Thought I'd take a short break from editing to post about one of the many time travel paradoxes out there. I find these things fascinating by the way, even though they fairly much establish that no meaningful form of time travel will actually be possible.
This paradox which I encountered through my time on the philosophy forums, isn't the classic killing your grandfather one, or the butterfly effect. It's the violation of the laws of conservation of mass and energy. (Sounds boring doesn't it - but it may still be the most telling damnation of time travel of them all.)
In essence we have all heard the old saw, energy can neither be created nor destroyed only transformed. And of course the same applies to matter, and thanks to Einstein we now know that matter can be converted to energy. Regardless what all of this means is that if in the universe there are x tons of matter and y joules of energy, the total of the two cannot change. There will always be x plus y in total. (No maths I promise.)
Time travel violates this law directly. In essence it says that if you live in a universe with x plus y and someone goes back in time, then suddenly you have less matter and energy in the present, and more in the past.
Ok, so that's the boring stuff part of the paradox. It seems minor even if it breaks the law if an eighty kilo man with so many joules of energy suddenly vanishes from now and arrives a hundred years ago. It's just a law isn't it? Well no.
To understand this problem let's look at it in an extreme example. So say we have one destination in the past where people want to go, and of course the crucifiction stands out for both the religious and the non. Now I get in my machine and travel back two thousand years. A little more then eighty kilos (I refuse to say how many more) of me leaves 2012 and arrives at year 0 AD. And of course because I've learned my ancient tongues and am wearing the right clothes no one notices me. Harmless right?
Not.
I go back from 2012. But so too does someone from 2013, and someone from 2014 and someone from 2015 - I think you get the picture. Now assuming that mankind continues sending one person back to Calvary every year and our civilisation survives another thousand years, that's now another thousand people wandering around that hilltop, all trying to pretend to be locals. But these as you will guess are extremely conservative figures. Assume now that once the machine is invented more then one person wants to go back each year, and then allow for the fact that the Earth will hopefully survive, with people on it, though they may not be entirely human, for the next billion years, and they all want to visit that one hill at that one time.
That's hundreds of billions of people, and getting back to the conservation laws, hundreds of billions of tons leaving the future one by one, and arriving at one time and place. A hundred billion tons is a small mountain, and the heat energy alone given off by so many people - well have you ever been to a rock concert in the cold night air and felt somewhat warm? Under that sort of weight the ground itself would start to fail and if they all jumped at once, it would shake too. If they cried out, the sound would be deafening. In short it would be a disaster.
But now take a more extreme example. The big bang. Scientists tell us that it began very, very quickly, a matter of a few seconds. And from it we know the entire universe was created. So now we have an event that people from every race and every world over all say thirty billion years of the life of the universe would want to visit. (In their spaceships naturally since there would be no air.) Can you imagine what the effect of trillions or more, spacecraft and passengers all arriving at one spot at the exact same moment would be?
I can't. Unless of course that was the cause of the big bang itself?
Anyway, just some random thoughts. I should get back to editing.
Cheers,Greg.
