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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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 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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Yesterday I talked about time dilation due to both gravity and relative motion. The reason for both of these effects is the fact that light in a vacuum always has a constant speed. The first experimental evidence for the constancy of light was performed in 1887 by Michelson and Morley, and by 1984 measurements of light speed had become so precise that it was defined as an absolute physical constant. But why is that? What makes light so special?

The short answer is that it just is. Experiments have found the speed of light is constant to within a few parts per quintillion, which is astoundingly accurate. Experimentally that’s just how light works. But on some level that seems an unsatisfactory answer, and there are underlying reasons why we would expect light to have a constant speed.

On the face of things it would seem that relativity should imply that all speeds are relative. If I’m on a train traveling at 10 km/hr (relative to the ground), and I throw a ball at 5 km/hr relative to the ground, then the ball should be moving 15 km/hr relative to the ground. Likewise, if I see water wave in a canal traveling at 4 km/hr, and I run with the waves along the canal at 3 km/hr, I would see the waves moving at 1 km/hr relative to me. In fact, this idea of “everything is relative” is known as Galilean relativity, and pretty well summarizes Galileo’s view of relative motion.

What makes light different is that it isn’t made of chunks of matter like little baseballs, nor is it a wave in the way water waves or sound waves are. Yes, light can be described as particle-like quanta we call photons, and yes, light can be described as a wave, but unlike other waves light doesn’t wave through a medium. Sound waves are vibrations that travel through air (or other materials). Without the medium of air, there would be no sound, hence the old saying that in space no one can hear you scream. Likewise, water waves can only travel through water.

But light waves occur because of a fundamental connection between electricity and magnetism. If you’ve ever played with battery-powered toys, you know the battery must be in place (and it must be charged) for things to work. When you connect the battery, it uses electrochemical energy to create an electric field in the circuit of the toy. If you’ve ever wrapped wire around a nail and connected it to a battery, you know that the nail becomes magnetized when the battery is connected. Thus, the electric field creating the current in the wire can induce a magnetic field. Likewise, if you take a magnet and shake it back and forth near a coil of wire, you can induce an electric field (and thus a current) in the wire.

The ability to create magnetic fields from electric fields and vice versa is central to our modern generation of electric power. But it also means that electromagnetic waves don’t travel through a medium. A changing electric field induces a magnetic field, and a changing magnetic field induces an electric field. So in essence, the electric and magnetic fields induce waves in each other. Basically, light, like all electromagnetic waves, simply waves itself. As a result, the speed of light can’t be relative to something else, because it doesn’t wave relative to anything else.

This inherent speed of light is built into the very nature of what light is. Since the fundamental leptons and quarks that comprise matter have electric charge, they are also subject to the fundamental nature of light speed. All the strange aspects of time dilation and warped gravity work in such a way that the speed of light is always preserved.

This is why light matters.

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After making a successful landing on 67P/C-G, the Philae probe gathered as much data as it could before entering “sleep mode.” It’s not clear at this point whether it will revive again. While the ESA team had hoped Philae would have remained active longer, the lander did complete all of its major data gathering, so we can call this a win. The reason Philae went into sleep mode is that it happened to land in the shadow of a cliff, which meant it wasn’t getting enough exposure to sunlight to keep its batteries charged. Like many spacecraft, Philae is solar powered. It needs light to keep going.

Generating electric power from sunlight is done by  photovoltaic cells, or solar cells. These work by using a process similar to the photoelectric effect. In the photoelectric effect, photons from the light kick electrons out of a metal, creating a net static charge within the metal. In photovoltaic cells, the electrons are aren’t knocked completely free of the material, but instead are kicked free of their atom into a region where they can flow easily through the material as a whole.

Usually the material used for solar cells is silicon, because it is a semiconductor. That means can sometimes act as a conductor where electrons can move freely through it, and sometimes act as an insulator that resists electron flow. This is important, because simply moving electrons into a conductive region isn’t enough to generate electric power.

