# Soft question - Why do theoretical physics ? and what is its motivation? [closed]

Okay some background - I am a Theoretical physics graduate student (and i would go even further calling myself a math person) and quite recently I have been into this question and am finding it hard to justify to myself as to why I choose it. And I "was" a member of one of those elite sects who declared that theoretical physics is the fundamental truth/science of nature.

So one day, I was the sole astrophysics grad student and in a CMT seminar, when the professor talked about making a model "x" for a system (if you want to know, it is for graphene) with which he initially started worked with the action and did stuff to it to later realize that it dint fit the experiment "y". so he had to modify his model from "x" to "x1" by bringing in new ideas to make it fit "y". So I asked him what would you do with this model "x1" ? He said this could predict the result of another set of values for exp "y1".

So after thinking about it the final thoughts - 1) Theoretical people are model builders in different scale be it Cosmology or biophysics or CMT only at different scales and using different tools.

2) Once these models are built, we are solely motivated to change our model until it fits the existing experimental data, and keep on modifying the model if it still does not fit future data. (expect for phenomenology where you do not have sufficient data eg.. Massive gravity, SUSY, String etc..)

Can someone explain to me the uses of these models if they cannot guarantee the results of a new experiments (I know this is impossible to do, like to build a theory that will for sure predict future exp results), but if you cant, what is the point of building just theories ?

PS : Now I have moved myself from the theoretical sect to accept the fact that exp is more fundamental than theory. (I guess someone answer here could change my mind?) Since I came to physics for the mathematical rigor I needed to understand stuff, I don't think exp caters to it. So I have decided to move to computational part of physics where again one makes model, but are not limited just by analytically solvable models and is inbetween exp and theoretical.

PPS : This question is not to condescend on theoretical physicists ( I am one) or experementilists (now a strong believer of exps!) but rather to reason out as to why I choose this field. It seems like everyone had big ideas about how physics works when in high school and one they reach the level of field theory and other hardcore subjects, get trapped in solving problems missing the big picture. I initially thought I choose theoritical physics (I was an engineer by undergrad and hated the lack of details and the fact that every other formula is empirical) because of the rigors structured mathematical convincing I wanted to understand things, but then latter realized if that's what I seek for, then I should have taken pure abstract math. Then came across this awesome book "feynmann's rainbow" by Mlodinow, where he talks about two kind of rigors that physicist have - "mathematical rigor" and "logical rigor" and I am now convincing myself that having a logically rigors proof is as acceptable as a mathematically rigors proof. While self searching as to switch to pure math or to continue on physics, I found myself trapped in this question.

## closed as unclear what you're asking by CuriousOne, Luboš Motl, knzhou, Gert, user36790 May 27 '16 at 3:00

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• For this kind of soft question, you might get a good answer asking in chat. – knzhou May 26 '16 at 17:36
• Yes, science is in the business of building models of nature. Now, somewhere in front of you there is a small machine on the table. It's called a computer. You are obviously enjoying using it. It was built by people who have been building models of nature for centuries. The food that you had for lunch was made by people who are using century old models of nature about how to grow food. Your house... built by people with centuries of modeling of how to arrange wood and bricks and plaster so that it doesn't collapse... need I go on? – CuriousOne May 26 '16 at 17:36
• This might be better asked in Physics meta? – jim May 26 '16 at 20:18
• @CuriousOne I do agree, applied physics has many implications in the social life we have. But from a pure theoretical perspective I have seen very few people that do physics to "help humanity" and their motivation is often isnt to make human life better (unlike other fields like Medicine, engineering etc..). And as far as i know, most of us do physics(especially theoretical)/math just because we love the abstraction and rigor involved in it. So i do agree there are lot of cosmetic advantages, but is that the reason people do theoretical physics? – user15173 May 26 '16 at 23:22
• @jim I actually wanted a take from people who are working in theoretical physics on this topic. I asked my fellow graduate students and none of them could give me a strong satisfying answer. – user15173 May 26 '16 at 23:24

Theory without experiment is mathematics. Experiment without theory is just doing random things.

Perhaps there is a "fundamental truth" to nature, perhaps there isn't. Theoretical physics is independent of that, because theoretical models are what gives science predictive power.

Theoretical models are things that give you formulae with which to quantitatively describe nature. This quantitative description is in terms of more-or-less observable quantities so we can test those predictions. If we have tested a theory over and over and it failed to be wrong we make the (logically unjustified!) leap of faith to believe the theory to be "true", and we take its predictions as given without checking them every time. Theories whose predictions turn out to be wrong are either discarded or modified.

Now, with the theories we "know" to be "true" we can do amazing things: Statics allows people to build bridges and houses that do not collapse without having to measure every component whether it really holds when put there. Mechanics allows us to know exactly how much fuel we need to pack onto a rocket for it to reach the moon. Thermodynamics tells us just how efficient our engines can be, and where we can stop looking for improvement. And many more things. The trust in our theoretical models (not only in physical models, but those of all natural sciences) is one important component of our engineering successes.

But there is more than this "material" benefit. The models allow us to "understand" the world. To tell overselves coherent, logical stories about how the world works, derived from a few simple (compared to the mess that is the real world) axioms, and to be as confident as we are in our theories that those stories are an accurate description of the world. Like the story of the Big Bang and the expanding universe, or the story of everything being made up from only a handful of fundamental components. This has no immediate material benefits. It is simply knowledge, and its pursuit is part of our societies just like the pursuit of art. But some day, someone might take a model without material benefit and design something that relies on it. All scientific knowledge is potential.

Sure, our models can turn out to be wrong. Sure, no theory is ever undoubtedly, 100% "true". This is the nature of scientific knowledge. If you don't like it, that's your prerogative, but the overwhelming success of science, both in its application to engineering and in its more cultural form, seems to suggest that this is a useful way of approximating the "truth".

Must admit, have found it sometimes difficult to justify why I did a PhD. Ideally because I have lust, errrr lust for knowledge

Most subjects get more interesting beyond the undergrad degree level, it just seems natural for me to say physics is more interesting than other subjects. As well, with theoretical physics you develop mathematical skills and this makes the subject more interesting.

But as far as "Experiment without theory is just doing random things.", this isn't true, I believe experiments came first and then there was a desire to try and rationalise.

• I agree about the "lust" part, there is certainly some of that going on in the knowledge culture (and not just in physics). I am not sure about the "experiment first" part. I agree with ACuriousMind that random experimentation gives very little information content, just like random theory building without observational data is a really poor way to spend ones life in the sciences, see the endless struggles of our sting theory and quantum gravity friends. Real progress can only be made when theory gives strong hints to experiments and when nature is willing to surrender information. – CuriousOne May 27 '16 at 0:34