Invisible wrote:
> I thought that QM simply doesn't describe gravity *at all*, and that was 
> the problem.

There are a number of different theories of "QM", depending on what you're 
looking at.

QM of photons interacting with electrons is QED, Quantum Electrodynamics. 
That covers everything but nuclear reactions and gravity. I.e., all of 
chemistry and everything that's based on chemistry, as well as why electrons 
"orbit" the nucleus, *why* the periodic table of elements is how it is, 
electricity, friction, lasers, pauli exclusion, partial and total 
reflections, diffraction gratings, polarization, etc.

There's a kind of QM (whose name I don't know) that accounts for the strong 
and weak nuclear forces. It has muons an mesons and stuff like that acting 
like photons, with neutrons and protons acting like electrons, and etc. (I'm 
probably messing all that up, but...) It's essentially the same formula with 
different numbers plugged in. This theory is not so well confirmed, because 
the numbers are a couple orders of magnitude bigger - i.e., the same 
difference between a chemical bomb and a nuclear bomb.

QCD is quantum chromodynamics, which I think accounts for all the different 
subatomic particles based on quarks. Again, it's essentially the same 
process, with a few extra kinds of numbers thrown in (but which behave the 
same way). Kind of like having polarization in other directions.

So given that every other "action at a distance" seems to follow the same 
basic formulas and seems to move at the speed of light, and that gravity 
itself seems to move at the speed of light, it's not surprising that gravity 
is thought to maybe follow the same basic formulas.

Just like people thought photons were waves because they followed the same 
basic formulas as waves do, but it turns out they're 100% always particles, 
it's sort of like gravity perhaps follows the same math as curved space 
time, but maybe is exchange particles after all.  I have never heard of us 
actually measuring actual curved space-time unambiguously. I'd be interested 
in hearing if we did.

Or, it's possible that gravity works in a way that's just incompatible with 
QM and the two physically cannot be unified regardless of theory chosen. :)

-- 
Darren New, San Diego CA, USA (PST)
   Human nature dictates that toothpaste tubes spend
   much longer being almost empty than almost full.

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