Solar panels on the International Space Station. Credit: NASA

What makes silicon so useful is that you can mix it with small amounts of other elements (what we call doping) to create a material that has more free electrons than there are places for them to settle (known as n-type). With other doping elements you can make a material that has more places to settle than there are free electrons (known as p-type). By placing these two types of material next to each other you can create a p-n junction. The extra electrons of the n-type can jump over to the p-type, creating a voltage across the junction. So with a photovoltaic cell, sunlight kicks electrons into the region where they are free to move through the silicon, and the voltage between the p-type and n-type layers causes them to flow, creating a current. This current can then be used to power anything from a pocket calculator to a comet lander.

Solar cells are used on spacecraft because sunlight is plentiful in the inner solar system, and it is a way to maintain power without the limitations of fuel. The only big downside is if you don’t have access to sunlight, which is what happened in the case of Philae.

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The supermassive black hole in the center of our galaxy, known as Sagittarius A*, is pretty quiet for a black hole. It does however flare up from time to time, when material is captured, as can be seen in images from the NuSTAR x-ray telescope. Of course, x-ray astronomy with enough sensitivity to observe x-ray flares at galactic center is a fairly recent development. It would be nice to have a range of data spanning decades, or even centuries.

While we can’t turn back the clock to see past activity of our galaxy’s black hole, we can see past activity through their reflection by gas and dust in the central region of the galaxy. Imagine striking a match in a dark room. When the match is struck, light streams away from it. Some of the light streams in your direction and reaches your eyes. But some of the light streams away from you. It reaches the far wall of the room, and then reflects back toward you.

This happens much too quickly in a dark room, but if you could watch in super super slow motion, you would see the light of the match, then later see the walls glow from the light of the match. At cosmic scales we can see such an effect. As the supermassive black hole flares up, x-ray light streams away from the black hole. Some of the light we would see directly, but some of it streams in other directions, and can travel for years or decades before reaching clouds and dust in the central region. These dusty regions then reflect some of the light, which we see as an x-ray glow years or decades after the initial flare.

This is exactly what a team observed, as presented in a recent article in Astronomy and Astrophysics. The team looked at x-rays coming from molecular clouds in the region of galactic center over a period of 12 years. What they found was a distinct flow of x-ray illumination as the x-rays from a central black hole flare struck closer regions of the molecular clouds first, then later regions.

Credit: NASA/NuSTAR

You can see this in the image from the article above, which shows the earliest x-rays in red, then green and blue. In a few of the circled regions you can see a distinct pattern of red to blue moving from right to left. Other regions are more varied. Since the x-rays have a distinct shift over time, this means the initial flare occurred over a brief time span (a few years or so). Given the estimated distance of the molecular clouds from the central black hole, the active period of the supermassive black hole would have been roughly a few hundred years ago.

With higher resolution observations, we should be able to map the dust clouds through reflected light, as well as gain a better understanding of the active and quiet periods of Sagittarius A*.

All from an echo of light in the night sky.

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There’s been a bit of press regarding “missing light” in the universe. It all starts with a recent paper in the Astrophysical Journal Letters. Most of the popular articles spin things as missing light because that was the spin of the press release, but the actual work is more subtle, and more interesting.

In this work, the team looked at the distribution of neutral hydrogen in the universe. They did this by looking at what is known as the Lyman-alpha forest, which is a series of absorption lines seen in light from quasars and other distant active galactic nebulae. Basically, as the light from a distant quasar (which is highly redshifted) passes through interstellar clouds that are closer (and less redshifted) a series of absorption lines appear in the spectrum.  This “forest” of absorption lines is known as the Lyman-alpha forest. The team did a statistical analysis of various quasar observations, and from that determined the amount of neutral hydrogen in the universe at various distances (redshifts).

Neutral hydrogen is the most common ingredient of interstellar gas, but it is also sensitive to UV light. Ultraviolet light can ionize neutral hydrogen, and the more ultraviolet light there is, the more ionized hydrogen you would expect.  There are lots of sources of UV light in the universe, and together they form what is known as the cosmic ultraviolet background. The team compared the amount of neutral hydrogen observed with the cosmic ultraviolet background, and found that there is more ionized hydrogen than could be accounted for by the cosmic UV background. Specifically, what they found was that the higher the redshift (thus, the greater the distance from us), the better the two agreed. It is only in the “local” region of the universe that they don’t agree.  They don’t agree by about a factor of 5, which is pretty significant.

