I was watching a sixty symbols video on Stephen Hawking: , and it got me thinking. Is there any practical value for theoretical physicists, if what they are doing is simply speculation not backed by any data? I understand observing a physical phenomena, and from understanding it predicting new phenomena, but what of just speculation? Is that what theoretical physics is all about?

Look, I like to make fun of theorists as much as the next experimentalist, but you really couldn't be more wrong.

Theorists are incredibly valuable - the whole game of physics is making good models of physical reality, and theorists are oftentimes the drivers of new models. In broad strokes, as an experimentalist, you provide input to new models, and evidence that new models are required. Theorists can take that evidence, and put it into new models, or refine old ones, and tell experimentalists where they should look next, increasing our total understanding of the universe. I think the best situation is where theorists and experimentalists work closely with each other.

Hey, just the other day, I had a theorist help me by doing calculations to refine my experimental design!

For some reason, a lot of non physicists get the idea that theorists just look at stuff like string theory/BSM/cosmology stuff. And while theorists do look at that (and so do experimentalists, for that matter), many many theorists are involved with all fields of physics - from climate models, to making better transistors, making better cleaning products (no kidding!) and so on and so on. Just the other day, I was at a talk about beer bubble formation from a theorist!

He wasn't talking about theoretical physicists. I work in a small R&D group. I'm doing simulations, while others are doing experiments. But we do simulations and experiments to understand what is going on. There is always some 'real physics', i.e. equations involved, although for our group it is not as fundamental as understanding quantum entanglement or something like that. For our group, sometimes it's enough to see that there is a correlation between x and y, or to find that the temperature stays below a certain value. But always, there is some theory, some equation, behind it. The measurements, together with the simulations, just show that you have taken the correct assumptions and that the theory you have is correct to use in the situation you have. The 'speculation' consists of a step where you decide which physical effects to ignore and which to keep, based on what you think will be important effects. I can safely ignore any quantum effects in my situation, and even gravity, but not for instance turbulence effects.
In the end, we always know (with a certain level of confidence) why there is a correlation between x and y because ... theoretical physics!

As a theorist would say: simulations and measurements are just tools to prove that the theories are right!