system
Given the following post, respond as if you're commenting on Reddit:
human
Is string theory still a prominent theory in understanding the universe?

I am reading Brian Greene's The Elegant Universe, is String theory still viable? If not, what are some of flaws?
gpt
Well, that depends on what you mean by "viable." It's viable in the sense that it _could_ be correct. But it's also not viable in the sense that we may not ever be able to know whether it's correct or not.

The thing about string theory is that all the predictions it makes (that we know of) either require impossibly huge amounts of energy to test, or are identical to predictions made by other, simpler theories. That's not a good status for a theory to be in. The way that a theory gets to be taken seriously is when it makes a prediction that differs from other competing theories, the prediction is tested with an experiment, and the results of the experiment decisively show that the prediction is correct and the competing prediction is not. For example, general relativity was tested by the prediction of how much starlight was deflected by the sun. Newtonian gravity predicted something like 0.87 arcseconds, GR predicted 1.75 arcseconds, and the Eddington expedition in 1919 measured 1.65 arcseconds. (Of course GR has been tested and confirmed much more accurately in other ways since then.) A lot of people are looking for some way to extract a distinctive prediction out of string theory which we can test with our current technology, but so far they haven't found anything, which is why string theory is still considered speculative.