One way to state this discrepancy is to say that there is “missing” ultraviolet light in the local universe, hence the popular articles. But it could also be the case that we haven’t accounted for all the local sources of UV radiation. There could be ionizing sources that we don’t see directly due to dust, for example. It could also be the case that the statistical models are wrong, though it is hard to see how they would be wrong by a factor of 5.

Of course whenever this kind of oddness shows up, there are always more exotic proposals, such as the idea that dark matter might decay to produce ultraviolet light. But that is really speculative. At this point what we can say is that something is ionizing hydrogen gas in the local universe. Whether it is some unseen source of UV light, or some other mechanism is yet to be determined.

Paper: Juna A. Kollmeier et al. The Photon Underproduction Crisis. ApJ 789 L32 (2014) doi:10.1088/2041-8205/789/2/L32

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A paper has recently been published in the New Journal of Physics claiming that the speed of light is wrong. This has triggered the usual headlines in the popular press, but as the saying goes, “extraordinary claims require extraordinary evidence.” So let’s look at the evidence behind this paper.

The motivation for the work comes from the 1987a supernova. This supernova appeared in the Large Magellanic Cloud, only 168,000 light years away.  It was also the first (and only) time it was detected by neutrinos as well as electromagnetic light.  About three hours before the supernova light reached Earth, neutrino spikes were observed at three separate neutrino observatories.

Now you might think it odd that the neutrinos were observed before the light. After all, nothing can travel faster than light (so it is claimed) so how is this possible?  It turns out that neutrinos are not hindered by regular matter since they interact so weakly with it. Light, on the other hand, interacts strongly with matter. Light  produced in the Sun’s core, for example, can take tens of thousands of years to reach the Sun’s surface due to all the material interactions. In a supernova the light and neutrinos are produced at the same time. The neutrinos pass almost immediately through the star, but the light is “trapped” in the star’s core until a shockwave starts to rip the star apart. At that point the density of stellar material is becomes low enough for the light to escape. Models calculate that this would occur about three hours after the initial reaction.

So the delay of light is due to the physics of a supernova, and not faster-than-light neutrinos.  But it turns out that a couple hours before the three neutrino observatories detected the event, a fourth neutrino observatory observed a small spike of neutrinos.  Since this spike wasn’t observed by the other neutrino detectors, it’s generally thought to be a random event. But the author argues that it is a valid detection from 1987a. Thus the initial reaction started earlier, and the light appeared a few hours later than models predict.

If you assume the first burst is valid, then there are a couple ways you could try to account for it. One is to look at the details of supernova models. You could argue, for example, that there is a double reaction where only the second reaction triggers a shock wave in the star. Thus you get two neutrino bursts, with a delayed appearance of light.  The author of this paper decided to take a different route. He proposes that the first neutrino burst triggered the shock wave, and that the light was delayed because it was traveling slightly slower than we would expect. So based upon a single observation, the author decides to rewrite fundamental laws of physics.

So how do you get light to slow down? To do this, the author argues that over cosmic scales light can be affected by vacuum fluctuations within space.  These fluctuations can produce “virtual” particles, and is a real effect in quantum mechanics. They can create things like the Casimir effect, and are thought to produce Hawking radiation in black holes. So the idea is that a photon traveling through space could split into an electron and positron for a tiny moment, then recombine into a photon.

Diagram of a quantum fluctuation. Credit: J D Franson

This has been looked at before, but the author takes a different view of the effect. Usually these virtual particles are not viewed as “real” in the traditional sense. This is because they are used as a part of a perturbative model where you have to add up all the the parts to get the correct answer.  It’s kind of like adding up bunches of bananas.  If you have a bunches of 6, 5, and 7, you would say you have 18 bananas total. You wouldn’t argue that physically first you have 6, then a moment later you have 11, then finally 18, even though you have counted them that way.  The counting of the bananas doesn’t matter, just the final answer. Basically the author argues that the counting does physically matter. So the photon travels for a while, splits into an electron-positron pair for a fraction of a second, then combines back into a photon.  While the photon is a particle pair it isn’t traveling at the speed of light, so over cosmic scales the travel time of light is slightly longer than expected.  Running the calculations, the author gets the extra few hours needed to account for the first neutrino spike.

