Time flies - but not if you're a fly. Flies avoid being swatted in just the same way Keanu Reeves dodges flying bullets in the movie The Matrix – by watching time pass slowly.
Like Reeves standing back and side-stepping slo-mo bullets, the fly has ample time to escape. And it is not alone in its ability to perceive time differently from us. Across a wide range of species, time perception is directly related to size. Generally the smaller an animal is, and the faster its metabolic rate, the slower time passes.
If we look at ourselves, we notice that a person’s subjective experience of the passage of time, or the perceived duration of events, can differ significantly between different individuals and/or in different circumstances. Although physical time appears to be more or less objective, psychological time is subjective and potentially malleable, exemplified by common phrases like “time flies when you are having fun” and “a watched pot never boils”.
It's clear that the perception of time and duration is crucially bound up with memory, as the time that seemed to fly because of the fun we had, gives us so much more memories than the time spent on watching a pot. It may therefore be good to realize that, paradoxically, a long life has more to do with the amount of experiences we have than with a lifetime measured in a certain number of years.
If that isn't enough to make time a confusing aspect of life, Einstein made things even worse when he showed that objective time, as measured by clocks and calendars, is an illusion as well. The human perception of time as well as the measurement by instruments such as clocks are different for observers undergoing a difference in gravity or speed.
Isaac Newton discovered that speed and distance traveled are dependent on the frame of reference of the observer. For example, if you are on a train and you roll a ball in the same direction the train is moving at a speed of 5 meter per second, you will observe the ball moving 5 meter in one second. Suppose however that the train is moving down the tracks at 10 meter per second. A person standing next to the tracks will observe the ball moving at 15 meter per second and traveling 15 meter in the same one second. So who's right? They both are, but from their own frame of reference.
When more than 100 years ago scientists measured the speed of light they expected the same thing to happen but instead, they found that it's a constant. Albert Einstein then concluded that since the speed of light is the same in all frames of reference, but the distance the light traveled differed between observers moving at different speeds, and since distance is speed multiplied by time, the only remaining possibility is that different observers must see time differently. Einstein theorized, and it was later proven, that good clocks will not always agree in what time it is because they move through time at different speeds.
If you got in a rocket ship and accelerated to almost the speed of light you could go what seems infinitely fast to you. You could travel to a star 100 light years away and get there by lunch, turn around, and get back to Earth the same day. But you will find that everyone else is 200 years (and one day) older than you are. From their perspective, you were traveling very close to the speed of light and it takes 200 years for light to get to that star and back. But to you, it was only a day. Your aging slowed down because you move forward through time faster.
Read more: Why space and time might be an illusion
In 1926, Einstein wrote in the Encyclopædia Britannica: "Until the theory of relativity was propounded it was assumed that the conception of simultaneity had an absolute objective meaning also for events separated in space. This assumption was demolished by the discovery of the law of propagation of light. For if the velocity of light in empty space is to be a quantity that is independent of the choice (or, respectively, of the state of motion) of the inertial system to which it is referred, no absolute meaning can be assigned to the conception of the simultaneity of events that occur at points separated by a distance in space. Rather, a special time must be allocated to every inertial system. If no co-ordinate system (inertial system) is used as a basis of reference there is no sense in asserting that events at different points in space occur simultaneously. It is in consequence of this that space and time are welded together into a uniform four-dimensional continuum".
This four-dimensional continuum is known as spacetime, a mathematical model that joins space and time into a single idea, with time as the "fourth dimension". The nature of spacetime is such that time measured along different trajectories is affected by differences in either gravity or velocity – each of which affects time in different ways. Clocks on the Space Shuttle run slightly slower than reference clocks on Earth, while clocks on GPS satellites run slightly faster.
But could time have more than just one dimension? If time is one-dimensional, like a straight line, the route linking the past, present and future is clearly defined. Adding another dimension transforms time into a two-dimensional plane. On such a plane, the path between the past and future would loop back on itself, allowing you to travel back and forwards in time. That would permit all kinds of absurd situations, such as the scenario in which you could go back and kill your grandfather, thereby preventing your own birth.
Although no well-known physicist or cosmologist has endorsed his ideas, John G. Bennett, an English mathematician and follower of mystic George Gurdjieff, posited a six-dimensional Universe with the usual three spatial dimensions and three time-like dimensions that he called time, eternity and hyparxis. Time is the sequential chronological time that we are familiar with. The hypertime dimensions called eternity and hyparxis were said to have distinctive properties of their own.
Eternity could be considered cosmological time or timeless time. Hyparxis is supposed to be characterised as an ableness-to-be and may be more noticeable in the realm of quantum processes. According to Bennett, the conjunction of the two dimensions of time and eternity could form a hypothetical basis for a multiverse cosmology with parallel universes existing across a plane of vast possibilities, while the third time-like dimension hyparxis could allow the theoretical existence of sci-fi possibilities such as time travel, sliding between parallel worlds and faster-than-light travel.
Read more: The emergence of gravity and dark matter
In 2007, theoretical physicist Itzhak Bars put forward the heretical idea that there are two dimensions of time, and even proposed a way to test his theory. Bars first found hints of an extra time dimension in M-theory and, when he looked into it, discovered the grandfather paradox and other fears could be overcome by using a new kind of symmetry in the quantities of position and momentum. Bars postulated that position and momentum are not distinguishable at a given instant of time. Technically, they can be related by a mathematical symmetry, meaning that swapping position for momentum leaves the underlying physics unchanged, just as a mirror switching left and right doesn’t change the appearance of a symmetrical object.
In ordinary physics, position and momentum differ because the equation for momentum involves velocity. Since velocity is distance divided by time, it requires the notion of a time dimension. If swapping the equations for position and momentum really doesn’t change anything, then position needs a time dimension too, requiring an extra time dimension.
It is this symmetry that might help reconcile the two mighty pillars of 20th-century physics, quantum mechanics and relativity. But simply adding an extra dimension of time doesn't solve everything: To produce equations that work with the new symmetry that describe the world accurately, a fourth dimension of space is needed as well, giving a total of six dimensions.
To find evidence, particles are smashed together in CERN's Large Hadron Collider at higher energies than ever before to create "supersymmetric" particles. However, the machine’s collisions have so far conjured up no new particles that could comprise dark matter, no siblings or cousins of the Higgs boson, no sign of extra dimensions, no leptoquarks — and above all, none of the desperately sought supersymmetry particles.