Time Travel and Christmas

Christmas is just around the corner. Mince pies are baking. Turkey (or Tofurkey) is ready to be basted. Presents are being wrapped. The Pogues are playing on mind-numbing repeat. Time, then, for a Christmas themed post: How does Santa manage to deliver all of his gifts in one evening?

In each timezone, Santa gets roughly the same amount of time. Therefore, for instance, he has roughly an hour to deliver his parcels throughout, not just the United Kingdom, but also portions of Greenland, all of Iceland, and numerous countries in Africa, from Burkina Faso to Senegal.

Christmas in Burkina Faso.

It’d be hard enough to do just the UK in one hour! Assume that the average distance between houses was ten metres. And imagine that only one million households were nice rather than naughty this year. That’d still mean Santa has to weave a six thousand mile path across the UK. His sleigh would have to average 6000mph, roughly three times quicker than the fastest airplane. Further, he has to stop and start a million times in that hour. My back of a stamp calculation indicates that this would involve a g-force of roughly 280 million gs. That would reduce his reindeer to pulp. (I’ve not googled how many g’s a reindeer can survive––I was worried what I’d find––but I’m fairly certain it’s at most 279.999 million less gs than that…)

The SR-71 Blackbird, with a top recorded speed of 2,193.2 mph.

But presumably Santa Claus has other powers at his disposal. After all, if the elves in his toy factory can ignore patent law, maybe Santa can ignore the laws of physics? Dr. Roger Highfield suggests that he might have access to technologies which allow him to warp spacetime. And if Santa could toy with the fabric of space and time, maybe he could deliver his parcels without reindeer puree being an issue.

What if Santa could freeze time? This is exactly how it works in the Disney movie ‘Twas the Night. By freezing time, the world around Santa remains stock still whilst he and his reindeer fly around, from home to home. He now has all the time in the world to deliver to a million houses. In fact, now it’s easy to see how he gets inside every house! No longer must Santa squeeze down your chimney; he has all the time in the world to take your front door off of its hinges and reattach it when he leaves.

Bryan Cranston taking over as Santa: ‘I am not the one who knocks.’

That time might freeze, but people might move around nonetheless, is not a new idea. It appears in Thomas White’s 1646 Institutionum peripateticarum. White identified time with the passage of the sun. As Emily Thomas explains in her book Absolute Time, were the sun to stop in the sky then “White states that time ‘would not, really, passe on’ although we would ‘make use of it as if it did.’”. It looks as if White had in mind that, were the sun to stop, whilst time stops with it we could all move around anyhow, active within that frozen instant. Santa would be doing the same thing, but it’d just be him and the reindeer which got to move around; everyone else would be as frozen as time itself.

Thomas White.

That someone might manage to move around a world frozen in time has appeared in various pieces of fiction. One of the more famous appearances is in The Twilight Zone’s ‘A Kind of Stopwatch’ where Patrick McNulty, the protagonist, finds a stopwatch that freezes time. He uses it to comic effect before trying to steal money from a bank vault. In the process the watch smashes, and McNulty is forever trapped in a single moment of time.

‘A Kind of Stopwatch’ probably took its inspiration from MacDonald’s The Girl, the Gold Watch & Everything, released the year before (and which got a film adaptation in 1980). And we see it in Saved by the Bell with Zack Morris inexplicably having the power to ‘Time Out’ and move around. In fact, it appears in countless other places!

Santa will have some challenges using this method. For instance, it’s unclear how physics works when time is frozen. Imagine we freeze time whilst a hurricane is taking place on the other side of the world. Doesn’t that hurricane stop in its path? But if that mass of air stops moving, shouldn’t that mean that every mass of air stops moving? And if that’s true, how am I moving air molecules into my lungs in order to breath? Or consider light rays. If time has stopped, won’t they stop too? Won’t I be plunged into darkness when I freeze time because I no longer have photons bouncing off of my eyes? How’s Santa meant to steer?

