Is the Planck Length real? How do we know?

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SUMMARY

The Planck Length, approximately 1.6 x 10-35 meters, represents the smallest measurable length in physics, beyond which classical concepts of space and time cease to apply. Stephen Hawking posits that at this scale, quantum effects dominate, and probing smaller lengths would require energy so high that it would create a black hole. The discussion highlights the limitations of current physics in addressing phenomena at the Planck scale and emphasizes that the inability to measure does not equate to non-existence. The debate also touches on the implications of quantum gravity and the nature of spacetime at these extreme scales.

PREREQUISITES
  • Understanding of quantum mechanics and general relativity
  • Familiarity with Planck units, specifically Planck Length and Planck Time
  • Knowledge of black hole physics and the uncertainty principle
  • Basic grasp of the Gibbs Paradox and its implications
NEXT STEPS
  • Research the implications of quantum gravity theories on the Planck scale
  • Explore the relationship between Planck Length and black hole formation
  • Study the Gibbs Paradox and its relevance to thermodynamics at microscopic scales
  • Investigate current experimental approaches to probing Planck-scale phenomena
USEFUL FOR

Physicists, cosmologists, and students of theoretical physics interested in the fundamental limits of measurement and the nature of spacetime at quantum scales.

  • #31
Is a Planck length relative, after all it does define a meter but it is based on time which is a variable in space-time depending on the position and age of the photon you are measuring.
 
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  • #32
petm1 said:
Is a Planck length relative, after all it does define a meter but it is based on time which is a variable in space-time depending on the position and age of the photon you are measuring.

I don't understand what you mean... for any given photon (its own frame of reference) there is no relative effect.
 
  • #33
The Planck length is derived from G, h-bar, and c. But how do we know if these constants stay the same at very small scales or in very tightly curved spacetimes?
 
  • #34
The only safe thing to say is that the Planck energy scale is the absolute upper bound of the region of validity of our current understanding of physics. It remains a possibility that our theory breaks down much faster than that. But there's absolutely no chance that it ever works beyond Planck scale.
 
  • #35
friend said:
The Planck length is derived from G, h-bar, and c. But how do we know if these constants stay the same at very small scales or in very tightly curved spacetimes?

If such spacetimes exist in accordance with String Theory, then there is nothing smaller than the constant, and if they exist in some other context... who the hell knows? One clarification: c is a constant... \hbar is not a fundamental natural constant, it's just a number.
 
  • #36
can anyone think of a way that one could PROVE that things can or can't exist smaller than the Planck length? imagine if you could see how gravity does work at these scales or what happens in place of gravity? would it require an electron microscope or does an electron microscope even view anything close to the Planck length?

ALSO...someone smart answer this question...can someone tell me the size of the Planck length and the size of the known universe? I'd be curious to know how close the sizes of things that humans deal with is to the middle of this gap. I mean.. how large is the Planck length relative to the size of the known universe...
 
  • #37
Kevin_Axion said:
Just for a clarification of scale, if an object of the order of Planck Scale were to expand in size to be equivalent to a tall tree then tall tree would be approximately 150,000,000,000,000,000 (150 quadrillion) light years long, much larger than the Observable Universe.

so, say the universe is 156 billion light years long... i'd be curious to know what is furthest away from the scales humans deal with... the size of the universe or the size of the Planck length. if that makes sense.
 
  • #38
The radius of the observable universe is about 1.3 x 10^26 meters. The midpoint between that and a Planck length is around 5 nanometers.
 
  • #39
Chronos said:
The radius of the observable universe is about 1.3 x 10^26 meters. The midpoint between that and a Planck length is around 5 nanometers.

Wow that is pretty amazing actually. To imagine how much SPACE there is in the universe and realize that it is almost minuscule compared to the amount of space below us... Thank you!
 
  • #40
sorad said:
can anyone think of a way that one could PROVE that things can or can't exist smaller than the Planck length? imagine if you could see how gravity does work at these scales or what happens in place of gravity? would it require an electron microscope or does an electron microscope even view anything close to the Planck length?

ALSO...someone smart answer this question...can someone tell me the size of the Planck length and the size of the known universe? I'd be curious to know how close the sizes of things that humans deal with is to the middle of this gap. I mean.. how large is the Planck length relative to the size of the known universe...

We're far from being able to observe anything at the Planck Scales... so no.
 
  • #41
We are down to about e-18, so we only have 25 orders of magnitude to go!
 
  • #42
Chronos said:
We are down to about e-18, so we only have 25 orders of magnitude to go!

Oh, so close! :wink:
 
  • #43
sorad said:
can anyone think of a way that one could PROVE that things can or can't exist smaller than the Planck length?

See my previous post https://www.physicsforums.com/showpost.php?p=2887431&postcount=25

If Hawking is right, that would make a Planck square the fundamentally smallest region of space, and nothing could occur within that space. Check out the wikipedia articles on the Holographic principle and black hole thermodynamics.
 
  • #44
RLutz said:
See my previous post https://www.physicsforums.com/showpost.php?p=2887431&postcount=25

If Hawking is right, that would make a Planck square the fundamentally smallest region of space, and nothing could occur within that space. Check out the wikipedia articles on the Holographic principle and black hole thermodynamics.

Much as String Theory makes that prediction, but as the previous posts make clear, it'll be a struggle to find even the most indirect evidence to support that in our lifetimes and then some.
 
  • #45
Besides no direct evidence, I'm a little skeptical in general of invoking a mini-black holes argument to define a minimum size scale. The only black holes we "know" are pretty big things, they tend to make our sun look puny. Now to (1) assume they behave the same at the microscopic level (2) assume any change in physics at that scale wouldn't change their dynamics or (3) even believe they exist on that size scale, are all leaps of faith.

If anything I think the best reason that we wouldn't trust physics at the plank scale is because we've never really tested it there. Its many many orders of magnitude away from what we've calibrated and verified, so I think its fine to chalk it up as unexplored territory. But I'm not an expert on this topic by any means.
 
  • #46
diggy said:
Besides no direct evidence, I'm a little skeptical in general of invoking a mini-black holes argument to define a minimum size scale. The only black holes we "know" are pretty big things, they tend to make our sun look puny. Now to (1) assume they behave the same at the microscopic level (2) assume any change in physics at that scale wouldn't change their dynamics or (3) even believe they exist on that size scale, are all leaps of faith.

If anything I think the best reason that we wouldn't trust physics at the plank scale is because we've never really tested it there. Its many many orders of magnitude away from what we've calibrated and verified, so I think its fine to chalk it up as unexplored territory. But I'm not an expert on this topic by any means.

Expert or not, I think you're taking a reasonable view of something that is so far beyond our ability to examine, even indirectly.
 
  • #47
I have long been of the opinion that Planck units were a path to false conclusions based upon cosmological numerology rather than physics - The article referenced in Post 20 was a great overview - and most appreciated.

Yogi
 
  • #48
It's a double edged sword. Not easy to dismiss a minimal length - as the author noted. In a universe composed of particles, it appears probable there is a finite limit on the number of subunits. Infinities suggest a logical error.
 

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