If I understand it, which I may well not, the quantum electrodynamics view of a vacuum, of “nothing”, is that there is no such thing. A vacuum is a field of probabilities, often exceedingly small, but never zero. These probabilities are reality. There is, in other words, no such thing as “nothing” at any physical level.
Now: in the classical world “probability” is a measure of human ignorance, not a measure of reality. If the probability of the card at the bottom of the deck being the five of hearts is one-in-fifty two, that is a measure of our ignorance. The bottom card is, in reality, not the five of hearts (usually) or is the five of hearts.
I realize that some aspects of quantum mechanics have probabilities that are the ultimate reality; the probability is not merely a measure of human ignorance, but is reality.
But here I am speaking of only the field of probabilities in a vacuum. If “probability” here is analogous to the classical view of probability, then the implication is that, despite the fact that the field has no zero probability, in reality there are some places where there is no particle in reality and, so, “nothing” does exist in reality.
However, if the probabilities expressed in the vacuum field are not merely measures of ignorance, but are reality itself, then there is no such thing as “nothing”.
Which is it? Or neither.
As I understand it there are 2 aspects to “nothing”: 1) at the subatomic level, particles continuously pop into existence and then disappear
- the universe is expanding through the creation of new “space” and this is constantly occurring everywhere in the universe. This creation of empty space is associated with some level of energy.
I think what you’re getting at is the idea that the unpredictability of QM is just our ignorance, and maybe deep down it has a definite state which we just can’t get to. This is the same as the idea that there are hidden properties, or variables, which are really there but we just have no access to.
Most physicists (I think) have accepted that there are no hidden variables, that nature itself doesn’t know what state something is in if it’s below the quantum observability. This is due to something called Bell’s Inequality, which you can read up on, but per my understanding, says that if there were hidden variables, then certain measurements would be statistically different than what we can observe. That is, unless things get weird in a way that’s far more disturbing than just simple quantum uncertainty.
Bell’s theorem rules out certain kinds of hidden variables, but by no means all of them—however, it tells you that you have to pay a certain metaphysical price if you want to keep up the idea of hidden definite properties, most commonly, that these properties can influence one another instantaneously at arbitrary separation (non-locality). Other choices are possible; for instance, you could have retrocausation (i. e. causal influences that propagate backwards in time, such as from measurement outcomes to preparations), or what’s called superdeterminism, which postulates that you’re not free to choose which measurements to make, in such a way as to agree with the statistics of Bell experiments. Another possibility is to deny that experiments have unique outcomes—this is the path the many-worlds interpretation takes.
In fact, Bell himself was a proponent of such a theory, namely Bohmian mechanics, where particles always have a definite position at all times, which influence one another non-locally via the so-called quantum potential. In this theory, probability is indeed a measure of uncertainty, but it’s essentially uncertainty about the original configuration of particle positions (as afterwards, everything evolves deterministically).
In Bohmian mechanics, there are indeed places where there’s just no particles, but it’s not quite clear whether there’s really nothing—the wave function, in most versions of this interpretation, is a physically real field, which dictates the form of the quantum potential. It has been argued, however, that one shouldn’t think of the wave function as a physical entity in Bohmian mechanics after all, but rather, as something that has the status of a physical law. But I’m not very familiar with the philosophical implications of that particular strain.
One should note that talk of quantum fluctuations should be taken with a grain of salt. They’re what a physicist calls ‘perturbative’ phenomena—that is, they arise as elements in a mathematical series that forms an approximation of the full physics of the theory. In a sense, they’re thus mathematical artifacts, which we never would have to talk about if we worked with the theory in its full form.
Perhaps it’s best to think of the vacuum in a quantum theory as simply the state of the lowest possible energy—which is still a quantum state, and thus, has the properties of a quantum state. As a quantum state isn’t nothing, one shouldn’t be too surprised that the vacuum itself has certain properties.
It’s also worth mentioning (as it is any time that anyone brings up interpretations of quantum mechanics) that all interpretations of quantum mechanics are completely scientifically equivalent. For any experiment you can come up with, every interpretation will make the same predictions for what you’ll observe. As such, most physicists prefer not to take any stand on the question of which interpretation is “correct”, and just “shut up and do the math”. At most, one interpretation or another might make it easier to figure out how to set up a particular calculation, but a physicist will still end up using different interpretations for different problems.
To be clear the “virtual particles” of a vacuum under QED are mathematical “bookkeeping devices” and as noted above the result of perturbation theory.
The problem is thinking these “virtual particles” are physical objects but they are not. Really it is more like an idealized definition of momentum as an excitation of a field. With our inbuilt human intuition it seems absurd but if you understand the math it makes sense and is a result of the uncertainty principal.
Note while these “virtual particles” are a tool created by perturbation theory, a fields modes not always being zero is also found in theories that don’t use perturbation theory like quantum lattice models. “Virtual particles” even in QFT don’t physically exist but when you do something like interact with a field by say measuring something there is a probability that the measurement will not be zero.
As far as we know a “vacuum” is “empty” from what we would call an physical particle but that filed also does not have zero energy. Unfortunately to mesh with classical theories this energy level is called “Zero-point energy” despite it having “energy”. What this “Zero-point energy” is or how to describe it is an open question in science.
This is actually a subtle issue in quantum foundations, and hasn’t been conclusively settled, although there have been some surprising developments. The two main opposing viewpoints are called ‘psi-epistemicism’ (the quantum state is just a bookkeeping device collecting our knowledge of a physical system), and ‘psi-onticism’ (the quantum state is something real, independently of any observer’s description of the world).
The latter has received strong support thanks to a theorem due to Pusey, Barrett, and Rudolph. Essentially, the PBR-theorem aims to show that one can’t interpret a quantum state as a statistical entity—that is, as something to which there exists a more detailed description, which merely gives us a probabilistic description of this description.
If you’re interested in an overview of that result, Matt Leifer has given it a good discussion on his blog.