Some other interesting examples are:
- The wave-function of quantum particles, given we can only take measurements that indicate it, never measure it.
- Dark-matter, which has no direct observations since it was posited in the modern sense in the 1970s.
- The Many Worlds of the Measurement Problem interpreted in the most ‘unadorned’ way.
- Places beyond cosmological event horizons (and regions of space in a Big Rip longterm cosmological picture), given there seems to be no possible worldline for us to interact with them.
- Hawking Radiation, which though very widely accepted will be extremely, extremely, difficult to observe directly (Hawking points may be evidence).
- Unruh radiation which may mean the existence of some photons depends on your rest-frame.
I would say this is an area where physicists do not think enough about philosophy. Sabine Hossenfelder for instance holds the view that science is only concerned with making predictions, existence being necessary parts of a model that makes correct predictions. But when you consider Poppers point that measurements decide between models, you can see if we haven’t imagined a model yet, we can’t know about a truly existing thing implied by our methods yet. We take for granted existence is a property out there, and forget that science is tentative, it’s results provisional, and it’s determinations generally within confidence intervals. And, fundamentally in a relationship with our models that we are testing between.
I thought Rupert Sheldrake made this point very clearly in a debate with Hossenfelder, where he talked about his paper Is The Sun Conscious? (PDF of the article from his website, it was published in The Journal Of Consciousness Studies), which she immediately dismissed. Sheldrake thinks the movement of stars by non-random solar flares could account for the missing ‘dark matter’. If we don’t open our minds to possibilities like this, we can’t look for evidence for or against them.
“the universe is not only queerer than we suppose, but queerer than we
can suppose” JBS Haldane, in his essay Possible Worlds
We are well to remember this. What we can use observations to understand, is limited by our hypothesis generation.
Existence is a simple word, and it deceives us into thinking it is a simple property. It is better to think about it having different qualities in different contexts, and to look at what our statements about it in those contexts are used to indicate. That is, a kind of parallel to the deflationary theory of truth, in regard to existence.
Virtual particles have no mass, they are only relevant within the bounds of the uncertainty principle related to their mass. But this has real consequences, including Hawking Radiation. I would say we should describe them as part of the potential of empty space, and related to the energy of it. That is, not as having existence separate to particles, but not as only a book-keeping exercise.
QFT is considered to have provided a more convincing formulation of the vacuum as empty, than the Dirac Sea model of This leaves the problem of the high vacuum energy which seems to indicate potential, eg in the Casimir effect.
In a deeper sense until QFT and GR are unified we don’t know what spacetime is, so we just don’t know yet.