Artificial light has transformed human society. It frees us from the darkness, and allows us to light our homes and communities. It has also made the night sky increasingly less dark, which poses a challenge to astronomers. And it’s gotten worse in recent years, thanks to an energy-saving light known as LEDs.

The earliest light bulbs (of Edison fame) were extremely inefficient. They produce incandescent light by electrically heating a thin wire of metal to the point that it emits light. But only a small fraction of the light emitted is visible. Most of it is infrared, which we feel as heat. The bulbs make much better heaters than lights, and for a time there were even toy ovens that used a light bulb to bake little cookies or muffins. Their big advantage was that they were cheap and reliable.

As energy costs rose, the quest for greater efficiency led to new types of light bulbs. The most popular were variations known as fluorescent lights. These involve a tube of low-pressure mercury gas. An electric current is passed through the gas, causing it to emit ultraviolet light. The interior of the tube is coated with a phosphorus powder that converts ultraviolet light to visible light. The efficiency of these lights made them ubiquitous in large lighting environments such as office buildings, but their greenish hue and flickering nature were often found irritating. What we really needed was a light that is highly efficient and emits a more sun-like light.

The latest answer to that challenge is the Light Emitting Diode, or LED. These are based on semiconductors. The same type of semiconductors used in computers and other electronic devices, except that they are built to emit light. They are highly efficient, but only emit light in a narrow color range. However by combining LEDs of different colors you can approximate the wide spectrum of colors produced by sunlight.

The spectra of different types of lights.

Early on LEDs were expensive, but even as costs came down they still failed to mimic sunlight well. That’s because there weren’t any good blue LEDs. Early LED lightbulbs were an odd yellowish-orange, since they didn’t emit much light in the blue spectrum. But in 1993 Shuji Nakamura developed an efficient blue LED. This was such a big breakthrough that he was awarded the 2014 Nobel prize in physics for his work. LED lightbulbs could now be produced that emit a more natural white light. But this breakthrough has had unintended consequences.

As the price of white-LED lights have dropped, they have become a favored choice for both interior and exterior lighting. For outdoor lighting in particular they have started to replace mercury vapor and sodium vapor lights. Their low cost and high efficiency has also resulted in a rise of exterior lighting. The white glow of LEDs has filled our communities and homes. But white-LED lights depend upon blue LEDs, and are brightest in the blue spectrum. And blue light pollution is bad for astronomy.

Low pressure sodium lights are often used in protected dark sky regions. Credit: Robert Ashworth

If you’ve ever wondered why the sky is blue, the answer comes from the fact that our atmosphere scatters blue light more than red. Sunlight comes in a rainbow of colors, but it is the blue that is scattered across the sky, giving it a blue hue. Since modern LED lights emit a lot of blue, it is scattered by the atmosphere, giving a diffuse blue glow at night. We don’t notice it with the naked eye, but for astronomers it is a constant glow of light pollution. As more lights are converted to LEDs, the astronomer’s sky becomes ever less dark.

There are ways to help astronomers, such as making sure lights are shielded to focus only at the ground, and using “warm” LEDs that don’t emit strongly in the blue. In particularly sensitive areas, one of the best solutions is to use low-pressure sodium light for exterior lighting. You can recognize these by their distinct yellow-orange glow. That’s because they emit light at at very narrow range of colors, which makes it easy for astronomers to filter out.

Given all the advantages, we aren’t likely to give up artificial lighting. But sometimes we have to balance the desire for a brilliant night with the desire to see the stars.

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Green is an interesting color in astronomy. Our eyes are more sensitive to green than any other color, and so it is a color that is often seen in the night sky. There are green comets, the momentary green brilliance of a meteor, the faint green glow of northern aurora, and even a green glow to some distant galaxies. These objects can have other colors as well, but green is a common color of the night sky. 

