Why Didn`t A Time Traveller Warn Us About 2020?

Time is a commodity I ve had somewhat of an excess of recently, thanks to the pandemic lockdown wiping out my tutoring business. Given that time was the thing I was always the most short of while I was a teacher, it has made a bit of a refreshing change to be able to slow down and really put a surfeit of thought into questions I otherwise would have been too busy to ponder in the rush to complete teaching the respective GCSE and A-level Physics curriculums. The abandonment of the exams this summer gave a couple of my Year 13 students and I the almost unique opportunity to push beyond A-level Physics into the really interesting stuff you ll find in an undergraduate degree course and inevitably in all the popular science books on Physics.

Having already covered the basics of quantum mechanics, we changed tack to look at that other colossal pillar of Physics, Einstein s Relativity. Relativity comes in two parts: Special Relativity, which deals with the very odd effects you encounter when comparing the observation of an event from the point of view of two different frames of reference, and General Relativity, which describes the fundamental force of gravity and how it affects matter, energy and the fabric of spacetime. The concept of spacetime is arguably one of the most difficult to grasp in the entirety of Physics, as it requires you to throw away a lot the basic preconceptions about the nature of the universe that are given to you by your science teachers at school (yes, everything you learned in science at school is just a pale, shadowy approximation of the real view, essentially because we don t want to traumatise you too early in your life with the terrible truth #sorrynotsorry).

The idea of space is relatively easy to wrap your head around. Imagine a flat, blank piece of paper. Using an infinitely small pen or pencil, mark a dot on the paper. This is a zero dimensional (0D) singularity I ll discuss this concept more thoroughly when I write an essay on black holes. Keep ahold of your pen and mark another singularity elsewhere on the paper. You can connect these singularities with a straight line, and suddenly we have our first dimension of space (1D), length. Now make a third mark on the sheet of paper anywhere that doesn t fall along this line. Connect this new dot to each end of the line and, hey presto! You ve invented a plane with two spatial dimensions (2D, length and width), commonly referred to by geometry enthusiasts as an area. Now grab a trio of cocktail sticks and prop them up together into a tripod, with one end of each stick settling over each of the dots you made on the paper. Consider the shape you ve created and you ll see that you ve utilised a third spatial dimension (height) to construct the simplest possible polygon (3D shape): a tetrahedron a volume consisting of four triangular 2D planes, joined together at four vertexes, using six edges.

Adding vertexes, planes and edges allows you to describe increasingly complex polygons, but they always remain steadfastly within the 3D realm. The fourth dimension Einstein required to describe his theory of Relativity was time, and it s here things start to get messy, because throughout your career as a scholastic scientist, you ve always been taught to treat time as an entirely separate quantity from space. Einstein s genius was to recognise that time and space are intrinsically linked, and indeed should not be treated as separate things when making observations of events in the universe. What makes this particularly difficult to accept is that the very nature of time itself is elusive, especially when you begin to consider the implications of Relativity and Quantum Mechanics when trying to describe what time is.

In fact, many of the equations used to describe processes in quantum mechanics don t really put much stock into the concept of time at all and have very little to say about the relevance of the passage of time during the descri ption of events, which is one of the reasons why Einstein s theories of Relativity has resisted the drive to become part of quantum theory for over a century: gravity, space and time (it appears) don t like to be quantised This is one of the reasons (along with our everyday experiences) that most people consider space as being separate to time. Within reason, we have the freedom to move wherever we like through space (length, width height), but time drags us along, kicking and screaming if necessary.

Edward Page Mitchell may have been the first person to come up with the concept of time travel as far back as 1881 and temporal paradoxes such as The Grandfather Paradox have been used as a narrative device in science-fiction ever since. Almost all of these kinds of stories seem to rely on the idea that you can navigate through the dimension of time separately from the dimensions of space. If you re not familiar with the Grandfather Paradox and can t be bothered to read the page I ve linked, here s a tl&dr executive summary: You invent a time machine and use it to meet your grandfather before he married your grandmother. Obviously, because you ve thought of no better way to test the integrity of the laws of causality, you kill him (couldn t you just try to rig the lottery, like a normal person?). Your grandfather will then never meet your grandmother, meaning one of your parents will never be born, and therefore should you not vanish in a puff of logic? But if you then disappear in a puff of logic, you ll have never existed to kill your grandfather, so you get the idea, round and round in circles we go.

