05/31/2008 05:26 pm ET Updated Nov 17, 2011

No Going Back

Beautiful mind, Stephen Hawking. Saw a question from him the other day - if there is life elsewhere in the universe, why haven't we stumbled across alien broadcasts by now? An alien "Wheel of Fortune" comes to mind, for example, or a radio soap opera starring lovelorn little green men and women. The implication of his question, if I may be so bold, is that there are no other intelligent life forms in the universe. Although I suppose you could argue that an intelligent life form would have better things to do than watch tv quiz shows or listen to radio soap operas, so perhaps the lack of such broadcasts simply means that we are the dumbest intelligent life form in the universe.

A similar question arises from the lack of space travellers arriving on Earth (except at Roswell of course). I can't remember if this was Hawking, or someone else, or me, but this lack of flying saucers also suggests that faster than light travel is not just theoretically but practically impossible, otherwise they would have been here by now. Or it may just mean there is a sign on the space lane to Earth warning travellers "Wrong way, go back".

It was certainly Hawking though who also pointed out another piece of evidence about space time mysteries - there are no people travelling back from the future. We don't get time travellers appearing in our major cities shopping for future antiques in Ikea stores, or sending our bookmakers bankrupt with certain knowledge about the winners of major horse races. Time travel seems to be theoretically possible, but the absence of arrivals from the future suggests that it is not practically possible, and has never been invented any time in the future.

In a sense these ideas are based on the old "mark and recapture" technique, one of the first things young ecology students are taught about field research. The idea is so simple that it makes you (well, it made me) want to hug yourself with delight at the sheer ingenuity of it. Want to know how big a population of wild animals is without the impossible task of counting them? Simple, catch a few, mark them in some way that doesn't adversely affect them (for example rings on the legs of birds) but lets them be instantly recognized when seen again, and then let them go. Let some time go by, and then go out and catch some number of animals, say 100, from the population. Count how many of the 100 have your mark on them. Then the proportion of marked animals in the sample is the same as the percentage of marked animals in the total population. You know how many animals you marked and released, how many you caught, hey presto, solve an equation with three known figures and one unknown and you can calculate the total size of the population.

So Hawking's thought experiment is equivalent to seeing whether space travellers can be "marked", and the absence of sightings means either they can't be, or they don't exist, there is no population of aliens elsewhere in the universe to be marked. In the case of time travellers, Hawking assumes that the absence of strangely dressed lottery winners in our street means that they can't be "marked", time travel simply can't be invented and never will be. But, just as with alien travellers from space, there is a second possibility. We know time travel is theoretically possible, and therefore should be invented by our clever descendants. So what if the absence of time travellers means that there are no people in the future of the planet?

Climate change denialists have lately taken to pretending that the Earth has started cooling. They have continued to claim that two lines trending upwards for fifty years have no implications for the future. I have taken to wondering how long the trend line has to be for these people to admit that the globe is warming as a result of the CO2 being poured into the air by their industrialist friends. At what point do they stop pretending that the modelling that projects that trend forward 10, 50, 100 years has no relevance because we don't know what the future holds (being unable, as they wrongly claim, to predict the weather tomorrow)?

Well, what if the lack of time travellers tells us that the denialists achieved their aim and prevented action on climate change for so long that the upward graph began to accelerate as tipping points were achieved, and that the next big extinction event on the planet included Homo sapiens? What if Stephen Hawking's observation doesn't show that time travel wasn't achieved, but that human wisdom wasn't? Not enough people saying "Wrong way, go back".

Big responsibility, being a denialist, but they have broad shoulders. And hey, the future is bunk, as Ford said.

There is a future (and not a bad past) on the Watermelon Blog, but for how long?