[Harri (OP)]

Is it critical that the quantum world and the the world of relativity be unified? Are there real world implications for there being unification or not? Or is it just a matter of the math/numbers don't add up and at the end of the day it doesn't affect anything?

[Eternal Student]

Hi.

We aren't going to lose anything we already have if we don't unify these theories.

     There is an obvious problem with physics in that relativity and quantum mechanics don't seem to match up and work well together.  They're both good theories which are pillars of modern physics.  So trying to get them to work well together is a sensible and natural thing to want to do.   So it's not a waste of time and effort, it is  "a big elephant" and it is right in the way of general developments in physics, rather than being some problem that Physicists have have gone looking for.  If we are only interested in developing our field of knowledge then this problem is a sensible one to analyse.
     Based only on history, advancements in theory do usually lead to all sorts of practical discoveries and new technologies.  However, on the short term, it's hard to see what practical benefit would come from a unified theory of quantum mechanics and relativity.  We would be able to analyse and hypothesize more on the early universe, understand black holes better (and other things that don't usually matter in your day-to-day life).   It's unlikely that this will quickly be turned into some new technology that gives us a better washing machine.   There's no piece of technology that I know of that is desperately waiting for a unified theory of quantum gravity - but maybe others on the forum will know of something.

Best Wishes.

[chiralSPO]

Our understanding doesn't influence how the universe works (except insofar as we can use our understanding to make parts of the universe do weird things).

That said, I was at a seminar yesterday which was discussing the need to include general relativistic terms into quantum calculations when trying to model very heavy elements: the methods that I use to model light atoms like hydrogen through chlorine do abysmally when trying to model heavier elements (like it would not get that gold is yellow, or that mercury is liquid at room temperature, and would apparently be off by nearly 30% when estimating the redox potential of lead), and only when quantum and relativity are combined can useful ab initio models of these elements (and compounds containing them) be done.

[hamdani yusuf]

Our understanding doesn't influence how the universe works (except insofar as we can use our understanding to make parts of the universe do weird things).

That said, I was at a seminar yesterday which was discussing the need to include general relativistic terms into quantum calculations when trying to model very heavy elements: the methods that I use to model light atoms like hydrogen through chlorine do abysmally when trying to model heavier elements (like it would not get that gold is yellow, or that mercury is liquid at room temperature, and would apparently be off by nearly 30% when estimating the redox potential of lead), and only when quantum and relativity are combined can useful ab initio models of these elements (and compounds containing them) be done.

Does the model that you use predict that liquid/solid chlorine is yellow?



Or that carbon can be either black or transparent?

[Colin2B]

That said, I was at a seminar yesterday which was discussing the need to include general relativistic terms into quantum calculations when trying to model very heavy elements

How well does that work without the need to have a quantised field theory of gravity fully integrated with the theory of relativity?

To answer the op, I think there are 2 things to consider:
First, it’s an un answered puzzle and it will keep niggling away until it is solved.
Second, just as relatively showed us that Newton’s laws are approximations integration of the two theories will give us more information on what is really happening. Who knows where it will lead, but knowledge of relativity has allowed gps to work.

[Harri (OP)]

So it's quite exciting that not only is a theory of unification to be discovered but also the discovery could lead to who knows what advancements in science. In particular I wonder if a unification would reveal some insight into dark energy or dark matter?

[Colin2B]

In particular I wonder if a unification would reveal some insight into dark energy or dark matter?

It might, I wouldn’t want to speculate as it’s not my field , but I’m sure there are some exciting discoveries on the way.

[evan_au]

Do the theories of quantum and relativity have to be unified?

Quantum theory and relativity are already unified quite well in the low to moderate gravity regime (eg in the solar system).
 It is in the immediate vicinity of a black hole that you get results that don't look right.

Einstein's theory allows for a cosmological constant which (so far) looks like a good model for the accelerating expansion of the universe. But it gives no clue about how to derive the value of this constant (apart from measuring it).

Some theorists suggest that Dark Matter and Dark Energy may both be quantum effects, requiring an extension of the current quantum theory.

Sean Carrol suggests that any new cosmological/subatomic physics cannot possibly affect our real-world experience on Earth, since the theory we already have explains the world around us to an amazing degree of precision. Any new physics would only show itself at extraordinarily high energy levels (more than the LHC can generate), so it won't affect life as we know it.

Of course, you can't predict what you don't know. X-Rays have very high energy levels that could not be produced before the 1850s. But once they were discovered, they found an amazing range of applications (many of which did not adequately protect against their very high energy levels!).

[chiralSPO]

Does the model that you use predict that liquid/solid chlorine is yellow?

Yes, it does ! And it gets that diamonds are a colorless and insulator while graphite is a 2-D metal.

How well does that work without the need to have a quantised field theory of gravity fully integrated with the theory of relativity?

I don't think there is any consideration of gravity in these calculations (thankfully). The immense electric field of the nucleus with a charge of 75 or more protons means that the electrons experience mass/time dilations—and the magnitude of those dilations depends on which orbitals they are in. And mass is an important part of the Schrödinger equation even though gravity is not involved.

[hamdani yusuf]

Yes, it does  ! And it gets that diamonds are a colorless and insulator while graphite is a 2-D metal.

That's great. Can we make an online calculator that takes type of element and structure as its input and produce absorption spectrum as output?

[chiralSPO]

Yes, it does  ! And it gets that diamonds are a colorless and insulator while graphite is a 2-D metal.

That's great. Can we make an online calculator that takes type of element and structure as its input and produce absorption spectrum as output?

well.... it's not that simple. It's easy to have online tools that can predict things about absorption spectra from simple models, like  the Woodward Fieser rules https://en.wikipedia.org/wiki/Woodward%27s_rules

if you want to calculate absorption spectra from first principles, that would likely involve DFT (density functional theory) or TD-DFT (time-dependent density functional theory) or MP2 (Møller-Plesset perturbation theory) or the like. Which is all well and good if you have a very powerful computer. (my calculations can often use >100GB or RAM....)