Can sub-planck-length exist?

[Obnoxious]

The only proof I know of that suggests sub-planck-length can't exist derives from the fact that we don't know currently anything smaller than the sub-planck-length, but wouldn't sub-planck-length be possible inside a black hole? Wouldn't the singularity inside a black hole be sub-planck-length?

[ydoaPs]

um, therin lies the problem. elementary particles are point particles(they have zero dimensions). iirc, the uncertainty of energy in a piece of spacetime smaller than planklenth causes extreme warpin such that space and time become meaningless.

 

that is one of the reasons i like string theory, but many don't go for it and string theory in its current form seems more and more unlikely.

[Obnoxious]

Well, if String Theory is correct, and strings are as basic as it gets, then wouldn't that mean singularities are at just planck length?

[ydoaPs]

strings are thought to be larger than plank length

[Obnoxious]

Isn't there massive tension on each string that causes them to shrink down to planck length?

[ydoaPs]

not that i know of

[Obnoxious]

I was reading the Elegant Universe by Brian Greene, he claims that a string's force is inversely porportional to it's tension, so gravitons and the like are very tense at 10^39 tons of force on one string alone, causing it to strink to plack lenght under this planck tension.

[Daecon]

I was reading the Elegant Universe by Brian Greene...

 

I have that book! That and "Just Six Numbers" by Martin Rees are two of my all-time fave books! XD

 

BTW - Exactly why is the Planck length supposed to be the shortest distance there is possible?

[Severian]

Since we don't have a working theory of quantum gravity we can't say for sure. If we were to formulate a quantum gravity it is most likely that the strength of the gravitational interaction would change with energy. Extrapolating to high energies, the Planck scale is the energy to which is is thought that gravity will become strong. The Planck length is the length associated with this energy.

 

People say that one cannot have a length smaller than the Planck length because in order to probe such a length you need so much energy that gravity becomes strong enough to 'foam' space-time - i.e it breaks it up into lot os little black holes. But this is all speculation. There is no reason to believe that the gravity coupling strength behaves in this way - there may be some other physical effect which prevents gravity from becoming this strong. Since we have no scientific experiments or observations to probe the physics of this high scale it is rather unscientific to speculate on the physics there.

[Johnny5]

The only proof I know of that suggests sub-planck-length can't exist derives from the fact that we don't know currently anything smaller than the sub-planck-length, but wouldn't sub-planck-length be possible inside a black hole? Wouldn't the singularity inside a black hole be sub-planck-length?

 

I don't think you are going to get a general concensus on the meaning of "Planck length."

 

I myself am not sure how to correctly interpret it yet, and I've been working on it.

 

Your question is centered around something existing and being smaller than the Planck length, but there is another possibility you might consider.

 

Changes in the position of the center of inertia must occur in jumps, over consecutive moments in time, and the Planck length is the smallest amount through which the center of mass of something can jump, when that something's motion is being viewed in an inertial reference frame.

 

Thus, Planck length would tie into the meaning of inertial reference frame, and possibly have nothing to do with actual size of objects.

 

But as I said, I haven't decided what it means yet.

 

Regards

[Daecon]

Wouldn't the Planck length imply that space is "digital" instead of "analogue"?

 

(If they're the right words to use in an anaolgy...)

[bascule]

Isn't there massive tension on each string that causes them to shrink down to planck length?

 

There is a massive tension, and most strings are roughly Planck length. As to the interrelationship between the two I have no comment.

[insane_alien]

Wouldn't the Planck length imply that space is "digital" instead of "analogue"?

 

(If they're the right words to use in an anaolgy...)

 

I was just thinking that myself. can something move by 0.5 plank legnths?

if it can't then it could mean we are living in a giant computer. Lol just kidding. i don't like the idea of a digital universe it shortens the possibilities infinitely

[bascule]

Wouldn't the Planck length imply that space is "digital" instead of "analogue"?

 

I think the words you're looking for are "discrete" versus "continuous"

 

Loop quantum gravity presupposes a discrete universe

[Daecon]

I think the words you're looking for are "discrete" versus "continuous"

 

Loop quantum gravity presupposes a discrete universe

 

Ah yes' date=' thanks. They're the words I'm looking for.

 

I have a question for insane_alien: What do you mean by "I don't like the idea of a digital universe it shortens the possibilities infinitely"?

 

Possibilities of what, exactly?

 

Surely a universe with limited possibilities is more favourable as it's more reliable/predictable/understandable etc?

[Severian]

There has been some interesting work on Black holes recently by Bena at al (http://arxiv.org/abs/hep-th/0402144 etc) which suggest that black holes may not be singularities at all. They were able to build objects which look like black holes out of more normal geometries. So there is really no experimental evidence to suggest that physics has problems beyond the Planck scale either.

[bascule]

There has been some interesting work on Black holes recently by Bena at al (http://arxiv.org/abs/hep-th/0402144[/url'] etc) which suggest that black holes may not be singularities at all. They were able to build objects which look like black holes out of more normal geometries. So there is really no experimental evidence to suggest that physics has problems beyond the Planck scale either.

 

Or to put it another way, string theory suggests a black hole may be nothing more than an inflated string... but to accept that you'd first have to believe in string theory...