Are all unobservables in physics very small?

Question

Are all unobservables in physics very small?

I think I've read that the interior of the sun is an unobservable, and so entity realists like Ian Hacking have to claim we can't know anything at all about it, since we can't directly manipulate it, which we can't.

Is that right? Are there any other big unobservables?

It seems odd to believe we don't know anything about such large things especially if the consensus in physics is that there are no large things we know nothing about. Is there anything like that in the universe?

Answer 1

Marc Lange discusses fields in An Introduction to the Philosophy of Physics. Fields would be useful in physics to avoid action at a distance. If one can avoid action at a distinct then one has spatiotemporal locality as he described it in Chapter 1.

However, fields are "unobservable" in the sense that one can't point to them like one can the sun or the moon. Furthermore, some physicists reject them as being real. For example, on page 42, Lange quotes Alfred O'Railly's 1965 textbook, Electromagnetic Theory:

The assertion [of the field's reality], taken by itself apart from the quantitative force-law is scientifically otiose....It is merely the physically irrelevant statement of a metaphysical conviction....This is certainly not a legitimate physical theory at all; it is the confusion of metaphysical belief with metrical physics....

A field, since it occupies all of space, could be viewed as very large.

So, to answer the question, Are there any other big unobservables?, one potential candidate would be an electromagnetic field.

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Reference

Lange, M. (2002). An Introduction to the philosophy of physics: Locality, fields, energy, and mass. Blackwell Publishing.

Answer 2

I suspect you mean quantum observable. These are limited not so much by size, as number and scale of interactions. See the TED talk Making Sense of a Visible Quantum Object. Low energy photons have huge wavelengths, which could be called size.

'Unobservable' in a non-quauntum general physics sense, is not a clearly defined term. Over an event horizon by definition can't be observed, so inside black holes and outside the visible universe, can't be observed. Parallel universes and higher dimensions may or may not be observable, but certainly can't currently be observed, and may turn out to be unfalsifiable, so unobservable.