‘A Kind of Stopwatch’ (1963)

You might also be wondering what freezing time has to do with this blog’s focus on time travel? Well, it turns out that the two are very similar and that Santa has some philosophical worries when he freezes time. Imagine we travel back in time. Some think that in doing so we make time two, contradictory, ways. For instance, one way would be that my grandfather was alive and well in 1930. But after I use my time machine I could shoot him dead back in 1930. Now the world is a second, conflicting way: In 1930 he is not alive and is not well at all. By using a time machine we can make the same instant in 1930 two different ways. How’s that meant to work, though? How can we change time?

Whatever the solution (and we will look at some in later posts!) we get exactly the same problem with freezing time. Imagine Zack Morris is in Mr. Belding’s office. Caught in a tricky situation, he freezes time. He runs from one side of the office to the other before  time restarts as he’s heading out of the door. At that moment in time, it seems he both is in Mr. Belding’s office and isn’t in it. Time is two, contradictory, ways!

So Santa would have to contend with the mysteries of time travel. If freezing time, and delivering parcels, needs time to be contradictory ways, Santa would need to figure out how that would work––how could time be changed to have him first at one house and then at another?

The Early Time Machines

How might we travel back in time? Perhaps we blip from existence and simply reappear in the past. Perhaps we travel through a wormhole (or a time gate, or a time corridor, or a time portal, or anything else that we stick the word ‘time’ in front of). Or perhaps we take some exotic drug and find ourselves able to possess someone in the past. There are lots of ways people have thought we might travel back in time.

The earliest sci-fi writers thought of time machines travelling back through time in the same way that we travel forwards through time. Just as I don’t manage to get to 2020 AD by teleporting there, and have to live through the intervening years, the fictional time machines of the late 19th century did much the same, but in reverse. Time travellers would have to sit in their time machines, watching as reality did an about-face and time rolled back.

Gaspar’s time machine: It travels back in time by flying against the Earth’s rotation.

This is how it is in one of the first stories to feature a time machine: Edward Page Mitchell’s 1881 The Clock That Went Backwards. (Prior to that, there had been time travel stories but no machine which took you back in time.) It was also the means of time travel for the first time travelling vehicle, from Enrique Gaspar’s 1887 El Anacronópete, Viaje a China-Metempsícosis.

And it’s exactly how one of the most famous time machines of all manages to work. In H.G. Wells’ 1895 The Time Machine, the Traveller clambers into his machine and as he pulls the lever he watches the world around him accelerate, and speed up, as he hurtles into the future. When he returns, it runs backwards.

Nor has the popularity of such machines waned. Ian Watson played with the idea in his 1978 ‘The Very Slow Time Machine’. Wells, Gaspar, and co., all imagined time travelling into the past taking very little time. The years would zoom past; you could get to the Jurassic Period in a few hours! But in Watson’s story, the time machine goes back only as fast as it’d otherwise have gone forward—if you want to travel back twenty years then it’ll take you twenty years! And the people in the world outside, who aren’t time travelling, get to watch the time machine plodding its way back into the past, its passenger all the while growing less insane from the isolation…

But it’s not all smooth sailing in the choppy ocean of time. This method of time travel would have a problem, namely yourself! Imagine it’s 8am and you’re in your workshop, sat in your time machine having a cup of coffee. It’s a big Venti-size from Starbucks, so it takes you an hour to drink it. Casting aside the coffee cup (it’s your workshop, so you can clear up the mess later) at 9am you flick the switch to send you back in time. The next moment you should find yourself at should be 8.59am.

But you’d still be in the workshop. Your machine moves in time and not in space. How can you travel back to the workshop at 8.59am? You are in the way! That is, your earlier coffee-drinking self! You can’t move back in time in the same way you can’t move through a brick wall—objects can’t interpenetrate one another, even when they’re one and the same object.

Rod Taylor, in the 1960 film version of The Time Machine

Wells noticed this problem. He hand waved it away. In The Time Machine, the Time Traveller talks about how it is that his time machine manages to avoid obstacles that lay in its path. He says ‘Don’t you know that every body, solid, liquid, or gaseous, is made up of molecules with empty spaces between them? That leaves plenty of room to slip through a brick wall’. That’s all guff, of course. It’s a literary flourish on a par with the Doctor explaining away problems with time travel as ‘wibbly wobbly timey wimey stuff’. But it is Wells’ novel, so he can ignore the problem if he wants.

