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Brian Koberlein
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Call Of The Wild

A potentially habitable, Earth-like world has been discovered around the nearest star from the Sun, Proxima Centauri. It sounds pretty cool when you say it like that. While it is a great discovery, there are reasons to be cautious.

To begin with, this particular planet, known as Proxima B, was discovered using the Dopper method rather than the transit method. This means it was discovered by looking at how the light from Proxima Centauri shifts due to the motion of its surface. While such Doppler effects can be due to the gravitational influence of an orbiting planet, it can also be caused by things like solar flares. Sometimes this leads to a false positive, so we really need more data to be sure it’s really a planet. Assuming the data holds up, calling the planet “Earth-like” is a bit optimistic. Since the Doppler method only measured stellar motion towards us and away from us (radial motion) it only gives the minimum mass of a planet. If the planet’s orbit is highly tilted relative to us, then its mass will be larger. The quoted size of 1.3 Earth masses is actually the low end, and it’s more likely to have a mass of 2 – 3 Earths, making it more of a super-Earth planet.

The planet is also in Proxima Centauri’s habitable zone, which again means less than you might think. The “habitable zone” of a star is just a simple calculation of an distance where liquid water could exist on a planetary surface. Actual surface temperature depends not only on distance, but also atmospheric density and composition. One need only look at the variation between Venus, Earth, and Mars to see how much that matters. Since Proxima Centauri is a red dwarf, there are other challenges to habitability. For one, any planet in a red dwarf’s habitable zone is likely to be tidally locked, with one side under constant noon while the other side is in constant night. Such planets are likely to experience constant extremes of temperature, rather than a reasonable Earth-like variation. Then there is the fact that red dwarfs like Proxima Centauri are quite active, with large solar flares and bursts of x-rays that could fry a close planet like Proxima B.

All that said, there is some reason to be optimistic. Computer simulations of the planet’s orbit indicate that it likely formed farther away from Proxima Centauri, and therefore is likely to have plenty of water. It’s mass is large enough that it could have a strong magnetic field like Earth due to an iron core and geologic activity, and this could protect the planet’s surface from solar flares and x-rays. Calling the planet potentially habitable is not too much of a stretch. There is a chance that if its mass is on the low end and conditions are favorable it could look somewhat like Earth.

But the real draw for Proxima B is that it is only 4 light years away. That’s still about 180,000 times the average distance from Earth to Mars, but it is close enough that we can imagine sending a space probe to Proxima B. The proposed Project Longshot mission envisioned just such a mission that would take about 100 years. It’s a long mission, but humans have undertaken century-long projects before. Just as the call of the Moon led to the Apollo mission, and Mars inspires near-future missions, Proxima B inspires a mission to the nearest star.

That’s the power of a planet like Proxima B. It allows us to hear the call of the wild.

A new planet discovered around Proxima Centauri begs to be explored.
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Oh wow, another earth like planet? Very surprising m8, jk, theres probably more earth like planets been discovered than me having girlfriends
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The Nitro Project

Our atmosphere is about 78% nitrogen and 21% oxygen, with traces of other things like water and carbon dioxide. It’s an odd mix compared to the atmospheres of other planets. Jupiter and Saturn are dominated by hydrogen and helium, The thick atmosphere of Venus is about 96% carbon dioxide, and only 3% nitrogen, which is about the same ratio as the thin atmosphere of Mars. So why is our atmosphere so dominated by nitrogen?

It wasn’t always this way. Like most planets Earth’s earliest atmosphere was dominated by hydrogen and helium. These two elements are by far the most abundant in the Universe. About 92% of the atoms created by the big bang were hydrogen, and most of the rest were helium. All the other elements on the periodic table are formed through astrophysical processes such as nuclear fusion in the heart of a star. To this day they make up only a small trace of cosmic elements. When planets initially form, their composition is mostly hydrogen and helium. Some of the hydrogen bonds with other elements, but most of it remains free hydrogen. Both hydrogen and helium are light elements, so they will tend to evaporate into space over time. A large planet such as Jupiter has enough gravity to hold on to most of its hydrogen and helium, which is why these elements dominate the atmospheres of gas giants. But the gravity of Earth isn’t strong enough, so Earth’s early atmosphere of helium and free hydrogen evaporated into space.

