Ok, so there's a problem in physics. General relativity and quantum mechanics both beautifully describe the universe at very large and very small scales respectively. However, they disagree with each other (general relativity breaks down when applied to quantum objects).

Many physicists since a long time have been believing that string theory would be the theory that would unify quantum mechanics and general relativity to get the theory of everything.

Why do so many ppl believe this? It's because the math of string theory is very elegant. Why is it elegant? It's because it strongly hints at unification.

But this is the problem - there is zero experimental evidence for string theory. In fact, certain requirements for string theory to be true have not been proven to be true yet (and have started to become less and less likely as experiments have progressed). This is why, string theory is just this incredibly complicated and mathematically intense theory without any practical applications.

The mathematician here hates her math to be practically applied. However, when she's told that it's being applied in string theory, she's relieved as she knows that it won't ever be practically applied. That's the joke lmao