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The concept of time feels so natural that we often forget just how peculiar it is.
What actually is time? Figuring it out remains one of the trickiest problems in physics.
Time governs our everyday lives. We keep track of it, waste it and wish we had more of it. It seems to go fast when we're having fun, and slow when we're not. Time anchors our references. Past, present, and future: these are the categories we use for events in our lives - births, marriages, deaths, parties, meetings, work deadlines, everything. Time seems to have a flow from the past to the present which we intuitively understand, yet don't think much about, though we recognise that we can remember things that happened in the past but cannot foresee future events.
Consider time in the context of physical objects. Smashed plates and cracked eggs are never seen to return to their original unbroken form. A spilt drink never flows back into a glass. These irreversible events are characterised by a concept called entropy, the amount of disorder in a system, which the second law of thermodynamics states can only ever increase. This Thermodynamic Arrow of Time is taken to have a strict irreversible direction from the past into the future: cause followed by effect.
For astrophysicists, time is important in understanding the makeup of the universe. Looking out into the night sky the light we see from distant stars has travelled for millions of years across the cosmos, giving us a view of how it was in the past. Light though never comes to us from future stars. This observation is described by the Electromagnetic Arrow of Time. Yet our theories of light and electromagnetism are unaffected by this arrow, they are time-symmetric, and strict conditions are imposed that ensure they concur with this one-way flow of time that we observe in nature.
There is the view that time is nothing but an illusion. This idea is not new or radical; it has been influential in physics for hundreds of years. Greek philosopher Zeno devised a set of philosophical problems, known as Zeno's Paradoxes, designed to support the view of his mentor Parmenides that "Time is not a reality, but a concept or a measure". Buddhism teaches this view that time is just an illusion, nothing but a manifestation of the mind.
Einstein fundamentally altered the way we think about time. His theory was that time is not only real, but that it is a dimension just like the three spatial ones, and they are all intertwined into a four-dimensional 'spacetime' that forms the fabric of the universe.
Moving through the time dimension is what gives us the feeling of time passing, similar to how we sense motion as we traverse through space.
Our intuition about time is that we live in the present. The past has happened and no longer exists. The future is yet to happen. We often dwell on the past, on times gone by, and like to think our futures are not already set. But if time is simply a dimension like those of space, as Einstein theorised, then we can come up with a counter-intuitive idea: that time and all events have always existed and always will. The past, the present, and the future all exist simultaneously and forever. In this world, rather than the flow of time, mathematics and logic rule.
Think of this as like your journey from home to your school or to your work. As you travel away from your home, it still exists in space, and similarly past events still exist in time. Your destination already exists and you are simply travelling through space to it. Similarly, future events already exist and you're travelling through time to when they occur. Travelling through time is no different to travelling through space.
This means the notion of ‘now' is one of the biggest misconceptions of time. There is no instantaneous moment but rather a seemless blurring of the past into the future. What we perceive as happening in the present is actually composed of things that have happened in the past. This idea comes from the fact that light does not travel instantaneously but takes time to reach us.
Not making much sense? Consider this. The Sun is about 93 million miles away from Earth (that's about 150 thousand million metres) meaning that even though light travels at just under 300 million metres per second through space it still takes light from the Sun just over 8 minutes to reach us. That means we actually see the Sun how it was 8 minutes ago. Looking into the night sky, we see the stars how they were millions or billions of years ago, as they are so far away the light has only just reached us. Even reading these words from your computer screen there is a delay of a few nanoseconds - so small you never notice it, but the fact is this intake of information is not instantaneous.
This is still a relatively large division of time - the smallest unit of time in current theoretical physics is the Planck time of 10-43 seconds. It's almost impossible to imagine how small the Planck length is. To put it in some kind of context, imagine each unit of Planck time is represented by the size of a grain of sand. If we also blew up real grain of sand in the same proportion it would be 10 thousand trillion times the size of the Milky Way.
The idea that the future and the past are as real and everlasting as the present is poignant but doesn't seem quite right. That's because it's a classical viewpoint and doesn't take into account quantum mechanics (QM). In the QM world nothing is certain, least of all the future. Einstein's ideas teamed with quantum mechanics should be able to give us a better grasp on the true nature of time.