Of the remaining elements, carbon, nitrogen, and oxygen are the most abundant. This is due to the fact that the main fusion reaction in large stars is the CNO cycle, which produces these elements as a by-product. These react easily with other elements, and produce gasses like water (H2O), carbon dioxide (CO2), and ammonia (NH3). Young Earth was much more geologically active than it is today, and volcanic activity released large quantities of these gases, and over time they came to dominate Earth’s atmosphere.

So why are the atmospheres of Venus and Mars dominated by CO2, while Earth’s is not? It all comes down to water. Earth’s vulcanism driven atmosphere was likely dominated by carbon dioxide like Venus and Mars, but Earth also has vast oceans of liquid water. Carbon dioxide dissolves easily in water, so our oceans absorbed much of the atmospheric CO2, leaving an atmosphere dominated by ammonia. It turns out that ammonia is unstable in Earth’s atmosphere. When struck by ultraviolet light from the Sun, it breaks apart into nitrogen and hydrogen. The liberated hydrogen evaporated into space, leaving nitrogen behind. Venus’ atmosphere likely followed a similar process, but without vast oceans to pull CO2 out of its atmosphere. While Venus’ atmosphere is mostly carbon dioxide, it is much thicker than Earth’s, and contains four times the nitrogen.

Even with it’s vast oceans, Earth’s atmosphere would likely have been dominated by carbon dioxide were it not for the appearance of life. Early cyanobacteria used sunlight and the carbon dioxide dissolved in Earth’s oceans to produce energy, and released oxygen as a by-product. Early oxygen bonded with iron to form a layer of rust, but eventually began to build up in Earth’s atmosphere. As carbon dioxide was broken down by cyanobacteria, more CO2 could be dissolved into the ocean. This gave rise to our modern atmosphere dominated by nitrogen and oxygen.

Our atmosphere is very different from that of planets such as Venus and Mars. What makes our atmosphere so special?
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+Libris Fidelis - You were warned.
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Here Be Dragons

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.

The laws of physics conspire to keep you trapped within a black hole. You could no more escape a black hole than you could walk backwards in time.
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Great post! Keep up the good work.

One comment: sorry if it sounds dumb; I am a computer programmer, not a black hole expert.

Could it be that the 'disappearance of information', or, more generally, breaking the time reversal symmetry, is simply because we are using time in the observer's frame of reference and not proper time? Put another way, an object falling into a black hole will come out on the other end, just as a ball falling inside the earth in a hypothetical tunnel dug radially through the center will come out on the other side, but it will do so in proper time, which is infinite in the observer's frame of reference? In Newtonian gravity the difference between proper and observer time is small and is ignored.
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Dawn Of Time

Although the Sun seems ageless and never changing, it is a star like any other. It’s only a bit older than the Earth itself, and like every star it formed from the gas and dust of a stellar nursery. As we’ve come to understand stellar evolution, it has become clear that stars get warmer as they age. Billions of years ago, our Sun was about 70% as luminous as it is today. That means young Earth received less heat from the Sun than it does today. So much less heat that it wasn’t enough to sustain liquid water. But geologic evidence clearly shows that there were oceans of water in Earth’s youth.

This is known as the faint young Sun paradox, and it remains a big challenge. Over the past few decades we’ve learned how atmospheric composition can drastically affect surface temperature on a planet. While Venus is warmer than Earth, it’s thick atmosphere makes it even hotter than Mercury. Mars, on the other hand once had liquid water on its surface due to a thicker atmosphere. But while Earth did have a thicker atmosphere in its past, that can’t fully account for young Earth’s oceans. It’s not just the amount of atmosphere, but its composition that plays a vital role in surface temperature. Greenhouse gases like methane and carbon dioxide are far more effective at trapping solar heat than other compounds. Measurements of Earth’s young atmosphere taken from air trapped in rocks show that methane and carbon dioxide levels weren’t high enough to maintain liquid water on Earth.

One possible solution to the problem is that Earth’s early atmosphere had high quantities of molecular hydrogen. Today our atmosphere has very little hydrogen. It’s so light that it can escape Earth’s atmosphere pretty easily. But it does so with the help of ultraviolet light. Since Earth’s young Sun was cooler it produced less ultraviolet light, making it more difficult for hydrogen to escape. Hydrogen is not a particularly strong greenhouse gas, but it can trap heat. As part of a thicker nitrogen atmosphere it might have been enough to maintain Earth’s early oceans. Other ideas propose that solar flares from our young Sun helped heat our atmosphere, or that tidal heating from a closer young Moon contributed to Earth’s warmth.

