Albert Einstein is easily one of the most brilliant physicists who ever lived. His theories of general relativity changed our understanding of the cosmos, as did his work on quantum theory. But his genius has also led many to hold him up as a poor stereotype of science. The lone genius who ignores the science of his day to overturn everything with a simple brilliant theory. He’s become the icon of every crackpot who feels compelled to send emails to scientists about their idea that will revolutionize science if we only take the time to listen (and work out all the math for them). But as revolutionary as Einstein’s ideas were, they weren’t entirely unexpected. Other scientists had similar ideas, and developed similar equations. Take, for example, Einstein’s most famous equation, E = mc². 

The equation appears in Einstein’s 1905 paper “Does the Inertia of a Body Depend Upon Its Energy Content?“, and it expresses a fundamental connection between matter and energy. Energy was long known to be a property of matter in terms of its kinetic motion, heat and interactions, but Einstein’s equation proposed that matter, simply by having mass, has an inherent amount of energy. It allowed us to understand how radioactive particles decay and how stars create energy through nuclear fusion. But the idea had been proposed by others before.

Like Einstein, J. J. Thompson wondered about the connection between light and matter. He thought that electromagnetism was more fundamental than Newton’s laws of motion, and tried to figure out how mass could be created by electric charge. In 1881 he showed that a moving sphere of charge would create a magnetic field, and this caused a kind of drag on its motion. This acts as an effective mass of the charge. Thompson found that the electromagnetic mass of the electron is given by m = (4/3) E/c², which is surprisingly close to Einstein’s equation. Thompson’s derivation was rather cumbersome, but other researchers found the same result with more elegant derivations.

Thompson’s model was not without it’s problems. For one, it only applied to objects that have charge, and only when they are moving. Another problem came from Thompson’s assumption of a uniform sphere of charge. If an electron were an extended sphere of charge, some kind of force or pressure must keep the electron from flying apart. This pressure would obviously have some energy. This led Henri Poincaré to propose non-electromagnetic stresses to hold the electron together. When he calculated the energy of these stresses, he found it amounted to a fourth of an electron’s total mass. Thus, the “actual” mass of the electron due to its electric charge alone must be  m = E/c². Poincaré’s paper deriving this result was published in June of 1905, just a few months before Einstein’s paper.

Although the equation is often attributed to Einstein’s 1905 paper, Einstein didn’t actually derive the equation from his theory of relativity. The paper is only two pages long, and only shows how the equation can arise from approximations to relativity. It’s more of a proof of concept than a formal derivation. It took other scholars to definitively prove that the equivalence between mass and energy is a consequence of special relativity.

None of this detracts from Einstein’s brilliance, but it does demonstrate that even radical ideas in science rarely come from a single individual. The ideas of Thompson,Poincaré, and others were on the right track, as were the ideas of Einstein. Over the decades the scientific evidence we’ve gathered has further confirmed Einstein’s theory as the best representation of reality. And in the end it’s the best models that win, regardless of who first thought of them.

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If you caught the eclipse this week, you probably took care to use eclipse glasses or a pinhole camera. But you may have caught a glance at the Sun with the unaided eye. Perhaps while looking at the Sun during totality you saw a bit of the Sun come back before looking away. If you did, you wouldn’t be alone. Lots of people have looked at the Sun before, including Galileo. 

Galileo is most famous for his telescopic observations. His discovery of the the moons of Jupiter and the phases of Venus changed our understanding of the solar system. But Galileo also made naked-eye observations of the heavens. As part of his work he looked directly at the Sun many times. This was not one of Galileo’s best ideas, but he did try to be careful about it. He never looked directly at the Sun when it was high in the sky. He looked when the Sun was low on the horizon at dawn or sunset, often when somewhat obscured by fogs or clouds. When the Sun is low in the sky its light must travel through much more atmosphere to reach us. This not only makes the Sun less intense, it gives the Sun its reddish glow. Galileo found the addition of fog particularly useful, because it allowed Galileo to observe sunspots on the Sun.

Although we don’t know how often Galileo looked directly at the Sun, we know from his letters that he made several observations. He may even have looked at the Sun through a low power telescope. Later, Galileo’s student Benedetto Castelli discovered the projection method for looking at the Sun (using a pinhole camera), which Galileo felt was a far superior method of solar observation. Toward the end of his life, Galileo did go blind, but this was more than two decades after his direct solar observations.

