Dark matter is tourists

Yes, the return of Batcave. This one is pretty technical, comes from spending too much time discussing the physics of Interstellar with serious physicists and so on, but persevere, dear reader, if nothing else it might help your insomnia.

I was at a summit/workshop for people in the pensions industry. It was astonishing. The 50 or so people in the room between them managed about 100 billion pounds in assets, but they were surprised, even stunned, by my description of changes in life expectancy over the last 100 years, and some modest (OK, wildly speculative) extrapolations of those changes into the future.

The gist of my talk was that average age at death has been increasing steadily since ~1920 for adult, middle-class type people, and there was no reason to think that it was going to stop. This can therefore be extrapolated to when someone dying in 2100 is born, and depending on the model you use this could be 2020, 2000 or 1965. Wow. Yes, you over there, and you, (not me) might live that long.

But that is not the really cool bit. Well, it is, but the really cool bit are other extrapolations. If this goes on, how long before we live long enough for an astrobiologist like me to travel to the nearest star? Well, rather depends how fast you travel, of course - if we could travel at 20% the speed of light then a PhD in astrobiology today could set off for Alpha Centauri and get back in time for their retirement party. So how fast is travel?

That turns out to be a more complicated extrapolation, as 'top speed' has gone down as well as up. Consider Concorde vs the Airbus 380. The returning Apollo astronauts vs the ISS. But pondering this, it occurred to me that an Airbus 380 is a lot bigger than Concorde. Maybe we put ever larger amounts of energy into transport, but we make different decisions in different epochs as to whether we chose to move a small number of people very fast (Concorde, Apollo) or a larger number of people more slowly (380, ISS), depending on tedious things like economics. So what if we plot the energy of transport with time?

So I am doing that, and bear in mind that this is a work in progress, but here is a plot of UK and Soviet military planes by energy [(max speed)2*take-off mass] vs time. It is quite amazingly regular. I am bringing this up to date with commercial planes, rockets etc., so watch this space. Shipping I expect to follow the same pattern. Tea clippers were as fast as a modern container ship, but much, much smaller. And so on.

But here is the cool bit. This is an order of magnitude increase in the energy in a single vehicle every decade. What if it goes on like this? Unlike the aging curve, there is no reason to suspect a limit. Or is there? How much energy is there to put into travel? If we go faster and faster, by the year 2200 a billion tonne aircraft will be able to go at 99% of the speed of light. I am not sure the Saudi oil reserves will take that sort of consumption. By 2350 a spacecraft the mass of the Earth will cruise from ... well, it is not quite clear where it will cruise from and to, but it will do it at 98% lightspeed. Or to put it another way. if current trends continue, the entire mass of the solar system will be converted to energy and used to move tourists around by about the year 2500.

If one takes the premise of astrobiology seriously, then we are not the only intelligent, Airbus-380-building beings in existence. All it would take is for one other intelligent, jumbo-jet- building, tourist-industry-running civilization to have arisen in our galaxy in, say, the past ten million years ago, and by now virtually all the mass of the galaxy would be tied up in relativistic travelers hurtling around at a hairsbreadth of the speed of light, watching bad movies and eating pre-packaged food and wondering when they were going to get to the galactic core for all the fun nightlife they hear goes on there. The same is true of any galaxy, of course. The same must logically apply to the whole universe, if we really believe that there is a possibility that life could arise elsewhere.

And the evidence is there, ladies and gentlebeings! We look into the night sky, and we see galaxies rotating as if they are permeated by unseen masses which the astronomers, superficial, unimaginative creatures that they are, call 'dark matter' and postulate must be dark particles of some sort. They speculate on the properties of these particles based on the calculation that they are mostly confined to galaxies. But we know better! They are the relativistic spaceships of a trans-galactic tourist industry at full throttle, arcing round for a quick trip to the Duty Free in the Orion nebula before a weekend break (relatavistic time - 50,000 years in our rest frame) in the Lesser Magellanic Cloud. The lights in the sky may be stars, but the darkness is tourists. Let's hope accidents are not too frequent. A collision at such speeds would cause not just a regrettable loss of life but a regrettable loss of everything within several trillion kilometers.

When does longevity come into this? Well, unlike normal air travel, speeding up and slowing down does take a while unless you want your passengers to be delivered to the sun-kissed beaches of Tau Ceti III as jam. We are talking cruise ships here rather than day hops to Titan.

One could even consider that Dark Energy, that unknowable force that is expanding the Universe, is the tourist industry pushing out into new markets, but that is just silly. Let's stick to the facts, entities of all genders, stick out our thumbs in true Hitchhiker mode, and hope that the passing HyperMegaSuperJumbo brakes before trying to pick us up.