Here’s How You Can ‘See’ Molecules—on a Whole ‘Nother Planet

So maybe you heard this thing about possible signs of life on Venus. Yes, it’s Venus this time and not Mars. Scientists have detected the signature of the molecule phosphine in the atmosphere of this planet using rotational spectroscopy. As far as we understand it now, the only way to get phosphine is to make it in a lab or as a byproduct of some types of bacteria. Oh, bacteria on Venus? That would be kind of a big deal.

Now, some pretty awesome physics are involved in the possible detection of this molecule. Let me go over some of the coolest ideas so that you can fully understand it.

Radio Waves Are Light Waves

The signal for phosphine is a radio wave with a wavelength of 1.123 mm. So, how you do “see” radio waves? Yes, you use a radio telescope. In this case, that telescope was the James Clerk Maxwell Telescope at the Mauna Kea Observatory in Hawaii. Although a radio telescope might look quite different compared to an optical telescope, they are basically the same thing.

Both visible light and radio waves are types of electromagnetic wave. An electromagnetic wave starts with an electrically charged particle like a proton or an electron. These electric charges create an electric field around the particles—this field allows charges to interact with other charges without even touching. But something else happens if you could actually hold this charge and accelerate this charge back and forth (which you can’t actually do with an electron—not with your hand). The accelerating charge changes the magnitude of the electric field. Here is the cool part—this changing magnetic field creates an electric field such that this changing electric and magnetic field can create a sustaining oscillation. I know, it’s crazy but that’s exactly what is known as an electromagnetic wave.

Then what makes a radio wave different than a visible light wave? The only difference is the wavelength. We typically classify radio waves as electromagnetic waves with a wavelength larger than 1 millimeter and smaller than the universe (that’s just sort of a joke). Visible light has a wavelength of 680 nanometers for red light down to 380 nm for violet light. But all electromagnetic waves travel at the same speed—the speed of light at 300 million meters per second.

Although radio and visible light are both electromagnetic waves, there is one thing that is very different—the way that they interact with matter. Of course you already knew this. You know that the radio waves that your radio receives can travel through solid walls, but the visible light from the Sun or a lamp cannot. But it also means that instead of a shiny parabolic mirror for your telescope, you can use plain painted metal for a radio telescope lens. This makes it much easier to build very large diameter lenses like the James Clerk Maxwell Telescope (yup, same guy as in Maxwell’s equations). Of course we always want a lens as big as possible for the best possible image, but you actually need the radio telescope to have a larger parabolic dish because the wavelength is bigger. The radio telescope would still work with a smaller lens, but you would get poor image resolution.

What the Heck Is Rotational Spectroscopy?

You obviously can’t literally “see” the phosphine in the atmosphere of Venus. However, you can see evidence of it from the radio waves phosphine absorbs—the exact radio wavelength phosphine absorbs is a function of phosphine’s particular rotational energy level.

Let me start with plain visible light spectroscopy for the simplest atom—hydrogen. Hydrogen consists of just a single proton in the nucleus and one electron in the orbital shell. Since the there is an attractive force between the negative electron and the positive proton, it’s common to depict this atom as though it were a tiny solar system with the electron moving around in a circular orbit around the much heavier proton.

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