. (nd). Time travel. Internet Encyclopedia of Philosophy. Retrieved April 5, 2022 from https://iep.utm.edu/timetrav/ (opens in new tab)
O’Connell, C. (2021, August 3). Time Travel: Five Ways We Could Do It. Cosmos. https://cosmosmagazine.com/science/physics/five-ways-to-travel-through-time/ (opens in new tab)
Rovelli, C. (1998). Loop quantum gravity. Live reviews in relativity theory, 1(1). https://link.springer.com/article/10.12942/lrr-2008-5 (opens in new tab)