Quantum Fields: the Universe Throbs and Sings
If you think the idea of wave-particle duality is rather vague idea, or that having to simultaneously juggle incompatible models is rather unsatisfactory, you are in good company. This dissatisfaction led physicists to search for and articulate a deeper description of reality. Underlying these surface-level descriptions of wave and particle is the concept of quantum fields.
Fields originally arose as a way to describe how one object can affect another. For example, a charged particle creates an electromagnetic field, which mediates the electric and magnetic forces it applies on other charged particles. These fields can be made to vibrate – that’s the wave aspect – and when we apply the rules of quantum physics to fields, we find a certain lumpiness in these vibrations – that’s the particle aspect (photons).
Far more radically still, quantum physics tells us fields don’t just mediate interactions between matter, but that matter itself consists of fields. Particles of matter are themselves just the localised excitations of underlying quantum fields. Quantum fields therefore are the true building blocks of nature.
For example, any particular electron you may encounter is merely an excitation of the one electron field that pervades all of space. You and I and the rocks of the earth are just a bunch of quantized vibrations of the universal quantum fields that pervade all of space.
Now you can think of quantum fields as fields that obey Heisenberg’s uncertainty principle. Remember that, for particles, the uncertainty principle says that the more you know where the particle is, the less certain you can be about how fast it is going. For fields it is similar: the more precisely you pinpoint the value of the field at one particular point and time, the less you can be certain about how fast it is changing, and thus the less certain you can be about the value at a later time. The more you know now, the more quickly that knowledge becomes irrelevant! Trying to pinpoint a quantum field is like trying to grasp a hand full of foaming bubbles: the tighter your grip, the faster they slip away.
One implication of this is that even when there are no particles, the underlying fields are still present and active – they can never be set to zero – forming a bubbling vacuum of quantum fluctuations.
Quantum History
What is time? We experience time as a kind of movement or transition from the past to the future. What distinguishes past and future? For one thing, the future is unknown to us, whereas the past is what has already been revealed. Even if our memory of the past is patchy, we feel that, in principle, we could sift through the evidence and work out the path that history took to bring us to the present moment.
But in the quantum world this is impossible. The past remains uncertain even when the present is known. This uncertainty is not just ignorance: quantum superposition tells us that many alternate histories in a sense coexist and intermingle to produce the configurations we see in the present.
The idea of many histories is made explicit in what we call the ‘path-integral’ formulation of quantum physics. To calculate the probability of a particle moving from one space-time coordinate to another, we add together the effects of all possible paths between the two.
When they think no-one is watching, human beings can be a bit devious and perhaps try to get away with things they shouldn’t. In the quantum world, it’s not just the case that you could get away with anything, but rather you do get away with everything. In the quantum world, if no one is watching, anything can and does happen. All unobserved paths contribute to the present moment.
Quantata 