(Not every author would ignore this problem and would instead make it into an interesting plot point. For instance, in Silverberg’s The Masks of Time the protagonist tries to send particles back in time but they keep bumping into their earlier selves and annihilating.)

Whilst sci-fi writers can put the blinkers on when it comes to issues that get in the way of their story, philosophers don’t think that they have the same luxury. They have thought about this problem in more detail. Call it the Double Occupancy Problem.

One solution uses motion to solve the problem. Wells’ time machine stays still, in space if not in time. But a time machine needn’t be motionless. For instance, Gaspar’s machine flies through the air and travels back in time by travelling counter-rotationally around the Earth. Similarly, another quasi-imitator of Wells, Harold Steel MacKaye, wrote a 1904 story, The Panchronicon, which goes back in time by flying tight loops around the North Pole.

MacKaye’s The Panchronicon

To see how this solves the problem, imagine a particle zooming along. Suddenly it starts travelling back in time. It won’t hit its earlier self because it’s moved on from where its earlier self was a moment ago.

But this only works if you imagine that the particle has no extension, that it’s ‘sizeless’ and ‘point-sized’. What works for things with no size, won’t work for things which are extended and have some volume to them. Imagine you’re in a car. It has a front end and a back end. Driving at high speed, at 9am you try and travel back in time. However, no matter how fast you go, the front end of your car from the moment before you travel in time will always get in the way of the back end of the car when you start travelling into the past!

Philosophers have suggested all sorts of fixes. Perhaps you start time travelling back, bit by bit, ending up being like Carroll’s Cheshire Cat, vanishing slowly over time. Or perhaps you can only travel back in time if there are extra spatial dimension that the time machine navigates. Or perhaps time travellers get their own personal bit of spacetime to travel back through.

Carroll’s Cheshire Cat, which vanishes bit by bit. Perhaps time travellers are like this?

Of course, maybe it’s all nonsense. Maybe time travel’s not possible. Or, if it is possible, it’ll be nothing like what H. G. Wells imagined. But there’s no harm in doing what the philosophers do, and thinking through what could be, rather than what must be.

And one reason to do this is that this method of time travel might have some reflection in real-world physics. When matter and antimatter meets, it undergoes ‘mutual annihilation’, vanishing in a puff of radiation. Richard Feynman once proposed that rather than being distinct things which destroy one another, particles and antiparticles are just time reversed versions of themselves. An electron is a particle going forward in time. A positron—the antimatter counterpart of an electron—is just the same particle, but going backwards in time. What’s actually happening in ‘mutual annihilation’ is that the forward travelling electron suddenly decides to reverse direction in time, becoming (from our point of view) a positron. That’d look the same to the ignorant observer! Thus, thinking about how something like Wells’ time machine would manage to travel back in time is not just a parlour game for metaphysicians to indulge themselves with. .

Tachyons: Small, but nippy

My last post was about travelling faster than the speed of light and going back in time. In it I said that nothing could accelerate past the speed of light. No matter how good your BMW, you’ll never get a car where you just hit the accelerator and break the light barrier. As you zoom faster and faster, light will always be going at 300,000 km/s. It always outpaces you. It always beats you. In a race, you’ll always be Dick Dastardly–light will always win. So travelling in time by going faster than light is never going to work.

Mutley: A dog who’d have loved to learn how to go faster than light

Sort of.

There’s a hole in orthodox physics which allows things to travel faster than light. Nothing can accelerate past the speed of light, but things can start faster than the speed of light and never slow down below it. These things are theoretically possible. They’re called ‘tachyons’.

Since you’re currently travelling slower than the speed of light, this means you’ll never go tachyonic. You, yourself, can never travel back in time by moving faster than light.

But that doesn’t mean that tachyons aren’t interesting. If you could find some, and manipulate them, then you could use them to communicate with the past. Because they travel faster than light, and move back in time, you could theoretically send them back in time to a detector and, say, tap out a Morse code message to your earlier self. Imagine it! Looking on Tinder you suddenly get the message:

… .– .. .–. . / .-.. . ..-. –

Consulting what that means, you hastily swipe left…

Tachyonic Tinder: The Future of Dating

This method of communication was dubbed ‘the antitelephone’ by Benford, Book, and Newcomb. Benford would also go on to write a famous novel, Timescape, which featured just such a device. Indeed, in fiction tachyons appear all over the place. Sometimes they’re used to communicate with the past (as in XFiles and John Carpenter’s Prince of Darkness). Sometimes they’re just inserted whenever some sci-fi writer needs something suitably timey-wimey and sciencey sounding (such as Star Trek or Babylon 5).