Looking directly at the Sun is always risky. This is particularly true during a total eclipse when the Sun is mostly covered. If you take an extended look at the Sun during a partial eclipse you can damage your eyes permanently. There are several recorded cases of this. But a quick glance isn’t likely to be harmful. If you caught a quick glance during the eclipse, you shouldn’t worry.

And if you still have your solar glasses, you can hang on to them until the next great American eclipse in 2024, or you can donate them to Astronomers Without Borders for others to use.

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Next month a total solar eclipse will be seen across the United States. It is one of the few eclipses to trace a path through several densely populated areas, and that means there’s plenty of opportunity to do some experiments, including one that’s stirred a bit of controversy for the past 60 years.

The most famous eclipse experiment is Arthur Eddington’s 1919 experiment showing that starlight is deflected by the mass of the Sun. It was the first experiment to confirm that Einstein’s theory of relativity was correct. But perhaps the second most famous eclipse experiment was performed in 1954 by Maurice Allais. Allais was an economist, and won the Nobel Prize in Economics in 1988. But he was also interested in alternative theories of gravity and electromagnetism. He thought that gravity could be an effect of a cosmic aether, and that effects of this aether could be observed during a solar eclipse. So in 1954 Allais conducted a simple experiment with a Foucault pendulum.

A basic pendulum is simply a mass connected to a cable or rod. When the pendulum is released, it swings back and forth at a regular rate. But given a bit of time time, the orientation of a pendulum will shift. The direction of its back and forth motion will change. This was first noticed by Léon Foucault in the 1850s. As Foucault demonstrated, the gradual shift of a pendulum is due to Earth’s rotation. Everything on the Earth moves around in a circle once a day. If you are on the equator, you would travel the entire circumference of the Earth in 24 hours. If you are near the north pole, you would travel only a small circle in 24 hours. This means that while everything on Earth moves in a circle once a day, things closer to the equator move faster than things closer to the Earth’s poles. Your speed depends upon your latitude. As a pendulum swings, it will be slightly closer to the equator at one part of its swing, and slightly farther away at another part. As a result, the motion of the Earth causes the orientation of the pendulum to shift slightly with each swing, an effect known as precession. The effect is very small, but it builds up.

Graph showing the precession rate shift during an eclipse. Credit: Allais, original paper.

Because the precession is due to Earth’s rotation, traditional physics says the rate of precession should be the same during an eclipse as any other time. But when Allais did his experiment, he found the rate of precession shifted during the eclipse. This came to be known as the Allais anomaly, or Allais effect (not to be confused with the Allais paradox, which deals with his economics work). This was unexpected, and it generated a lot of criticism. One of the main arguments was that Allais was not an experimental scientist. Although the experiment seems simple, it could be influenced by things such as atmospheric changes of temperature, pressure and humidity which can occur during a total eclipse. Eliminating these factors is challenging, even for an experienced experimentalist. A second criticism was that there is no clear mechanism for such an anomaly. Even Allais didn’t have a claim a mechanism beyond some vague non-traditional effect.

Of course the real proof of the pudding would be to repeat the experiment. Either the effect is real or it isn’t, and further experiments should lead to the truth. But total solar eclipses are not overly common, and they don’t often occur over university research labs. So only a handful of experiments have been done, and they’ve been done with equipment of widely varying precision. The results have been mixed. Allais repeated the effect in 1959, and found the effect again. In 1965 an experiment using a more precise gravimeter device found no effect, while a pendulum experiment in 1970 found some effect, though the cause was unclear. One of the more precise pendulum experiments was performed in 1990 by Tom Kuusela. Kuusela found no effect to within 1 part in 4 million. Another one by Horacio R. Salva in 2010 also failed to see any effect. So it seems pretty clear that the effect isn’t real. But that hasn’t stopped the controversy. There are a few experiments that claim to observe the effect, though they tend to be published in less mainstream journals. Supporters of the effect have cited it as evidence for everything from dark matter to the electric universe and a flat Earth. If you delve deep enough you find accusations that NASA covered up research from several eclipse experiments.