In a slightly less bloodthirsty example, consider the following paradox: you re listening to your favourite piece of classical music, say Lust For Life by Iggy Pop and if you don t think Iggy Pop is classical music, get in the sea, because you re dead to me so you download a copy of the sheet music (clearly credited to Iggy Pop David Bowie), jump in your time machine and travel back to 1976 (the year before they wrote the song) and give it Iggy telling him this is going to be a huge hit when they use it on the Trainspotting soundtrack twenty years from now . Iggy duly shows the score to his buddy David while they re hanging out in Berlin and the rest is history. Except that there s one problem. Who actually wrote the song? The song did not exist in 1976 when I took the sheet music back in time to before Iggy and David collaborated to write the song in 1977, but the lyrics and melody were already documented when I stepped into my time machine in the present day. So where did the music originate from?

While these kinds of temporal paradoxes make for interesting science-fiction, they re of no use at all when it comes to building a coherent picture of the nature of the universe. Which leads us back to the opening question that titles this post: why didn t a time traveller warn us about 2020 and the multitude of horrors awaiting us within it? Secondary questions riffing off from this starting point include such profound contemplations as just what the heck is time anyway? and is time travel even possible?

As you might expect, these are not exactly simple questions to answer. But I will try regardless, referring liberally to the two predominant scientific theories of our time that describe reality: Einstein s Relativity and Quantum Mechanics. When it comes to the nature of time, Relativity has some very interesting things to say, which (to use that perennially favourite turn of phrase of physicists around the world) are counterintuitive at best, and at worst, downright bizarre. As I mentioned earlier, the principle of Relativity states that time is inseparable from space in the way it creates a framework for reality in the universe. If that weren t hard enough to grasp, it also says that all of time and space are relative, and the passage of time or the distances travelled in space can only be quantified within defined frames of reference. This leads to all of the wonderfully bizarre phenomena we associate with Special Relativity, all of which stem from the rather curious assertion that the speed of light is the same (invariant), regardless of your own motion and frame of reference. If you would like to know how and why this statement is true, I will refer you to this excellent tome by Professors Brian Cox and Jeff Forshaw, because there are only so many explanations you can steal before the cease-and-desist letters start to flood in.

Put simply, this means that moving clocks accrue time at a slower rate than stationary ones, and also that clocks in a stronger gravitational field will also run slower than clocks in a weak gravitational field. Even in everyday life we have to take this effect into account, otherwise the accuracy of our GPS satellites would drift due to the lack of synchrony in the timing of ground-based and space-based clocks by over a kilometre per day. The idea that the passage of time isn t the same for everyone poses a rather interesting challenge to the concept of time travel. The invariance of the speed of light, regardless of your own motion, means that travelling through time cannot be separated from the way you travel through space.

Instead you have to consider your motion through the universal fabric of spacetime. Given that everyone has to observe the speed of light as being (approximately) 300 million metres per second or 186,000 miles per hour relative to their own velocity, irregardless of their own motion, if you re not able to keep track of the flow of information from one frame of reference to another, very odd things start to happen with causality and whether two people agree on the same order of events. One of the best known thought experiments to examine the implications of Relativity on cause effect is the so-called Twin Paradox, where two identical twins age at different rates, according to the speed of their relative motion. Thought experiments lend themselves to extremes and the following version of the Twin Paradox is one of my favourites.