As it stands there is no definitive answer. So the faint Sun paradox remains a challenge, as it has since the dawn of time.

Next time: Cosmic rays are powerful. Too powerful, in fact. The discussion heats up tomorrow.

Stars get warmer as they age, which means there was a time when our Sun was too cool to liquify water on Earth. But the evidence is clear water existed on Earth for much longer. What gives?
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The Infinity Paradox

No matter what direction you look in the night sky, it looks basically the same. In astronomy terms we say the Universe is homogeneous and isotropic. Sure there are areas where galaxies cluster together, and other areas where galaxies are rare, but on average the distribution of stars is pretty even. Because of this, an early idea for the cosmos is that it is the same everywhere forever. It seems both ageless and infinite in expanse. But if that’s the case it raises a few troubling paradoxes.

The first paradox is perhaps the most famous. Known as Olber’s paradox, it questions how an infinite ageless universe could be mostly dark. At first glance it might seem obvious. The more distant a star, the dimmer it appears, so stars very far away are simply too dim to be seen. But the apparent brightness of a star follows a specific relationship known as the inverse square law. A single star some distance away is as bright as four similar stars twice as distant, or nine three times farther away. But if stars are distributed fairly evenly, then there are four times the number of stars twice as far away, and nine times more that are three times away. So while stars appear dimmer with distance, there are more stars at greater distances. So an infinite ageless universe should have a sky as bright as the Sun.

On the other hand, Clausius’ paradox argues that the sky should be completely dark, with no stars in the sky at all. First postulated by Rudolf Clausius, the paradox is based upon thermodynamics. One of the basic laws of thermodynamics is that heat flows from hotter regions to colder regions until they equalize in temperature. In other words, your morning coffee will always cool down until it reaches room temperature. It will never spontaneously heat up by cooling the surrounding room slightly. According to thermodynamics, even the stars will eventually cool. In an ageless universe the stars should have faded long ago, and the vast cosmos should be a sea of completely uniform temperature. So why is the universe not cold and dark?

Of course you might argue that stars still shine because gravity causes clouds of gas and dust to collapse in on themselves. New stars are being formed all the time, so naturally the Universe won’t be completely dark. But this raises another paradox: why does gravity work at all? As with light, gravity obeys the inverse square law. An object some distance away pulls upon you gravitationally with a force four times larger than an object of the same mass twice as far away. With distance a gravitational force gets ever weaker, but it never completely goes away. In an infinite universe the amount of mass at a particular distance also follows the square law. For every gravitational pull in one direction, there will always be enough mass in the other direction to balance it out. This is known as Seeliger’s paradox, and it means that gravity shouldn’t be able to act on anything. Gravitational forces should always balance out, so stars shouldn’t form and planets shouldn’t orbit stars. And yet they do.

The solution to these paradoxes is pretty clear. The Universe is not ageless, nor is it stationary. We now know it is only about 13.8 billion years old, and ever expanding. Because of expansion and a finite age, the observable universe doesn’t extend to infinity, so Olber’s and Seeliger’s arguments don’t apply. Since the Universe is finite in age, Clausius’ argument is also invalid. It seems an obvious solution to us, but it’s an excellent example of how incorrect assumptions are difficult to overcome. Before Hubble’s observation of cosmic expansion, it seemed obvious that the Universe must be ageless and stationary. The idea that it might begin with a primordial fireball seems downright creationist in comparison. But in the end, evidence for the big bang became overwhelming, and the paradoxes of an infinite cosmos were finally solved.

Next time: Nothing can be colder than absolute zero, or can it? Consider an ancient cold white dwarf. It’s temperature is near absolute zero, but it’s matter is tightly squeezed by gravity. If you took a chuck of the white dwarf away, would that chunk expand and cool even further? Arthur Eddington wrestles with stellar thermodynamics in tomorrow’s post.

In an infinite and ageless cosmos, how is it possible that the Universe is cold, dark and dominated by gravity?
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IMAGINE an explosion in SPACE... What would slow it down, what would organize it, what would place every gigantic mass into perfection
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Get Off My Lawn

Cepheid variable stars are most commonly known as a standard candle for measuring galactic distances. That’s because they vary in brightness at a rate proportional to their average brightness. But they can also tell us something about how young stars are distributed within our galaxy, and a recent study raises an interesting mystery.

