Most times an experiment isn&#x27;t possible&#x2F;practical to perform, but the experiment could at least be described. The Higgs particle &quot;If we build an accelerator that accelerates particles to energies pf X electron volts, we should see traces of it&quot; 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.<p>Clearly if you propose a theory that requires an experiment so advanced (or circumstances so rare) we can&#x27;t hope to do it in 20 years, it&#x27;s still a valid scientific theory. What about 50-100 years? What if it requires technology so advanced it&#x27;s nearly unthinkable that we will ever attain it? This is when it becomes a philiosophical gray area. It&#x27;s not a clear cut case what is verifiable and what isn&#x27;t, since the theory is valid before our ability to verify it.