While all these objects can share a common color, the mechanism behind their greenish glow can vary widely. Comets, for example take on a green hue because of the gas tail that forms as they approach the Sun. Most of the gas consists of hydrogen, but other compounds such as cyanide (CN₂) and carbon (C₂) can be contained in the tail as well. These molecules emit light at green wavelengths, and can be bright enough to be seen with the naked eye. In fact, cyanide (specifically cyanogen) was the first in Halley’s comet in 1910, and even led to a baseless panic that Earth might be poisoned by Halley’s tail.

The green glow of comet Lovejoy. Credit: Paul Stewart.

Meteors are green for a completely different mechanism. As a meteor enters Earth’s atmosphere, it is heated to the point where its outer layer is vaporized. The metals in the meteor glow with particular colors. Green comes from nickel. The most common metallic meteors are iron-nickel, so green is a common color. This glow tends to be brightest when meteors hit the atmosphere at high speed. For example, fast moving Leonid meteors can often have a green glow.

Galaxies that glow green are known as green pea galaxies. They are compact, and have a strong green emission line from oxygen, making them look like a green pea. The oxygen surrounding the galaxies glows green when it is ionized by the galaxy’s starlight. In order for the oxygen to be bright enough to give the galaxy a green color, there has to be a lot of ionized oxygen, and thus a lot of ultraviolet light produced by young stars. So green pea galaxies are young galaxies where lots of stars are forming, and may have played a role in the reionization of the early universe.

The solar radiation spectrum. Credit: Robert A. Rohde (CC BY-SA 3.0)

Since we’re most sensitive to green light, why are there no green stars? They can be red, yellow, blue, and white, but not green. It all has to do with the way stars produce light, which is very different from other astronomical objects. Comets, meteors, and the gas around galaxies all give off color when particular atoms or molecules give of light. The electrons in these atoms jump from one energy state to another, emitting a particular wavelength of light. But the Sun and other stars produce light from internal heat. Rather than emitting specific colors, stars emit a range of colors known as a thermal blackbody. The brightest color of a star depends upon its temperature. For cooler stars, the brightest color is red, and thus a star appears reddish. Hotter stars are brightest in the blue range, and so appear blue. But a star that peaks in the green is also bright in red and blue. Thus we see red, green, and blue light from the star, which our eyes interpret as white.  Since a thermal blackbody can’t peak at green without being bright in the entire visible spectrum, a green star simply isn’t possible.

But since green is so common in the sky, its absence from the stars is not a big loss.

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Why is the sky blue? It’s a common question asked by children, and the simple answer is that blue light is scattered by our atmosphere more than red light, hence the blue sky. That’s basically true, but then why don’t we see a violet sky?

The blue sky we observe depends upon two factors: how sunlight interacts with Earth’s atmosphere, and how our eyes perceive that light.

When light interacts with our atmosphere it can scatter, similar to the way one billiard ball can collide with another, making them go off in different directions. The main form of atmospheric scattering is known as Rayleigh scattering. If you imagine photons bouncing off molecules of air, that’s a rough approximation. But photons and air molecules aren’t billiard balls, so there are differences. One of these is that the amount of scattering depends upon the wavelength (or color) of the light. The shorter the wavelength, the more the light scatters. Since the rainbow of colors going from red to violet corresponds with wavelengths of light going from long to short, the shorter blue wavelengths are scattered more. So our sky appears blue because of all the scattered blue light. This is also the reason why sunsets can appear red. Blue light is scattered away, leaving a reddish looking sunset.

But if that’s the case, why isn’t the sky violet? Sure, blue light is scattered more than red or green, but violet light has an even shorter wavelength, so violet should be scattered more than blue. Shouldn’t the sky appear violet, or at least a violet-blue? It turns out our sky is violet, but it appears blue because of the way our eyes work.