Long story short, the Allais’ effect was likely due to experimental error, but it remains an interesting story.

Paper:  Maurice Allais. Should the Laws of Gravitation be Reconsidered?, Aero/Space Engineering 9, 46–55 (1959)

Paper: Kuusela T. Effect of the solar eclipse on the period of a torsion pendulum, Phys. Rev. D 43, 2041–2043 (1991)

Paper: Horacio R. Salva. Searching the Allais effect during the total sun eclipse of 11 July 2010. Phys. Rev. D 83, 067302 (2011)

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From 1645 to 1715 the Sun entered an extended quiet period known as the Maunder minimum. During that time there was almost no observed sunspot activity on the Sun. It was also a period of abnormally cold winters in Europe, known as the Little Ice Age. A similar, but smaller period of solar quiet from 1796 to 1820 (known as the Dalton solar minimum) correlates to the period of Charles Dickens’ childhood, whose books engrained the idea of a “White Christmas” in English society. These events support the idea that sunspot activity could have an effect on global temperatures. But a new look at the Little Ice Age finds that the connection isn’t quite so clear. 

The existence of the Maunder minimum is well established. Sunspot observations of the time were quite good, and they agree with indirect measurements of sun activity such as the levels of deuterium and oxygen-18 found in ice cores of the time, which are lower in periods of minimum solar activity. But what about those record cold temperatures?

A measure that’s often used is the number of frost fairs held in London. These were Winter festivals held when the Thames would freeze over. There’s a higher number of recorded frost fairs during the Maunder minimum than in other times. Add to that historical reports of bitter cold at the time, and you have an image of an unusually cold few decades. But when a team looked at the details of these historical records things got a bit more fuzzy. For one thing, there were years where the Thames froze over, but no frost fair was held. Things like widespread disease, food shortages, and cultural or religious opposition could prevent heavily influence whether frost fairs were held. Likewise, reports of an unusually cold period of Winter often speak of a brief period of unusual cold rather than an extended Winter. Then there are reports of Summer temperatures that would seem to contradict the “ice age” idea. Reports from both London and Paris tell of an unbearably hot Summer of 1701, which was in the center of the Maunder minimum. If sunspots really had an ice age effect, both summers and winters should be unusually cool.

CET data shows a period of colder temperatures during the late Maunder minimum. Credit: Wikipedia

Fortunately we do have some objective temperature data. The Central England Temperature record (CET) has made monthly temperature reports since 1659, and daily reports since 1722. It is the oldest set of instrument-based temperature measurements we have. We also have indirect measurements such as isotope ratios, tree rings and ice cores for the period. What we find is that there was a decrease in temperature for the period of about 0.5 °C. The colder temperatures were also centered around Northern Europe, and global temperatures were not affected in the same way. So it was a real and noticeable effect, and it agrees with reports of the time, but is hardly large enough to make the “little ice age” label very accurate.

It is still possible that solar activity does have some correlation with global temperatures, but it isn’t as strong or direct an effect as is implied by the legend of the Little Ice Age. And that’s perhaps the most important takeaway from this work. Personal experience of a cold winter or hot summer can be extremely compelling when talking about an issue like global climate change. It’s hard to accept global warming when you’re in the middle of a cold February. That’s why objective data matters, and that’s why the science of global climate change works.

Paper: Mike Lockwood, et al. Frost fairs, sunspots and the Little Ice Age. Astronomy & Geophysics (2017)

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Modern astronomy has a rich Islamic history. As with many cultures, the motions of the Sun, Moon and stars are an important part of Islam, and so Muslim astronomers needed to develop sophisticated astronomical techniques. To this day, many stars in the northern hemisphere continue to bear the Arabic names given them by Muslim astronomers. 

Using a quadrant to measure the stars.

Some of the biggest achievements in astronomy stem from the Islamic golden age, spanning the 9th to the 12th century. During this period, sophisticated mathematics was developed to accurately determine the first appearance of the crescent moon. Tools such as the quadrant were developed to make precise measurements of stellar positions. In the 9th century, Sunni astronomer Alfraganus wrote The Compendium of the Science of the Stars, where he gave a summary of Ptotemaic astronomy, giving revised values for the motions of the Sun and Moon, as well as the circumference of the Earth.