Gregory Benford’s Timescape

Back in the real world, though, we’ve searched for tachyons. Back in the 70s someone thought they may have found them but they decided that there was nothing there after all—although more recently there’s been some hope for their existence thrown out there. (Indeed, in my own book I argue that the current failures to detect them don’t really cast doubt on their possibility at all–but that’s another story.)

There have been two main methods for detecting tachyons.

One is to look for ‘Čerenkov radiation’ which is emitted when things travel faster than the speed of light in a given medium. Notice that last bit: ‘in a given medium’. Whilst light goes at 300,000 km/s in a vacuum—and nothing can beat that pace—it goes slower when it enters other substances (such as water or those tatty Perspex blocks you use at school to demonstrate refraction). If something passing through such a substance manages to beat the speed that light can do in that substance, it promptly starts emitting the radiation. People thought tachyons would emit the same radiation. When we went looking for it, we found nothing.

But it makes no sense to think that tachyons ever would have emitted such radiation. Think of the world from their perspective. From the point of view of a tachyon, it’s us who are the ones moving faster than the speed of light (and us who are going back in time). It’s long been thought that there’s no sense in asking which of us is ‘really’ moving at such-and-such a velocity. If true, we shouldn’t be able to figure out which of us or the tachyon is the one who is really moving faster than light. But if tachyons emitted Čerenkov radiation then we’d be able to tell which it was––the person going faster than the speed of light would be the guy emitting tonnes of radiation! And since we’re not emitting Čerenkov radiation, it can’t be us going faster than the speed of light. So if you believe in tachyons you’ve got a choice: Give up on that long-standing principle of physics (i.e. that we can’t determine absolute velocities) or give up on thinking that tachyons would blast out radiation as they blitzed around the cosmos.

(Indeed, buried away in the literature is a paper arguing for exactly this sort of thing,  claiming that the equations don’t bear out tachyons emitting Čerenkov radiation after all.)

The other way to detect tachyons assumed that they played a role in ‘proton decay’. You’re probably familiar with decay in general. Radioactive atoms decay every now and again, turning into something else––for instance, uranium decays and turns into thorium. When it decays, it emits ‘alpha radiation’ in the form of an alpha particle (and it’s that which makes the uranium radioactive). When the alpha particle shoots off one way, the resulting thorium atom gets ‘kicked back’ and goes the other way.

Proton Decay

This ‘kick back’ means that the thorium and the alpha particle spontaneously start moving. But the energy for their movement doesn’t come from nowhere. The mass of the thorium plus the alpha particle is less than the mass of the uranium. The energy for the alpha particle going one way and the thorium atom going the other comes from that difference in mass––by shedding mass, the uranium atom can generate the energy for them to move.

Some people think protons might decay if we left them for long enough. The proton would get ‘kicked back’ by whatever particle it emits when it decays. But here’s the twist: When a uranium atom decays, it ceases to be. But a proton cannot stop existing––that’s because there’s a law called ‘conservation of baryon number’ which means that the number of protons that exist throughout the universe can’t change. And if the proton can’t stop existing, it can’t shed mass to generate the energy to ‘kick it back’. (Similarly, where would the energy come from to create the particle it emits when it decays?)

So if protons decayed, we’d have a quandary. Do we give up on the conservation of energy and say that things can spontaneously start moving? Or do we give up on the conservation of baryon number and say that the proton stops existing? Or what?

The answer is the humble tachyon. I said above that tachyons can’t slow down below the speed of light. Anything that starts off travelling faster than the light barrier stays there! But what happens when a tachyon tries to leave the gravity well of the Earth? Wouldn’t they slow down as gravity pulls them back?

The answer––some people think––is that tachyons will have negative mass. That means gravity works in reverse when it comes to tachyons. They slow down when they approach the centre of the Earth and then speed up when they try and leave Earth’s gravitational clutch! And the negative mass means that, if they were emitted during proton decay, the energy produced would be counterbalanced by the negative mass-energy of the tachyon.

