By Vidya Rajan, Columnist, The Times
When his dear friend Michele Besso passed away, Einstein wrote a condolence letter: “Michele has left this strange world a little before me. This means nothing. People like us, who believe in physics, know that the distinction made between past, present and future is nothing more than a persistent, stubborn illusion.” Whether time is real, or whether it is “nothing more than a persistent, stubborn illusion” is what I will be interrogating in this article.
We are familiar with the way time seems to slip by, smoothly and continuously. However, to give ourselves a sense of control over this “flowing” of time, we impose artificial quantities on it – dates, hours, minutes, seconds – thereby giving us discrete units which we can measure, monitor, and regulate. But time is actually unfettered by these actions because it does not stand alone as an independent entity. According to Einstein’s special theory of relativity, time relates to space in a construct called space-time, which permeates everything in the universe. Light, planets, and we ourselves move through space-time at different rates. This leads to something called time dilation: the faster something moves through space, the slower it moves through time. Imagine an observer on a hill who sees two people covering a certain distance using two different modes of transport: Person 1 is sitting[1] in a regular car driving along a road, whereas Person 2 is sitting in a rocket-propelled car traveling the same distance along the same road. To the observer, Person 1 will appear to be moving slowly through space but more time will be consumed. Person 2 in the rocket will be moving fast through space, with less time used. Since Person 2 uses up less time, they will age a little slower than Person 1.[2]
Einstein’s general relativity uses this concept of space-time to also describe gravity. In a similar, but opposite manner, to how a rubber sheet distends when a bowling ball is placed on it, space-time warps in the presence of mass. But it stretches thinner farther away from the mass and contracts to greater thickness closer to it[3]. Because of this warping, time passes more slowly in proximity to a mass than away from it, and the bigger the mass, the slower time passes. Thus, if you are on top of a mountain, time passes more quickly than if you are at sea level. If you are on the surface of a black hole, time barely passes at all. Curiously, it seems biological processes also slow down proportionately. In the case of two identical twins delivered at the exact same moment, but separated instantly, the one who grows up on a mountain will be a little older, biologically, than the who spends his life down a mineshaft. Thus, time stretches or shrinks with velocity and gravity.
Time dilation due to relativity, intangible though it seems to our senses, has a real impact on technology. For example, when you plug a destination into your direction-finding app, the satellites that coordinate and map your location on the Earth are orbiting way up in the sky where time passes faster. To understand how time computes to distance, consider that the formula: distance = velocity x time. Since satellite-based clocks measure time faster (by 38 microseconds) than on the ground, the distance values on land would accrue errors of up to 6 miles per day. But physicists from the US Department of Defense, who first developed GPS in the 1980s, took relativity into account and made the corrections necessary to land you on “X marks the spot” that you had chosen, rather than over a cliff or into the river alongside. Nor is this potential positional disaster merely hypothetical (as far as history goes). In 1707, four British Royal Navy vessels ran aground, killing up to 2,000 men, because their imprecise navigation system had guided them 15 miles off course onto lethal rocks off the Isle of Scilly. No one wants that sort of thing to happen today; today, we have social-media outrage and ambulance-chasing lawyers.
There is still more weirdness afoot. According to the British astrophysicist Sir Arthur Eddington[4], the second law of thermodynamics is the reason that time appears to move forward “like an arrow”. This law (simplified) posits that every contained system in the universe, incorporating matter or energy – or politics (just kidding!) – tends towards increasing disorder or “entropy”. To reverse the direction of time, disorder would have to spontaneously revert to order, and that is not empirically observed[5]. Entropy can be seen as a manifestation of heat[6], and time mirrors the probabilistic movement of heat from hot objects to cold ones rather than the other way around. This observation is also stated as an “entanglement” between the two objects one uses to observe the passing of time, such as the cooling of a hot cup of coffee in a room. The hot coffee demonstrates quantum entanglement with the cooler room atoms, and equilibrates over time[7]. Discrete chunks of this state of entanglement relate to time moving forward as the coffee cools. Another related idea is that the forward movement of time is due to the emission of radioactive particles from atoms, which obviously can go in only one direction – particles do not spontaneously enter atoms and make them radioactive. The past is gone, the future is not yet here; we exist in the now.
Carlo Rovelli, a contemporary physicist and author who brings a poetic clarity to arcane theories, shatters the rather satisfying link of time to increasing entropy by showing that our perception of entropy is misleading. Using a pack of cards as an example, he describes an orderly pack, with the first 26 cards being “red” cards and the next 26 being “black” cards. If shuffled, they become disordered from the perspective of card color, but only card color. Other types of order may arise from the shuffling – the first 26 cards of the deck may be a jumble of black and red, sure, but they may manifest a different order. For example the first 26 may be more dog-eared, or may be picture cards, or have increasing numerical value. It all depends on the perspective. Thus entropy does not necessarily increase on shuffling, and therefore the concept of time as such does not even exist. Go figure. This brings us to the concept of the block universe where the past, present and future are all concurrent: they all exist simultaneously[8]. To mathematicians and physicists, this is reality. We just cannot perceive it with our senses.