We have two identical beryllium and aluminium quantum stopwatches, perfectly synchronised together and accurate to within one second per billion years. We keep one on Earth and place the other on a spaceship that we re going to fly to the Andromeda galaxy and back (a round trip of some 5 million light years) at as close to the speed of light, c, as we can. We start both stopwatches simultaneously as the spaceship begins its journey to our nearest intergalactic neighbour (for the pedants in the room, I am not considering nearby dwarf galaxies such as the Magellanic clouds, because I m sizeist deal with it). From the point of view of the occupants of the spaceship, the amount of subjective time that passes is 100 years, according to their stopwatch. How much time passes on the Earth-bound stopwatch? 6,000,000 years, a time period that s rather larger than the potential drift in the accuracy of the two clocks. From a certain point of view, you could describe this phenomenon as time travel , as you have made a journey through 100 years of time and 5 million light years of space to effectively travel forward through 6 million years of time and return to the same point in space. Whether this is a useful form of time travel is a different proposition entirely, because you cannot make the return journey relativistic spacetime will not permit it.

This is the consequence of having space and time woven together into a fabric where only relative measurements between frames of reference can be made. Time travel would be a much simpler proposition if we could assign absolute coordinates to locations, moments and events in spacetime. The premise of time travel in a lot of (good, bad, indifferent or otherwise) science-fiction is seemingly predicated upon the idea that you can lay in coordinates to visit a specific point in space and time to witness (for example) the assassination of JFK from the apex of the grassy knoll in Dallas. Relativity emphatically defenestrates this notion out of the hotel window because it is impossible to travel through time without travelling through space.

Consider the following situation to illustrate the difficulty in assigning absolute coordinates in spacetime. In theory, it s easy to do, because all you need are three spatial and one temporal coordinates, provided that you can assign a fixed, absolute grid of space and time across the whole universe. Let s say for the sake of argument, the JFK assassination happened at the coordinates (x,y,z,t), where the x,y,z correspond to the position in space on the Dallas grassy knoll and t refers to the instant of the first gunshot. So if you placed your time machine exactly on the top of the grassy knoll at the position you d like to view the event from, all you d need to do is set your time dial to roll your TARDIS back to 12.30pm on the 22nd of November 1963 and you re all set to get that killer (pun very much intended) video on your smartphone that proves Lee Harvey Oswald wasn t JFK s assassin, right?

Well, I m afraid it s not that simple, because it never is with Physics. In the 57 years since the Kennedy assassination, neither the Earth, Sun nor the Milky Way have stayed put. So those spatial coordinates of (x,y,z) that would place you on the grassy knoll in Dallas in 1963 will not put you in the same place in 2020, or at any time other than 1963, for that matter. The problem is two-fold. Firstly, the Earth orbits the Sun, and the Sun is in relative motion to the centre of the Milky Way. So even on the same date from year to year, when the Earth is in the same relative position to the Sun, it s neither in the same relative or absolute position in comparison to the centre of the Milky Way. Not only that, the Milky Way itself is not in a static position in space: it s moving relative to clusters of galaxies around it. And to make matters even more complicated (because at this point, why not?) spacetime itself is expanding. It s impossible to impose an absolute coordinate system of space and time in such circumstances. So if a method of imposing a definitive set of coordinates upon events in space and time is required for time travel is necessary, then we are doomed to failure, because it s impossible to travel through time without motion through space as well.

But wait! Because it gets weirder!

Even if you accept that you have to treat spacetime as a single entity, rather than being distinct dimensions of space and time, there is still room for (a hotly contested) debate and interpretation as to how time manifests itself in the universe, which still has not been universally agreed among physicists, despite over a century of trying. The first model is commonly known as the block universe , where all of spacetime is considered to be a continuum where all of space and time co-exist within a single block where it may be considered that all past and future events are fixed by the deterministic rules of quantum mechanics. In this model, the possibility of time travel is left open, as past and future spacetime always exist relative to now . The issue is how do you go about accessing them, as to do so would require you to break relativistic limits, potentially causing all sorts of issues with causality.

