Light comes in a rainbow of colors. Starlight typically looks white because all those colors are mixed together to make white light. Astronomers love to study the different colors of starlight because they contain a great deal of information. How different colors are absorbed, shifted or otherwise changed tells us about the composition, history and evolution of stars. Fortunately there are a couple of ways we can spread apart the colors of starlight in order to study them. 

Pink Floyd’s famous album cover shows a prism.

One way is through a simple piece of glass known as a prism. When light enters a material such as glass, its effective speed decreases. Once consequence of this is that if light enters the glass at an angle it changes direction slightly. If you’ve ever driven a car where one side hits gravel or snow (making that side of the car move more slowly) you might notice how the car tends to pull toward that direction. In the same way, the light is pulled to one direction because of the slower speed. It turns out that the change in speed depends upon the color of the light, so each color is deflected by a slightly different amount. As a result, white light entering a prism is spread out into a rainbow. This effect has been known throughout history, and inspired Newton to study the relation between color and light.

Red laser light passing through a diffraction grating. Credit: Wikipedia

Another way to split light into colors is through a diffraction grating. This device relies upon the wave properties of light. A diffraction grating consists of a pattern of gratings that allow light to pass through certain parts but not others. It’s kind of like a picket fence with evenly spaced gaps between each slat. One of the properties of light waves is that they superpose. If two waves happen to be waving in the same way (in phase) they add up and make the light brighter. If they are waving in opposite ways (out of phase) they cancel each other out and might the light dimmer. As light passes through a diffraction grating it spreads out in different directions. In some directions the light is in phase and makes the light bright, and in other directions it’s out of phase and cancels out. Different colors of light have different wavelengths, so the directions producing bright light vary with color. As a result, white light entering a diffraction grating spreads out into a rainbow of colors. Diffraction gratings have an advantage over prisms in that the angle for a particular color depends upon the spacing of the grating, so we can control how much spread the colors have.

A grism combines the effects of a prism and diffraction grating. Credit: Benjamin Weiner

While both prisms and diffraction gratings have been used in astronomy, they pose a challenge for modern telescopes. In order to produce high resolution images, modern telescopes use sophisticated optics to focus light precisely onto detectors. Both prisms and diffraction gratings spread colors apart by changing their direction, making the light out of focus. So astronomers have devised the grism, which can spread colors into a spectrum while maintaining a reasonable focus. A “grism” is a portmanteau of grating and prism, since it uses both a diffraction grating and a prism to spread light into colors. It turns out that while both devices spread light into colors, they do so in opposite ways. A prism deflects violet light more than red, while a diffraction grating deflects red more than violet. By combining the two light can be spread apart while canceling out the overall direction. As a result the light simply spreads into colors without changing its overall direction. So you get a rainbow of colors without significantly affecting the optics of your telescope.

It’s a clever bit of engineering that has greatly improved our study of the stars.

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A small telescope is often described as a rather simple device. By place two lenses in a tube at the right distance, Galileo changed our understanding of the universe. But in fact the interaction of light and glass is extraordinarily complex, as you can see in the video above. 

The video simulates a pulse of light striking a series of lenses. You can see how some of the light reflects off the surface of the lenses rather than simply passing through and how the effective speed of the light slows down while passing through the glass. You can also see how the colors of the light begin to spread apart, which is a process known as chromatic aberration. It’s a subtle dance we can’t see directly, but the effects of this dance makes telescope design challenging.

While it is fairly easy to make a basic telescope, making a truly good one is a big challenge. It’s forced us to learn how to make lenses and mirrors with precision, and even to use computer modeling to create more useful telescope designs. We’ve come a long way, but we are still learning about the ways light interacts with material, and how we can use that dance of light to better see the cosmos.

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While the idea of time travel gives rise to discussions of topics ranging from science fiction to ethics, understanding the possible effects of time travel gives us a better grasp of the foundations of general relativity and quantum theory. Most work in the area has focused on the theoretical aspects of time travel, but there are also attempts to simulate the effects of time travel experimentally.

