Thursday, July 22


Units Of Measure

Today on Pixy's Science Theatre: Tales of the Very Small!

A micron, or micrometre, is one millionth of a metre, 10-6 m. A typical human hair is about 80 microns thick (the range is rather large, from 18 to 180 microns). A human red blood cell is 6 to 8 microns across; the average bacterium is between 1 and 10 microns. The wavelength of visible light is around half a micron - ranging from 0.38 microns for violet to 0.74 microns for red. The smallest features of today's computer chips are just 0.09 microns wide.

An Ångström is much smaller, one ten-thousandth of a micron, one ten-billionth of a metre, 10-10 m. Atoms are around one Ångström wide - half an Ångström for hydrogen, the smallest of all atoms. X-rays have a wavelength of around an Ångström. The double-helix of DNA is about 20 Ångströms across.

The Fermi is far smaller still, one one-hundred-thousandth of an Ångström, one quadrillionth of a metre, 10-15 m. A proton or neutron is about one Fermi in diameter.

The Planck Length is really ridiculously small: about a tenth of a billionth of a quadrillionth of a Fermi, 1.6 x 10-35 m. According to current theoretical physics, that's as small as you can go: any distance smaller than a Planck Length doesn't actually exist.

62 orders of magnitude in length
83 orders of mangitude in mass

Posted by: Pixy Misa at 01:32 PM | Comments (8) | Add Comment | Trackbacks (Suck)
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1 So a half a Planck Length is not physically possible or is it not metaphysically conceivable?

Posted by: Jim at Thursday, July 22 2004 02:32 PM (IOwam)

2 Not physically possible - if the theory is correct.

Posted by: Pixy Misa at Thursday, July 22 2004 08:21 PM (+S1Ft)

3 The (untested) string theory explanation is that since particles are not points, but extended strings, about planck length long, it doesn't make sense to talk about distances or sizes smaller than the strings. There must be a conventional quantum explanation, but I don't know it. I think it relates to the uncertainty principle, that even with the largest energy uncertainty a particle can have, its position uncertainty is still nonzero, about planck length. You can't locate a particle to within a fraction of a planck length, so distances that small don't matter. Finite sized strings just make more sense to me, though. I remember reading somewhere that it's even more weird than that, in that in some weird 11 dimensional way, distances smaller than planck length really aren't smaller. Half a planck length in some sense is the same as two planck lengths. I have enough trouble with 3 dimensions, though. I think 11-dimensional physics will remain forever beyond me.

Posted by: Anonymous Coward at Friday, July 23 2004 02:23 AM (8M5Yp)

4 As I understand it - which is to say, not very well - distance may be quantized just as (say) electrical charge is. And that quantum is the Planck length. Your explanation also makes some sense. I'm really not comfortable dealing with distances below a picometre or times shorter than an femtosecond, so I'm a bit out of my depth. :)

Posted by: Pixy Misa at Friday, July 23 2004 03:28 AM (kOqZ6)

5 1035 planks are about a mile. Maybe we can reform the metric system to bring it into line (so to speak) with traditional measurements.

Posted by: David Boxenhorn at Friday, July 23 2004 03:56 AM (Zblwc)

6 At least in all my experience, Angstrom is becoming deprecated. It is 10^-10m, and so isn't one of the standard fractional units. The nanometer (10^-9m, 10 Angstroms) is the preferred SI unit of distances in the former Angstrom range. Pretty soon we are going to need a shorthand for nanometer like "micron" for micrometer. Nanon? (Picon for picometer?) In my laser physics classes, we always just called it a nano -- "The laser wavelength is 633 nanos." And yes, we would routinely deal with nanos, especially when dealing with interferometry. We could measure the length (ok, the change in length) of an interferometer or optical fiber to within a few nanos. We had one device which could measure the change in wavelength of a laser to better than 0.1nano. Now that the chips are getting down to 90 nanos, the nano is becoming an increasingly important unit of distance. The only problem with calling them nanos is that the chip guys probably use nanoseconds at least as often.

Posted by: Anonymous Coward at Friday, July 23 2004 06:20 PM (sCYzS)

7 I don't have notes in front of me, but I'm pretty sure that 10^-15m is the radius of a proton, not the diameter. I'm truely truely sorry for this comment. ;P

Posted by: Tuning Spork at Saturday, July 24 2004 09:18 PM (5TORB)

8 Next time on Pixy Science Theater 3000... Exponential growth, as exemplified by the interval between posts.

Posted by: Anonymous Coward at Tuesday, July 27 2004 11:04 AM (sCYzS)

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