In the 10th century, Azophi wrote the Book of Fixed Stars, which contains the earliest recorded observation of the Andromeda galaxy, which he describes as a small cloud. He also recorded the Large Magellanic Cloud, which wasn’t identified by Europeans until the 16th century. In the 11th century, the great scientist Al-Biruni wrote Canon Masudicus, where he discussed Earth as a moving body, and the ideas of a heliocentric universe, five centuries before Copernicus’ model was widely adopted by astronomers.

Although it’s common to focus on Islamic scientists of the golden age, Muslim scholars have continued made significant contributions to astronomy and other sciences. Farouk El-Baz worked on NASA’s Apollo mission, and helped to select the landing sites. Kerim Kerimov played a central role in the development of Russia’s space program. Abdus Salam won the Nobel prize in physics with  Sheldon Glashow and Steven Weinberg for his work on the electroweak theory. Human progress and understanding has been made richer for the efforts of these scientists.

Unfortunately, the leadership of my country has seen fit to paint people such as these as dangerous and unwelcome because of their heritage and place of birth. Such a view not only goes against the ideals of scientific advancement, it goes against basic human decency. It is to our loss and shame that such Muslim contributions to humanity are buried with racism and hate.

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One of the classic images of the Christmas holiday is that of three wise men or kings traveling to Bethlehem, over which hangs a brilliant star. The Star of Bethlehem has its roots in the opening verses of the Gospel of Matthew, which states: 

Now when Jesus was born in Bethlehem of Judaea in the days of Herod the king, behold, there came wise men from the east to Jerusalem, Saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him.

The wise men, or magi in the original Greek, are often represented as kings, but in context were likely astrologers. According to Matthew, they saw some astronomical event as a foretelling of Christ’s birth. There is debate among scholars as to the historical accuracy of event. Matthew is the only gospel that mentions the magi, and many scholars feel it’s a bit of pious allegory to show that the destiny of Jesus was written in the stars. But if we assume for the moment that Matthew’s account is accurate, it raises the question of what exactly the Star of Bethlehem could have been.

Although it’s referred to as a star, it’s clear that’s the one thing it couldn’t have been. On human time scales, stars are fixed and unchanging, and astrology of the time didn’t focus on the stars themselves. Instead it focused on astronomical events, such as the last appearance of a particular star before sunrise, or the conjunctions of stars and planets. One clue is buried in the verse itself, where “in the east” could also be interpreted as “at the rising.” This could be a heliacal rising, where a constellation or planet appears in the sky just before sunrise. For example, the first appearance of Venus as the morning star. Since Venus was long known to astronomers, its heliacal rising would not have been special by itself, but it could have been seen as significant if paired with another bright planet such as Jupiter. One idea proposed by Craig Chester is that it could have been the morning of Venus near the star Regulus (in the constellation of Leo, the lion) followed by a morning conjunction with Jupiter about nine months later. This occurred around 2 BC, but Herod likely died in 4 BC, meaning it didn’t occur “in the days of Herod.”

Adoration of the Magi, by Giotto (1266–1337), represents the Star of Bethlehem as a comet.

Another possibility is that the Star of Bethlehem was a comet. Bright comets do appear in the sky from time to time, and have been described as “hanging over” particular cities or lands, as the Star of Bethlehem is often represented. We know that Halley’s comet was visible in the region in 12 BC, and would have been bright enough to be described as a star. But the writers of Matthew would likely have known the difference between a comet and a star, and specifically noted the event as a star. There’s also the fact that comets were generally seen as bad omens, rather than good ones, so it would be unlikely that a comet would mark such an auspicious birth.

A third possibility is that it could have been a nova or supernova. These appear in the sky as “new stars” and are sometimes brighter than even Venus or Jupiter in the night sky. This would match the Biblical description, and might have been interpreted an a good omen. Chinese and Korean astronomers noted the appearance of a nova in 5 BC, which would be around the right time frame, but this nova wasn’t noted by astronomers in other regions, so it likely wasn’t particularly bright. A truly bright nova or supernova, such as the one observed by Tycho Brahe in 1572, would have created a remnant that we could observe today, and there is no known remnant that can be dated to the time of Jesus. It’s possible that there could have been a supernova in the Andromeda galaxy or the Magellanic Clouds, but there is no astronomical record of such an event.

