In 2009, Stephen Hawking held a party in Cambridge. No one came. After the event, he sent out invitations to time travellers from the future to attend. Since none of them had shown up, he claimed this as experimental evidence that time travel is not possible.
He raises a fair question: if time travel is possible, then where are all the time travellers?
Hawking’s light-hearted experiment is far from conclusive, of course. Here are four possible reasons for the absence of visitors from the future, one or more of which might explain the no-shows.
1) Time travel takes a lot of energy.
Science fiction focuses on the idea of sending actual people, bodies and all, back in time. It’s obviously a great advantage from a storytelling perspective, but viewed scientifically, it makes little sense.
“That meeting could have been an email.” These days we don’t, as a rule, send people halfway around the world on an aeroplane when a video conference works just as well. Why? Energy.
The argument holds up even more strongly with time travel, which we’d have to assume would be far trickier – and, without entering into the possible mechanics of it, more energetic – than geographical travel.
An extraterrestrial observer of Earth might ask, “If humans have discovered nuclear technology, then where are all the nuclear explosions?” If time travel turns out to be as energetic and potentially dangerous, it’s perfectly conceivable that it will be regulated by the same sort of restrictions we place on nuclear power and weaponry. Hence no trips to Cambridge 2009 for a party, however congenial the host.
2) Time travel kills the time traveller.
Let’s run with this energy idea. We’d rather send an email or text message than a human messenger carrying bits of paper. For similar reasons, it’s not really plausible that we’d send an actual human back in time. Presumably at some point in the process we’d have to disassemble the molecules of the traveller’s body and put them back together. Putting aside the Trigger’s Broom question of whether that would actually be the same person, or just a copy, I struggle to think of a method via which ripping someone’s entire body apart to the level of individual molecules wouldn’t kill them.
Okay you say, but what if we could digitise consciousness, and send that back in time, to be placed in another brain and body, whether that would be artificial or some sort of permanent or temporary donor? In that case, you’d still be left with the original person at the transmitting end of the process – effectively, you’ve produced a clone, rather than an actual time traveller.
So if the process leads to the death, or at least the problematic cloning, of a human being, it’s not something you’d do for frivolous reasons.
In fact, the more you think about it as a serious proposition, the less sense it makes that you’d decide to send an actual human back in time, whether you’re talking about their entire body or just their consciousness. Why not just send a message? What are you trying to achieve?
Which brings us to our third question.
3) What’s the point?
So it’s likely that time travel would be a difficult, costly (in terms of energy and money), and dangerous process. The question would therefore be: what’s the object of the exercise?
Again, with the aim of constructing interesting stories, science fiction tends to concentrate on a “many-worlds” or “multiple timelines” approach, where a time traveller can go back to a point in history and change something, thereby altering the future and perhaps avoiding some global catastrophe.
Our experience of time doesn’t really back that up though. The past has already happened. If time travellers have gone back in time, whatever the consequences of their actions, we’re already living with them. It’s a tricky concept for us to accept on a philosophical level – what becomes of free will for those time travellers, if whatever they do leads to a future which they know is already fixed? But that’s more of a problem for us rather than Physics – from the point of view of the latter, a “fixed single timeline” model makes far more sense. Indeed, quantum physicists are finding and discussing instances of potential “retrocausality” – events in the present being determined by events in the future. Free will may not come into it, on a subatomic level at least.
“One of the major problems encountered in time travel is not that of becoming your own father or mother. There is no problem in becoming your own father or mother that a broad-minded and well-adjusted family can’t cope with. There is no problem with changing the course of history—the course of history does not change because it all fits together like a jigsaw. All the important changes have happened before the things they were supposed to change and it all sorts itself out in the end.” – Douglas Adams, The Restaurant at the End of the Universe
If that is the case, then you’d have to ask: what exactly would be the point of travelling backwards in time? If history can’t be changed, then what is there to be gained in a perilous and costly trip to visit it?
Any or all of these might well be a factor. But there’s one more point which provides sufficient explanation on its own.
4) Time travel requires a receiver as well as a transmitter.
When Alexander Graham Bell invented the telephone, he didn’t immediately have the ability to contact anyone in the world remotely. He could only talk to the one man who also had a telephone. Since that’s a far simpler process than time travel, why wouldn’t the same principle hold?
Again, science fiction is largely fixated on the ability to send people back to any point in space and time. But how would that location be identified and fixed – especially the spatial element? It would clearly require a second set of equipment, which by definition hasn’t been invented yet. Just as Professor Bell couldn’t call anyone who lived beyond the reaches of his nascent telephone network, so time travellers can’t reach any location where time travel hasn’t been invented yet, because there’s no one to pick up the phone.
In other words, time travel into the past is – or should I say, will be – only possible as far as the point in history at which time travel is invented, and not before.
And that’s why no one came to Professor Hawking’s party.
Further reading / viewing, which deal with plausible models of time travel, or other aspects discussed here :
Books: Altered Carbon (Richard K. Morgan) , Re:Zero (Tappei Nagatsuki), The Order of Time (Carlo Rovelli)
TV: Westworld (HBO, Seasons 1 & 2)
Movies: Bill and Ted (1989/1991), Predestination (2014)