Why Don't the Laws of Quantum Mechanics Work at Planck Scales?

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SUMMARY

The discussion centers on the breakdown of quantum mechanics at Planck scales, specifically below 10^-33 meters, where the laws of physics, particularly gravity, have not been quantized. Participants highlight the challenges of reconciling general relativity with quantum mechanics, emphasizing the uncertainty principle's role in this breakdown. The conversation also touches on string theory as a potential unifying framework, although its ability to quantize gravity and the existence of gravitons remain unproven. Recommendations for further reading include Brian Greene's works, which provide clearer insights into these complex topics.

PREREQUISITES
  • Understanding of quantum mechanics and general relativity
  • Familiarity with the uncertainty principle
  • Basic knowledge of string theory and its implications
  • Awareness of Planck length and its significance in physics
NEXT STEPS
  • Research the implications of the uncertainty principle in quantum mechanics
  • Explore the fundamentals of string theory and its claims about gravity
  • Study the concept of quantization in physics, particularly regarding gravity
  • Read Brian Greene's "The Elegant Universe" and "The Fabric of the Cosmos" for deeper insights
USEFUL FOR

Physicists, students of theoretical physics, and anyone interested in the intersection of quantum mechanics and general relativity will benefit from this discussion.

jr@kg
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Hello,
I do not understand why the laws of Quantum mechanics do not work once you get down to distances below Planck's length.
 
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Sounds to me like you have been reading Brian Greene's books.

Of my knowledge, they break down when you try to combine general relativity and quantum mechanics on such small scales, because gravity (space and time) has yet to be quantisized.

I believe that they also break down (correct me if I am wrong) because of the uncertainty principle. The smaller the space, the more erratic it is.

All of the above leads us to believe that time and space must be quantisised.

As for quantisised, I haven’t the slightest clue how to spell it. :)

This was a bit of a quick post, but I have to go eat.
 
ΔxΔp≥ћ/2 said:
Sounds to me like you have been reading Brian Greene's books.

Of my knowledge, they break down when you try to combine general relativity and quantum mechanics on such small scales, because gravity (space and time) has yet to be quantisized.

I believe that they also break down (correct me if I am wrong) because of the uncertainty principle. The smaller the space, the more erratic it is.

All of the above leads us to believe that time and space must be quantisised.

As for quantisised, I haven’t the slightest clue how to spell it. :)

Try: "Quantized". Or: "Quantised", if you are British.

Not "quantisisisisised"

This was a bit of a quick post, but I have to go eat.
 
Also, I do not understand why they suppose the existence of superstrings resolves the problems between quantum mechanics and general relativity. It is considered to be a unifying theory, that seems to imply that superstrings do quantize gravity. But that would mean that strings composed gravitons, the existence of which has yet to be proven.
 
Quantum mechanics breaks down long before the Planck length scale. This is why the same equations that predict the correct spectrum for hydrogen, fail with a heavier nucleus. The Compton wavelength is inconveniently small for a heavy nucleus.

10^-33 is 20 or more orders of magnitude smaller than reality as we experience it, so it seems fairly irrelevant ( like string theory ?).
 
But our immediate perception of reality is so limited that it would be utterly conceded to consider other orders of magnitude "irrelevant". Things as small as that can not be simply disregarded due to their inferiority of size. They still affect us constantly.
 
Well, they probably don't really- so long as the laws of physics keep working, that's one distance scale we could probably safely remain ignorant about :-p but it's interesting!
 
Also, I do not understand why they suppose the existence of superstrings resolves the problems between quantum mechanics and general relativity. It is considered to be a unifying theory, that seems to imply that superstrings do quantize gravity. But that would mean that strings composed gravitons, the existence of which has yet to be proven.
A lot of theories say gravitons exist, this is the consequence of the quantisisisisisisisisisation of space.

As for why string theory claims to solve those problems you mentionned, I really suggest you read again (I had to) to understand what Dr. Greene is talking about. He can explain these things much better than myself.

As far as I know, it has been proven that string theory cannot possibly be the theory of everything. However, the more physics books I read, the more I appreciate Dr. Greene's explanations of SR, GR and QM (and some other interesting topics). I recommend The Elegant Universe and The Fabric of the Cosmos to absolutly everybody, even if you only read the non-string chapters.

But our immediate perception of reality is so limited that it would be utterly conceded to consider other orders of magnitude "irrelevant". Things as small as that can not be simply disregarded due to their inferiority of size. They still affect us constantly.
Most physicist will argue that if something cannot be observed than it is not physics. That does not mean smaller scales are irrelevant though.
 
Thanks for clearing that up. It will certainly help me understand my current readings (though I am not reading Greene, that is where most of my knowledge has come from).
 
  • #10
muppet said:
Well, they probably don't really- so long as the laws of physics keep working, that's one distance scale we could probably safely remain ignorant about :-p but it's interesting!

Safely, perhaps, but it'd be a pretty boring world if all we could interact with was the macro-universe.
 
  • #11
Safely, perhaps, but it'd be a pretty boring world if all we could interact with was the macro-universe.
If this was the case, physicists wouldn't have jobs. :(
 
  • #12
Posted by peter0302:
Safely, perhaps, but it'd be a pretty boring world if all we could interact with was the macro-universe.
You miss the point. We interact with the micro-world down to quark/gluon scales. But Planck scales are 100000000000000000000 times smaller at least. We're a long way from that and I'm not in the least bored.

As Pop-eye said, 'a man who is tired of QM is tired of spinach itself.'
 

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