Is the Planck Length real? How do we know?

[sorad]

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...

 

[sorad]

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.

 

[Chronos]

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.

 

[sorad]

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!

 

[nismaratwork]

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.

 

[Chronos]

We are down to about e-18, so we only have 25 orders of magnitude to go!

 

[nismaratwork]

We are down to about e-18, so we only have 25 orders of magnitude to go!

Oh, so close!

 

[RLutz]

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.

 

[nismaratwork]

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.

 

[diggy]

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.

 

[nismaratwork]

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.

 

[yogi]

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

 

[Chronos]

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.