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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As we prepare for the Great American Eclipse, you might have your eclipse glasses ready. If not, another great way to view the eclipse is with a pinhole camera. They are extremely easy to make, and you just need a cardboard box, some tin foil, scissors, tape and a toothpick. 

Two holes are cut into the box. One for the pinhole, and one for viewing. The pinhole should be placed on the short side of the box. Credit: Brian Koberlein

The basic idea is to allow light to shine through a small opening. The hole acts as the lens of your camera. It doesn’t focus the light like a regular camera does, but since the hole is small it creates an image of your light source (like the Sun). Since light travels in a straight line, light coming from the left of the hole will reach the inside of the box slightly to the right. Light from the right will travel through the hole and to the right, and so on. As a result and image will appear on the inside of the box.

Schematic of a pinhole camera. Credit: Wikipedia

Pinhole cameras are an ancient technology. They were known by the Chinese since 500 BCE. Arabic astronomer Ibn al-Haytham first used one to view an eclipses around 1000 CE. The first modern cameras were also based upon pinhole cameras. They aren’t used as much today because of a major drawback. The sharpness of your image depends upon the size of your pinhole. The smaller the hole, the sharper the image. But a smaller hole also means the image is more dim. For everyday images this means a camera would need a long exposure time to get a decent image. Modern lens cameras create brighter images. But for a solar eclipse the pinhole cameras are perfect. The Sun is so bright that you need a much dimmer image to see it.

An image of the Sun and nearby clouds as seen through a pinhole camera. Credit: Brian Koberlein

To make a pinhole camera, any size box will do, but a larger box will work best. You’ll need to cut at least one square hole on the short side of the box. You then tape tin foil over the hole, and use a toothpick to poke a small hole in the foil. When you face the pinhole side of your box toward the Sun, the image will appear on the far side of the interior. To see your image you need a viewing hole. If your box is big enough you can cut it on the same side as your pinhole, but for smaller boxes you can cut it on an adjacent side. Some folks use big boxes and cut a hole in the bottom so kids can place the whole box on their heads. Any way works, as long as you don’t block the light coming through the pinhole.

You should build your box before the day of the eclipse, just to make sure it works well. It’s a great project for kids, and a way to teach them a little about optics before the main event. Regardless of your design, stay safe and have fun on eclipse day!

The post How To Build A Pinhole Camera, And Why It Works appeared first on One Universe at a Time.

If you play Scrabble, you might know syzygy as a great high-point word, but in astronomy it is the term used to describe three celestial bodies in a line. It’s derived from the Ancient Greek word meaning, “yoked together.” 

Astronomer Katie Mack’s famous twitter pun.

Nowadays syzygy is often used more broadly to denote an interesting arrangements of planets, such as when three planets appear to cluster together, but the original meaning leads to interesting arrangements on its own. The Moon is in syzygy with the Earth and Sun every full moon and new moon, since that is when the three bodies are roughly in a line. But usually the Moon is a bit above or below the Earth-Sun line due to the orientation of the Moon’s orbit. However when the Moon’s orbit comes into line with the Earth and Sun, then a lunar or solar eclipse can occur. Solar eclipses aren’t more rare, but they can only be seen along a narrow path on Earth due to the Moon’s smaller size.

When the inner planets Mercury and Venus are between the Earth and Sun, they can transit the Sun. As with the Moon, Mercury and Venus usually enter syzygy a bit above or below the Sun, so transits are rare. Although we usually only think of transits seen from Earth, others can occur, such as the transit of Phobos as seen from Mars. The outer planets can’t be seen to transit the Sun from Earth, but enter opposition when they are in syzygy. At such a time a planet is high in the sky around midnight, and is as close to Earth as it can be, making for excellent viewing conditions.

There are also times when the Earth, Moon and a planet are in syzygy, which can lead to an occultation. In this case the Moon passes in front of the planet, hiding the planet from view for a time. Capturing an occultation is high on the list for many amateur astronomers.

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Our Sun is adrift among the stars. As our home star moves through the galaxy, so to do the other stars. This means that the apparent positions of the stars change over time. Because of the great distances of stars this shift is minuscule and difficult to measure. For years we have only been able to measure the motion of a few close stars. But that’s beginning to change. 

From 1989 to 1993, the Hipparcos spacecraft made high precision measurements of more than 100,000 stars, cataloging their positions and distances, as well as a measure of their proper motion across the sky. This data was compiled in the Hipparcos Catalog in 1997. A less precise catalog of more than 2 million stars, known as the Tycho catalog was also published. While the accuracy of the Hipparcos data for some stellar clusters has been debated, it has proved to be quite accurate for most stars.

