Talk:Planck length

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[edit] Statements moved to talk page

Moved these statements here since they seemed wishy-washy. Please move them back if you can go into detail about which physicists make these statements and why.

His paper says nothing about its being "the smallest meaningful length in quantum mechanics" although some contemporary physicists talk like that. In 1899 quantum mechanics had not been invented yet. It might or might not be helpful to say "two points separated by less than the Planck length are indistinguishable from each other". This is an issue for today's physicists irrelevant to the original definition of the Planck length a hundred years ago.

It might or might not turn out to be useful to think of it as "the smallest meaningful division of time." One hears speculation about that, but the jury is still out. —The preceding unsigned comment was added by 24.93.53.199 (talk • contribs) on 15:51, 25 February 2002.

[edit] Nature article

Here is an article from Nature that seems to raise doubts about the Planck Length:

http://www.nature.com/nsu/030324/030324-13.html

—The preceding unsigned comment was added by 203.218.79.78 (talk • contribs) on 06:46, 24 June 2004.

[edit] 1.6blabla(12)*10^-35?

What does that (12)-thing do there? It seems to be totally out of place. Crakkpot 15:11, 10 March 2007 (UTC)

If I recall correctly, a number in parentheses in a figure tells you what the standard deviation is in the measurement (where it's pretty likely to be within one standard deviation, very likely to be within two deviations, etc). So, it's telling you how accurate the value given is. The deviation value is in terms of the last place in the original number, so 1.61624(12) means 1.61624 with a standard deviation of 0.00012. --Christopher Thomas 20:28, 10 March 2007 (UTC)
The numbers in parentheses are the uncertainty of a measurement or result of a calculation of two or more measurements. So, for instance, 1.650(25) is the same as saying 1.650, plus or minus 0.025. Uncertainty and standard deviation are two completely different things. Standard deviation is a statistical measure of the variation within a sample. Uncertainty represents the accuracy of a measurement. Mtiffany71 (talk) 20:13, 19 April 2008 (UTC)

[edit] Uncertainty in Momentum

Uncertainty in momentum is not a momentum, but a delta momentum, in the case of Heisenberg's equation the delta momentum is a range of possible momentums. So I think it should read something like "precision of position of an object to the plank length would mean that it would be impossible to distinguish if the object was a something moving like an electron, or having the capacities of a black hole." This is also meaningless because black holes do not necessarily have momentum.

== Compton Length== question: Is this meant to be the same as Compton Wavelength? Also, if one knew the sum total of all energy in the universe, would the corresponding wavelength be the Planck length?

Yes, same as compton wavelength. I don't know what you mean. If you mean the corresponding compton wavelength for all the energy in the world. I'm pretty sure that the answer would be no. The mass that has a compton wavelength equal to the planck length is equal to the planck mass. And the sum total of the energy in the universe is much larger than the energy in the planck mass.McKay

[edit] Consequences

The article does not distinguish, but I presume it is not whether or not the baseball is at rest or moving that matters, but that the speed can only be estimated within ±51 mph--JimWae 04:50, 2004 Nov 25 (UTC)

Yes essentially. The uncertainty of velocity in this case would be 51 mph. I don't think it's a +- 51, but that the range is 51, so its like +=25 McKay 00:49, 28 Nov 2004 (UTC)
Does this phenomenon appply to footballs as well as baseballs? Indeed, how about any other type of ball? Arcturus 16:36, 30 Mar 2005 (UTC)
Yes, the phenomenon works fine with any object, but I'll bet the masses are different. The article on uncertainty principle covers the ground nicely. Note my recent change to this article though. If you've further questions about the uncertainty principle, feel free to ask (here or my talk page works fine).McKay 23:26, 30 Mar 2005 (UTC)
So perhaps object would be a better word to use than baseball? I'll change it unless anyone disagrees. Thanks, Arcturus 16:34, 31 Mar 2005 (UTC)
Object doesn't work, because the uncertainty in this case is in the momentum. Since we can probably safely assume the mass of the baseball is unchanged, the uncertainty is in the velocity (the typical case). The momentum is the uncertainty, so you can't just say "object" but you could say an object of 34kilos (or whatever the size of a baseball is, I forget), like a baseball if you want to.
OK let's stick with baseball. However, not being a specialist in these matters I found it difficult to understand the concept as it is currently written. Could you elaborate within the article on the point about the mass? Arcturus 16:52, 4 Apr 2005 (UTC)
Mabey it should say "something with the same mass as a baseball" so people know it doesn't work with all objects.Daniel 19:05, 11 Apr 2005 (UTC)
A baseball with a mass of 34 kg? I don't know whether that was a parody of Americans not understanding the metric system or a genuine mistake. JIP | Talk 04:25, 11 April 2006 (UTC)