So whilst we might not be able to detect the tachyon itself, some physicists figured that they might see the protons decaying, and moving about spontaneously. That’d then be evidence that tachyons existed.

But when we looked, we did not see any protons decay and didn’t see them  spontaneously move about. And so, presumably, they weren’t emitting tachyons.

So tachyons might, in some sense, be allowed by the laws of physics. But even though they are, they’d both be pretty weird (negative mass? what the hell does that even mean?) and, as far as we can tell, they don’t appear to be out there.

Going Faster Than Light!

In fiction, things sometimes go faster than light. Superman’s at it all the time. People are often boggled by the end of the original Superman movie. Superman circles the Earth faster and faster until he spins it backwards. Suddenly, time runs in reverse––which makes no sense! Why would spinning the Earth clockwise reverse time itself? Of course, that’s not what’s happening. Instead, he’s going so fast he’s going back in time and we get to see the world from his point of view, reversing backwards as he tunnels into the past.

The Space Eagle Operation Doomsday.jpg
The Space Eagle Books (NB: Not as good as Harry Potter…)

It isn’t just Superman. In the Space Eagle novels there are machines which travel faster than light, turning them into time machines. In Red Dwarf’s ‘Future Echoes’ the ship manages to break the light barrier and time becomes warped, allowing the crew to catch glimpses of the future.

But why is travelling faster than light connected to time travel?

According to physics, if you start slower than the speed of light, you can’t ramp up your speed to get past the speed of light. This fact about the universe is deeply, deeply weird and the key to understanding why people connect faster than light travel with travelling back in time.

Let’s start at the beginning. Given physics, you can’t find out how fast you’re moving. No, really, you can’t. You can discover similar things, though. You can find out how fast you’re moving relative to something else. If you’re on a train, moving away, you can find out how fast you’re moving relative to the platform (and how fast the platform is moving relative to you). And you can find out if you’re accelerating. When you’re on a roller coaster or an airplane, you might feel all giddy and stomach-turny. That’s not because you’re in motion but because you’re accelerating i.e. your velocity is increasing or decreasing. When you’re at a constant velocity (that is, a constant speed in an unchanging direction) you won’t feel a thing.

Readers are advised to keep inflight smoking confined to thought experiments.

You’re familiar with these things already. Imagine smoking on a plane. This used to be a thing you could do. It isn’t now––so I recommend trying this only if you actually have a time machine to take you back to the 70s. For now, just picture it in your mind. Watch the trace of smoke. It curls behind you and around you in exactly the same way whether you’re sitting on the ground or travelling at 1000mph in the air. It’s not as if, when you’re in flight, the smoke zooms to the back of the plane, gathering at the back like a bizarre cancerous wall, pinned to the bulkhead.

So you can detect whether you’re accelerating. And you can detect whether you’re moving at a certain velocity relative to, say, the surface of the Earth, or the sun, or the Milky Way. But you can’t detect what your velocity is full stop. From this, many people believe that, not only is it undetectable, but there’s simply no fact of the matter about it. You don’t have a ‘velocity full stop’; you only have ‘relative velocities’.

Then Maxwell came along. He does some research in electromagnetism. It turns out, he says, that light moves at roughly 300,000 km/s. Here’s the kicker: That’s not 300,000 km/s relative to something, that’s just 300,000 km/s. ‘Aha!’, thought some, ‘Now we have a way to determine who’s really moving or not. We’ll simply measure how fast we’re going relative to light and then we’ll know! Now we can figure out our real velocity!’.

James Clark Maxwell.

Sketch that out a bit more. Imagine I know my velocity relative to the train platform. I know it relative to the surface of the Earth. I know it relative to the sun. I know it relative to everything else. But I don’t know what my objective, non-relative velocity is. Then someone tells me that they’ve got God on the phone. Chatting to God, He tells me that there’s a comet which is at rest––not rest relative to anything, but just simply, really, objectively at rest. At rest full stop! Now I can figure out what my velocity is! I take my velocity relative to that comet and that’s my velocity—my objective, non-relative velocity.