Color sensitivity of the cones and rods of the human eye. Credit: Wikipedia

We don’t see individual wavelengths. Instead, the retinas of our eyes have three types of color sensitive cells known as cones. One type is most sensitive to red wavelengths, while the other two are most sensitive to green and blue wavelengths. When light we look at something, the strength of signal from each type of cone allows our brains to determine the colors we see. These colors roughly correspond to the actual wavelengths we see, but there are subtle differences. While each type of cone has its peak sensitivity at red, green, or blue, they also detect light of other colors. Light with “blue” wavelengths stimulate blue cones the most, but they also stimulate red and green just a little bit. If it really was blue light that was scattered most, then we’d see the sky as a slightly greenish blue.

We don’t see the greenish hue, however, because of the sky’s violet light. Violet is scattered most by Earth’s atmosphere, but the blue cones in our eyes aren’t as sensitive to it. While our red cones aren’t good at seeing blue or violet light, they are a bit more sensitive to violet than our green cones. If only violet wavelengths were scattered, then we would see violet light with a reddish tinge. But when you combine the blue and violet light of the sky, the greenish tinge of blue and reddish tinge of violet are about the same, and wash out. So what we see is a pale blue sky.

As far as wavelengths go, Earth’s sky really is a bluish violet. But because of our eyes we see it as pale blue.

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One of the big mysteries of modern cosmology is the fact that the Universe is so uniform on large scales. Observations tell us our Universe is topologically flat, and the cosmic microwave background we see in all directions has only the smallest temperature fluctuations. But if the cosmos began with a hot and dense big bang, then we wouldn’t expect such high uniformity. As the Universe expanded, distant parts of it would have moved out of reach from each other before there was time for their temperatures to even out. One would expect the cosmic background to have large hot and cold regions. The most common idea to explain this uniformity is early cosmic inflation. That is, soon after the big bang, the Universe expanded at an immense rate. The Universe we can currently observe originated from an extremely small region, and early inflation made everything even out. The inflation model has a lot going for it, but proving inflation is difficult, so some theorists have looked for alternative models that might be easier to prove. One recent idea looks at a speed of light that changes over time.

The idea that light may have had a different speed in the past isn’t new. Despite the assertions of some young Earth creationists, we know the speed of light has remained constant for at least 7 billion years. The well-tested theories of special and general relativity also confirm a constant speed of light. But perhaps things were very different in the earliest moments of the cosmos. This new work looks at alternative approach to gravity where the speed of gravity and the speed of light don’t have to be the same. In general relativity, if the speed of light changed significantly, so would the speed of gravity, and this would lead to effects we don’t observe. In this new model, the speed of light could have been much faster than gravity early on, and this would allow the cosmic microwave background to even out. As the Universe expanded and cooled, a phase transition would shift the speed of light to that of gravity, just as we observe now.

Normally this kind of thing can be discarded as just another handwaving idea, but the model makes two key predictions. The first is that there shouldn’t be any primordial gravitational waves. Inflation models predict primordial gravitational fluctuations, so if they are observed this new model is ruled out. But it might be the case that primordial gravitational waves are simply too faint to be observed, which would leave inflation in theoretical limbo. But this new model also predicts that the cosmic background should have temperature fluctuations of a particular scale (known as the scalar spectral index n_s). According to the model, n_s should be about 0.96478. Current observations find n_s = 0.9667 ± 0.0040. So the predictions of this model actually agree with observation.

That seems promising, but inflation can’t be ruled out yet. This current model only explains the uniformity of the cosmic background. Inflation also explains things like topological flatness and a few other subtle cosmological issues this new model doesn’t address. The key is that this new model is testable, and that makes it a worthy challenger to inflation.

Paper: Niayesh Afshordi and Joao Magueijo. The critical geometry of a thermal big bang.  arXiv:1603.03312 [gr-qc]

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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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One of the predictions of special relativity is that the speed of light in a vacuum is a universal constant. This prediction has held up so well that we now use the speed of light to define part of the metric system. The first verification of special relativity is typically seen as the Michelson-Morley experiment, which demonstrated there wasn’t a luminiferous aether. But this experiment was actually done before Einstein proposed relativity, and so it wasn’t technically a prediction. It took two other experiments to completely verify Einstein’s model.