Most physicists argue that this method of dealing with vacuum fluctuations is completely wrong, but there is some debate about it on the fringes. The author of the paper does note at the end of the paper that this is a tentative model, but even so it has serious issues, such as the fact that the model violates conservation of energy.  It seems to have been published as an exploration of an idea, which is fine. But it is hardly convincing as an argument that special and general relativity is wrong.

In the end it is an extraordinary claim with very little evidence to support it.

Paper:  J D Franson. Apparent correction to the speed of light in a gravitational potential. New J. Phys. 16 065008 (2014)

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One of the properties of atoms and molecules is that they interact with light in an interesting way. If you heat up atoms or molecules in a gas, they will give off light. But they only give off light at specific wavelengths (colors).

The particular colors they emit depends on the type of atom or molecule it is. So one type of atom might give off red, orange and blue, while another might give off yellow and green. We can look at the full range of colors a particular atom emits, which is known as an emission spectrum. You can see an example of such a spectrum in the figure below.

Credit: Moussas Xenofon and Strikis Iakovos

If you put a cool gas in front of a light source you get what’s known as an absorption spectrum. Basically this looks like a full rainbow of colors with dark lines at particular wavelengths where the gas absorbs that color. The colors a cool gas absorbs are the same colors a hot gas emits.

Each type of atom or molecule has a specific emission spectrum. It’s kind of a chemical fingerprint that allows us to identify the type of atom or molecule makes it. That is particularly useful in astronomy, because when we observe a particular pattern in starlight we know a particular atom or molecule is in the star’s atmosphere. The spectra are also affected by things like temperature and magnetic fields, so we can learn lots of things about a star by looking at its spectra. The closest star to us is our Sun, so we can make very detailed observations of its spectra. One of the tales these spectra tell is a bit of a mystery.

The Sun’s atmosphere can be divided into the photosphere, chromosphere, and corona. The photosphere is the layer where light is emitted from the sun. Within the photosphere we see an absorption spectrum This makes sense because we would expect the atmosphere of the Sun to get cooler at higher levels. The absorption spectrum means the upper layer of the photosphere is cooler than the lower layer.

One would expect the chromosphere to be cooler still, but within the chromosphere we see emission spectra. This means the chromosphere is actually hotter than the photosphere. At its lowest region the chromosphere is about 4,500 K, but at its upper region it is about 25,000 K. The corona is very diffuse, but its temperature is even higher, on the order of a few million Kelvin.

So what’s going on? Why is the upper region of the Sun’s atmosphere so much hotter than the lower region? We aren’t entirely sure, but we have a few ideas. A driving factor is that the solar atmosphere is a plasma. This means it interacts strongly with magnetic fields. The magnetic field lines of the Sun can be twisted by the motion of the plasma up to a point, but eventually snaps back into place, releasing energy. (I wrote about this in a post earlier this week). Another factor is that the chromosphere is very active with solar flares, prominences, etc., and the energy from these tends to heat the chromosphere.

What we aren’t entirely sure of is what mechanism causes the corona to become so extraordinarily hot. Plasma physics is complex, and we’re still figuring it out.

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Yesterday I wrote about the Alcock-Paczynski cosmological test, and how it narrowed the field of cosmological models down to two broad choices: an expanding universe with dark matter and dark energy, or a static universe that exhibits what is known as “tired light”.  Now you might think that adding “tiredness” to light is no worse than inventing “dark matter” and “dark energy” to fit observational data.  From a theoretical standpoint you’d be right.  So why do astronomers accept dark matter and dark energy rather than tired light? 

The short answer is that it’s where the evidence has led us, but let’s look at the details.

The idea of tired light was first proposed by Fritz Zwicky in 1929 (pdf here), soon after Edwin Hubble discovered the relation between the distance of a galaxy and its redshift (now known as Hubble’s law).  The basic idea was that rather than galactic redshift being due to an expansion of the universe, as Hubble suggested, the redshift could be due to another mechanism such as gravitational interaction with galaxies, or collisions with diffuse gas between galaxies.  The further away the galaxy, the more of these type of interactions the light has, and the greater its redshift.

This is not a crazy idea.  In fact, gravitational redshift does occur, and there is a reddening of light due to diffuse gases in intergalactic space.  But we now know these are small effects compared to the redshift for several reasons.

