by Tim James
Couder built a double-slit experiment in a water tank and put tiny droplets of oil onto the surface to see how they moved. The oil droplets represented particles and the ripples in the water were the guide wave.
Couder reported that the oil droplets followed the landscape of the water waves and accumulated at the other end in clumps, just like photons and electrons did.2 Was it possible that particles had locations after all, and they were just riding guide waves like classical objects?
It was an exciting conclusion and seemed to finally knock the Copenhagen interpretation off its throne, but it was too good to be true. Further research by physicists John Bush and, appropriately enough, Niels Bohr’s grandson Tomas, failed to recreate the results,3 and they concluded that Couder had made a few honest mistakes in his setup.
If you try recreating the double-slit experiment with actual particles on actual waves, you get classical results and no zebra stripes. You cannot explain the results of the double-slit experiment using Bohmian mechanics unless the guide wave is some special kind of wave that behaves in a non-classical way. That might be the answer, of course, but all that does is shift the weirdness away from the particle and stick it into a guide wave which, conveniently, we can never observe.
THE HANDSHAKE
The transactional interpretation, which we will look at next, wipes the floor with the Copenhagen interpretation and sticks a finger up at common sense, but it is deliciously cool so it is worth having a look.
This time the original idea came from Richard Feynman, who pointed out that at the quantum level physics works just as well forwards as backwards. A particle moving to the left is the reverse of a particle moving to the right and both processes are equally permitted. Particles do not have a preference for which way time is flowing. An electron emitting a photon can be viewed just as neatly in reverse as an electron absorbing a photon. Both are equally real events.
Fifty years later, the physics professor and sci-fi author John Cramer decided to take Feynman’s idea and move forwards with it. And, I suppose, backwards as well.
A particle’s behaviour is described as a wavefunction but, remember, it has to be multiplied by itself to get an answer. Cramer wondered if the reason we need two identical wavefunctions is because there genuinely are two of them but we only see one because its partner is moving backwards in time.
Let us say our particle is coming up to a double slit. Its normal wave-function (unfortunately called the ‘retarded’ wave in the literature) moves forward through time, scoping the possible paths it could take. But at the same time(ish), particles in the detector screen are sending backward wavefunctions (called ‘advanced’ waves) towards our particle from the future. Whichever future particle happens to send the strongest backward-time signal is the one with which our particle ends up interacting.
Cramer imagines it like a business exchange in which a particle emits an offer and the detector emits a confirmation. The particle approaching the slit and the particle in the detector synchronise wavefunctions in what he calls a quantum handshake, leading to an entanglement between past, present and future.
Delayed choice quantum eraser experiments, those weird ones where Alice knows what Bob is going to do in the future, are suddenly easy to explain. A particle can tell whether it is going to be detected in the future because it gets a message from the future telling it so.
Interestingly, Cramer has said he does not think his interpretation gets rid of human free will.4 He uses the analogy of a debit card being used to pay for something in a supermarket. The offer wave is the card and the confirmation wave is the bank, but you still get to choose what to buy.
You could, however, make the counterpoint that when you thought you were choosing to buy something such as almond milk, you were actually getting signals from those almonds in the future telling you what to do. Perhaps every decision you think you make is actually the result of a future event guiding your present life choices. Did you really choose to buy this book or was I influencing you from this chapter? WoooOOOOoooOOOooooOOOO (spooky ghost noise).
AND I WILL ALWAYS LOVE HUGH
The same year David Bohm published his guide-wave theory, Erwin Schrödinger was giving a lecture in Dublin on why he still did not accept Copenhagen. In this talk he explained that although he would sound like a lunatic, his equation was never intended to describe a situation where particles chose a property when measured.5
In a superposition, two sets of properties hover around the same particle, that much is known. But where do we get this idea that we have to chuck one of the properties away the moment a measurement is taken? There is nothing in the equation that says that has to happen. In fact, if we take the equation (which always seems to work) literally, it tells us that after a measurement both outcomes are still there, even if we cannot see one of them for some reason.
The Schrödinger equation is a smooth one which describes things evolving gradually, but when a particle interacts with a detector, Bohr insisted we suddenly switch physics and start using chunky particle equations instead. Why should that be?
If we trust the Schrödinger equation it says nothing of the sort. All outcomes happen and there is no such thing as wavefunction ‘collapse’. When we measure an electron as up-spin, the down-ness still exists somewhere. We just have to figure out where it is hiding. Enter Hugh Everett III.
A chain-smoking science-fiction obsessive, Everett had a Bachelor’s in chemistry and a Master’s in mathematics, and decided he should also get a PhD in physics to complete the trio. Under the supervision of John Archibald Wheeler, Everett was charged with trying to come up with a new version of quantum mechanics that did not involve probability.
