B Why we don't have several theories

[kent davidge]

I wonder why we have actually only two theories of gravity, only one "theory of the microscopical world". There are a lot of brilliant people working on physics. I would expect new theories (that work) appearing like every year... I would expect we having now dozens of theories of gravity, each one as valid as Newtons and Einsteins.

Why this doesn't happen?

 

[fresh_42]

I wonder why we have actually only two theories of gravity, only one "theory of the microscopical world". There are a lot of brilliant people working on physics. I would expect new theories (that work) appearing like every year... I would expect we having now dozens of theories of gravity, each one as valid as Newtons and Einsteins.

Why this doesn't happen?

Why re-invent the wheel? Newton works perfectly on most occasions, and for very high speeds, calculations have to be more careful and precise, that's where Einstein works. A bit more complicated, but it gave us navigation systems.

What we don't know is a theory which combines gravity with the microworld, and in this case we do have a few dozens of theories. All waiting to be announced the one and only theory which gets the Nobel prize.

 

[romsofia]

There are at least 3 different theories of gravity i can think of off the top of my head that aren't exactly GR. We know GR works, and it encompasses a lot of scenarios, so as they say, if the shoe fits... wear it!

If you take away one, or both the conditions of the Levi-Cievta connection, you have two new theories of gravity (Cartan, Weyl). The issue with these two is... they also only pop up in niche scenarios that most physicists won't encounter!

That is what it comes down to, why bother teaching graduate students/undergrad about some niche theory that only pops up in rare scenarios that we can't even verify experimentally? There are loads of theories out there that cover such small areas that unless you're doing a PhD in that area you won't hear about them!

 

[sweet springs]

For macroscopic world, Newton and Einstein theories work pretty well. For microscopic world where QM works, we do not have a good one yet. Many theories are postulated e.g. super string theory or loop quantum gravity.

 

[Herman Trivilino]

I would expect we having now dozens of theories of gravity, each one as valid as Newtons and Einsteins.

What does "as valid" mean to you? To me, Einstein's theory is more valid than Newton's.

 

[PeterDonis]

I wonder why we have actually only two theories of gravity

We don't. We have only one theory of gravity, General Relativity. Newtonian gravity is not a separate theory; it's just an approximation that is derived in GR when speeds are slow compared to the speed of light and gravity is weak.

Why this doesn't happen?

Because all of those other theories don't match the experimental data. Experimental data is what constrains us to have a narrow set of theories, and the more experimental data we collect, the narrower the set of theories gets.

 

[PAllen]

I would say Newtonian gravity (and mechanics) are a separate theory from GR/SR if treated as exact. It could have been true that they would never have been found faulty. As it turns out, they remain a highly accurate, approximate theory, whose bounds of utility are derivable from GR/SR.

 

[Sandeep T S]

What does "as valid" mean to you? To me, Einstein's theory is more valid than Newton's.

So there maybe few theory's valid than Einstein's theory