The Michelson-Morley experiment focused on determining the speed of the Earth through the aether. It wasn’t designed as a test of special relativity, and so it only tested that the speed of light was the same with different orientations. No matter which way you orient your device, the travel time back and forth along your experiment is the same. That’s certainly a prediction of relativity, but the theory goes further to claim that light speed is the same even if you’re moving at different speeds.

It took two other experiments to fully pin down the veracity of relativity. One, known as the Ives-Stilwell experiment looked at the time dilation effects of the model. In order for the speed of light to be the same in every reference frame, the clock of an experiment moving relative to you must appear to tick more slowly than that of an experiment sitting next to you. This effect is known as time dilation, and is one of the stranger aspects of relativity.

The Ives-Stilwell experiment looks at the light emitted or absorbed by fast moving particles and compares them with the transverse Doppler effect. If an object speeds past you from left to right, when it is directly in front of you would you see any Doppler shift of its light? Since the relative motion along your line of sight at that moment is zero, you might think there would be no shift. But since the object is speeding past you, its time should be dilated. As a result there should be a Doppler shift. The experiment confirmed the Doppler shift just as relativity predicts.

But relativity also predicts that space and time are connected, so a time dilation must also create a change of apparent length (known as length contraction). In other words not only must the clock of a moving experiment appear slower, then length of the experiment must appear shorter. Ives-Stilwell confirmed the first part, but not the second. To do that took a different test known as the Kennedy-Thorndike experiment.

Schematic of the Kennedy-Thorndike experiment.

The Kennedy-Thorndike experiment is similar to the Michelson-Morley. A beam of light is split to travel along two different paths. The separate beams of light are then recombined to create an interference pattern. The main difference is that the path length of the two beams is radically different. Since (according to Michelson-Morley) the speed of light is independent of orientation, the travel time of each path is different. Since Ives-Stilwell verified time dilation, as the apparatus moves with Earth, the amount of time dilation along one path is different from the other. This would produce a shift in the resulting interference pattern unless the lengths of the two paths also contract as relativity predicts.

The Kennedy-Thorndike experiment found no apparent shift in the interference pattern. Combined with the results of Michelson-Morley and Ives-Stilwell, this confirms that the speed of light is constant, and time dilation and length contraction both occur in agreement with special relativity.

And that’s why relativity is the strangest theory we know is true.

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The more distant a galaxy, the more its light is redshifted. This effect, first observed by Edwin Hubble, is one of the main points of evidence for cosmic expansion. But this redshift effect is often misunderstood. After all, if light from a distant galaxy is redshifted, and the energy of a photon depends upon its wavelength, doesn’t that mean the photon is spontaneously losing energy? What about conservation of energy?

While the energy of a photon does depend upon its wavelength, and its energy is conserved, that only tells part of the story. While energy is conserved in a particular reference frame, the amount of energy you observe can vary depending upon your frame of reference. Imagine, for example, that you are riding in a train, and you place a cup of coffee on a table. From your vantage point the coffee is just sitting in place. From your perspective it has no kinetic energy, and it will continue to sit on the table as long as the train continues to move at a constant speed. From a pedestrian standing outside the train the coffee mug (and the train) are moving along at some speed. It has some kinetic energy because of its motion. From both vantage points the kinetic energy of the coffee cup is constant, but the amount of energy seen from each viewpoint is different. The same is true with light.

Of course the redshift from cosmic expansion is different from that of relative motion. With cosmic redshift space expands between the time light is emitted and the time it is absorbed, so it seems as if the light spontaneously redshifts as it travels. What’s really going on is that spacetime is warped, giving the illusion of spontaneous redshift, just as the warping of space can cause the path of light to bend in space even though light always follows the shortest path through spacetime. The energy of a photon is still conserved along its path, even though the galaxy that emits the photon and our observation of the photon will perceive different energies.

This is why the cosmic redshift model is very different from the so-called tired light model, which assumes that light spontaneously loses energy rather than the universe is expanding. Not only does tired light violate conservation of energy, it makes very different predictions about how the universe would look. When you compare it to observation, the tired light model fails.