Then in 2013 the Gaia spacecraft was launched, with the goal of measuring the position and motion of more than a billion stars. In 2014 the Gaia team published its initial data, including measurements of more than 2 million Hipparcos stars. This gave us the opportunity to see just how far these stars had moved over the course of 25 years.

Fortunately the data from both Hipparcos and Gaia are freely available. So the United States Naval Observatory (USNO) analyzed the data to calculate both the location and motion of these 2 million stars, giving the most accurate proper motions thus far. They then went one step further, and compared the positions of these 2 millions stars with the positions of million that had been measured by the USNO in 1998 and 2004, and were able to determine the proper motions of millions more stars. A new catalog containing this data will be released soon.

We’ve long known the stars moved over time, but we are now able to determine this motion accurately for millions of stars. This will help us understand not only the dynamics and evolution of our Milky Way galaxy, it could also provide clues to things like dark matter.

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When news of the TRAPPIST-1 system blazed across headlines, one of the common questions I got was what the planets really looked like. After all, if we can discover planets around other stars, we surely must be able to see them. But we can’t. In some ways can barely see the star. But this demonstrates how what we actually observe (and what data is important to astronomers) is very different from the common perception of what astronomers observe. 

The apparent sizes are accurate, but the surface features are pure fiction.

Part of this misconception comes from the way we tell the story of astronomy. When articles came out talking about seven Earth-sized worlds, there were plenty of pictures of the planets as rich worlds with complex surface features. These artistic imaginings make for great images, but they are just imagined possibilities. We don’t know anything about the surface features of these planets, because we can’t even see the planets. But we don’t have to observe planets directly to know that they are there.

The TRAPPIST planets, like most exoplanets, were discovered using a technique known as the transit method. Basically we measure the brightness of a star over time, and watch for little dips in brightness. You can see a graph of these measurements in the image above, which shows 500 hours of data gathered from the Spitzer space telescope. Every dot on the graph is a brightness measurement. You can see how most of the time it seems to fluctuate randomly along a common average, but every now and then it dips in brightness for a bit. That dip occurs when one of the planets passes in front of the star, blocking some of the light.

Animation showing the actual brightness variation of the few pixels of light from TRAPPIST-1.

If you look on the vertical scale, you’ll notice that the variation in brightness is actually quite small. It only dips in brightness by about 1% when a planet passes by. This is actually pretty large for an exoplanet, and is due to the fact that TRAPPIST-1 is a small star, only about the size of Jupiter (though 80 times more massive), so the planets block about 1% of the light. This is why we need to make sensitive measurements of a star to detect exoplanets.

But even this graph is a bit misleading. We don’t just point a telescope at the star and measure “brightness.” What the telescope actually does is focus the image of a star on a digital camera detector known as a CCD. Each pixel of the detector measures the amount of light it gathers as a number, where a higher number means more light struck the pixel. TRAPPIST-1 is a small, faint, 18th-magnitude star, so even on a good telescope its light only strikes a few pixels at a time. You can see an animation of its actual image here. If you want to know what the TRAPPIST-1 system looks like from Earth, that’s it.

Technically we don’t even see that. Since the CCD pixels just produce a number, what we really have are an array of numbers for each observation we make. The pixel numbers for each observation are then combined to create an overall brightness measurement. From that we analyze the dips in brightness to calculate the orbits, sizes, and masses of the planets. It’s complex work, which is what makes exoplanet discoveries so amazing.

Now certain skeptics might argue that since we don’t have images of the planets, we don’t really know they exist. But that goes back to the misconception about astronomy. While there are lots of great astronomical images, astronomy is really about data. Even when we gather images, our focus is not about making them pretty, but about making them useful. That’s why, for example, most astronomical images are black and white rather than color, and why we observe things at a range of wavelengths to see different features.

So while astronomy can discover entire solar systems, and those distant worlds would undoubtedly be wondrous to behold, that’s not what we really see.

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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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Kitt Peak is the oldest national observatory in the United States. It was founded in 1958, when the National Science Foundation signed a lease with the Tohono O’odham Nation, upon whose land the observatory resides. 

Before Kitt Peak, observatories were either privately funded, such as Lowell Observatory, founded by Percival Lowell, or university managed, such as the Harvard College Observatory. But with the rise of the Cold War in the 1950s, there was a desire to have an American space program, which would be supplemented by a national astronomy program. Kitt Peak was chosen as the location because of its high altitude and clear calm skies. It was also reasonably close to the University of Arizona, which had (and still has) an excellent astronomy program.

