Comments on: Common Sense for Uncommon Physics

By: Paul Murray

Paul Murray — Wed, 13 May 2009 05:16:31 +0000

3db is approximately double.

1 bit of information at room temperature is about 1/55th of an electron volt. At liquid nitrogen temperature, its about 1/200th .

To cycle through all values of a 128-bit key at liquid nitrogen temperatures will need about 10^19.6 J, about 2.5 seconds of the total amout of solar radiation hitting the earth.

Roughly. Of course, you’ll need one mother of a heatsink to keep that comfy liquid nitrogen temperature. And that’s simply to cycle through the values. If you want to do some computations as well, that’ll cost you more.

By: philip

philip — Mon, 04 May 2009 03:29:41 +0000

A billion seconds is about 30 years.

By: Jonathan Thornburg

Jonathan Thornburg — Sun, 03 May 2009 19:22:21 +0000

(a) kT = 1 electron volt at a temperature of just over 10^4 Kelvin
(b) 1g (acceleration) * 1 year = approximately the speed of light
(c) 2^10 = 1024 = approximately 10^3 (useful for quicly converting powers of 2 to powers of 10)
(d) another astronomy one: 1 arcsecond = 1/206265 radians,
or equivalently, 1 parsec = 206265 AU
(e) and one more astronomy one, 1AU (= radius of Earth’s orbit around sun)
= 149 million km (but astronomers tend to write things like 10^13 cm,
which always makes me stop and convert to meters then kilometers than AU)

By: Heather

Heather — Sun, 03 May 2009 19:20:59 +0000

A barn is 10 fm by 10 fm.

An erg is about 600 GeV and the Planck mass is of order 10 nanograms.

The local density of dark matter is about one WIMP [of 100 GeV] per coffee cup [300 mL].

By: Gavin Flower

Gavin Flower — Thu, 30 Apr 2009 01:41:26 +0000

oops!

“36 Km per hour = 20 metres per second”

should be

“36 Km per hour = 10 metres per second”

does no one check these approximations???

By: melior

melior — Wed, 29 Apr 2009 19:53:31 +0000

60 miles an hour is also a mile a minute, or 88 ft/second.

By: rob

rob — Wed, 29 Apr 2009 18:17:06 +0000

i like hc~1234 ev nm

a megaparsec-barn is about the volume of your fingertip.

pi squared is ~10 (or as mentioned above, g)

By: Peter Erwin

Peter Erwin — Wed, 29 Apr 2009 11:51:13 +0000

Another shortcut from astronomy: 1 km/sec is roughly 1 parsec/million years. (Useful because typical motions of stars are measured in km/sec and distances are measured in parsecs.)

Example: how long does it take the Sun to make one complete Galactic orbit, given that its orbit is roughly circular with a radius of 8 kiloparsecs (kpc) and a velocity of about 200 km/s? Circumference of orbit = 2 pi x 8 kpc = 50,000 parsecs; divide this by 200 and you get 250, so the answer is “about 250 million years”.

By: citrine

citrine — Wed, 29 Apr 2009 01:11:31 +0000

The weight of an average apple is about 1 Newton.

By: just another astronomer

just another astronomer — Tue, 28 Apr 2009 22:35:36 +0000

Here are two rules of thumb that I’ve used in the last week or so (though not, I hasten to add, for anything very important):

– Most chemical reactions yield about an eV per bond, so typically a few eV per molecule. This is useful for calculating the energy density of all kinds of things (like, say, gasoline), particularly if you remember that 1 eV is about 1.6×10^-19 Joules, and that a mole is about 6×10^23 whatevers. A mole of carbon weighs about 12 grams.

– Less generally useful, but interesting: the flux of solar radiation at Earth’s distance is about 1400 Watts per square meter. Fifteen lightbulbs.