So even though the light from distant galaxies are redshifted, the light’s energy is still conserved.

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One of the great things about science is how we keep testing our assumptions. Even when a phenomenon has been rigorously tested, we still push the limits of observation. Take, for example, some recent research on the speed of light.

According to relativity, the speed of light in a vacuum is an absolute constant. We’ve observed this fact for about a century, and by 1989 our measurements had gotten so precise that it’s been used to define the length of a meter ever since. As far as we can tell, the speed of light should be the same in all directions, and in all places in the universe. It’s a property often known as Lorentz symmetry or Lorentz covariance. If Lorentz symmetry is violated, then relativity is wrong. Some models trying to unify general relativity with quantum theory actually predict a violation of Lorentz symmetry by a tiny amount, so precise tests of it could provide clues to a truly unified physical model. But according to recent results presented in Nature Communications, relativity still holds true.

The usual interferometer method vs the cavity resonance method. Credit: Nagel, et al.

The most popular method of demonstrating Lorentz symmetry follows the original work of Michelson and Morley, where a beam of light is split into two beams and reflected back so that the beams interfere with each other. This interferometry method is typically the method done by undergraduate physics students. But if you’re more interested in whether light speed varies with orientation, then a more accurate method uses cavity resonance. Laser beams are bounced within a chamber. This can create a resonance in the chamber.

The accuracy of Lorentz symmetry measurements over the past 140 years.

If you’ve ever swirled a wet finger around the rim of a wine glass, you know you can cause the glass to “ring” at a particular tone. Swirl your finger too quickly or slowly and it won’t work. There’s a “sweet point” where your finger moves just right, and the glass sings. A similar effect occurs with the resonator cavity. Light at just the right frequency will cause the cavity to resonate. If the speed of light is truly the same in all directions, then the frequency that works should be the same no matter what the orientation of the chamber. To eliminate any background vibrations, the team cooled their experiment to a mere 4 K.

In the end the team found no variation of the speed of light in different directions. Specifically, they confirmed Lorentz symmetry to 9.2 × 10⁻¹⁹, which is an incredible precision. Once again, relativity passes the test.

 Paper: Moritz Nagel, et al. Direct terrestrial test of Lorentz symmetry in electrodynamics to 10⁻¹⁸. Nature Communications 6, Article number: 8174 (2015)

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The speed of light, c, is an absolute physical constant. No matter where you are in the Universe, or how fast you’re moving relative to something else, the speed of light in a vacuum is always the same. That’s often taken to imply that nothing can travel faster than light, but things aren’t quite so simple. It turns out there are several ways things can travel faster than light, depending on what you mean by a “thing,” “faster-than-light,” and “travel”.

One way is to note that the immutable speed of light only applies to light in a vacuum. When light travels through a material, its effective speed is reduced. This is often given by an index of refraction, n, where the effective speed of light is c/n. (And n is pretty much always greater than 1.) For example, when light travels through water, its speed is about 0.75c. Because of this, it is possible for particles to “break the light barrier” in a material while still traveling less than c.

The blue glow of Cherenkov radiation. Credit: Matt Howard

For example, in nuclear reactors electrons are emitted with so much energy that they are traveling at nearly the speed of light c. When those electrons travel through the coolant (water) surrounding the reactor they travel faster than light can travel through the water, thus breaking the light barrier. You’re probably familiar with a sonic boom that occurs when a plane travels faster than sound, which is caused by a shock wave of air. A similar effect occurs when an electron breaks the light barrier. The electron causes an optical “shock wave” known as Cherenkov radiation, which gives nuclear reactors their blue glow.

