Have you ever met a time traveller? I haven't. Or at least, not one who identified themselves as a time traveller. Why haven't we met any time travellers? Is it because time travel is impossible? Or, given the many choices of time period where a time traveller might choose to go to, perhaps our time just isn't worth visiting? Perhaps we just aren't that interesting or important to people from the future. These are the questions that Stuart Mills (University of Leeds) grapples with in this recent and interesting article published in Seeds of Science (open access). Mills' argument is simple, and summarised at the end of the introduction:
I argue the main economic benefit which our descendants may receive via time travel is knowledge which we currently possess, but they have lost. Furthermore, this knowledge must be sufficiently critical to our descendants to justify the costs of time travel, which are likely to be dominated by energy costs. I posit that even assuming the energy requirements for time travel are met by a human civilisation in the future, it is highly unlikely that that same civilisation will come to depend on a piece of knowledge which we currently possess, but they have lost and cannot rediscover by other means. In other words, I argue that even assuming time travel is possible, our epoch is unlikely to offer any economic benefit to a future, time travelling civilisation.
In other words, our time simply isn't worth travelling to. Mills supports this argument with a rough cost-benefit analysis. The costs and benefits are largely unknown, but the point is not to perfectly evaluate a benefit-cost ratio for time travel, but to establish an explanation for the empirical observation that time travellers have thus far never been observed. As Mills notes:
...the reader is encouraged to regard the given inequality as a proposal along the lines of the Drake equation for estimating the number of intelligent civilisations in the universe—not necessarily accurate, but sufficient for provoking some thought and discussion.
The paper is interesting, but I wonder if the premise itself is faulty. Just because time travellers have not been observed, that doesn't mean that there have been no time travellers. It just means that any time travellers have not been observed. A civilisation that is advanced enough to have developed time travel is almost certainly also advanced enough to have developed some form of cloaking technology, such that time travellers can avoid detection. Paraphrasing the American astronomer Carl Sagan, we could be over-run with time travellers, and we wouldn't even know it.
To be fair to Mills, Footnote 1 in the paper does make note of a lot of reasons why we might not observe time travellers, but misses Sagan's suggestion. To some extent, I found that footnote to be one of the highlights of the paper, especially the seventh reason (which is something I have wondered about on occasion):
Seventhly, time travel may only affect time, not space. As the Earth is constantly moving around the Sun, and the Solar System shifting around the galaxy, and the galaxy moving throughout the universe, a time traveller may very well travel to attend the party, only to find themselves on the opposite side of the universe.
Time travel is an interesting problem to ponder. I suspect we don't know enough to really answer this question yet. Mills has provided a starting point, but there are a lot of unjustified assumptions, and alternative assumptions would likely be equally unjustified. If you are interested in time travel, you should read the article. But also, read the comments at the end of the article, and Mill's response.
[HT: Marginal Revolution]
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