Parallel realities, the Big Bang, a cat that's dead and alive at the same time -- physics is full of seemingly unsolvable paradoxes. Jim Al-Khalili, a quantum physicist, explores nine massive contradictions in his new book, Paradox. But here's the good part: you don't have to be a rocket scientist to enjoy his take on mind-bending conundrums. Here, he makes the case that some of the biggest stumpers in history can be good, clean fun for the rest of us.
The reason I am so passionate about science, and theoretical physics in particular, is because I want to make sense of the universe, which is full of so many remarkable, puzzling, infuriating, amazing, mind-boggling mysteries. So, to pose some of the deepest questions of reality in terms of paradoxes that just cry out for a resolution is a great way to unpick where our scientific knowledge is incomplete or breaking down. You see, I firmly believe that nature is never paradoxical -- everything should make sense. So forcing a paradox is one way of highlighting where we are missing something in our scientific understanding. In this book, I bring together some of the most famous paradoxes in physics as well as a few other lesser known but equally delightful ones. But while scientists use paradoxes for the serious business of furthering their understanding, for most people, the paradoxes in this book can just be enjoyed as brain-teasers, each of which will, in turn, be resolved (I hope) to your satisfaction.
What is a paradox?
Strictly speaking, a paradox is a statement or circular argument that either contradicts itself or leads to a situation that is logically impossible and to which people’s reaction can range from delight to bafflement to downright frustration. But there is a important difference between a true logical paradox and the myriad of scientific puzzles that have gained notoriety by being labelled as such. Take the simple assertion: "This statement is a lie." The sentence is grammatically correct and appears straightforward enough. But work carefully through its implications and you soon see it is anything but straightforward. In announcing itself to be a lie it is stating that it cannot be a lie, in which case it is true, which is to say that it really is a lie, which means it is not a lie, and so on in an infinite loop. This is a clear paradox, and there really is nothing more we can say about it since it cannot be resolved. Far more satisfying of course is when we know there is a way out.
Some paradoxes can be resolved
A perceived paradox is one that is much more interesting because it can be resolved with careful consideration. Here is a fun example well known in Great Britain: "Every Scotsman who travels south of the border to England raises the average IQ of both nations." Impossible? Well, no – albeit more satisfactorily resolved if you happen to be Scottish. The point is this: since all Scotsmen claim to be smarter than all Englishmen, then any one of them surely would enhance the average IQ of England by moving there. But to have left his beloved Scotland in the first place is deemed to be such a foolish act that he would have to be less intelligent than most Scots, and so will leave behind a slightly higher average IQ.
Many paradoxes in physics are of this type. While seeming impossible when first described, they will turn out to be missing some subtle consideration which, when taken into account, knocks out one of the pillars on which the paradox is built and brings the whole edifice toppling down. But despite having been resolved, many of such scientific paradoxes continue to be referred to as such, partly due to the notoriety they gained at the time of inception (before we had figured out where we were going wrong) and partly because they are a useful tool in helping scientists gain a better understanding of the laws of nature.
Take a simple version of the famous time travel paradox: what if you were to travel back in time and kill your younger self? What then happens to the older you? Do you pop out of existence because you stopped yourself from growing older? And if so, and you never did reach the age at which you became a murderous time traveller, who killed the younger you? The older you has the perfect alibi: you never even existed! So if you did not live long enough to reach the point in time when you travel back, then you do not end up killing your younger self, and so you will have survived to travel back in time and kill yourself, so you don’t, and so on. This would appear to be the perfect logical paradox. And yet physicists have not ruled out the possibility, in theory at least, of time travel to the past. But it turns out that this can only be achieved if our universe is but one of many parallel realities, so that multiple versions of history can be acted out.
My favorite paradoxes in physics
Some paradoxes will be familiar to many, if only in name. Take the quantum paradox of Schrödinger’s Cat, in which the unfortunate feline is locked inside a sealed box with some poison that is released if triggered by the decay of a radioactive atom. Since the rules of quantum physics state that while we are not observing the atom we are forced to consider it to have both decayed and not decayed at the same time, then so the fate of the cat must hang in an ‘in between’ state of being simultaneously dead and alive until we open the box.
My personal favorite is known as Olbers’ paradox and is posed as a simple question: why does it get dark at night? This famous conundrum was first set in the 17th century and asks why the night sky is not ablaze with light, since we should see, in an infinite universe, a star at every point in the sky, however far away it might be. Many of the greatest minds in science grappled with this puzzle until it was resolved, not by an astronomer, but by the writer Edgar Allan Poe. The reason of course is that our universe had a beginning, and therefore distant stars and galaxies are invisible to us because their light has not yet had time to reach us. So the fact that the night sky is dark turns out to be the most profound confirmation of the Big Bang itself. Isn’t that just delicious?