[Cyph0n]

A large chunk of physics is theoretical in nature. Sometimes you simply cannot experimentally validate a phenomenon, so you instead propose a theory that attempts to explain it using physics. Then in the future, when the phenomenon is testable, your peers determine whether your theory was correct or not.

[alkonaut]

Most times an experiment isn't possible/practical to perform, but the experiment could at least be described. The Higgs particle "If we build an accelerator that accelerates particles to energies pf X electron volts, we should see traces of it" must have sounded almost like a thought experiment when the Higgs boson was theorized. Same thing for e.g. the General Theory of Relativity and the Mercury passage that was one of the first validations of it.

Clearly if you propose a theory that requires an experiment so advanced (or circumstances so rare) we can't hope to do it in 20 years, it's still a valid scientific theory. What about 50-100 years? What if it requires technology so advanced it's nearly unthinkable that we will ever attain it? This is when it becomes a philiosophical gray area. It's not a clear cut case what is verifiable and what isn't, since the theory is valid before our ability to verify it.

[legulere]

Atomism is a good example of how philosophy first made up a theory that was later confirmed (in a changed version).

https://en.wikipedia.org/wiki/Atomism

You could also derive the Atomic nature of matter through math though, as infinitely devisable sets end up with paradoxes that don't match up with reality (think banarch-tarski)

You can do all this without an idea how to test it.

[pdkl95]

Given the knowledge available 2000+ years ago, atomism was a surprisingly good idea. Yah, they got a lot of the details wrong, but that basic idea that reality is made up of basic "building blocks" that combine into the larger structures we can observe is basically correct.

The philosophers that invented atomism probably thought about how the might see their atoms directly. I wonder how many "crazy" ideas were dreamed up that sounded impossible, that are now easy experiments to do today.

Theory is important, even if it sounds impossible today, because we have a history of redefining what is "possible" throughout the history of science.

[Figs]

Well, there is the case of Aristarchus who came up with the heliocentric model of the solar system about 1800 years or so before Copernicus. His model was rejected because of experimental evidence -- the theory predicted stellar parallax which observation at the time could not detect! (Of course, we can detect it now that telescopes have been invented -- the stars are just so much farther away than anyone could have possibly believed at the time.)

[XorNot]

No sometimes you can't build a practical, on-demand test. You can predict what future observations should look like.

[spooningtamarin]

theory is by definition a tested hypothesis