There is an alternative theory that time is not the smooth dimension proposed by Einstein, but rather that it is granular. Intervals of time are like grains of sand, with the passing of events able to be thought of as like sand flowing through an hour glass. It's a novel way to think about time, with the analogy providing a fitting visual representation of the flow of time that we experience. This granular theory takes into account the uncertainty of quantum mechanics which we know defines how the universe behaves at its smallest scale. If spacetime is grainy then it could grow grain by grain, event by event. There is a reassuring uncertainty in our futures. This is not possible in Einstein's vision, where spacetime is a continuum with all events already and always in existence.
One of the well known laws of physics is that you can't travel at, or faster than, the speed of light. More accurately, you can't travel through space at the speed of light. This restriction doesn't apply to that of time. Because space and time are together as 4D Spacetime, the faster you travel through space, the slower you travel through time. This means that if you're stationary in space, you're travelling through time at the speed of light.
The nature of time is even stranger. It's not just about how fast you're moving, but also what you're close to that influences the time on your clock.
In the 1960s Irwin Shapiro conducted experiments that showed the Sun's warping of spacetime causes a signal sent between Earth and Mercury to travel at a different time to that expected without this effect. This validation of Einstein's theory is called the Shapiro Time Delay. On earth the effect is miniscule, but out in the universe the vast mass of the stars and galaxies warp spacetime so much that time ticks all over the place.
There are even things in the universe that warp spacetime so much they appear to not just slow down time, but stop it altogether. Black holes are the ultimate perpetrator of twisting time. Observing someone's watch as they moved towards a black hole it would be seen to tick slower and slower, until when they reach its event horizon, time would appear to stop completely.
The notion of time being just an illusion may well seem strange, and that's because it is. How can it be that something that seems so real and is fundamental to our world potentially be unconnected to it at all. The alternative notion is that time, and its one-directional flow from the past to the future, is real. This oddly presents us with more problems in our understanding of the universe.
If time is real and influential to how our universe works, rather than just being an illusion, then it will dramatically affect our current theories thought to represent nature at a fundamental level. General Relativity, Quantum Mechanics, the Standard Model of particle physics, would all need revaluating. If the arrow of time is found to be real, then these would all be time-symmetric theories of a deeper time-asymmetric reality. It's an intriguing and slightly unnerving question that takes us right to the crux of reality.
Understanding the true nature of time continues to perplex physicists. Perhaps time is just an illusion we will never be able to fully comprehend. It could well be that ultimately the strangest thing about time is not that it is an illusion, but that it is real. Theories need experimental validation, something that may not just elude us now but possibly be beyond our reach.
Time is a fundamental concept, dictating not only our everyday lives but the entire universe. It's remarkable that we still know so little about it. Will we ever understand the true nature of time? Only time will tell.
Image credits:
Newton's cradle: DemonDeLuxe (Dominique Toussaint)
Travel motion blur: matrianklw
Balloon burst: Amyn Kassam
Hour glass: openDemocracy
Twisted time: Fdecomite
As far as the quote on how much bigger a grain of sand would be compared to the Milky Way if scaled up by the same proportion that a unit of Planck time is compared to a grain of sand, this is the quick calculation I did. It does rely on not worrying about the units too strictly. A unit of Planck time is 10^-43s. A grain of sand is taken to be 1mm (10^-3m). That's a difference in magnitude of 10^40. This makes a grain of sand 10^37m which taking the size of the Milky Way to be 100,000 light years across (10^21m) makes it 10^16 times bigger than MW. This is equivalent to 10 thousand million million, or 10 thousand trillion.
"To put it in some kind of context, imagine each unit of Planck time is represented by the size of a grain of sand. If we also blew up real grain of sand in the same proportion it would be 10 thousand trillion times the size of the Milky Way."
This doesn't parse and I can't figure out what's meant. If we blew _what_ up to the same proportion ? The size of a grain of sand, expressed in seconds of light travel time (about 1/3 picosecond)? For a popular article you cannot assume that people will get this shifting between length and time, at least, not without explanation, and, anyway, I get that the grain of sand would become 70 million times the size of the Milky Way, so I don't think that's right either.