The strangest theory in the history of science is one hundred years old today. On December 14, 1900 the German physicist Max Planck informed the Berlin Physical Society of the existence of h, a new constant of nature. Planck's constant, as it came to be known, is tiny, a mere 0.00000000000000000000000000000000066, but its existence would uproot the entire edifice of classical physics and give birth to quantum mechanics. Before the twentieth century was out, h would be used to explain phenomena as diverse as the structure of atoms, life, and even the origin of the universe.
But a hundred years ago, scientists believed that they already had all the answers. Lord Kelvin, one of the greatest physicists of the nineteenth century expressed the confidence of his age, claiming that, "There is nothing new to be discovered in physics." Even Planck had been advised to choose another career. Yet, as Lord Kelvin noted, there were "two small clouds" on the horizon. One had to do with the speed of light and its dispersal by Albert Einstein led to his theories of relativity and the demise of our simplistic concepts of space and time. The second cloud involved a problem with the colour of hot objects. Classical physics predicted that they should emit lots of high frequency radiation. But they didn't. Something was wrong.
Planck stumbled upon an equation that gave the right prediction - the correct spectrum of radiation - but it held within it this previously unknown number, h. Why? It seemed to imply that the atoms inside hot matter could spin only at certain speeds and the frequency of the emitted radiation corresponded to those speeds. But this was profoundly puzzling. Atoms were thought to be like spinning tops that should have been able to spin at any speed. Planck's equation allowed them to spin only at speeds that were multiples of h. This would be like your car's engine only being allowed to revolve at speeds that are of multiples of 10 revs per second. But then how could your engine (or an atom) get from one speed to another without spinning at the in-between speeds?
Four years later and the problem deepened. Einstein showed that light energy also came in tiny h-sized packages, which we now known as photons. A further two decades of head scratching followed before the Danish physicist Neils Bohr discovered that all our descriptions of the physical world involved two complementary properties (for instance, position and momentum or energy and time), whilst his student, Werner Heisenberg, showed that our ability to measure these properties is forever limited to h-sized chunks of uncertainty. Heisenberg's famous 'uncertainty principle' prevents us from accurately measuring one property without uncertainty popping out in the complementary property. Like squeezing a wet bar of soap, the uncertainty always slips from our grasp. It is no exaggeration to say that quantum mechanics has been the most successful theory in the history of science. The equations cobbled together by the quantum architects underpin nearly all (hard) high technology, from your microwave oven to the hydrogen bomb. Scratch beneath the circuit boards of your PC and you will find Planck's constant. Peek into the lens of your CD player and Einstein's photons peer back at you.
But what does it mean? This is the question that troubled Planck in the winter months of 1900 and a century later, scientists still can't agree on the answer. At its simplest, h is telling us that we cannot divide the world into packages smaller than h. That, in itself, wouldn't be too revolutionary. We could simply equate the smallest packages with 'fundamental particles' and relegate them into the dark interiors of matter. The problem is that these h-sized packages can be big. In 1935, the Austrian-born physicist Erwin Schrödinger let the cat out of the quantum bag by proposing a simple thought experiment.
Schrödinger placed a metaphorical cat inside a box with a 'diabolical device' that contained a vial of poisonous gas. Release of the gas was triggered by a quantum event; say the radioactive decay of an atom of radon to yield an atom of lead. Here, the quantum uncertainty pops out in the time it takes for the atom to decay. It could be a minute, a day or a year. If during the course of the experiment the diabolic device detects the decay event, then the poison is released and its curtains for our feline friend. But if the radon atom refuses to budge then the cat enjoys another day curled up between Professor Schrödinger's slippers. Quantum mechanics insists that before the system is examined, the h-shaped package of uncertainty includes both the radon atom and its decay product, the lead atom. But remember the fate of the cat is entangled with the state of the atom. If the atom isn't allowed to decide whether or not it has decayed, then quantum mechanics must be similarly ambiguous about the fate of the cat. Our feline heroine, tied to the uncertain atom, is left in the perplexing condition of being both alive and dead, at the same time. Clearly cats can't be both alive and dead but equally clearly atoms can. Thousands of experiments have demonstrated that atoms and other simple particles do indeed exist in multiple states. Between the atom and the cat, something has to give. What that something is, has been the subject of one of the great scientific debates of the twentieth century (that looks to continue into the twenty-first).
The architects of quantum mechanics sacrificed objective reality. In the 'Copenhagen School' (named after the Institute of its mentor, Neils Bohr), nothing is real until it is measured. The atom, the cat and the diabolical device simply don't really exist until someone peeks into the box and makes them real. And if you find that weird, try the 'many world's' interpretation. Both the atom and cat are now allowed to exist in their dead and alive states, but each state inhabits a separate universe.
Many worlders believe that the universe is split into parallel universes whenever a particle is forced to make a decision. Our world becomes only one strand within a vast 'multiverse' of every possible reality. And if that wasn't enough, quantum mechanics also removes causation from the world. Consider our undecided atom. Say it does decay. What caused that decay? In Lord Kelvin's world, there would have been a cause: perhaps some tiny atomic vibration. But the uncertainty principle denies the existence of any influence smaller than the atom's own h-sized chunk of reality. When the atom decays, it just does; entirely randomly, entirely without cause. The decay event just pops out of the h-shaped atomic uncertainty. And the absence of causation isn't limited to the world of atoms. Everything we see ultimately depends on quantum events that have popped out of h-shaped packages of uncertainty. Quantum mechanics offers us a world without reason. Einstein could never abandon causation ("God does not play dice") and spent most of his later career trying to find a way out of the quantum world.
But quantum mechanics prevailed, even against Einstein's intellect, because of its vast explanatory power. Quantum mechanics accounts for phenomena from the hardness of metals to why the sun shines. It explains things that you probably didn't think needed explaining, like why you don't fall through the floor. In my book, 'Quantum Evolution' I propose that quantum mechanics was also the missing ingredient in the primordial soup that sparked the origin of life. Cosmologists believe that h can even account for the origin of the universe. Heisenberg's principle tells us that even empty space contains enough uncertainty to include the possibility that there is actually something there. In quantum mechanics, matter and energy simply pop out of the h-shaped packages of uncertainty that exist in empty space. And that's precisely how quantum cosmologists describe the big bang, as 'a random quantum fluctuation from nothing'. So on h's birthday, we should remember that good things come in small packages.
Johnjoe McFadden