There are basically two types of Cepheid variable stars. Classical cepheids are large bright stars, typically with a mass 5 – 20 times that of our Sun. Since larger stars have shorter lifetimes, a classical Cepheid is typically no more than 100 million years old. Type II Cepheids are small, old stars, with masses much less than our Sun. They are typically around 10 billion years old. Distinguishing between these two types of Cepheid variables is straight forward, because they have very different metallicities (traces of elements other than hydrogen and helium). So we can distinguish them by looking at their spectra and the way they brighten and dim (their light curve). Because classical Cepheids are brighter, they are typically used to determine the distances to galaxies, and helped establish the Hubble law for cosmic expansion. The dimmer type II Cepheids are typically used for distances within our galaxy, such as determining the distance to the center of our galaxy.

Since the ages of these different types of Cepheids are very different we can use them as a gauge for the age of surrounding stars. For example, if a globular cluster contains type II Cepheids, we know it is billions of years old. If a star cluster contains a classical cepheid, we know that stars formed there relatively recently. A new paper in MNRAS uses this fact to look at the distribution of young stars in our galaxy, and found a rather puzzling void.

Mapping young stars in our galaxy can be a challenge, particularly in the direction of the center of our galaxy, where high amounts of gas and dust obscure most of the visible light from distant stars. Fortunately infrared light isn’t absorbed as strongly, so an infrared survey of Cepheids gives us a good view of the central region of the Milky Way. This new study found some classical Cepheids clustered very close to the center of our galaxy, but found a region about 8,000 light years in radius where there aren’t any classical Cepheids. This would seem to indicate that this region hasn’t produced stars in at least 100 million years. This is in agreement with infrared and radio surveys of the central region of our galaxy, which also find a lack of star producing regions in that area.

We don’t know why stars don’t form in this region. There is certainly plenty of matter in the region, and older stars are clearly present there. For some reason the conditions for young stars are lacking there, producing a cosmic “get of my lawn” effect.

Paper: Noriyuki Matsunaga, et al. A lack of classical Cepheids in the inner part of the Galactic disc. MNRAS 462 (1): 414-420. (2016) doi: 10.1093/mnras/stw1548

There's a surprising lack of young stars in the center of our galaxy.
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Slower Than Light

China recently launched a satellite to test quantum entanglement in space. It’s an interesting experiment that could lead to “hack proof” satellite communication. It’s also led to a flurry of articles claiming that quantum entanglement allows particles to communicate faster than light. Several science bloggers have noted why this is wrong, but it’s worth emphasizing again. Quantum entanglement does not allow faster than light communication.

This particular misconception is grounded in the way quantum theory is typically popularized. Quantum objects can be both particles and waves, They have a wavefunction that describes the probability of certain outcomes, and when you measure the object it “collapses” into a particular particle state. Unfortunately this Copenhagen interpretation of quantum theory glosses over much of the subtlety of quantum behavior, so when it’s applied to entanglement it seems a bit contradictory.

The most popular example of entanglement is known as the Einstein-Podolsky-Rosen (EPR) experiment. Take a system of two objects, such as photons such that their sum has a specific known outcome. Usually this is presented as their polarization or spin, such that the total must be zero. If one photon is measured to be in a +1 state, the other must be in a -1 state. Since the outcome of one photon affects the outcome of the other, the two are said to be entangled. Under the Copenhagen view, if the entangled photons are separated by a great distance (in principle, even light years apart) when you measure the state of one photon you immediately know the state of the other. In order for the wavefunction to collapse instantly the two particles must communicate faster than light, right? A popular counter-argument is that while the wavefunction does collapse faster than light (that is, it’s nonlocal) it can’t be used to send messages faster than light because the outcome is statistical. If we’re light years apart, we each know the other’s outcome for entangled pairs of photons, but the outcome of each entangled pair is random (what with quantum uncertainty and all), and we can’t force our photon to have a particular outcome.

The reality is more subtle, and vastly more interesting. Although quantum systems are often viewed as fragile things where the slightest interaction will cause them to collapse into a particular state, that isn’t the case. Entangled systems can actually be manipulated in a variety of ways, and you can even manipulate them to have a specific outcome. I could, for example, create pairs of entangled photons in different particular quantum states. One state could represent a 1, and the other a 0. All my distant colleague needs to do is determine which quantum state a particular pair is in. But to do this my colleague would need to make lots of copies of a quantum state, then make measurements of these copies in order to determine statistically the state of the original. But it turns out you can’t make a copy of a quantum system without knowing the state of the quantum system. This is known as the no-cloning theorem, and it means entangled systems can’t transmit messages faster than light.

