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Xkcd confuses planck length with some kind of fundamental limit, and I'm sad

2 points by JoeAltmaier ↗ HN
Today's xkcd implies, I think, that the Planck length is a kind of measurement boundary which is a common misconception. It's just a convenient unit when calculating things around that size, like an inch or a light-year. Makes me worry that the author Randall Munroe may need to get out more. #2671

7 comments

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h2odragon
"the Planck length, at which the concept of distance may break down" ... seems reasonable enough for the purposes of the joke, anyway. It's true enough at this scale that the error bars are negligible.

Perhaps re-evaluate the yardstick which you're using to measure pick-able nits?

beardyw
xkcd has certainly lost top place in my list of scientific reference material.
JoeAltmaier
It's untrue. The Planck length is utterly unrelated to the concept of distance breaking down, just like the meter or the furlong. That's not nit-picking; this was a repeating of an old erroneous trope that the Planck length is some minimal possible distance, which as I said in the title, makes me sad. Whatever joke is being made.

I hold Mr. Monroe in high esteem, as his humor is often smart and spot-on with the science. I hope this lapse is temporary.

h2odragon
I think a reference to a common misunderstanding is fine for the joke; which is about a possible failure mode of a user interface quirk.

Since you're obviously passionate about the subject tho, perhaps you could help me understand the error better? I dunno much more about the Planck length than that most places offer them in 8 and 12 foot, and pressure treated costs extra.

So just what is it then and how is it we can still shoot the tortoise with an arrow?

gjvc
xkcd is not as clever as the hype said it was. boo hoo.
raxxorraxor
I doubt that "getting out more" would help in getting tighter definitions. The Planck length is probably a pretty effective method for contraception for what it is worth.

What would you propose? I think it is primarily a boundary, but conceptually I can see it used as a measure and rhetorical vehicle as well. Without being a physicist that is.

jfengel
It is a common misconception with a grain of truth to it.

The Planck length is more than just a unit. It can be derived as the point where quantum mechanics and general relativity conflict. Planck-wavelength light is its own black hole.

Which does not mean that the Planck length is a fundamental unit, but it's the starting point for some beyond-the-Standard-Model physics. Most popularly, Loop Quantum Gravity (the main alternative to string theory) does use the Planck length as its fundamental granularity of space-time.

The fact is that most people have no business having an opinion on String Theory vs Loop Quantum Gravity. But it makes good copy to play it as a David-vs-Goliath story in popular science publications. And those publications are a double-edged sword. For a few, they are a crucial stepping-stone into deciding on science as a career (even though vanishingly few will do fundamental physics). For the many, they are entertainment, and leave them squarely at the top of Mount Stupid[1].

Munroe is well past Mount Stupid, and knows everything I've just said. But it's a trope he'll play with on his way to some other educational/entertainment end. He's explicitly referring to "quantum foam", a property of Loop Quantum Gravity, which does include P_l as a fundamental unit.

As I said, I'm always wary of that kind of popular science. It's hard for me to tell whether it's a net benefit, whether it informs more than it misinforms. But I can say that I would not have gone into science without starting there, even though I was told an awful lot of lies in my early science education.

[1] https://www.ascpsychological.com/blog/mount-stupid/