Parallel universes, or alternative universes or mirror universes have had a long run of popularity in science fiction and science fantasy, in both print and visual formats. One need only look at an “Alice in Wonderland” or look no further than the “Star Trek” universe (our Universe in less than obvious disguise) to view the near endless plot variations that such parallel / alternative / mirror universes provide our heroes and heroines. While there are some serious reasons to suspect that parallel universes do exist, there’s another, albeit more theoretical proof of the parallel universe concept. Time travel is the name of the game!
Why go back in time at all? Just to observe? That’s okay, as long as you look and don’t touch. Just to get rich? Probably! Just to do good deeds? I’d have to advise against that. Travelling to the past from the future, armed with the knowledge of what that future holds, brings a ‘wealth’ of opportunities – accent on the wealth. I mean go back in time and there’s this new company called IBM or Microsoft just starting to sell shares. You’d make a killing! You know in advance about the Global Financial Crisis. Put that to your advantage! Knowledge is power. Maybe all our billionaires are time travellers from the future who came back to make a killing! Sorry for taking such a cynical view. Actually the danger is the ‘butterfly effect’. Go back in time, be a goodie-goodie two shoes and correct something – kill Hitler in 1935 say – and the ripple effect could have so changed the future from which you came from that you might never have existed. Say that soldier in WWII, killed by the Nazis, lived instead because you assassinated Hitler in 1935 and he married your Mum instead of your Dad. You were then never born. Or say you assassinated Hitler in 1935, and someone else took over the leadership, but that someone started up Germany’s nuclear program almost immediately. Thus, Germany had the A-bomb when she started WWII – Germany won! Of course maybe somebody else goes back in time and assassinates you just before you bump off Hitler! Lesson: You do not muck around by changing the past since you can’t predict the rippling effects on down the line. Ray Bradbury’s excellent short story (also filmed) that illustrates this point nicely is “The Sound of Thunder”.
Be that as it may, I can immediately think of four actual, possible or theoretical ways of doing the time warp – again!
The first is your tried and true traditional time travel into the future, and one which you have no control over – we all travel into the future at the speed of one second per second.
Secondly, there’s a possible albeit highly theoretical equivalent for an antimatter you. You’d have a tried and true traditional time travel into the past, again one which you have no control over, and again (presumably) at a rate of one second per second. That’s because of the idea that antimatter is just normal matter, but normal matter going backwards in time at the rate of one second per second. I don’t rate that too highly on the reality scale.
Thirdly, when you view any object, you’re time travelling back into the past. That’s because it takes a finite amount of time for light to reach you from whatever it is you’re looking at. What you actually see is how it looked that finite amount of time ago. Since you always view what happened in the past, I guess that’s a form of time travel! By the way, even if you directly touch or taste something, it still takes a finite time to travel on up the nerves to the brain and a finite time to register and be processed.
Fourthly, there’s the concept that if you travel and accelerate outwards at a significant percentage of the speed of light, stop, reverse, accelerate and come home again, when you land, you will have aged less than those stay-at-homes, or translated, you’ve travelled into the future a lot quicker than otherwise would have been the case. You and your stay-at-home twin are now both at the same point in time again; it’s just that you have biologically aged far less. That’s the ‘twin paradox’ of relativity physics, but it’s no paradox – it actually happens.
Interestingly, at the micro, hence macro levels, the laws of physics do not forbid other ways and means of time travel, for example using rotating Black Holes, wormholes, etc. While these are theoretically possible, in practice it would be difficult in the extreme, very energy intensive, and certainly way, way beyond our current, or even projected, level of technology, and may indeed remain forever out of our practical experiences and thus remain a parlour game for physics professors. But maybe, just maybe…
However, for starters, there’s a catch – there’s always a catch! Travelling backwards in time is restricted. You can’t travel back in time beyond the point where the time travel device was first constructed. If we should construct such a device in say the year 2050, nobody will be able to visit 2949 or any earlier date. Thus, since no such terrestrial device yet exists, we’re not overrun by tourists from our future. Our descendants can’t reach us. We cannot have time travelling visitors from the future who can tell us our future because they are not from our future, but from the future of an elsewhere – you guessed it, from the future of a parallel universe where a time travelling devise exists.
The reason, as I understand it, that you can’t time travel in your time travelling device before the device itself existed, is that your device isn’t a machine or a device you sit in – it’s a medium. It’s likely to a natural object, even if manipulated – rotating Black Holes or wormholes. A tunnel allows you to go from one side of the mountain to another, but in doing so you don’t take the tunnel with you. A ship travels through the water – it doesn’t take the water along with it. A tennis ball gets hit by the tennis racket; it doesn’t take the racket along with it. Instead, it relies on finding another racket in its new ‘universe’ or ‘time frame’ or court, able to return it home! You probably get the drift. That’s why you can’t go back in time further than the medium itself you used. If your mountain tunnel was built in 2009, you can’t go through and exit it in 2008! But you could go through and exit it in 2010, so you’re not of necessity stuck in the past if you get the analogy. The tunnel allows travel both ways, but just not before it was constructed. Anyway, forget your time travel movies with machines – no machines allowed. Probably the best movie to illustrate the point that the device is a medium and not a machine is “The Final Countdown”.
That of course says nothing about extraterrestrial intelligence(s) time travellers who may have had a working time travel devices eons ago, so we certainly can’t rule out, or preclude such a possibility – there’s those pesky UFOs again!
But there’s also a second catch – and here’s where parallel universes enter the picture. To avoid all those time travel paradoxes sci-fi writers love so well, if you travel back in time you also have to travel between parallel universes!
