CAPTCHA, ergo sum

Bad Astronomy Newsletter<br>Posts<br>CAPTCHA, ergo sum

CAPTCHA, ergo sum<br>I guess robots don’t know about Planck’s Law

Philip Plait<br>May 18, 2026

The Trifid Nebula and environs. Credit: RubinObs/NOIRLab/SLAC/NSF/DOE/AURA

May 18, 2026 Issue #1037

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Mea culpa!

Oops.

Last week, in BAN #1035, I mistakenly wrote that the Psyche spacecraft was swinging past Mars to steal some of its orbital energy and speed up to meet up with its asteroid target. That’s usually why these missions get a gravity assist from planets, but not in this case! The spacecraft was using Mars to change the inclination of its orbit — the tilt it has with respect to the plane of the solar system — to match up with the asteroid Pysche’s* inclination of about 3°. Changing the tilt of a spacecraft’s orbit is very energy intensive and takes a lot of fuel… unless, that is, you steal that energy from a planet that will never miss it. Anyway, sorry for any confusion.

* Yes, the mission has the same name as its target. Yes, it really bugs me.

I might be a robot

Another reason to hate those CAPTCHA thingies

If you’ve spent any time on the internet, then you’ve run across those “Prove you’re not a robot” popups. They’re called CAPTCHAs, and they ask you to click all the images in a grid that belong to some category. They’re all irritating, though the ones that constantly refresh as you click the pictures are the worst. Is that a tiny corner of the crosswalk in that frame? Does that photo of a city intersection have a fire hydrant two pixels high in it? Arg.

The idea behind them is that the ‘net is infested with bots, automated software that goes through pages and harvests info about them. The bots do this to aggregate stories, or steal them for plagiaristic purposes, or whatever. They’re a curse on the web, and I understand the desire for some sites to try to curtail them.

But I came across one recently that was more confounding than usual, all because I understand science.

It was one of those “Pick everything in this category” ones, and it really did throw me:

I’d rather walk the planck. Credit: A random damnable web page somewhere.

I’ll admit I was initially baffled by this one, and it got worse the more I looked at it. “Pick everything that produces heat”. OK, well the sun does, obviously. But the steaks don’t; they are hot but don’t produce heat. And if you count the steaks, then you have to include the chair, the ice skates, and the mountain, too!

They’re not “hot” by colloquial standards, but they do emit heat in a scientific sense. I wrote about how this works in BAN Issue 859 in 2025. Basically, a fundamental principle in physics is that anything above a temperature of absolute zero (-273°C or -460°F) emits light. That light has a spread of wavelengths (think of them as colors if you like) that we call a blackbody curve, shaped something like a bell curve with a steep dropoff at shorter wavelengths and a long, declining tail at longer ones. This is also called Planck’s law, if you want to be more mathy about it.

If this article made you laugh or think or somehow pass the Turing test, then please share it with a fellow human! Just click/tap here; no picking out a bicycle or a traffic light necessary.<br>The peak is at a certain wavelength (sometimes called the Wien wavelength) that depends on the temperature of the object. The lower the temperature, the longer the wavelength of that peak. The sun is quite hot at roughly 5,500°C, and emits its peak light in the visible wavelengths. But you emit light too! You have a temperature of about 37°C, more or less, and emit light in the thermal infrared, peaking at a wavelength of about 9 microns. You can’t see that light because it’s outside the wavelength range our eyes are sensitive to, but it’s still light. You’ve probably seen thermal infrared photos or video of people (sometimes called “heat vision”); that works because you emit light at that wavelength.

Even objects we think of as cold do this. Dry ice — frozen carbon dioxide, used to keep frozen products cool for transport — is at a temperature of about -80° C and peaks around 15 microns. Some astronomical objects are so cold they emit very long radio waves! You get the picture.

But CAPTCHA literally doesn’t. That chair, the skates, the mountain, the meat — they all emit light, which in a sense means they all emit heat.

OK, fine, I may be being a little pedantic here, but I can at least claim I do it because I love to point out fun science coolness (so to speak). But in real life nothing there but the sun actually generates heat; they just are all at some temperature and passively emit energy because of it. And for the record, the “correct” answer for that CAPTCHA included the steak but not the other objects, so it was wrong either way.

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