[wtallis]

What do you mean by "testable"? There's a pretty big spectrum of "untestable" ranging from requiring too many trillions of dollars to requiring more resources than this planet has to requiring equipment or energy levels we don't know how to produce even in theory to requiring stuff we're pretty sure is impossible like looking beyond the range of the observable universe or requiring more energy than is contained in the observable universe.

And at what point in its development and study do you abandon a theory that nobody has yet invented a way to test?

[shas3]

You've actually summarized the article in your comment!

[mahranch]

Except the question remains unanswered. "Untestable" is basically a grey area. There's "untestable right this minute, but in 5, 10 or 20 years it will be testable", then there's "we can probably test it in 100 or so years..." then there's the "it would take 1,000 years or more (maybe never) to test it".

Theories like String Theory tend to fall into the first, possibly dabbling with the second option. Researchers are already starting to work on ways to test string theory, with promising results. It's quite likely that within 20 years, we might be able to do just that. While things like the many worlds interpretation of quantum mechanics or multiverse theories tend to fall on the "maybe in 1,000 years, but probably never" category.

I think the real question is: How patient are we?

[guscost]

> How patient are we?

If you mean we're going to be hearing about the Many Mystical Worlds of Quantum Physics from Deepak Chopra types for the next 1,000 years, I'd say "not very" is a reasonable answer.

[smegel]

> And at what point in its development and study do you abandon a theory that nobody has yet invented a way to test?

Perhaps a better question is at what point does philosophy become physics? Finding a way to test the theory, and doing so, may be such a point.

[TheOtherHobbes]

An even better question - is this the most interesting of all possible untestable theories?

String theory's critics are sure that it isn't. I've even heard it said that it only has the prominence it has because a noisy generation of physicists elbowed out competing projects.

If you don't have a testable theory in the short term, should you carry on trying to refine that theory in the absence of evidence, or should you spend more time on competing theories that perhaps have more chance of being open to experimental verification in a reasonable time frame?

The list of open, unsolved, and unexplained issues in physics isn't small. Continuing to prioritise one theory and gambling that it works out eventually is a classic sunk-cost error.