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Comment by tsimionescu | Hacker News Reader

QM and GR are mutually exclusive: if one is right, the other must be wrong (of course, it's very much still possible that both are wrong). If you use GR to predict the movement and interactions of photons in a medium, it's just wrong. If you use QM to predict the movement of light around a huge body, especially a black hole, it's just wrong. And what's worse, there is no limit or term you can add such that you could say "QM works only for objects up to size X, GR works for objects larger than size Y".

Also, we're nowhere near explaining 99.99999999% of the behaviors we see in the universe. In fact, we're not even able to explain 6% of the things we see in cosmology - as is often explained, dark matter accounts for 27% of all energy in the universe, and dark energy for 68% - and we have no ideas what these actually are, if they exist at all.

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Well, actually we have developed a very good feeling when to apply which theory. From subatomic particles to the large scale structure of the universe the right theories applied give us excellent results. lambda CDM models very well fit to the observed structures and the standard model describes all the particles that have been observed so far. We are desparately looking for effects that are unexplained by the standard model in particle physics so far. That is the reason why it is so difficult to justify yet another particle accelerator at CERN.

my 2 cent

Sure, we have a feeling for when to use one and when the other, but a feeling is not a physical theory. Also, the Lambda CDM literally doesn't explain what 95% of the universe is made of and how it behaves, it simply posits that it exists. So even if the Lambda CDM is perfectly correct, it's still extremely far from a complete model.

Not to mention, the reason the tension between GR and QM is not very prominent is that we don't know how to conduct "medium scale" experiments, even though the vast majority of physical objects on Earth are in this medium scale: far too big to count particles, too small to measure observe gravitational bending effects. Basically, both QM and GR are completely useless for telling you what happens in compel scenarios like two billiards balls colliding on a frictionless table. They both have equations that are far too complex to actually solve for anything like this. And QM is even worse - even if you could solve the equation, it doesn't tell you what the balls will do, it only tells what chance they have atof being at some position with some velocity and spin if you were to measure that, whatever "measurement" might mean.

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