If time travel will one day be possible, why has nobody from the future dropped in? The usual answer is that time travel must be impossible. Economist Stuart Mills offers a humbler one: we may simply not be worth the trip.
Mills, an assistant professor of economics at the University of Leeds, set out the argument in “The Economics of Time Travel,” published in Seeds of Science on December 19, 2023. It is a thought experiment built on a cost-benefit analysis, and Mills is open that one of its goals is to entertain.
The party nobody attends
The paper opens with a test. Stage a huge, unforgettable welcome party for time travellers, say in Greenwich on New Year’s Eve 2023, and advertise it so well that it is remembered for all of human history. If no chrononaut shows up, the reasoning goes, time travel is not possible.
Mills argues this assumes something big: that our era is worth visiting. Earlier economic writing on time travel, much of it tongue-in-cheek, imagined future travellers exploiting cheap past labour, arbitrage or stock tips. He objects that a civilisation able to time travel would likely be centuries or millennia ahead and would have little reason to think like a 21st-century investor. Cheap labour and raw materials, he adds, would hardly be scarce for a society that has harnessed its home star.
Huge costs, a narrow benefit
On the cost side, Mills argues energy would dominate. Travel near or beyond light speed, or warping spacetime, implies vast amounts of it. He points to the Kardashev scale, which ranks civilisations by the energy they capture, and notes estimates that Earth sits at about 0.73, short of even Type I. On that basis he judges time travel unlikely to be achieved in the next few centuries without a monumental rise in energy production.
On the benefit side, the one thing the past might uniquely hold is knowledge the future has lost. Mills focuses on “system critical” knowledge, meaning know-how a future society depends on but can no longer reproduce. History offers examples: Greek fire, Roman concrete and fogbank, a classified material for US nuclear warheads whose production process was so poorly recorded that engineers spent over a decade rediscovering it. Banks running old code that few people still understand show how complex systems can come to rely on fading expertise.
Why the numbers rarely add up
Mills writes this as an inequality: a trip happens only if the energy cost is lower than the value of the knowledge, multiplied by the probability that it has been lost and the probability that it is system critical. The catch is that those two probabilities pull against each other. Knowledge a whole civilisation relies on is widely known and recorded, so it is rarely lost. Knowledge that gets lost is usually niche, so it is rarely critical. Only odd cases like fogbank meet both conditions.
He is candid about the limits. A far-future society may not run cost-benefit sums at all, and motives such as curiosity or “chrono-tourism” could justify trips his model ignores. Reviewers published alongside the paper pressed the same point, and one noted that the claims cannot be falsified.
His bottom line is blunt. If the Greenwich party comes and goes without a guest, Mills writes, it may be because “we as a civilisation are unlikely to be important enough to warrant visiting.”
Study Details:
- Title: The Economics of Time Travel
- Authors: Stuart Mills
- Journal: Seeds of Science
- Publication Date: December 19, 2023
- DOI: 10.53975/7t49-r566