In physics, a time machine is known as a closed timelike curve (CTC). Basically, an object makes a loop through spacetime to interact with its past self. In a recent work published in Nature, a team simulated the possible effect of a time machine using polarized light. Since they couldn’t actually make a beam of light travel back in time, they used two separate beams of light, with one beam mirroring an earlier state of the other. Their focus was to study how quantum computers might be affected by a CTC.

The DWave chip is promoted as a quantum computer. Credit: DWave

Quantum computers use the fuzzy aspects of quantum mechanics to perform calculations. Rather than discrete bits of 0s and 1s, a quantum computer uses quantum states or q-bits. The challenges of quantum computing are huge, but they have the potential to perform some incredibly difficult computations with relative ease. In the early 1990s, David Deutsch demonstrated that if a CTC is self-consistent on a quantum level, then quantum computers could solve computational problems known as PSPACE-complete. In other words, it would be the supercomputer of all supercomputers.

Deutsch’s model is controversial because it relies on an interpretation of quantum mechanics that invokes “parallel universes.” And without a real time machine, his ideas are impossible to prove. For this simulated time machine, the team tweaked the states of their light beams to see what results they could get. They found that the results were self-consistent as Deutsch proposed, and they also completely agreed with relativity. This doesn’t mean that Deutsch is right, but rather if Deutsch is right the effects would work as he claims. There are other quantum models that would also prevent time-traveling paradoxes, but wouldn’t allow for the construction of a super-duper supercomputer.

The results of this work aren’t particularly surprising, but it’s an excellent demonstration of just how subtle and sophisticated optical experiments can be. And until someone is able to make a real time machine, simulated time machines like this one are the only way we can study time travel experimentally.

Paper: Martin Ringbauer, et al. Experimental simulation of closed timelike curves. Nature Communications 5, Article number: 4145 (2014)

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If you’ve ever looked up in the night sky you’ve seen the twinkling of the stars. This twinkle is not due to the stars themselves, but to the turbulent motion of the Earth’s atmosphere. As starlight enters our atmosphere, the variations in density in turbulent air cause the light wave fronts to distort. So instead of reaching the telescope evenly like even rows of a band on parade, the wave fronts come in uneven and wobbly. This wobbly behavior is why stars appear to twinkle.

One way to overcome this problem is to simply put your telescope in space. This not only eliminates the twinkle effect, but also allows you to make observations at wavelengths our atmosphere absorbs, such as most of the infrared. The big downside is that space telescopes are very expensive, and their size is fairly limited. The Hubble telescope, for example, has a mirror diameter of about 8 feet. In contrast the Keck telescopes have a diameter of about 32 feet, which means their mirrors have about 16 times the area of the Hubble. Because of the cost, there are also only a handful of optical space telescopes, while there are dozens of large ground-based telescopes.

There is a way that you can minimize the twinkling effect for ground-based telescopes, and it’s known as adaptive optics. The basic idea is to correct for the wave distortion by adjusting the mirror in real time to account for it. The main way this is done is by using a tip-tilt mirror to realign the image, or if the mirror is segmented (as many modern large telescopes are), adjust each segment to correct for the distortion.

Of course to make this correction you have to be able to distinguish between distortion caused by the air and any real variance in what you are observing. There are a couple of ways this can be done. If what you are observing has a fairly bright star in your field of view, then you can adjust the mirrors to keep that particular star in perfect focus. By accounting for the distortion of the bright star you account for the distortion in your image.

But often what you are observing doesn’t have a sufficiently bright star in your field of view. In this case you can use a laser to simulate a star. You can see this trick being used with a sodium laser in the image above. The laser itself is not particularly bright, but there are small amounts of sodium atoms about 60 miles up, and the laser excites them, causing them to glow. Since the glowing atoms are in the telescope’s field of view, it looks like a star. And since 60 miles is higher than most of the Earth’s atmosphere, the distortion of the sodium’s glow is about the same as the distortion of star light. So by keeping your artificial star in focus you can keep your image in focus.

With adaptive optics and a simulated star ground-based telescopes can obtain images that rival those of a space telescope.

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