Time Travel and Aliens

The Question

Why haven’t we made contact with an alien civilisation?

There may be many reasons, but the far-ranging implications for such a question have encouraged us to think long and hard on the long-term nature of civilisation itself.


Broadly, the first consideration is how common intelligent life might be in the universe. I am of the opinion that while it may not be common, in that a planet capable of supporting intelligent life is much more likely not to support it (consider that our own planet has only had a recognisable civilisation on it for maybe 10,000 years in its 4.5 billion year history), the number of intelligent civilisations should still be very large simply because of the vast (astronomical, even) number of suitable planets and star systems that exist in the observable universe.

There are also far more uncertain considerations, such as how long a civilisation can last. This consideration comes up because of our own necessarily anthropomorphic lens – we’re still at risk of wiping ourselves out with nuclear weapons, and we’re very much hurtling towards making the planet uninhabitable by us through climate change and chemical contamination of the biosphere. There are many unknowns here – is this “hump” a necessary period in advanced civilisations? Can a civilisation which doesn’t come close to destroying itself ever become advanced enough to achieve interstellar communication and eventually travel? Is the speed of interstellar travel such that the time it takes to travel to another civilisation exceeds the reasonable lifespan of a civilisation?

Lastly are the distances involved, and how they impact the nature of possible interstellar civilisations beyond the scope of simply staying alive long enough to make the journey. The variables here are great, and thus it is extremely difficult to know what the landscape (spacescape?) might look like. possibilities range from Star Trek-like interstellar governance structures with interconnected trade and occasionally warfare, which we have been deliberately excluded from because we are too primitive, to the other end of the spectrum where there are many interstellar civilisations who never connect because they exist in a “dark forest” universe where, owing to the exponential nature of progress, the only valid strategy when encountering another civilisation, no matter how primitive or advanced, is to destroy it, or else be destroyed.1

Time Travel

Daydreaming while taking a shower, I happened across another theory for why we have not yet encountered alien life. I’ve not yet come across this yet, so I present it here 1) in case someone else has, and 2) to generate discussion and general feedback on the theory itself. The theory relates to time travel, and I’m afraid it’s rather bleak.

It’s become quite the trope in SciFi for something to go “wrong” and then for the protagonists to travel back in time to fix it. This obviously works well for plot since it creates situational conflict/tension, but it’s also a little cheap because it allows a writer to credibly reset literally any situation but with the same characters – short cutting the effort needed for additional character development. A lot of suspension of disbelief is required, but since we know so little about time travel, this is not such a great logical leap as a lot of other bad writing that exists out there.

The most obvious problem is that if you travelled to the past to change events, assuming you were successful, you would then have no reason to travel back in time. This is occasionally addressed (often confusingly, like in the recent German Netflix TV series Dark), with the time traveller somehow making sure that a trip to the past still occurs (seemingly for no reason) creating what is known as a “causal loop”. Travelling back in time just to observe past events, “time tourism” if you will, doesn’t present this problem, but instead raises a different problem in that small perturbations in past events can have much bigger effects down the line (a problem which also exists in the first in the original instance but then un-exists when the reason for your travelling back in time ceases to exist, causing you not to make the trip in the first place).

How does this affect our (non) observation of alien civilisations, I hear you thinking.

The Theory

My theory is as follows:

Any civilisation which invents time travel will eventually erase itself from existence

Like all technology, it’ll start off ineffectual and likely inconsequential. Only small jumps back and forth will be possible, and with who knows what kinds of limitations. Eventually though, progress will be made, and very meaningfully-sized jumps back in time will be possible. Would they threaten civilisation? Not initially, no. But as we say in mathematics, as T approaches infinity, the probability of an extremely unlikely event approaches 1 (absolute certainty). That event is the one where someone travels back in time far enough and either deliberately or accidentally does something which wipes out civilisation before it really exists.

As T approaches infinity, the probability of an extremely unlikely event approaches 1

But surely a civilisation advanced enough to invent time travel wouldn’t be stupid enough to destroy themselves with it. As a civilisation, probably not. But as we’ve seen with nuclear weapons, you only need one crazy person to put everyone in danger, and as technology marches on, the barriers to acquiring any technology get lower, and potentially put it within the reach of more and more crazy people.

The disturbing thing about this theory, as opposed to the myriad other theories that exist which posit that sufficiently advanced civilisations will end themselves, given enough time (something that I don’t believe any reasonable person would doubt), is that the means by which a civilisation ends itself means that they don’t really have a beginning and an end, but rather are erased from the timeline altogether. i.e. To an outside observer, it wasn’t a great civilisation that came and went – it never existed in the first place.

So Those Aliens…

The implications for the universe are clear – if time travel is a technology that generally comes before interstellar travel, then chances are that no civilisation will ever encounter another one for immediately obvious reasons. Even if it tends to occur afterwards, it only needs to be discovered, then it will be as if that civilisation had never existed. When all is said and done, the only civilisations who leave any sign that they ever existed will be those who were primitive enough to have run their course without ever inventing time travel. Perhaps this has played out a number of times, even on our own planet, but we wouldn’t know because the timeline kept resetting until the only remains that survived were the magnificent, but technologically unsavvy dinosaurs (or a version of them that never developed time travel).

I suppose it is possible for a civilisation to become interstellar and never invent time travel, but I believe that achieving interstellar travel is such a potentially civilisation-lifespan-extending achievement that doing so significantly increases the odds of discovering time travel. The crux of the theory is that meaninful interstellar travel and time travel are reasonably close together in terms of a civilisation’s developmental timeline. (“Close” is a relative term here – the steam engine and quantum computers may be separated by hundreds of years in human civilisation, but that is easily within the time frame of an interstellar trip.) In fact, the discovery of time travel need only be possible for a civilisation which is capable of interstellar travel for them to be doomed.

The Elephant in the Room

Of course, it is also possible that time travel is not.

(Or that I have very significantly underestimated how difficult it might be because I watch too much Doctor Who)

Perhaps there is an implication here that since our civilisation seems to still exist (although many events in 2020 have had me questioning whether or not we are living in a simulation) that either humans are too stupid to ever eventually discover time travel (if it is possible), or that time travel is impossible and that there are other reasons we haven’t encountered any aliens.

Anyway, those are my shower thoughts. Let me know what you think in the comments.


  1. The Dark Forest theory is very well-described in a SciFi novel “The Dark Forest” by Lui Cixin, the second book in his Three Body Problem series which I can highly recommend

1 Comment on Time Travel and Aliens

  1. Hi Daniel,

    This seems quite different from your usual fare. While Doctor Who is a wonderful show to watch, it doesn’t take itself seriously, which frees it from any constraints regarding scientific accuracy.

    The physics of interstellar travel are a harsh mistress. So are those of flux capacitors. Alas, your theory is therefore destined to be true for dull reasons.

Leave a comment

Your email address will not be published.