Title text: Bonus points if you can identify the science in question
: Bonus points if you can identify the science in question.
This was the forty-eighth comic originally posted to LiveJournal. The previous one was 50: Penny Arcade, and the next one was 51: Malaria. It was among the last eleven comics posted both on LiveJournal and on xkcd.com after the new site was launched. This comic wasn't published on the same day across both sites, but most of them shared the same posting day. It was released on xkcd.com on January 25, 2006, seven days after originally being posted on LiveJournal. See the trivia section below.
The solid line represents the theoretical radiation for a blackbody at 2.73 K according to Planck's Law (derived as early as 1900 by Max Planck). The formula, almost as written in the graph, can be found here. The only changes are that on Wikipedia, the frequency f is represented by the Greek letter ν (nu) and the temperature T is included as an independent variable, so I(f) becomes I(v,T). However, I(v,T) still represents the spectral radiance (similar to energy density). In this formula, h is the Planck constant, c is the speed of light in a vacuum, and k is the Boltzmann constant. The frequency (f or v) along the x-axis is measured in gigahertz. The curve peaks at 160.4 GHz. There is no scale or unit on the energy density on the y-axis.
The theory is that the blackbody in question was the universe at the point when it had cooled down enough to allow photons to escape, 0.38 million years into its 13.8 billion years history. The photons that reach us today are the ones that have been travelling to us at lightspeed since then. As the light from astronomical objects suffers from redshift due to the expansion of the universe, and this shift becomes more pronounced with distance from the observer, this light displays in the infrared range.
The title text praises viewers who can identify where this equation and corresponding graph come from (without consulting this wiki, of course).
This comic was made into a T-shirt but is no longer available. On the xkcd store, there was an explanation both for the title and for the graph in the comic:
Science: We finally figured out that you could separate fact from superstition by a completely radical method: observation. You can try things, measure them, and see how they work! Bitches.
The babydoll shirt is a slightly lighter green. The graph on the back of the shirt is data from theCOBE mission which looked at the background microwave glow of the universe and found that it fit perfectly with the idea that the universe used to be really hot everywhere. This strongly reinforced the Big Bang theory and was one of the most dramatic examples of an experiment agreeing with a theory in history -- the data points fit perfectly, with error bars too small to draw on the graph. It's one of the most triumphant scientific results in history.
- [A graph with a curve that begins at zero, then peaks at a given frequency, indicated via a thin vertical line, and then fades down towards zero. It is possible to see the data points, which fit the curve perfectly. The y-axis is labelled. Along the x-axis, the zero point and the frequency where the peak has its maximum are labelled and close to the arrow the unit of this axis is written.]
- y-axis: Energy Density
- Along the x-axis:
- [Above the graph to the right is the following formula, with the last inner parentheses only included to make the formula clear, since in the drawing the fractions are written above and below horizontal lines:]
- I(f) = (2hf3/c2)(1/(ehf/kT-1))
- [Below the graph is written the following:]
- It works, bitches.
- For unknown reasons, on January 18, 2006, this comic was posted on LiveJournal on the same day that 51: Malaria was released on xkcd.com. Three days later, on January 21, 2006, 51: Malaria was posted on LiveJournal, thus forcing the next two comics (52: Secret Worlds and 53: Hobby) to be released on xkcd.com two days before LiveJournal. Four days later, on January 25, 2006, this comic was finally posted on xkcd.com, which fixed the date discrepancies and allowed the next comic, 55: Useless, to be published on the same day across both sites.
- This comic used to be available as a T-shirt at the xkcd store.
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