It is not the speed of the basaeball that is uncertain, but its velocity. One can be certain of a baseball's speed yet still not know what its velocity is. That is, uncertainy in velocity can come from uncertainty in speed, uncertainty in direction, or a combination.Flarity

You would be correct in saying that it is the velocity that is uncertain, but if the velocity is that uncertain, can you really know the speed? What I'm saying, is that if you certainly know it's speed, you do know something about its velocity, so can there actually be that imprecise about velocity? Its easier to visualize the variation on a constant, rather than a vector. McKay 20:24, 30 October 2006 (UTC)

[edit] Schwarzschild radius and Compton length are not equal

"The Planck mass is a mass whose Schwarzschild radius and its Compton length are equal distances. This distance, called the Planck length, is equal to:"

The above statement is wrong.


m = mC / lC


m is the mass

mC is the constant of proportionality

lC is the compton wavelength


Mass is inversely proportional to the Compton wavelength.

The constant of proportionality, mC, is about 2.2102188e-42 kg.m


This shows that the Compton wavelength of the Planck mass is equal to the Planck Length times 2.pi

Confirmed. We should probably update all of the Planck unit pages accordingly. They, and compton wavelength, state that the Schwarzschild radius is equal to both the Compton length and the Planck length, whereas it's twice the Planck length and (1/pi) the compton wavelength.
The error most likely originally arose because some texts (including the one I'd first seen Planck units in) _define_ the Planck length in this manner, while Planck units defines it as the length that, with the Planck mass, makes G = 1. --Christopher Thomas 06:59, 25 Jun 2005 (UTC)

[edit] Reference frames and black hole information

I have two problems with the explanation given in this article. First, doesn't the issue of whether something is less than the Planck length depend on the reference frame? Suppose there is a ship traveling at .9c relative to me, and they are trying to measure a distance that, to them, is 1.2 Planck lengths. Wouldn't I measure it to be less than a Planck length?

Also, how would being absorbed by a black hole mean that the photon can't give information about the particle's position? Wouldn't the black hole carry information from the photon, such as mass and momentum?Flarity 21:27, 28 October 2006 (UTC)

[edit] What is it the length of?

Most lengths are defined as the length of some physical item. I see that the Planck length is 10^-20 x the diameter of a proton... but why? I do understand that the length falls out of other constants, it doesn't lead to the other constants, but not how the length was defined to be what it is.Garrie 05:05, 30 August 2007 (UTC)

It's not the length of anything in particular. In any theory that has G, h and c as constants, the Planck length, or small multiples of it, is likely to turn up just because of dimensional analysis — it's the only way to get a length from those constants. There's no reason to believe that it's a quantum of length or a minimum meaningful distance or anything like that, unless some theory of quantum gravity predicts that it is. There's actually some reason to believe that area is more fundamental than length — Loop quantum gravity has a quantum of area, the string theory Lagrangian is proportional to the surface area of the world sheet, and the Bekenstein entropy bound is roughly one bit per Planck area. -- BenRG 21:13, 30 August 2007 (UTC)
So "It is the scale at which classical ideas about gravity and space-time cease to be valid, and quantum effects dominate" is nonsense. Quantum effects dominate at much higher scales. --Rumping 00:04, 11 September 2007 (UTC)
That's talking about quantum effects on spacetime specifically. But, yeah, it's nonsense to claim that spacetime is dominated by quantum effects at the Planck length when the truth is that nobody has the faintest idea what happens at the Planck length. I rewrote that paragraph. -- BenRG 14:41, 11 September 2007 (UTC)