More than anything else, this shows the problems of astrological prophesy. While there isn’t a single event that stands out as a clear origin to the Star of Bethlehem, there are lots of options that “kind of” fit after the fact. This is even true of the Gospel of Matthew itself. Matthew was written around 80 AD, decades after the events it describes, so the astronomical event it mentions would have been interpreted long after the Crucifixion and the rise of Christianity. Even if the author of Matthew felt the Star of Bethlehem was accurate history and not pious fiction, we’ll likely never know the particular event they had in mind.

Paper:  Chester, Craig. The Star of Bethlehem. Imprimis. December, 22(12) 1993.

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In my last post, I talked about how historians can help us understand aspects of astronomy, such as the rate at which Earth’s rotation is slowing. The same thing can happen in reverse, where astronomy can confirm aspects of history. Take, for example, the Early Middle Ages, and the theory of phantom time. 

One of the wilder things that has been proposed in Medieval history is the idea that about 300 years in the Middle Ages was simply made up. Gap of history and phantom time. Proposed by Heribert Illig in 1991, the idea is that the Catholic Church, led by Pope Sylvester II, rewrote the calendar to put themselves at 1000 AD, rather than around 700 AD. As a result, the years 614 – 911 are simply made up. If this were true, historical figures such as Charlemagne were simply made up.

The idea has never been accepted by mainstream Medievalists, but it raises an interesting aspect about the challenges of reconstructing history, namely that that contemporary authors of history don’t always tell the truth. Victors of a battle may overplay the strength of their enemies to make their success all the more glorious. Or the case of the Egyptian Pharaoh Thutmose III, who tried to expunge his Aunt and predecessor Hatshepsut from the records.

Usually such revisionist history is unsuccessful, since we can compare different historical accounts to get an accurate view of events. One of the biggest criticisms of the phantom time is that simply adding three centuries to European history would make it disagree with other historical regions, such as the Islamic expansion and the Tang Dynasty of China, and we see no such discrepancies. But another way to disprove phantom time is to look at astronomical records.

Throughout history, humans have recorded major astronomical events. We have, for example, observations of solar eclipses from both before and after the early Middle Ages. Pliny the Elder mentions a solar eclipse in 59 AD, which agrees with our current dates. Astronomical observations of the Tang Dynasty also confirm our current date.

So both mainstream historians and astronomy agree that phantom time is an idea that simply doesn’t hold up. If 300 years of history were simply added to the record, the forgery would be written in the stars, and this simply isn’t the case.

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A day on Earth is longer than it used to be. The increase is tiny. Over the span of a hundred years the Earth’s day will increase by only a few milliseconds. It’s only been in the past few decades that we’ve been able to measure Earth with enough precision to see this effect directly. Using atomic clocks and ultra-precise measurements of distant quasars, we can measure the length of a day to within nanoseconds. Our measurements are so precise that we can observe various fluctuations in the length of a day due to things like earthquakes. Those fluctuations make it a challenge to answer another question. How has Earth’s rotation changed over longer periods of time?  

Variation of the length of a day in recent years. Credit: Wikipedia.

Part of the reason Earth’s days are getting longer is due to the gravitational pull of the Moon on our oceans. The tides slosh against the Earth, gradually slowing its rotation. Over millions of years this means Earth’s day was hour shorter than it is now, thus there were more days in a year than today. We see this effect in the geological record, which tells us an Earth day was about 22 hours long 620 million years ago. Trying to measure the length of a day between the recent and geological era, however, is difficult. Hundreds of years ago clocks weren’t accurate enough to measure this variation, and the length of a day was fixed to its rotation, making any such comparison impossible. But recent work has found a way to study Earth’s changing days.

Although our ancestors of centuries past didn’t have accurate clocks, they were good astronomers. They observed and documented astronomical events such as the occultation of bright stars by the Moon, as well as solar eclipses. The occurrence of these events depends critically on when and where you are. If, for example, an astronomer in one city sees the Moon pass in front of a star one night, an astronomer in a nearby city will only see the Moon pass close to the star. By comparing the observations of these astronomical events with the actual time of their event as calculated from the orbital motions of the Earth and Moon, we know exactly when and where they occurred. Fitting a history of observations together, we can get an average rate for the increase of a day. That turns out to be about 1.8 milliseconds per century.