A random path of a photon through the Sun. Credit: ATNF

Another phenomenon that can travel faster than light through a medium is sound waves in a star. In the Sun (as with any star) light is produced in its core through nuclear fusion. Traveling at the speed of light, it should be just a two-to-three second journey to the surface of the Sun. But the Sun’s interior is packed so densely with charged particles that light can’t simply travel in a straight line. On average, a photon in the Sun’s core will travel less than a centimeter before colliding with an ion. It is then scattered in an almost random direction. Imagine a photon trying to leave the Sun, but getting bounced in a random direction every centimeter. This random walk of a photon through the Sun means that it actually takes about 20,000 to 150,000 years for light to travel from the Sun’s core to its surface.

Animation of the l=3, m=0 mode of oscillation (shown greatly exaggerated). Credit: D. B. Guenther

But sound waves propagate in a different way. They are pressure waves that transfer energy through a material rather than transporting material itself. As a result, they aren’t hampered by the ions in the core. Sound waves can travel through the Sun at thousands of meters per second, and they cause the Sun as a whole to vibrate. The study of these sonic vibrations is known as helioseismology, or in the case of other stars, asteroseismology. By analyzing these sounds we can determine things such as the density and pressure of the Sun’s interior.

While that’s all fine and good, you might argue, neither of these phenomena are actually traveling faster than light. What about something moving faster than the speed of light in a vacuum? It turns out that even that is possible in a way, thanks to general relativity.

Since the 1920s we’ve known that the more distant a galaxy is, the greater its apparent redshift, and thus the faster is appears to be moving away from us. This relation between redshift and distance is known as Hubble’s law. Over time we’ve come to understand that this relation is not due to galaxies racing away like an initial explosion from a single point, but rather it’s due to the fact that space itself is expanding.

Astronomy Magazine, 2007

The rate of cosmic expansion is determined by what is known as the Hubble constant. Currently our best measurement of the Hubble constant is about 20 km/s per million light years. This means that two point in space a million light years apart are moving away from each other at 20 kilometers per second. Since all of space is expanding, the greater the distance between two points in space, the faster they move apart. Because of this, if you consider two points far enough apart they will be moving away from each other faster than light. Since the speed of light is about 300,000 km/s, with our current Hubble constant that means that critical separation distance is about 15 billion light years.

A galaxy 16 billion light years away is moving away from us faster than light, but that distant galaxy isn’t defying relativity. After all, from that distant galaxy’s perspective we are moving away from it faster than light, speed being relative and all. The key point to remember is that this relative motion is due to cosmic expansion, not galactic motion. Relativity requires that nothing can move through space faster than light. It places no constraint on the expansion of space itself.

Credit: Take 27 Limited / Science Photo Library

Perhaps the most bizarre faster than light interaction involves quantum entanglement. Suppose we have a mischievious mutual friend. She decides to prank us by sending each of us one member of a pair of gloves. She packs each glove in a box and mails one to each of us. We find out about the prank, so we both know that we’re getting one glove of a pair. But until either of us open our respective box, neither of us know which glove we have. Once the box arrives at your door, you open it up, and find you have the left glove. At that exact moment, you know I must have the right glove.

This is the basic idea of an experiment known as the Einstein-Podolsky-Rosen (EPR) experiment. For gloves it isn’t a big deal, because gloves are not quantum things. In quantum theory, however, things can be in an indefinite state until you observe them. It would be as if your boxes contained a pair of something (gloves, shoes, etc.) but it is impossible to know what specific something (left/right, spin up/down, etc.) until a measurement is made.

In quantum theory we would say the boxes contain a superposition of possible things, and the outcome only becomes definite when observed. This means the outcomes of opening our boxes are entangled. Knowing the contents of one box tells us the contents of the other. We’ve actually done this experiment with photons, atoms and the like, and it really works.

Schematic of the quasar EPR experiment. Credit: Jason Gallicchio, Andrew S. Friedman, David I. Kaiser

The experiments we’ve done have pretty much eliminated any possibility for things like hidden variables or pre-determined results. Usually this is done by waiting until the two “boxes” are sent on their way, and then using a random number generator to determine whether your test for “gloves” or “shoes” or whatever. Since the choice of what to look for is random, and that choice is made after the experiment has started, there is no way for the system to have advance knowledge of the outcome. In principle, you could do an EPR experiment across billions of light years. One proposal is to use quasars as a trigger to determine what observation each person makes instead of a random generator.

