Is time travel possible? Image Source: Just 5 Minutes.
In his Discovery Channel series, Into the Universe, Stephen Hawking recently hypothesized that if time travel to the past is possible, then he could throw a party to welcome chrononauts from the future. He would publicize the invitation, and wait. Daily Galaxy provides a transcript of his comments on the result: "Let's imagine I'm throwing a party, a welcome reception for future time travellers. But there's a twist. I'm not letting anyone know about it until after the party has happened. I've drawn up an invitation giving the exact coordinates in time and space. I am hoping copies of it, in one form or another, will be around for many thousands of years."
Hawking's party for time travellers. Image Source: CommunitiesCanada.com.
Hawking continues: "Maybe one day someone living in the future will find the information on the invitation and use a wormhole time machine to come back to my party, proving that time travel will, one day, be possible. In the meantime, my time traveller guests should be arriving any moment now. Five, four, three, two, one. But as I say this, no one has arrived. What a shame. I was hoping at least a future Miss Universe was going to step through the door."
Why would the time travellers not show up? Hawking argues that it is due to something called the Grandfather Paradox. Basically, time travel backward is not possible because a man travelling to the past could kill his grandfather, thereby eliminating the birth of one of his parents, and hence of him. Hawking thinks that this logical fallacy forces a glitch in any system - a sort of ghost in the machine interferes - to prevent the time paradox from happening. This is the case, despite the fact that Hawking claims that theoretically near-instantaneous time travel from one point in time and space to another is conceivable through a manipulated wormhole. This Space Trek-like warp is discussed and explained in relation to Hawking's series when it was first broadcast in May of 2010, here, here and here.
Image Source: Daily Mail.
Yet in attributing the glitch to the Grandfather Paradox and his associated expectation of the failure of backward time travel, Hawking casually mentions another potential 'ghost in the machine.' In describing the prevention of the paradox, Hawking assumes that time, the entire Fourth Dimension, must follow the rules of cause and effect. Incidentally, the principle of causality underpins the entire conception of western civilization, so it's interesting that Hawking has run headlong up against that brick wall and steadfastly backed away from it.
Having decided that a wormhole of the kind that would enable time travel to the past is impossible, he hypothesizes that what would make it not work is radiation: "It just doesn't make sense. It's the sort of situation that gives cosmologists nightmares. This kind of time machine would violate a fundamental rule that governs the entire universe - that causes happen before effects, and never the other way around. I believe things can't make themselves impossible. If they could then there'd be nothing to stop the whole universe from descending into chaos. So I think something will always happen that prevents the paradox. ... And in this case, I'm sorry to say, the wormhole itself is the problem. In the end, I think a wormhole like this one can't exist. And the reason for that is feedback. If you've ever been to a rock gig, you'll probably recognise this screeching noise. It's feedback. What causes it is simple. Sound enters the microphone. It's transmitted along the wires, made louder by the amplifier, and comes out at the speakers. But if too much of the sound from the speakers goes back into the mic it goes around and around in a loop getting louder each time. If no one stops it, feedback can destroy the sound system. The same thing will happen with a wormhole, only with radiation instead of sound. As soon as the wormhole expands, natural radiation will enter it, and end up in a loop. The feedback will become so strong it destroys the wormhole. So although tiny wormholes do exist, and it may be possible to inflate one some day, it won't last long enough to be of use as a time machine. That's the real reason no one could come back in time to my party." You can see videos from Hawking's episode on time travel here. In my next post on this topic (going up on December 9th), I'll blog about the logical principle of causality that Hawking takes for granted to motivate his ghost in his machine, and ask how that principle shapes our understanding of time.
See my earlier posts on Time Travel.
See my posts on Space Exploration.