Recently, physics twitter exploded with a seemingly innocent question, one that physicists should be able to answer easily: are electrons real?
The electron has been with us as a concept since the early 20th century. J. J. Thomson won the Nobel prize for proving that electrons are particles using cathode ray tubes, the very same technology that once powered television sets. Thomson was able to calculate the charge-to-mass ratio of the electron using the degree of deflection achieved with a particular electric field. Millikan’s oil drop experiment later measured the electron charge directly.
These experiments showed that electricity came in integer multiples of some value, which is a smoking gun for a particle.
Later quantum field theory would incorporate the electron and explain how it could be both a particle and a wave. Dirac, who came up with the theory, also inadvertently predicted the positron, a particle that, until then, no one had even conceived of.
Since then, electrons have become part of our popular lexicon. If they are not real, then, how do you explain all the experiments that show that they are real? Isn’t experiment the sole arbiter of truth?
We can invoke a number of anti-realist arguments to suggest that electrons are not real but only at the expense of questioning whether anything is real. At some point, we end up at Descartes’ famous line: I think therefore I am. The only things we know are real with absolute certainty are ourselves.
But that is not what we want to do here.
Let us, instead, look at some definitions of what is real to try to come to some consensus on the electron. One definition of real is that some things are real, like you, me, trees, the Earth, and the stars. Things are real because we can perceive them with the senses as distinct entities and no matter how we move around or change our state, they don’t vanish. In addition, multiple observers, from multiple points of view, can all agree that these things exist. That last part is crucial. If I have a dream or an hallucination, only I can perceive what I am seeing or hearing. If multiple people all see what I am seeing and hearing, then it must be real.
That does not tell me anything about what is being seen. If multiple people all see a bright object in the sky and conclude it is an alien spacecraft, that proves that the bright object was real but not that it was an alien spacecraft. In order to determine what something is, you have to have more evidence to distinguish it from other things that could be real. That is the process of the scientific method.
It turns out that the statement that something can be detected by multiple observers has a name: Poincaré covariance. If something has Poincaré covariance, that means that no matter what our position or state of motion is, that thing will never disappear. If something is Poincaré invariant, then it doesn’t change at all, but this is generally untrue for almost everything.
If something does not have Poincaré covariance, such as the magnetic field, then we can actually question whether it is real. We can choose coordinate systems, which means observers can have states of motion, where fields vanish. That means that I can have many observers all agree that a magnetic field exists, but I can also have many who disagree that the same field exists.
This is especially true of the gravitational field. In free fall, an observer has no way of knowing they are in a gravitational field except by observing their motion relative to some other body. The only reason that you can do that is because the field by which we observe things most often, light which is the particle manifestation of the electromagnetic field, does not obey the equivalence principle!
Since electrons possess Poincaré covariance, they should qualify as real based on my definition above.
Some physicists, however, have argued that particles are not real but fields are. This relies on another definition of real: that of persistence. While particles are Poincaré covariant, they do not last forever. They can decay or enter an interaction with other matter, turning into other particles. While particles do not persist, fields do. Particles are simply manifestations of fields that satisfy the mass constraints of those fields. This causes them to become “real” in physics parlance, while particles that do not satisfy those mass constraints are called “virtual”.
A magnetic field produces virtual photons spontaneously because of the uncertainty principle. When two magnets are attracted to one another, they exchange virtual photons.
While physicists say that particles are “real” if they meet mass constraints and virtual otherwise, we can make the argument that all particles, real or virtual, are simply manifestations of the underlying fields.
Richard Feynman argued that we have to accept fields as real because if not we have to accept action at a distance. That is, without the concept of virtual particle exchanges, we would be forced to assume that particles just act as if they are exchanging virtual particles but they are really influencing each other at a distance.
If we require persistence for reality and get rid of Poincaré covariance, however, we run into trouble almost immediately because none of the objects we conventionally accept as real meet this criteria. You, me, trees, the Earth, and the stars are all made of particles and will all vanish over time. From this point of view, the only real thing are the fields from which we manifested.
This ontology might be useful for understanding quantum field theory, but it seems a stretch for trying to explain why electrons aren’t real to non-physicists.