If we use the standard Copenhagen interpretation of quantum theory, an observation of one state “collapses the wavefunction” of the entangled system. This collapse occurs instantly, and thus is clearly faster than the speed of light. But this again doesn’t violate relativity, since information about the system doesn’t travel faster than light. In other words, your knowledge about the measurements the other observer will make in no way gives information to anyone faster than light; the observations they can make will all be unaltered from their indeterminate state until they receive information from you about what’s in their entangled system.

Credit: Les Bossinas (Cortez III Service Corp.), 1998 / NASA

For an actual experiment of entangled photons, each observer has to decide what to measure (typically something like polarization). If I decide to measure vertical polarization and get an up result, the only thing I know is that if you also measured a vertical polarization you would get down. If you measure a different orientation, say horizontal, I don’t know what your result will be. Even if we agree ahead of time to both make vertical measurements, knowing the other’s outcome doesn’t tell us anything, because the outcomes are random. What this all means, unfortunately, is that there’s no way to use this to send a faster than light message.

Lots of things can travel faster than light, and it’s pretty straight forward to create devices that do this, but that’s because relativity doesn’t prevent FTL travel. What it really forbids is the transfer of information or objects through space at a speed greater than that of light in a vacuum. So — unless we master the extreme curving spacetime and passing objects through wormholes — it will still take us years, or centuries, or millennia, to get to the stars.

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What would the universe be like if the speed of light were infinite? It might seem like a silly question, since the speed of light clearly isn’t infinite, but questions like these are a good way to explore how different aspects of a physical model are interrelated.

For example, in our universe light is an electromagnetic wave. It not only has a speed, but a wavelength. If you think of a wave as an oscillation, then at infinite speed light would have no time to oscillate. So infinite light can’t be a wave. Since the wavelength of light determines its color, that would also mean it has no color. But it gets worse because in classical physics light is produced when electromagnetic waves cause the charges in atoms and molecules to oscillate. Without waves, atoms can’t be induced to emit light, the universe would be a sea of darkness.

But real light actually has both wave and particle aspects, so let’s suppose that for infinite light it’s just some kind of particle so we can still have light and color without all that meddling wave business. What else would change?

Relativity is an obvious choice. Einstein’s theory of relativity depends upon a finite speed of light. With an infinite light speed, all those fun things like time dilation are thrown out the window. So is Einstein’s most famous equation, E = mc². The main consequence of this equation is that matter can be transformed into energy and vice versa. It’s central to things like nuclear fusion, which powers the stars and creates the heavy elements. Stars could still be powered by gravitational contraction, but they would only last for a million years rather than billions of years. They also wouldn’t have any mechanism to explode as supernovae, so there would be no way to make new stars from old ones.

Since Einstein’s theory of gravity is a generalization of special relativity, it goes away too. Our model of the universe, beginning with a big bang and expanding through dark energy, depends upon Einstein’s theory. Without it the universe look very different. No dark energy, possibly no big bang.

Of course this is all just a game of pretend. If you made different assumptions about physical phenomena you would derive different effects. We have no way of knowing what an infinite light speed universe would really be like. But what this shows is just how interconnected different aspects of a physical model actually are. Any tweak to the model has consequences that can ripple into widely different areas, or even cause an entire model to collapse.

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Large radio telescopes are not solid dishes, but rather an open mesh. This is largely done to save on weight, since the dish doesn’t need to be solid to reflect radio waves. You see a similar thing on a microwave, where the door is covered with an open mesh so you can watch what’s cooking. The mesh blocks microwaves from reaching you, but not visible light. But how is that possible? Aren’t photons supposed to be point particles? Radio, microwaves are light, so how does a mesh prevent their photons from passing through it?