Everyone in the Copenhagen club was enamoured with randomness, but Wheeler wanted to fight back so he put the challenge to his brightest student and was not disappointed. Everett came up with an answer that not only did away with chance, it also solved the measurement problem.
When a particle is measured in an experiment, all the possible outcomes are actualised. The one we observe gets recorded in our lab book but the other possible outcomes are still there. They just exist in different universes.
Everett figured that superposition was the main headache of quantum mechanics so he eliminated the idea by having overlapping realities instead of overlapping properties. When a particle is offered a choice, the universe splits and each choice is taken in parallel by a different parallel version of the same particle.
A superposition was not really a particle existing in contradictory ways, it was whole universes sitting on top of each other like images on tracing paper.
Provided a particle does not entangle with its environment too much, all the universes involved in a quantum experiment will remain in contact. But as soon as an entanglement takes place (with a detector screen, say) they diverge and become independent realities.
In one universe a particle could go through the left slit and in the other universe it could go through the right. Both outcomes remain mixed together, giving us an interference pattern mid-air, but when the detector screen is reached, every version of the particle hits the screen in a different place – each in its own world.
If you measure the slits with your ‘camera’ you are not really finding out which slit the particle chooses, you are learning which universe you are in. So when you measure a particle coming through the left slit, a parallel version of you is also measuring it through the right. In Everett’s view, known as the ‘many worlds interpretation’, we no longer have to deal with probability and measurement. We just accept that we are seeing a single slice of a bigger universal cake.
Imagine that 40 per cent of parallel universes have a particle destined for the central stripe of our zebra pattern. Logically, you have a 40 per cent chance of being in one of those universes but you do not know for definite at the start of the experiment where you are. All you can say is there is a 40 per cent chance of the particle hitting the centre of your screen.
The electron is not deciding where to go at random at all. Parallel electrons are going everywhere at the same time but because we only see a single page of the options it looks like an unpredictable outcome. This is a lot more elegant than the Copenhagen interpretation because we do not have to chuck out half the answer for no reason. We simply acknowledge that it is happening in another plane of reality.
THE CAT FINALLY LIVES
This is good news at last! When your cat is in a superposition of dead/alive that really means it is alive and dead in parallel universes. If you happen to open the box and find a kitten stew, you do not need to be sad because an alternate version of you is finding the cat alive and well.
The many worlds interpretation also accounts for the EPR paradox while allowing special relativity to survive untarnished. When two entangled particles are sent to opposite ends of the solar system their properties are pre-decided, just as Einstein believed, but there are two universes with opposite outcomes.
In one universe Alice is up and Bob is down but in the other universe, Bob is up and Alice is down. Prior to measurement we do not know which is which, so we consider the outcome random but that is because our universes have not decohered yet. When we take a measurement and find ourselves to be in the Alice = up universe, both universes split, meaning the universe where Alice is down and Bob is up gets carried away somewhere into the multiverse.
Nobody knows how this splitting works but it seemingly occurs every time a particle is given an option about what to do. In Everett’s opinion, measurement was not the issue, choice was.
Right now, as you sit reading this, particles in your body are being given choices about which way around their nucleus to vibrate. In one universe they pick one direction and in another universe they pick the opposite. If you consider how many particles there are in the universe now, and how long it has been around for, the sheer number of choices that have been made is so enormous it is impossible to even give it a name.
Every nanosecond your body, just by existing, is causing trillions of universes to tear away from each other and decohere for ever. The number of parallel realities is so big nobody has even attempted to do a calculation on how many there would be today. Which brings us to…
THE MOST IMPORTANT PART OF THE BOOK
Over the years such luminaries as Richard Feynman and Stephen Hawking expressed their support for Everett’s many worlds interpretation6 but, when it was unveiled, Everett was met with ridicule and chose to abandon the whole thing to work for the intelligence division of the Pentagon.
When he died, he insisted his ashes be tossed into the garbage because he was completely unsentimental7 and, besides, even if he died in this universe he was still alive in numerous others.
We can make that statement because every pathway gets chosen in at least one reality, so different brain processes and events get picked in different worlds and thus every parallel universe has its own version of history. Anything you can imagine will probably have happened somewhere.
The laws of physics would still hold true, of course. There will not be a universe where human skin is made of clouds and frogs glow when you whistle at them. But provided you stick to the standard rules, everything is happening at least somewhere.
There is a universe where America did not win the War of Independence. There is a universe where the Berlin wall is still standing. There is a universe where the Osmonds stuck with rock music after ‘Crazy Horses’ and never returned to sugary love ballads. And, most crucially of all, there is a universe out there, far into the distant reaches of multi-reality, where I, Tim James, am Batman.
DECISIONS, DECISIONS
The debates rage. At the time of writing there are no experiments that distinguish one interpretation of quantum mechanics from another, so none of them have much right to call themselves definitive.