There are two things that are interesting about this result. The first is that it’s pretty amazing to be able to determine this rate from historical documents. The observations span more than two and a half millennia, and are written in various languages and locations. Gathering them all together and verifying them is an amazing effort. The other is that this rate is actually less than the rate theorized from the tidal effects of our Moon (about 2.3 ms/century). This is likely due to changes in Earth’s overall shape. We know, for example, that the melting of ice since the last ice age (about 10,000 years ago) has released pressure at the Earth’s poles, allowing it to return to a more spherical shape. This would tend to shorten Earth’s days a bit. The combination of these two effects give us the historical rate we see.

Overall this work is a great demonstration of how history can speak to us. If we listen closely, we can even see the changes of time.

Paper: F. R. Stephenson, et al. Measurement of the Earth’s rotation: 720 BC to AD 2015. Proceedings of the Royal Society A. DOI: 10.1098/rspa.2016.0404 (2016)

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Dark matter is one of the great unsolved mysteries of modern astronomy. We’ve reached the point where we know most matter in the cosmos is made of matter that interacts weakly with light if at all, but drives much of the gravitational interactions between galaxies. While it’s often portrayed as a modern idea added simply to shoehorn observations into the standard model, it actually has a history spanning more than a century, and the theory of dark matter has been refined and improved as we’ve learned more about our Universe. 

The origins of dark matter can be traced to the 1600s. Soon after Isaac Newton presented his theory of universal gravity, some astronomers began to speculate about the existence of objects that might emit little or no light, but could still be known by their gravitational tug on bright objects like stars and planets. This idea was strengthened in the 1700s when Pierre Laplace argued that some objects might be massive enough to trap any light they emit (a simplistic idea of a black hole), and by the 1800s Urbain Le Verrier and John Couch Adams used gravitational anomalies in the motion of Uranus to predict the presence of Neptune. By this point astronomers had demonstrated the presence of dark nebulae, seen only by the light they absorb from bright objects behind them. It was clear that there was more in the Universe than could be seen by visible light.

Our modern take on dark matter as a major contributor to galactic mass can be traced to Fritz Zwicky. In 1933 he studied the motion of galaxies within the Coma Cluster. The Coma Cluster is a galactic supercluster containing more than 1,000 galaxies. Since these galaxies are gravitationally bound, the speed of these galaxies can provide a measure of the cluster’s mass. Basically, the more mass the cluster has, the wider the distribution of galactic speeds following a relation known as the virial theorem. A few years earlier Edwin Hubble had estimated that the Coma Cluster contained about 800 galaxies, each containing about a billion stars. Using the virial theorem Zwicky calculated a cluster mass more than 500 times larger than that of Hubble. Zwicky noted that if his measurements held true “dark matter is present in much greater amount than luminous matter.” Over the next couple decades the virial theorem was applied to other galaxy clusters with similar results. Not everyone accepted these results, largely because the virial theorem is a statistical calculation that depends upon certain assumptions. For example, it assumes the clusters are gravitationally bound. Perhaps the galaxies in these clusters are actually flying away from each other, so that the virial theorem simply doesn’t apply. But there was another line of evidence to support dark matter. One that couldn’t be so easily dismissed.

Dark matter shows its presence in large galactic clusters such as Abell 1689. Credit: HST, ACS, WFC, H. Ford (JHU)