The short answer is it’s complicated. Although photons are often referred to as particles, they aren’t particles in the traditional sense. They are light quanta, which have both particle and wave properties. Depending on the situation, sometimes the particle aspect is useful, and sometimes the wave aspect is. While photons don’t have a physical diameter, and can be treated as point particles, their quantum behavior gives them a probabilistic size. As a photon gets closer to another object, the chance of it interacting becomes greater. This is often represented as a cross section given in terms of area.

Different cross sections vs energy for light with lead. Credit: J. H. Hubbell

You could say the cross sectional area represents the “size” of a photon, but the problem with this is that a photon’s cross section varies depending on what it is interacting with. Photons interact pretty strongly with electrons, and have a relatively large cross section, but with neutrons the interaction is smaller. Under this definition there is no absolute “size” to a photon. The cross section also depends upon the energy of the photon and things like its polarization. There are also different cross sections for absorption vs scattering.

Despite all this complexity, there is a basic way to talk about the effective “size” of a photon, and that is through it’s wavelength. Very roughly, the interaction range of a photon scales with its wavelength, so light of longer wavelength is more likely to interact with a conductive material than light of a shorter wavelength. In this way you could say the “size” of photon is basically the width of its wavelength. The  wavelength of green light is about 500 nanometers, or two thousandths of a millimeter. The typical wavelength of a microwave oven is about 12 centimeters, which is larger than a baseball.

It’s a bit odd to think of a microwave photon as being larger than a baseball, but it is a simple way to explain how mesh reflectors work. The “size” of microwave and radio wave photons is simply too large to fit through the mesh, and so are reflected. Visible light is much smaller, so it easily passes through the mesh. It’s important not to take this model too literally, but it’s good enough for rough estimates.

And that’s about the size of it.

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Often in astronomy we’ll show false color images of celestial objects to enhance the visual appeal, or to show how an object appears at different wavelengths. While it might look like the above image is a similar multi-wavelength view, it’s actually much more sophisticated. Rather than simply being a few at different wavelengths, the four images of the Milky Way are each focused on the origin of the light in question, and each tells a particular story about our galaxy.

The red image in the upper left is light that follows a thermal spectrum (what we call blackbody radiation). In this case the source of the light has a temperature of about 20 K. This is the temperature of much of the interstellar dust in our galaxy, so this image shows the distribution of dust in the Milky Way. The central band is the plane of our galaxy, and is where most of the dust lies, though it’s clear that most of the sky is somewhat dusty. This is the main reason why efforts to observe effects of inflation in the cosmic microwave background have been so problematic.

The yellow image in the upper right is light emitted at a specific radio wavelength emitted by carbon monoxide. Wherever you see that wavelength, you know carbon monoxide is there. Carbon monoxide is abundant in stellar nurseries, and where stars are actively forming. Carbon monoxide is much less abundant than hydrogen in these regions, but light from hydrogen is much more dim.

The green image in the lower left is a bit more complicated. It shows thermal bremsstrahlung light, formed when particles collide with each other. The light is also known as free-free emissions, because it is produced by electrons colliding with hydrogen ions and not getting captured to form neutral hydrogen. Thus the electrons and ions are “free” both before and after the collision. Like thermal blackbody emissions, thermal bremsstrahlung can be identified by its overall spectrum. Free-free emissions typically occur where there is hot, ionized gas, such as near massive stars.

The last image in blue shows a type of light known as synchrotron radiation. When charged particles move through a magnetic field, they begin to move in a helix along the magnetic field lines. As they are accelerated in a circle they emit synchrotron radiation. This radiation is brightest when high-energy electrons are trapped in strong galactic magnetic fields. So the image show how the Milky Way’s magnetic field traps high energy electrons produced by things such as supernovae.

With all these images combined we can see where material is, where it’s collapsing under gravity, where it’s heating up, and where it’s energized. It’s a view of our galaxy that extends far beyond simply looking at images from different wavelengths. From the cosmic light we can learn not only what’s there, but also what it’s doing.

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