The Copenhagen interpretation was the big tamale for most of the twentieth century and is still the most popular. But it is also the most infuriating since it requires you to add extra stuff and accept several things on faith.
The de Broglie–Bohm interpretation is the most mathematically complicated and forces us to include hidden variables and hidden guide waves, but if true it would give us a classical explanation for the measurement problem and that is enough to keep it in the loop.
The transactional interpretation is unique among all interpretations because it is the only one to explain why the Schrödinger equation needs two wavefunctions rather than one, but it does not predict a universe where I am Batman so we should reject it on those grounds alone.
The many worlds interpretation does not require you to add anything new to the equations (just new universes) and is by far the most elegant. But is it too much to stomach the idea that all of existence can be splitting constantly into parallel versions?
Picking which interpretation is true is not yet possible. We have equally valid hypotheses so endorsing one over another is a matter of preference and nothing more. But that does not mean we cannot be allowed to have them.
In 2013 Maximilian Schlosshauer asked a group of thirty-three quantum physicists at a convention which interpretation they favoured.8 Fourteen picked Copenhagen, six went for many worlds (in this universe at least, in another they chose differently), nobody went for transactional or de Broglie–Bohm and a few voted for options we have not covered.
Four people gave no answer and maybe they were the purest scientists of the bunch. After all, if a question is raised for which there is no evidence one way or the other, the honest thing to say is ‘I do not know.’
But, as Isaac Asimov once pointed out, humans are emotional beings as well as intellectual ones9 and, provided we are not committed to saying our choice is absolute, there is no harm in having a favourite. So, pick whichever one makes your brain hurt the most and go with that.
CHAPTER ELEVEN Far Afield
TO ASK A DIFFICULT QUESTION
We have been using the word ‘particle’ throughout this book with casual abandon so far. But it is time we got specific and nailed things down. If we are going to talk about particle physics seriously we need to decide what exactly we mean by a particle.
The neatest definition physicists sometimes use is ‘something which holds itself together and does not fall apart spontaneously’. Your body is a large particle in this sense because your arms do not detach at random (usually) so you fit the definition.
Your body is a composite particle, of course, because while it tends to stay together it can be broken into smaller particles – your organs – which also hold together. Organs can be broken down into particles called cells, and then they can be broken into molecules, then atoms and finally protons, neutrons and electrons.
But an electron, as far as we can tell, is not composed of anything smaller. It is not made of sub-electron particles, yet holds together. It is a very special type of particle, distinct from atoms, molecules and cells. It has no substructure and is therefore truly fundamental.
FANTASTIC MR FARADAY
The story of fundamental particles starts during the 1800s with the greatest scientific showman of the age, Michael Faraday. Faraday had grown up the poor son of a blacksmith but believed science should be made accessible to anyone curious enough to pursue it. To that end, Faraday began delivering public talks on science at the Royal Institution, dazzling his audiences with chemical reactions and physical phenomena, and it was at these lectures that he first revealed the discoveries he had made about magnetism, a force like no other.
Magnetism can act through a vacuum and attract or repel other magnets without anything in between to communicate the signal. Magnetism can also work through solid barriers, which is peculiar as all other forces have to be in direct contact with whatever they are pushing or pulling.
Today nothing about magnetism seems remarkable because we live in a world of mobile phones and wi-fi routers, but in the 1800s the notion of causing an effect on something without touching it was tan
tamount to witchcraft. In order to explain how magnets could do their thing, Faraday urged people to think of magnetism as being made from a ‘field’ rather than matter itself.
Every magnet creates an invisible distortion in the geometry of the space surrounding it, which other magnetic objects can lock onto. These regions of distorted space dictate the behaviour of particles moving through them, but are not themselves composed of matter.
A field is therefore a non-material, fluid-like substance that tells particles how to move as they pass through. It can be stronger in some places than in others, but you cannot see it directly, only its influence.
The idea is a bit of an eyebrow raiser because we are used to stuff being made from other stuff, and a thing existing without being made of stuff at all is not easy to stomach, but that is the way a field seems to be. Empty space, it would seem, can have properties.
Which reminds me of a joke: did you hear about the scarecrow who won a Nobel Prize? He was out-standing in his field.
I am not going to apologise for that joke, just as Michael Faraday made no apology for introducing fields to physics. There is just no way of accounting for things such as magnetism, electric charge or gravity without fields. And what’s more, fields can talk to each other.
ONE FIELD OR TWO, VICAR?
If you take a magnet and waggle it up and down you create a vertical ripple in the magnetic field, but that is not all you create. Faraday discovered that in doing so, you simultaneously generate a disturbance in the electric-charge field, at right angles to the original magnetic disturbance.
This electric field ripple kicks back against the magnetic field, however, which returns the favour by shaking the electric field once again, which agitates the magnetic field in reply, back and forth indefinitely.