The McMath-Pierce solar observatory. Credit: Harvey Barrison

Over the years some of Kitt Peak’s status as the flagship U.S. observatory has faded a bit as newer and larger telescopes have been built elsewhere, the history of Kitt Peak is still evident its wide range of telescopes. There are optical telescopes ranging in size from 0.9 meters to the 4-meter Mayall telescope. There are radio telescopes, including a 25-meter telescope that is part of the Very Long Baseline Array, and there is even the McMath-Pierce solar observatory, which observes the Sun during daylight hours.

If you are ever in the area, the observatory does have daily tours and night viewing sessions. It’s one of the more accessible major observatories, and well worth the visit.

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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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On August 21 of 2017 the shadow of the Moon will trace a path across the United States. It’s a total solar eclipse that many will have the opportunity to observe. But whether you will observe totality or not depends on where you are. Astronomers have been able to predict the path of solar eclipses for millennia, but this new video demonstrates just how precise our predictions have become. 

The video combines lunar terrain data with that of the Earth’s terrain and the predicted positions of the Earth, Moon, and Sun. Data collected from a variety of sources and combined to create an extremely accurate prediction of the 2017 eclipse. Much of this data is publicly available, so you could do your own calculations as well.

If you have a chance to see this eclipse (or any solar eclipse) take it. We already know where you need to be to see it. The only other factor is to ensure a clear sky, but when we get within a few days of the eclipse we’ll have that prediction as well.

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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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Take a mass, any mass. Compress it into an ever smaller volume. As its density rises, the gravity near its surface with increase. Squeeze it into a small enough volume and the surface gravity will become so strong that nothing can escape, not even light. Squeeze anything into a small enough volume at it will become a black hole. The defining feature of a black hole is its event horizon, which defines the volume of no return. But the event horizon also marks a region where our basic understanding of physics breaks down. It is perhaps the greatest paradox of modern astrophysics.

The event horizon of a black hole is often defined as the point where the escape velocity becomes greater than the speed of light. It turns out the truth is a bit more subtle. Mass curves space around it, and for a black hole space is curved to the point where it basically folds into itself. The event horizon doesn’t mark an escape velocity, it marks a region that is isolated from the rest of the Universe until the end of time. The laws of physics conspire to keep you trapped, and you could no more escape a black hole than you could walk backwards in time.

However the existence of a one-way path to oblivion flies in the face of the most basic tenets of physics: phenomena should be predictable. If you throw a baseball in a particular direction at a particular speed, you can figure out where it’s going to land. Just determine the initial speed and direction of the ball, then use the laws of physics to predict what its motion will be. The ball doesn’t have any choice in the matter. Once it leaves your hand it will land in a particular spot. Its motion is determined by the physical laws of the universe. We can also work backwards. Knowing the speed and direction of the ball we can work out where it was in the past. If that’s true, then knowing something about the Universe now allows us to determine its past and future. But an event horizon breaks that rule. Once something crosses the event horizon, all you can possibly know about the object is its mass, charge and rotation. Was it a car or a spaceship? No idea. What path did it take to enter the black hole? No idea. All that information we’re supposed to know about the object, seems to simply disappear. This is known as the information paradox.

Now some of you might point out that quantum mechanics isn’t deterministic like a baseball, so perhaps information isn’t conserved after all. But it turns out that quantum theory does conserve information, it simply conserves the probabilities of certain outcomes. Knowing the state of an object we can still predict what it’s likely to do next, and what it likely did in the past. But it’s possible that quantum theory might provide a way out of the information paradox. After all, Stephen Hawking showed that quantum theory allows matter to escape a black hole through Hawking radiation. If matter radiates from a black hole, perhaps it also allows information to escape the black hole.

Unfortunately quantum theory isn’t an easy fix. Hawking radiation as it is typically defined is completely random, so while matter and energy can escape a black hole, information can’t. Theoretically you can make Hawking radiation non-random, but doing so turns it into an intense firewall near the event horizon. This flies in the face of the principle of equivalence, which says that a small region of space near an event horizon shouldn’t be any different than a small region of space anywhere else. Thus trying to solve the information paradox gives rise to another problem known as the firewall paradox.

So how do we solve this problem? The short answer is we don’t know. Lots of very smart people have tried to crack this problem, and while there are some interesting ideas there is no definitive solution. To really address this issue will require a quantum theory of gravity, which we don’t yet have. There have been some arguments that the way around the paradox is to simply declare that black holes can’t exist, but now that we’ve detected gravitational waves we know they absolutely do exist.

There’s no easy way around these paradoxes, and until there is, event horizons will remain a clear marker of the great unknown.

Miss the beginning of this series? It all starts here.

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