In the early 1900s astronomers began to look at the spectra of galaxies. From this they could determine the speeds of stars as a function of their distance from galactic center, known as a galactic rotation curve. Seen in visible light, most galaxies have a bright center, dimming as you move away from the center. This would imply most of the stars (and thus most of the mass) is located near the center of a galaxy. If that’s the case, one would expect stars far from the center to move much more slowly than stars near the center, just as in our solar system Earth orbits the Sun much more quickly than distant Pluto. When Max Wolf and Vesto Slipher measured the rotation curve of the Andromeda galaxy, they found it was basically flat, meaning that stars moved at the same speed regardless of their distance from galactic center. One solution to this mystery was that Andromeda is surrounded by a halo of dark matter so that its mass is not concentrated in the center. While other galaxies showed similar rotation curves, seeming to support the presence of dark matter, even Fritz Zwicky was skeptical. Gas and dust within a galaxy might exert some kind of drag on fast moving stars, he argued, thus flattening the rotation curves. But by the 1950s radio astronomy had progressed to the point where it could detect monatomic hydrogen through the famous 21 centimeter line. Radio observations of both the Andromeda galaxy and our own Milky Way galaxy showed similarly flat rotation curves. Since hydrogen is by far the most abundant element in the Universe, the results proved that not only stars, but the gas of any dark nebulae were orbiting the galaxies at similar speeds. Either galaxies contained significant dark matter, or our understanding of gravity was very wrong.

As the evidence for dark matter grew, it soon became clear that there was a serious problem. Assuming our gravitational theories were correct, dark matter must be far more plentiful than luminous matter both in galaxies and among galactic clusters. If this dark matter consisted of things like dark nebulae, their presence should be detectable by the light they absorb. If so much dark matter exists, it must not only be non-luminous, it must not absorb light either. It couldn’t simply be regular matter that is cold and dark, but must be something very different. This was such a radial idea that many astronomers questioned the validity of Newtonian gravity. By the 1980s several alternative gravitational models, the most famous of which was Modified Newtonian Dynamics (MoND), proposed by Mordehai Milgrom. While these models did work well for things like dwarf galaxies, they worked horribly with things like galactic clusters. Dark matter models were not without their problems, but they agreed more readily with observations.

Dark matter in colliding galaxies like the Bullet Cluster show us how dark matter behaves. Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.

In the past couple decades data gathered from gravitational lensing and deep sky surveys have allowed us to further refine our dark matter models. From the large scale distribution of galaxies we know that dark matter must be cold and slow moving, so the countless neutrinos that zip through the cosmos at nearly the speed of light cannot account for dark matter. From gravitational lensing we know the distribution of dark matter within galaxies. By observing the distribution of dark matter within colliding galaxies we know that not only does dark matter not interact with light, it also doesn’t interact strongly with regular matter or itself. While this further verifies the existence of dark matter, it also makes it more difficult to determine just what dark matter is.

The most recent challenge for dark matter has been to determine its composition. The most popular idea is that they are Weakly Interacting Massive Particles (WIMPs), but these particles should be detectible by the same experiments used to observe astrophysical neutrinos. So far, no evidence for these particles has been forthcoming. Direct efforts to detect dark matter have only served to eliminate our options for dark matter. After studying dark matter for more than a century, it continues to elude us.

And so the dark history of dark matter continues.

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In the early 1970s, as the Apollo missions to the Moon were coming to a close, there were plans to explore even further into the Universe. Not simply to Mars, or even the outer solar system, but a mission to another star. It became known as Project Daedalus. 

Project Daedalus was hugely ambitious. In order to reach Barnard’s star within 50 years, Daedalus would rely upon nuclear fusion rather than chemical rockets. Pellets of deuterium and helium-3 would be detonated 250 times a second, and the plasma exhaust would be directed away from the rocket by a magnetic field. As a two-stage rocket this would accelerate the ship to 12% of the speed of light.

To gather the 50,000 tonnes of fuel necessary for the journey, there were plans to harvest helium-3 from the atmosphere of Jupiter using hot air balloons. The helium-3 could also be mined from the lunar surface. Construction of the spacecraft itself would require the development of new materials capable of surviving a range of temperatures from 1,600 K to the cold of deep space. Since there would be no crew for the mission, robotic technology would need to be developed to explore the Barnard system.

Needless to say, the Daedalus mission never got off the ground. It was so ambitious that it was intended more as a proof of concept rather than a mission feasible for its time. But the project inspired later ideas for interstellar missions, and when the first human spacecraft reach the stars their success will be based in part on the efforts of wild ideas like Project Daedalus.

As we focus more practical ideas on a return to the Moon and a mission to Mars, it’s worth keeping in mind that big dreams like Daedalus can spur us to keep pushing the envelope of what is possible.