Which brings us back to the experiment China just launched. The no cloning theorem means an entangled system can be used to send encrypted messages. Although our entangled photons can’t transmit messages, their random outcomes are correlated, so a partner and I can use a series of entangled photons to generate a random string we can use for encryption. Since we each know the other’s outcome, we both know the same random string. To crack our encryption, someone would need to make a copy of our entangled states, which can’t be done. There are ways to partially copy the quantum state, which would still improve the odds of breaking the encryption, but a perfect copy is impossible.

So entanglement doesn’t give us faster than light communication, but it may make it a bit easier to keep our secrets secret.

Quantum entanglement won't let us communicate faster than light, but it might help us keep our secrets.
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When You See The Flash

A nova occurs when a star brightens by several magnitudes over a very short time. Like supernovae, they’ve been recorded throughout history. We now know novas are caused by a dance between two stars, where a white dwarf orbits close enough to a companion star that it captures material from its companion until it reaches a critical limit and it’s outer layer explodes. Studying the details of this phenomena is difficult, because a nova is usually too faint to be noticed until it brightens. But thanks to large sky surveys, that’s starting to change.

Recently a team observed a nova in their data, and knew they had captured that region of sky before. So they went back through their data and were able to document the binary system before, during and after the nova occurred. They found the two stars orbit each other once every five hours, putting them at a distance roughly equal to the diameter of our Sun. Before the explosion, the white dwarf was capturing material at an irregular rate, causing its brightness to “sputter” slightly. After the nova the white dwarf captured material at a more regular rate. This would support the hibernation model, where the white dwarf captures material early on, then the rate of capture dies off. It should be stressed however, that the aftermath of the nova is still young, so we’ll need to collect more data to be sure.

In addition to helping us understand novae, observations like these could also help us understand supernovae. Type Ia supernovae in particular are caused by a similar dance between a white dwarf and companion star, but instead of just the outer layers exploding, the entire white dwarf is ripped apart by a cataclysmic explosion.

Paper: Przemek Mróz, et al. The awakening of a classical nova from hibernation. Nature doi:10.1038/nature19066 (2016)

Astronomers have observed a star before it became a nova.
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wow it's so interesting and amazing! God created.
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Too Big To Fail

Earth is showered with cosmic rays. They are protons, electrons and atomic nuclei traveling at nearly the speed of light, and strike our atmosphere to create the most power particle collisions ever observed. As a particle approaches the speed of light, it’s energy increases exponentially, so it might seem that there is no upper limit to just how much energy cosmic rays can have. But it turns out there is a limit, at least in theory.

The limit is imposed by the cosmic microwave background (CMB). This thermal remnant of the big bang fills the Universe with a sea of microwave photons, which is why we observe the CMB from all directions in space. But because of relativity, a cosmic ray moving at nearly the speed of light will observe this radiation greatly blue shifted. Instead of a sea of faint microwaves, these cosmic rays observe CMB photons as high energy gamma rays. Occasionally the cosmic ray will collide with a photon, producing particles such as pions and taking some of the energy from the cosmic ray. This will continue until the cosmic ray isn’t powerful enough to produce pion collisions. As a result, over the vast expanse of intergalactic space any really high energy cosmic ray will be lowered to this cutoff energy.

This cutoff is known as the GZK limit, after Kenneth Greisen,Vadim Kuzmin, and Georgiy Zatsepin, who calculated the limit to be about 8 joules of energy (a proton traveling at 99.999998% of light speed), and that any cosmic ray traveling at least 160 million light years will have dropped below this limit. While that’s a huge amount of energy, there have been observations of cosmic rays with even higher energy. The highest energy cosmic ray had an energy of about 50 joules. So how is this possible?

The short answer is that we aren’t sure. High energy cosmic rays are more powerful than any particle accelerator we have, so these kinds of particles can’t be recreated in the lab. One possibility is that our measurement of high energy cosmic rays is somehow wrong. We don’t observe cosmic rays directly, but instead observe the shower of particles they create when striking our atmosphere. From this we infer its energy and composition. While that’s certainly a possibility, the observations we have seem pretty robust.