By all means go back to early April 1912 (I’m ignoring Catch #1 for sake of illustration here) and warn the Captain of RMS Titanic of what’s going to shortly happen. Assuming you convince him of the pending disaster (but more likely as not you’ll be thrown straight into the loony bin), and the disaster is avoided, you’ll have saved the ship and passengers and crew in a parallel universe – not in our Universe. Our RMS Titanic sank. People died. That cannot be changed, at least not by anyone from our Universe.
Because parallel universes don’t have to be carbon copies of ours, you might find, of course, that the ship maybe known as HMS Titan, and she’ll be due to strike a mine instead of a iceberg because the First World War started a decade earlier in that universe.
Maybe one could go back and warn parallel dinosaurs that a parallel asteroid was coming – only I doubt they’d listen (unless of course in this parallel universe they had evolved both an intelligence and technology to deal with the issue).
Or, by all means go back to 1912 and meet (and chat up) your now young and future great grandmother (or great grandfather as the case may be). You both fall in love and get married and thus your real great grandparents never meet and therefore one of your grandparents never was born and thus one of your parents never made it into this world and thus neither did you. So how can you exist therefore to have gone back in time and meet your great grandparent? (Note that this is a more humane version of the grandfather paradox where you kill one of your ancestors before they got their reproductive act together). I assume here that by doing so, that is going back in time and meeting your great grandparent, that you’re not actually going to be able to parent or father or produce your own grandparent, hence parent, hence yourself. You’re not your own great grandparent. Not in this Universe.
If you had done the above in our Universe, you’d have a paradox. You have prevented yourself from being conceived and born. You can’t be your own ancestor. There’s no paradox if you went back to a 1912 in a parallel universe. In that universe you indeed prevented your potential future existence in that universe, but not your actual existence which happened in our Universe.
But maybe you didn’t really travel back in time at all (eliminating Catch #1). If there is such a thing as a universal ‘now’, then our 2012 might correspond to another universe’s 1912 (and our 1912 corresponded to their 1812, and so on). Therefore, that you in our Universe in 2012 can perhaps see the living, albeit ghostly, image of someone/something that exists in that other universe’s 1912 (and maybe vice versa).
Now it doesn’t follow that our Universe of 2012 is actually a century older than a parallel universe whose ‘now’ is 1912. Evolution just developed at an ever so slightly slower pace in our parallel universe – that’s all.
There’s apparently yet another catch in all this. If you do time travel to a parallel universe, whether its real time travel, or pseudo and just apparent time travel, once you go, you’re stuck – there’s no coming home again. Of course you might in turn time travel to a third and fourth, etc. parallel universe, but you can’t come back to our Universe again, for that could mean one universe, our Universe, contains two copies of identical bits of matter – matter (in the form of you) has been created out of apparently nothing, in violation of basic conservation laws.
Why can’t you go home again? You could conceivably return to the time just before you originally left and therefore there would be two identical copies of you existing at the same time and vicinity. Another universe could of course contain more than one copy of you. A universe can lose matter (you) to another parallel universe. A universe therefore can gain matter (you again), but no universe can have more than one copy of identical matter. That doesn’t mean that 50 parallel universes couldn’t lose their own version of you, and all 50 versions of you ends up in say a 51st universe. There are no exact duplications, so no problems.
There’s also what’s known as information paradoxes within time travel scenarios that are also resolved via parallel universes. Say, for example, that your name is Richard Wagner (I’m sure there are lots of people in our 21st Century with that name), and you were named after the famous opera composer. And say, like millions of others, you’re a fan of Wagner’s operatic music and productions, and further say you have the ways and means to travel back in time to the 19th Century and have a desire to meet your famous namesake. And so off you go to meet the young budding opera composer – maybe get his autograph!
Upon arrival in the 19th Century however you find that there is no such person! Well, being a fast thinker ready to seize the moment, you figure you won’t be missed in the 21st Century (you’re an only child, an orphan, unemployed and unmarried). So you decide to step in and fill history’s breach and become somebody famous. You know exactly what to do, and so you become THE Richard Wagner. The operas (which you know so well) are duly composed by you and performed and all’s well with established history and the world – or is it?
The question arises, and remains, where did the operas (music, plots, etc.) actually come from? You learned and eventually knew them by heart when living back in the 21st Century, yet you apparently composed them yourself based on that knowledge in the 19th Century. Something is screwy somewhere. Original opera music and productions have apparently been written and created without any creativity or intellect involved.
Of course there is no paradox if you invoke a parallel universe. In our Universe there really was a 19th Century opera composer known as Richard Wagner, and a 21st Century enthusiast of his music and productions with the same name. The 21st Century Richard Wagner does travel back in time to a 19th Century, but a 19th Century in a universe where the composer Richard Wagner never existed. Our 21st Century fan of Richard Wagner brought our 19th Century’s Wagner’s music to a parallel universe where it hadn’t or wouldn’t otherwise have existed. So, no paradox! There was a point of origin or an original intellect and acts of creativity in our Universe and in our 19th Century.
And by the way, there are dozens of variations on this theme in the literature, especially books that highlight the paradoxes of time travel.
As you see, you can have a lot of fun with proposals that give credibility to parallel universes, which is all the better as these proposals are thoroughly grounded in hard science. Lastly, we need the concept of parallel universes to help keep our beloved sci-fi authors in a profitable job!
Summary: Because time travel is allowable in relativity physics, to avoid time travel paradoxes, one requires parallel universes to exist.