Maxwell seemed to be saying that we could do the same thing using light. We figure out how fast we’re going relative to light and BINGO! We’d know how fast we’re going ‘full stop’! But the experiment to do this turned up weird results. Light was going 300,000 km/s. Which meant we were at rest. And that’d be lucky, right? Then it got weirder still: Measurements showed that, no matter how fast something was going, the result was always that light was going 300,000 km/s. Stationary relative to the Earth? That was how fast it was going. On a plane at 1000 mph? Still going the same speed!

This is super bizarre. Imagine you’re watching Mr. Slow and Ms. Fast race one another. Mr. Slow is going at 20 mph. Ms. Fast is going at 200 mph. You phone Mr. Slow. Mr. Slow says that Ms. Fast appears to be going at 180 mph (for his 20 mph means that’s how fast she is going relative to him). Mr. Slow accelerates until he appears to be going at 199 mph relative to you. Again you phone him. He now says that Ms. Fast is going only 1 mph relative to him.

That all sounds fine.

But that’s not how light works. Imagine Dr. Weird is driving a weird car that works like light does. Dr. Weird appears to be going 500 mph relative to me. I phone Ms. Fast. I expect her to say that, from her point of view, Dr. Weird is going 300 mph. But she instead says he’s going 500 mph. Weird. Stranger still, when I tell her to accelerate, and she is now bombing along at 400 mph according to me—and Dr. Weird is still going only 100 mph faster—she still reports that Dr. Weird is going 500 mph to her.

How the hell does that happen?

The explanation is that when things are moving fast relative to one another, time starts to slow down (it also turns out that things change shape, size, and mass, but I’ll ignore those changes for the purpose of this post). Think how that’d work with Dr. Weird and Ms. Fast. Imagine I watch them for an hour. Dr. Weird covers five hundred miles; Ms. Fast covers four hundred; Dr. Weird outpaces Ms. Fast by one hundred miles. But imagine that time has slowed down for Ms. Fast five-fold. When she uses her watch to measure time, it takes her watch five times as long as my watch to move forward. When she measures an hour, five hours will have passed for me. And in that period of time, Dr. Weird will have moved—not one hundred miles ahead of her—but five hundred miles. So if I communicate with Ms. Fast, she’ll tell me that Dr. Weird is moving five hundred miles per hour—exactly the same rate I have!

This is what’s going on with light and things moving closer and closer to light speed. When Ms. Fast starts moving faster and faster, she appears (from my point of view) to be going slower and slower. And that’s where we get the connection to time travel! Billions of years could take a single second to pass by from your point of view if only you went fast enough! And if you managed to hit light speed, time would stand still (although you never will hit light speed—remember, no matter how fast you go, you’ll always find light outpacing you at 300,000 km/s!). And if you somehow managed to go faster still, then time would start going backwards.

I’ll talk more about superluminal travel in future blog posts. For now, let the weirdness of light speed settle in.

Red Dwarf: Shockingly, not an accurate guide to real-world physics. 

Before ending, consider one mistake that people make. They think that when Ms. Fast appears to be going slower and slower in time from my point of view, that when she looks back she’ll see me all sped up. (You get this sort of thing with Red Dwarf’s ‘White Hole’.) That only makes sense, right? WRONG!

Think of the train: You’re on a train moving relative to the platform but the platform is moving relative to you. There’s no way to tell who’s ‘really’ in motion. The same applies here. If I sped up and Ms. Fast slowed down, we could tell who was ‘really’ in motion—it’d be Ms. Fast! And that’s not what happens. Instead, we both see one another ‘slowing down in time’ as each of us has a high velocity relative to the other. And if that isn’t the weirdest thing you’ve thought about today, you’re either a physicist already intimately acquainted with relativity theory (and probably grinding your teeth in rage as I run roughshod over important details for the sake of a good blog post) or you take too many hallucinogenic drugs.

The Road to Nowhere

I step into the Timeambulator, ready to travel back to 1930 AD. Cogs whirr. Gizmos spin. Lights flicker. I blip out of existence. And I go… where? How can I go to 1930 AD? It doesn’t exist! It’s not there for me to arrive at! I can no more travel to 1930 AD than I could charter a plane to Neverneverland, clamber aboard a train to Hogwarts, or hike to Wakanda. They’re non-existents; they’re fictions and nothings. Along the same lines, I can’t travel back in time because the past doesn’t exist. A time machine is a road to nowhere.