Back to now. What sort of now? This also depends on the perspective, called the “frame of reference”. When two objects or people share the same “frame of reference”, then things happen simultaneously. You can observe a car arriving and picking up passengers and leaving. As long as you all share the same “space”, that event would occur at the “same time” for both observer and passengers. However, if you are way up in the atmosphere, the frames of reference differ, and the timing of the event would not be shared between an observer on the ground and one on the space station. In other words, the event would not happen simultaneously. This felt weird for me to contemplate. Think about this: If the same event – say the assassination of John F. Kennedy – were viewed from the Space Station and Dallas, they would not happen simultaneously for the observers; the now would be offset. The assassination would happen at different times for the different observers. Isn’t that just crazy? But this is the mind-bending consequence of relativity. Note: This is different from the effect of the distance light has to travel. Since light travels at 186,282 miles/second and the space station is 250 miles above in the atmosphere, it would take about 0.001342 sec[9] for the information to be transmitted. This is really very little “clock time”. The difference in the time-perception is due to the lesser warping of space-time around the space station and greater warping near the Earth.
To go back to the original question: Is time real, or an illusion? It depends. Specifically, it depends on your frame of reference. If you are in it, it feels real. If you are outside it, it is warped by the effects of speed, gravity, or the block universe. Since we are inexorably and inescapably trapped in our frame of reference, the construct of time for us feels real.
To end with another quote: The past is history. The future is a mystery. Today is a gift. That is why it is called the present.[10]
Bibliography and Notes:
[1]. Sitting is defined as a “stationary” action, but that is also an illusion. We are sitting on the surface of a planet spinning on its axis at about 1,000 mph, while flying around the sun at 66,000 mph (which would take us from Washington DC to San Francisco in 3 minutes), while the solar system orbits our Milky Way galaxy at 483,000 mph.
[2]. The fun part of this time dilation is the extrapolation that if humans could be accelerated to near light-speed velocities[2], it would have the same effect as slowing down time, so no one need be put into suspended animation for interstellar travel because they would barely age, at least for the duration that they were traveling at near light velocity!
[3]. This may seem counter-intuitive, because the imaginary rubber sheet will get thinner where it is distended by a weight. For space, imagine it to be anti-rubber, such that it stretches and expands more away from mass, and becomes thicker and denser closer to the mass. In the case of a black hole, the density is so massive that it crushes everything into subatomic soup and causes a bending so steep that even light cannot emerge from the depth of the hole caused by the warping. On the surface of our imaginary black hole, time will stop.
[4]. Eddington, along with Frank Watson Dyson. demonstrated the deflection of light calculated by Einstein due to the warping of space-time by observing the bending of light during a solar eclipse. There is a story of Eddington at a meeting. “Eddington recalled that as the meeting was dispersing, Ludwig Silberstein (the author of one of the early books on relativity), came up to him and said,’ Professor Eddington, you must be one of three persons in the world who understands general relativity’. On Eddington demurring to this statement, Silberstein responded, ‘Don’t be modest Eddington’. And Eddington’s reply was, ‘On the contrary, I am trying to think who the third person is!’“ Reference: https://www.physicsforums.com/threads/only-three-people-understand-general-relativity.935624/
[5]. The second law of thermodynamics defines a “closed system” where no energy is input. Energy can reverse disorder, but the universe is a closed system with no energy coming into it from an external source. Recently scientists did “unboil” an egg, but they only made the proteins uncoil rather than return the egg to viability to hatch. But then, more recently, scientists created a “time crystal” which reverts to its original condition without absorbing energy, effectively acting like a perpetual motion machine. It is now considered a new phase of matter. Is this time crystal reversing entropy and breaking the second law? Probably, at least at minute scales. https://www.quantamagazine.org/first-time-crystal-built-using-googles-quantum-computer-20210730/. Watch an explanatory video from Physics Girl here: https://youtu.be/ieDIpgso4no?si=kSzj3re_e2M5LWD0
[6]. Heat is related to entropy. When a cup of water is hot, it is in a state of higher disorder because the atoms are moving faster. But some of the heat is dissipated into the atmosphere so it becomes part of the background. Therefore, even as the water is cooling, it is increasing the entropy of the universe. To impose order on the water, it would have to be turned to ice, with energy extracted from the water, dissipating that heat into the universe, increasing its entropy further.
[7]. WIRED Staff (2014). New Quantum Theory Could Explain the Flow of Time. [online] WIRED. Available at: https://www.wired.com/2014/04/quantum-theory-flow-time/
[8]. You can only understand this as a concept by imagining yourself stepping out of the space-time dimension and looking back. You can look to the present of your dog occupying your armchair in your absence, to the past where the dog looked around for you and then eyed your armchair covetously, and to the future when you return to turf the dog out and retake possession of the armchair. Or maybe a different future where the dog refuses to yield. You will know when you step out of our constraining space-time.
[9]. Loo, W.B. (n.d.). Speed of Light Calculator. [online] Available at: https://www.omnicalculator.com/physics/speed-of-light
[10]. Attributed to a speaker at the 1993 Rutgers Preptheoni graduation. https://quoteinvestigator.com/2023/07/26/past-history/
Suggested further reading:
NPR. (2022). Researchers say time is an illusion. So why are we all obsessed with it? [online] Available at: https://www.npr.org/2022/12/16/1139780043/what-is-time-physics-atomic-clocks-society.