One prediction of tired light is that any type of collision or gravitational interaction that causes a redshift would have the light to lose energy.  This would also cause the momentum of the light to change,  and that would not only cause the wavelength of light to get longer (redden), it would also smear the light slightly.  This means that distant galaxies would not only appear more red, they would also appear blurry.  The more distant the galaxy, the more blurry it would appear.  We don’t see that at all.  Galaxies near or far can appear sharp.

Another prediction is that the redshift we observe is not due to cosmic expansion.  Instead the universe is fairly stationary.  If this were the case, then a particular kind of supernova known as type 1a should brighten and fade in the same way regardless of their distance.  Distant supernova would appear more dim, but the rate at which they brighten and fade should be the same as that of a close supernova.  What we actually observe is that distant supernova actually brighten and fade more slowly than closer ones.  The more distant the supernova, the slower that process is.  This is exactly what is predicted by cosmic expansion.  If distant galaxies are moving away from us faster than closer ones, they should be time dilated by their relative motion to us.  The more distant the galaxy, the more dilated the time, which is exactly what we see.

Perhaps the most convincing evidence that the tired light model doesn’t work can be seen in the cosmic microwave background (CMB).  In the standard model the CMB is the heat remnant of the big bang.  As the universe expanded, it cooled to a temperature of about 2.7 Kelvin.  Just as a hot piece of iron glows red due to its heat, the universe glows due to its 2.7 Kelvin temperature.  It just mainly glows at microwave wavelengths instead of visible ones.  If the CMB is from an early hot period of the universe, then its brightness at different wavelengths should follow a very specific function known as the blackbody curve.  If instead the cosmic background is due to the scattered remnants of tired light, it will follow a different curve.

In the image below, the observational data of the CMB is plotted.  The error bars plotted on the graph are 400 sigma.  Scientific studies usually use 6 sigma as the cutoff for “certainty”, meaning that 99.9999998% of your data falls within that range.  Just for fun, I calculated 400 sigma in Mathematica to see how many 9s I would have in the percentage.  The result came back as 100% to the limit of what Mathematica can calculate.  Basically, if your theory doesn’t fall within those error bars it is so abysmally wrong it can’t be quantified in Mathematica.  The black line shows the blackbody curve expected if the universe is expanding.  The red curve is the curve predicted by tired light.  It’s not remotely close.

So even though tired light agrees with the Alcock-Paczynski test, we can throw it out because of its disagreement with other evidence.  Tired light is an example of a neat model that just doesn’t hold up to the evidence.

And in the end, the evidence wins.

Note: You can also check out this link for more details and images about tired light and why it fails to match the evidence.

Paper:  Zwicky F. On the red shift of spectral lines through interstellar space. Proceedings of the National Academy of Sciences 15 (10): 773–779 (1929)

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In astronomy and astrophysics, much of the information we gather comes to us in the form of light.  Visible light, radio waves, x-rays and the like.  We gather information not only from the type light emitted from an object but also how that light appears to us as an observer.  The two are not often the same.

One way in which light is affected is known as the Doppler effect.  In our everyday lives, we’re familiar with the Doppler effect as it applies to sound.  You might notice when a car or train passes you, its sound shifts downward as it passes.  This is because the sound waves from an object are bunched together as it moves toward you, and stretched apart as it moves away from you.  The bunched together waves sound higher, and the stretched waves sound lower, hence a shift in tone known as the Doppler shift.

For light a similar thing occurs.  Waves of light coming from an object are compressed as an object moves toward us, making them look bluer (blue shifted).  They’re stretched as the object moves away from us and look more red (red shifted).  We don’t notice this color shift in our daily lives because nothing moves very fast compared to light, however stars and galaxies move fast enough for us to measure their redshift or blueshift.  This tells us how fast they are moving relative to us.

Redshift and blueshift can also be caused by gravity.  If you were to shine a beam of light into space, the light has to escape Earth’s gravity.  As it does this, it loses a little bit of energy, making it slightly more red than it was initially.  If you were to shine a light down to Earth from space, the light would gain a bit of energy, making it slightly bluer as it reaches Earth.  This means that we can use the Doppler shift of light to measure both relative motion and gravitational strength.

Of course there is another way that light is redshifted, and that is due to the expansion of the universe.  But I’ll talk about that next time.

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