Another solution is that these cosmic rays are produced locally (in a cosmic sense). Most cosmic rays have traveled billions of light years before reaching us, but if a cosmic ray was produced less than 160 million light years away it could have more energy than the GZK limit. The problem with this idea is that there is no known source of high energy cosmic rays within 160 million light years, so this answer simply replaces the GZK paradox with the mystery of their origin. Another possibility is that the highest energy cosmic rays might be heavier nuclei. About 90% of cosmic rays are protons, and another 9% are alpha particles (helium nuclei), with the rest mostly electrons. It’s possible that a few cosmic rays are nuclei of heavier elements such as carbon, nitrogen, or even iron. Such heavy nuclei might be able to sustain their energy over greater cosmic distances, thus overcoming the GZK limit.

But one other option is perhaps the most tantalizing. Since these cosmic rays have more energy than anything we can create in the lab, they are a test of really high energy particle physics. It’s possible that the GZK limit is simply invalid. It’s based upon our current understanding of the standard model, and if the standard model is wrong so could the GZK limit. The answer to the GZK paradox might be new physics we don’t yet understand.

The energy of the most powerful cosmic rays might just be too big to fail.

Next time: The event horizon of a black hole marks a one way trip to oblivion. It also seems to defy some of the most foundational ideas of physics. We look at the hottest paradox in physics tomorrow.

Cosmic rays are powerful. Too powerful, in fact.
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Beyond The Cold

The discovery of white dwarfs in the early 1900s was deeply perplexing for astronomers. From their temperature and brightness it was clear white dwarfs are roughly the size of Earth. Since some white dwarfs orbit other stars, we can also determine they are about as massive as the Sun. How is it possible for so much mass to be compressed within such a small volume without collapsing on itself?

The most popular idea at the time supposed that under great pressure electrons would become free from atoms, producing a super dense plasma of free electrons and atomic nuclei. Since electrons are extraordinarily tiny, they would act like an ideal gas with the usual temperature and pressure relations. The “electron gas” of a white dwarf would therefore have enough pressure to keep the star from collapsing.

While that seems reasonable, Arthur Eddington noted it gave rise to a paradox involving thermodynamics. A fundamental law of thermodynamics states that nothing can be cooled below absolute zero. This applies to a gas of electrons as well. Since white dwarfs emit heat and light, over time they would cool. But Eddington noted that white dwarf matter only existed because it is under pressure. If you removed the pressure the material should expand back into regular atomic matter. So suppose you found a particularly cold white dwarf. The gas of electrons and nuclei would be above absolute zero, but it’s energy per mass would be less than that of regular matter at absolute zero. If you scooped up a bit of that white dwarf and remove the pressure, what would happen? Theoretically it should be colder than absolute zero, which isn’t possible.

The paradox was finally solved in 1926 by R. H. Fowler. The problem, he argued, stemmed from treating electrons as classical objects like atoms. Electrons follow the rules of quantum theory. Because of the Pauli exclusion principle there is a limit to how closely they can be pushed together. A gas of electrons in a white dwarf therefore can’t cool below absolute zero because the laws of quantum mechanics don’t allow it. Within a few years Subrahmanyan Chandrasekhar expanded upon this idea to show that white dwarfs can never have more mass than about 1.4 Suns. This upper limit on size became known as the Chadrasekhar limit.

What began as a paradox of thermodynamics became the first demonstration of the quantum connection between the very large and the very small. It pointed us toward the direction of modern astronomy.

Next time: Stars get warmer as they age, which means there was a time when our Sun was too cool to liquify water on Earth. But the evidence is clear water existed on Earth for much longer. What gives? The paradox of the faint Sun heats up tomorrow.

Nothing can be colder than absolute zero, or can it? Arthur Eddington wrestles with the paradox of stellar thermodynamics.
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A spring can be at absolute zero and still expand without violating any principles of thermodynamics
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Brian Koberlein

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Riddle Me This

Anyone practicing science needs to get comfortable with uncertainty. Often the questions raised lead to an answer that is simply “we don’t know.” But there are times when we are instead faced with a contradiction. One set of evidence and theoretical reasoning leads to a conclusion in contradiction with another set of evidence. Usually these contradictions resolve themselves pretty quickly, but there are times when these contradictions grow into a paradox. While some of the most famous astronomical paradoxes are now used to demonstrate where our reasoning went wrong, others still challenge us with no clear resolution.

