This one’s for all the physicists out there (but they knew that from the title). I’ve been told that the Planck length is the smallest scale on which measurements can be meaningfully interpreted. I assume that this isn’t due to a lack of precision on the part of our measurements, given the equation relating the Planck length to other fundamental constants. So is the Planck length the “smallest distance”, or can we just not measure what’s smaller?
The Planck length is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate. This is the ‘quantum of length’, the smallest measurement of length with any meaning.
And roughly equal to 1.6 x 10-35 m or about 10-20 times the size of a proton.
From -
That’s 10E-20 times (or 10 times ten to the minus 20 times ) the size of a proton.
It’d be a strange world if we had subatomic particles that were measured to be smaller than the smallest measurable smallness 
[The Planck Length](http://physics.nist.gov/cgi-bin/cuu/Value? plkl|search_for=Planck+length)
Value 1.6160 x 10[sup]-35[/sup] m
Relative standard uncertainty 7.5 x 10[sup]-4[/sup]
Tris
“A scientific truth does not triumph by convincing its opponents and making them see the light, but rather because its opponents eventually die and a new generation grows up that is familiar with it.” ~ Maxwell Planck ~
Thanks for the replies. This raises a couple more questions:
- What exactly does it mean to say that “classical ideas about gravity and space-time cease to be valid”?
- Does this shed any light on the old question of whether the universe is discrete or continuous?
What the Planck length is sounding like to me (IANAQuantumPhysicist) is nothing more than the *approximate scale * at which quantum effects become too big to ignore. Like in many things, the quantum effects gradually become dominant as the size of things goes down - at some point, one realizes that they’ve pretty much taken over.
From this site:
This doesn’t sound at all to me like the “smallest size possible in the universe”. It simply reads like the approximate lengthscale at which it’s necessary to take into account both general relativity (large-scale) and quantum theory (small-scale) effects. It sounds like the transition between the two is gradual, and the Planck length lies somewhere in the middle of the regimes.
Many problems in physics (I deal mostly with fluid mechanics, and it’s certainly true there) can be separated into various regimes in which one effect or another is so dominant that something else can be ignored. Naturally, things get messy when you’re working kind of “in between” the regimes - you need to deal with both effects. It’s pretty common to obtain a rough value to denote the location of the “in between” region, and it often comes about from mucking about with known constants of the situation. It seems to me like that’s what was done here.
brad_d wrote:
Quantum effects become too big to ignore at much larger scales than the Planck Length. As an example, one Ångstrom is a whopping 10^-10 meters (nearly 10^25 times bigger than the Planck length), and is the approximate “diameter” of a hydrogen atom, right? Well, what do we mean when we say that a hydrogen atom is about an Ångstrom “across”? We mean that the uncertainty in the position of the electron as it orbits the nucleus is about one Ångstrom. Thus, quantum effects are too big to ignore even when your scale is still 10^25 times the size of the Planck length.
The Planck length is more like the approximate scale at which classical effects are too small to matter.
But an electron is held in place around a nucleus by the electromagnetic force. The Planck Length is the approximate distance over which quantum properties of the Gravitational force become too big to ignore.
Let me see if I can put this right and please o please let this be right. It is all very confusing but I think I got it.
planck length is the length at which changes to a photon’s wavelength after striking a planck mass occur which are smaller then that mass’s event horizon. Since an event horizon is supposedly unsurpassable by anything, including light, obviously Einstein’s gravity theory is failing, and a new one needs to arise to explain how a photon can enter this object’s event horizon and then reemerge with its data intact. whew, this stuff always made my head hurt, which was why I stopped after Physics 452. This first part is correct. My reasoning about why gravity needs to be reworked is most likely correct. The explanation I was given was rather confusing, as all of this stuff is but hey, that’s what I can say about that.
This is the way we found Planck length.
The event horizon L = (Gm)/(c[sup]2[/sup]
The change in wavlength for a photon striking a mass is the Compton wavelength = h/mc
You equate the Compton wavelength with the event horizon and you get the Planck mass. The event horizon and Compton wavelength for that mass is the Planck length. And Planck time is the amount of time it will take a photon to transverse that length. Thqt equated equation is given in brad’s post.
well, he gave the length version of it, not mass
I’m not going to contribute much to the thread (everything I know has already been said), but I must say that I’ve found these quantum physics threads extremely enlightening, and I offer a huge thanks to everyone who participates.
Also, I should note that I plan on using the word/name “Planck” sometime in the next 48 hours.
So long as you don’t cheat, SPOOFE, by quoting Matthew 7:3:
“Why do you look at the speck of sawdust in your brother’s eye and pay no attention to the Planck in your own eye?”
I’d like to hear some thoughts about my explanation. Does it make sense?
brother rat, it does make sense. The problem I see is that the gravitional radius of mass m is L = 2m (G/c[sup]2[/sup]). There’s that factor of two. I’m not sure where it comes from. (Actually, it comes from my copy of Gravitation by Misner, Thorne, Wheeler. But I haven’t figured out where they get it from.) But, I wouldn’t let a factor of two stand between me and a good explanation.
Groan. Figures, I try to ask a factual question about QM, and next thing I know, someone’s quoting the Bible at me.
Thanks, everybody. This makes a lot more sense now.
Usually about eight feet, but you can cut them smaller.