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On a calm November evening in 1988, the 300 foot radio telescope at Green Bank Observatory collapsed. While the collapse was a huge blow to radio astronomy, it is somewhat surprising that it lasted as long as it did. The radio telescope was proposed in 1960 as a way to fill the observational gap between earlier radio telescopes and telescope arrays such as the VLA, and was intended to operate for about five years. In a way it was meant to nurture success out of failure. 

The 140 foot telescope at Green Bank. Credit: NRAO/AUI/NSF

At the time, the major radio telescope under construction was Green Bank’s 140 foot telescope. This telescope was polar-aligned, and had a tracking mechanism that could follow objects as they moved across the sky. This would allow for high-precision observations of radio objects such as pulsars. Unfortunately the gearing necessary to move such a large telescope was plagued with flaws, and the construction of the telescope faced increasing delays and costs. While the 300-foot telescope was larger, it was also lighter and had limited mobility, making it cheaper and easier to build. It depended upon the rotation of the Earth to bring objects into its view for about 40 seconds before drifting out of range, but that was enough to make good observations of things like pulsar remnants. It was also able to make a survey of the radio sky at a higher precision than ever before. When the 140 foot telescope was finally completed in 1965, it was able to further these discoveries, and even made radio observations of complex molecules in space, opening the door to astrochemistry.

The Robert C. Byrd Green Bank Radio Telescope. Credit: NRAO/AUI

If the 140 foot telescope hadn’t faced delays, the 300 foot telescope would likely not have been constructed. What began as a stop-gap solution became a powerful telescope in its own right. Because it lasted much longer than its original design, astronomers came to depend upon it, upgrading the telescope over the years. That’s why its collapse was such a blow. But the radio telescope had more than proven its value, so in the years following its demise a new telescope was proposed. This would be only slightly larger than the 300 foot telescope, but would be fully steerable and capable of tracking objects through the sky. Basically it would combine the best features of the 3oo foot and 140 foot telescopes. It was completed in 2001, and came to be known as the  Robert C. Byrd Green Bank Telescope. To this day it is the largest movable land structure on the planet. Of course it wouldn’t have been built if the 300 foot telescope hadn’t collapsed.

So one of the most powerful radio telescopes we have was built because of the structural failure of a telescope that was built because of the design problems of another telescope. It’s a classic example of how sometimes failure can lead to greater things, which is often what science is all about. Science is about pushing past boundaries, and often that means using a failure to move forward.

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Throughout most of history the stars were thought to be fixed in place. Sure, they rise and set as the great celestial sphere moves around the Earth, but relative to each other their positions never change. We now know that stars do move relative to each other, but because they are light years away their motion isn’t easily noticed. It wasn’t until Edmund Halley measured the positions of stars that we began to understand their motions. 

The motion of any star relative to us can be divided into two parts: radial motion, which is motion along our line of sight, and proper motion, which is motion across the sky (perpendicular to our line of sight). We can now measure radial motion fairly easily using the Doppler effect, but proper motion is more subtle and difficult to measure. In the early 1700s Halley set out to determine the precession of the equinoxes. At the time it was known that the celestial sphere shifted alignment over time, but the rate of that shift hadn’t been measured. To do this, Halley measured the latitudes and longitudes (we now use right ascension and declination) of the roughly thousand stars listed in Ptolemy’s Almagest star catalog. Halley compared his measurements with the results listed in the Almagest, as well as those listed in another star catalog by Hipparchus. Ptolemy’s catalog was published around 300 BC, and Hipparchus’ was published about 170 years later, so Halley could compare stellar motion over about 2,000 years.

Halley found that overall the stars shifted in longitude by about 50 arcseconds per year, but he noticed that Aldebaran, Sirius and Arcturus shifted in latitude differently than other stars. Over 2,000 years they had shifted relative to other stars. Thus, he argued, these stars must be moving through the heavens. To support his claim he noted that Tycho Brahe’s star table of the late 1500s also showed a shift for Sirius. It was a pretty radical claim for the time, which just goes to show that careful measurements can sometimes lead to revolutionary ideas.

Paper: Edmund Halley. Considerations on the Change of the Latitude of some of the principal fixt Stars. Phil. Trans. 1717 vol. 30 no. 351-363 736-738 (1717) doi: 10.1098/rstl.1717.0025

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