We all know what knots are, but there’s a lot more to them than tying your shoelaces together. The entire field of topology concerns itself with how mathematical spaces get tangled up when they deform. As spotted by Live Science, a team of scientists from the Universities of Ottawa, Rochester, Birmingham, and Bristol have wielded the sorcery of this field and managed to “tie” split laser beams into a series of knots.
Before we explain exactly how this magic trick was pulled off, let’s recap a little bit about what light actually is.
Light is an electromagnetic wave, generated by the vibration of electrical charges. Although the movement of the wave may be from, say, left to right, the vibration of these charges is up and down, perpendicular to the wave propagation.
Now, if that light wave is vibrating in more than one “plane” – as opposed to just straight up and down, say – then you would call that unpolarized light. If they’re all vibrating in the same plane, then that’s linearly polarized light. Polarization is pretty easy to achieve – after all, your sunglasses do it. All you need to do is filter out multiple planes of vibration, and let just one in.
There are other ways to polarize light too: You can also get circular polarization, whereupon you have two vibration planes going on that are at right angles to each other, and the associated electrical fields then appear to form a circle as the light approaches you. Elliptical polarization is very similar, but the highest points of both vibration planes are unequal.
Now, the scientists mentioned above are extremely interested in the properties of light. In order to get a closer look, they decided to twist a laser beam into knots. Light can’t be tied up in the same way as a piece of string, though; instead, it’s those field lines along those planes that get all twisted up.
Those electric and magnetic field lines have already been shown to form "knots" when the original laser beams have been focused, with the most complex forms being a torus shape. This time around, the international team upped the ante, and, as reported in Nature Physics, created a figure-8 knot for the first time.
This wasn’t easy. First, they had to fire a linearly polarized light beam through a gauntlet of modulators that split it into two, before their set-up manipulated and superimposed them. An optical filter then made them both circularly polarized, which allowed their fields to get tangled up.
This may all seem a bit trivial, but these knots affect both the properties of the light and the space around them. There's a chance that one day they'll form part of light-based information systems, so the more we understand about them, the better.
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