What makes paradoxes so powerful is that they force us to reconsider both the evidence and our reasoning. If the Universe is self consistent (and we assume that it is) then there must be a solution to the paradox. So this week we’ll look at five major astronomical paradoxes. A couple have been solved, but most challenge even the most cutting edge research.

To Infinity And Beyond – Olber’s paradox is perhaps the most famous example, but there are similar paradoxes involving gravity and thermodynamics. They all raise the same question: How can our Universe possibly be infinite?

Cold Equations – When a white dwarf cools over time, can it actually get colder than absolute zero?

Over The Edge – The event horizon of a black hole is a point of no return. But if nothing can escape a black hole, isn’t the fundamental nature of physics violated?

Icy Sunrise – Our Sun was much cooler in its youth. So how is it that liquid water existed on a young Earth?

Bigger Bang – There is an upper limit to the amount of energy a cosmic ray can have. So why do we observe cosmic rays that have even more energy than that limit?

We’ll start by confronting the assumption that even Einstein failed to challenge. In an infinite and ageless cosmos, how is it possible that the Universe is cold, dark and dominated by gravity? The paradox series starts next time.

What happens when one set of evidence contradicts another set of evidence?
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Brian Koberlein

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The First Ones

There’s a lot of potentially habitable worlds in the Universe, and yet we haven’t found any evidence of intelligent civilizations other than our own. Why is that? Lot’s of ideas have been proposed, such as the idea that aliens are being intentionally silent, or that intelligent life kills itself off in a short time. But another idea is simply that we’re the first civilization to appear. Someone has to be first, so why not us?

It’s generally thought that the existence of intelligent life should become more likely over time. As the Universe evolves, more heavy elements are created and become available, and stellar systems with heavy elements (like our solar system) are more likely to form. Life also takes time to arise and evolve, and over time it has a greater chance of achieving the complexity necessary for intelligence. So it seems reasonable that the odds of sentient life increase with cosmic age. Of course, after trillions of years star production will have died off, and even small red dwarfs will start to cool and fade, meaning that the likelihood of life arising at that point is basically zero. So somewhere between the big bang and the ends of time there should be a period of time where intelligent life is most likely to evolve.

A new paper looks at just when this “peak sentience” might occur. In this work they formulate an equation calculating the probability for life to form on a potentially habitable planet in a particular volume of space. It’s similar to the Drake equation, and includes similar factors such as the number of stars, and the number of habitable planets, but looks at how the overall probability changes over time. All things being equal (and only assuming life similar to that on Earth) the equation predicts that life is most likely to arise about 10 trillion years from now around small red dwarfs. In the grand scheme of things, the appearance of life on Earth occurred quite early, so we might just be the first civilization to arise.

All that said, there are reasons not to take this work too seriously. Key to the conclusion is the idea that all things are equal. Specifically that potentially habitable planets around small red dwarfs are just as likely to have life than Earth-like planets around Sun-like stars. That skews the data a bit, because small red dwarfs are much, much more common than stars like our Sun. But red dwarfs are also known to have large solar flares that could seriously harm any life on a close planet, and red dwarfs are so cool that habitable worlds would need to be very close to the star. So close that they would likely be tidally locked, with one side always facing toward the star. It’s quite likely that red dwarfs aren’t very life friendly, so they really shouldn’t be included in the tally. If you just include Sun-like stars, then the peak occurs roughly around now, which would mean life on Earth could be rather typical, and arose at a pretty typical time. So this work doesn’t answer the question of where life is out there as much as it raises an interesting question about the origin of life over time.

Still, it’s fun to imagine that trillions of years from now an alien species might find remnants of a great intergalactic civilization they refer to as the first ones, never knowing that we called ourselves human.

Paper: Abraham Loeb, et al. Relative Likelihood for Life as a Function of Cosmic Time. arXiv:1606.08448 [astro-ph.CO] (2016)

Are we the first civilization to arise in the cosmos?
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Very interesting article. Thanks.
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Astrophysicist, Professor, Author
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The Universe is amazing, let me tell you.
An astrophysicist and physics professor at Rochester Institute of Technology.  Author of "Astrophysics Through Computation" with David Meisel.  Creator of the science outreach project Prove Your World, developing a science television show for children.

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