This is the first of the many arguments against the possibility of time travel that this blog will examine. It is the No Destination Argument.

It’s been picked up in fiction a few times, albeit with some modification—after all, a time travel story where you simply can’t travel in time is going to be a tall order to pull off! In numerous fictions we find that, whilst travel to the past is possible, travel to the future isn’t because it hasn’t happened yet, it isn’t here yet, and it doesn’t exist.

For instance, in Arthur C. Clarke and Stephen Baxter’s The Light of Other Days it’s dressed up in some scientific lingo that one can use wormholes to see into the past but not the future.

Or in Stephen King’s The Langoliers the characters manage to get trapped back in time. But the past turns out to be second-rate and teeters on the edge of oblivion. Soon enough, the Langoliers turn up to wipe it from existence, eating everything away.

Stephen King’s ‘The Langoliers’

That the past and present doesn’t exist is captured by a philosophical theory called ‘presentism’. Presentists tend to think that their theory is intuitively true (indeed, maybe you think that too!) but it turns out that not everyone is a presentist. A popular alternative view is ‘eternalism‘: the past, present, and future all exist.

Eternalism’s not quite as weird as it sounds. The claim isn’t that the past or future exist now, only that they exist. Eternalists think ‘now’ functions a bit like ‘here’—the Eiffel Tower doesn’t exist here but that doesn’t mean it doesn’t exist! Similarly, the past and future exist, but they just don’t happen to be close and local—they’re ‘out there’, even though they’re not around you right now.

If eternalism were true, we’d have a solution to the No Destination Argument which allows for time travel. And, fortunately, there are reasons to think eternalism is true quite apart from time travel. Some motives are purely philosophical (e.g. presentists accept that there are truths about the past—such as ‘Ghenghis Khan was a warlord’—but what would make them true if the past doesn’t exist?). But the mainstay of support is from physics. Advances in understanding space and time led to relativity, which many physicists take to indicate that all times are ‘on a par’ i.e. eternalism is true (and many philosophers have followed suit). Einstein and Minkowski, amongst others, held an eternalist view. Einstein even once comforted the family of Michele Besso, a close friend of his, by writing:

People like us, who believe in physics, know that the distinction between past, present and future is only a stubbornly persistent illusion.

For Einstein, Besso hadn’t gone anywhere. He just wasn’t nearby anymore.

Lee Smolin; physicist who thinks the No Destination Argument may be right.

But there’s another way to attack the No Destination Argument, which is fortunate because not everyone—not even every physicist—is an eternalist; Lee Smolin, for instance, is minded to think the No Destination Argument works. Philosophers Simon Keller and Michael Nelson have written an excellent article explaining why even the presentist should believe in time travel. The gist of the position is that presentists will readily accept that things travel in time on a regular basis! Consider you and me. We’re both travelling to the future right now! Sure, we’re only heading there at the sedate pace of one second per second, but don’t deny that in a little bit you’ll be in the future. (Want to see what a world without Trump looks like? Live long enough and you will!) Since you can travel into the future even though it doesn’t exist—and who can deny that?—you can travel into the far future or the past.




My name’s Nikk Effingham; I’m a Reader in Philosophy at the University of Birmingham. My specialisation is metaphysics, with a focus on the philosophy of time. Having just completed a book on time travel–not yet published!–I thought I’d share my research in a user-friendly, easy-to-read way, via this blog.

Over the course of the next two years, I’ll be posting one post every two weeks on areas within time travel: its philosophy (what’s the Grandfather Paradox? how does probability work in time travel cases?); its history (when did people first think about time travel?); its books and films (what were the first time travel stories? what are the themes that time travel movies cover?); as well as dipping into some speculative physics (how might we time travel, if it were physically possible?). If all goes well, I’ll rope in some friends to do guest posts–don’t worry, I’m not the only philosopher involved with time travel!

Alongside this blog I invite you to add yourself to its associated twitter feed.

I hope you enjoy it!

Nikk Effingham