If we leave aside the practical and theoretical difficulties of constructing a functioning time machine for the moment, another ugly paradox rears its head at the notion of re-enacting the Grandfather Paradox. In a block universe you would be unable to alter any events that occurred in the past or future: they already exist in the continuum, so can be said to have already happened. So just by the the very fact that you re their to try to shoot your grandfather dooms you to failure. But there is yet another sting in the tail. While the events that preceeded your birth in the block universe are immutable and unchangeable within that block of spacetime, by travelling backwards in spacetime you have disrupted that block by adding extra mass-energy into that point in spacetime.

This in turn will change the new most-probable path of events within the spacetime, fundamentally altering the block, as you and your time machine have now become entangled with their surrounding environment. Remember, observation in quantum physics is not a passive process. In effect, by time travelling you have in fact created a new block of spacetime, which is consistent with the Many Worlds interpretation of quantum mechanics. The fact that you re in a new block of spacetime means that you can quite happily go on a killing spree throughout your entire ancestry, should you be so inclined, and you would never be born at a future point of spacetime within this new block. With your bloodlust sated, you would be then free to step back into your time machine and wibbly-wobbly-wend your way back through the quantum foam of multiverse to the exact point in your original spacetime block when you left and have afternoon tea with grandpa (assuming he was still alive at the point which you started playing around with your time machine).

The implications from this thought experiment are fairly clear: you can never change (or even observe) past or future events from within your own block of spacetime. If you were to make the attempt, the conditions of the spacetime at the point you wanted to travel to would be changed, spawning an entirely new, parallel reality where your appearance in your time machine was always the most probable event to occur at that point. So if you ever do bump into a time traveller, they re not actually from your universe. (That may be worth remembering, as they might like to eat human brains. Don t say you weren t warned.)

The block universe model of spacetime isn t the only way physicists think time may work, however. One of the consequences of a fully deterministic universe where the continuum of spacetime is set in stone (as it were) is that it completely eliminates the possibility of free will. And some people are a bit touchy about that. They don t like the idea that we just have the illusion of free will and that some unseen dice roll behind the Dungeon Master s screen actually picked the outcome for us. In other words, the decisions we make (or we think we make) were always going to be the decisions that we would choose, regardless of the circumstances.

In order to get around this rather thorny concept, physicists have another way of considering how time weaves its way into the fabric of spacetime. As the spacetime of the universe expands, time only exists at the point of now. There is no past and future in spacetime, no continuum or block. Just an ever-expanding bubble of spacetime now. Interestingly, this model of time is still completely consistent with observation and the laws of Physics, as it allows us to account for information travelling across the universe (such as ancient starlight) as well as account for the effects of entropy and the other laws of thermodynamics. It also opens the door to free will nice and wide, because although events are still random and deterministically decided by the underlying statistical laws of the universe, the future is unknowable because there is no future that exists. The price of free will is being trapped in an eternal now with no past and future at all. The passage of time is an illusion that results from our ability to receive, store, recall and synthesize information. We can remember events from the past, and use our experiences to predict how events might evolve in the future, but otherwise our sense of time is limited to right now. We cannot perceive time in any other way, let alone travel through it at will. This time on the fly model categorically prohibits the phenomenon of time travel, because there is nowhere (or should I say nowhen) else to go.

So to answer my opening question why didn t a time traveller warn us about 2020? , maybe they tried. It just wouldn t make any difference in this block universe because the path of events through the continuum is already fixed. I take some measure of comfort in imagining that a future time traveller from this block universe managed to send word back to a parallel reality and forewarn them of all the potential calamities that have befallen us this year. The other possibility can be inferred from an application of Occam s Razor to the throat of the question why haven t I met a time traveller? the simplest (and therefore most likely) explanation is that time travel is impossible. So even if someone wanted to warn us, they couldn t. On that cheery note, you re probably wishing that you had a time machine so that you could take back all the time you ve spent from reading this essay. Sorry about that. The best I can do is direct you to the Back button of your web browser.

But let s not end this missive on a depressing thought. Let s set a challenge instead. Any scientist worth their chocolate chip cookies is ready and willing to be proved wrong. So I will make the following statement: I will never meet a time traveller in my lifetime because time travel is impossible. It s up to the time travellers to prove me wrong. It s okay. I ll wait