Shared posts

15 Mar 20:15

Dark matter is missing from young galaxies

by John Timmer

Enlarge / The European Southern Observatory's Very Large Telescope, based in Chile. (credit: ESO)

One of the earliest indications of the existence of dark matter came from an examination of the rotation of nearby galaxies. The study showed that stars orbit the galaxy at speeds that indicate there's more mass there than the visible matter would indicate. Now, researchers have taken this analysis back in time, to a period when the Universe was only a couple billion years old, and the ancestors of today's large galaxies were forming stars at a rapid clip.

Oddly, the researchers find no need for dark matter to explain the rotation of these early galaxies. While there are a number of plausible explanations for dark matter's absence at this early stage of galaxy formation, it does suggest our models of the early Universe could use some refining.

The measurements at issue here are what are called the "galaxy rotation curves." These curves track the speed at which stars rotate as a function of their distance from the center of the galaxy. If regular matter were all that was present, it would be easy to predict what we'd see. Close to the galaxy's center, stars would only feel a portion of the total galactic mass, so they would orbit at a relatively sedate speed. Any faster, and their orbits would shift outward.

Read 12 remaining paragraphs | Comments

13 Mar 19:59

Chat Systems

I'm one of the few Instagram users who connects solely through the Unix 'talk' gateway.
22 Feb 20:41

40 years of data show immigration decreases or stabilizes crime rates

by Roheeni Saxena

Enlarge (credit: Alex M. Hayward)

According to the White House Office of the Press Secretary, a recent Executive Order on Border Security and Immigration Enforcement was intended to address the issue of “significant increase in violent crime” due to immigration driven by “transnational criminal organizations.” These claims directly contradict the results of academic work on immigration and crime, and a recent study published in the Journal of Ethnicity in Criminal Justice reinforces that. It shows that immigration is not linked to increases in crime—in fact, this study suggests it's linked to reductions in certain types of crimes.

This study builds on previous findings on arrests and criminal offenses. That previous data showed that foreign-born residents of the US were less likely to commit crimes than native-born Americans. The new study looked at 200 major metropolitan areas as defined by the US Census Bureau. The researchers then used Census data and FBI crime reporting data from 1970-2010 to look at trends for these regions.

The authors were interested in increases in crimes that might be attributable to an influx of immigrants who decreased economic opportunities or ended up in jobs that might otherwise have gone to local-born residents. To that end, they looked at violent crimes and property crimes, including rates of murder, non-negligent manslaughter, aggravated assault, robbery, burglary, and larceny.

Read 4 remaining paragraphs | Comments

16 Feb 21:21

Bad Map Projection: Time Zones

This is probably the first projection in cartographic history that can be criticized for its disproportionate focus on Finland, Mongolia, and the Democratic Republic of the Congo.
10 Feb 19:44

A Danish astronaut has captured the best-ever images of rare blue flashes

by Eric Berger


Scientists don't know much about the mysterious, powerful electric discharges that sometimes occur in the upper levels of the atmosphere in conjunction with thunderstorms. The first photograph of the phenomenon—which can occur as high as about 90km above the surface of the Earth and are known variously as sprites, pixies, elves, or jets—was only taken from Earth in 1989.

Fortunately for scientists interested in these storms, the International Space Station offers an excellent vantage point at an altitude of about 400km. So Danish researchers devised a "Thor experiment"—named after the hammer-wielding Norse god—to study the phenomenon. As part of the experiment, an astronaut on board the station would image thunderstorms under certain conditions, and these observations would be correlated with data collected by satellites and ground-based radar and lightning detection systems.

Read 5 remaining paragraphs | Comments

08 Feb 20:41

Shootings at school follow trends in the unemployment rate

by Roheeni Saxena

Enlarge (credit: Thomas Hawk)

Part of why we struggle to understand school shootings is because there isn’t enough data available about these extremely rare events. A recent study published in Nature Human Behavior describes a carefully curated dataset for US school shootings between 1990-2013, created from existing data and original data sources.

In their analysis, the authors of this paper found that the rate of school shootings increased from 2007-2013. They also found data that suggested increased shooting rates were correlated with increases in unemployment rates. This finding indicates that high levels of economic distress may lead to increases in school-related gun violence.

Previous research on school shootings has resulted in contradictory claims because there hasn’t been a single, coherent dataset. Instead, multiple datasets with different inclusion criteria have made the resulting findings difficult or impossible to compare since they analyze fundamentally different information. To solve this problem, the authors made a new collection of school shooting data, which resulted in the inclusion of almost 400 events. Their criteria for inclusion in this dataset are:

Read 8 remaining paragraphs | Comments

08 Feb 20:32

Milky Way is not only being pulled—it’s also “pushed” by a void

by Xaq Rzetelny

Enlarge / Labeled 3D model. The little arrows are galaxies, and the lines coming from them depicts their velocities (with the influence of the Universe's expansion on their velocities removed). Our galaxy, the Milky Way, is located in the Local Group, near the center of the image. The motions of all the galaxies seen here are dominated by the Shapley Attractor and the Dipole Repeller. (credit: Hebrew University of Jerusalem)

You may not notice it, but our Milky Way galaxy is cruising along at 630 kilometers (~391 miles) per second. That speed is often attributed to the influence of a single gravitational source. But in a new study, a group of researchers has found that the motions of the Local Group—the cluster of galaxies that includes the Milky Way—are being driven by two primary sources: the previously known and incredibly massive Shapley Supercluster and a newly discovered repeller, which the researchers dub the Dipole Repeller.

Shapley’s contribution was already known, but the Dipole Repeller’s hadn't been recognized prior to this study.

The researchers plotted the motions of many galaxies in the nearby Universe in a 3D model, using data from the Cosmicflows-2 database. Since the Universe is expanding, most galaxies are moving away from ours, creating a red-shift in the light they emit. But since the researchers were more interested in the other influences on a galaxy’s motion, they simply subtracted the expansion’s contribution. The resulting plot shows what the motions of galaxies would look like if space wasn’t expanding.

Read 7 remaining paragraphs | Comments

07 Feb 19:36

Matter-antimatter asymmetry confirmed in baryons

by Chris Lee

The LHCb detector. (credit: Fermilab)

Everyone, at some point in their lives, wonders why they are here. Existential questions don't stop at the personal level, though. Why is there a Universe, and why is it filled with matter? The last question is a puzzle that has gainfully occupied the minds of and employed physicists for many years. The time spent pondering such questions has not been wasted, as it turns out, as researchers from the LHCb detector report that one of the theoretical paths that allows matter to outnumber antimatter is open for business.

An overly simple reading of the Standard Model of physics predicts that matter will be produced at the same rate as antimatter. The antimatter and matter should, through simple statistics, collide and wipe each other out, leaving only energy. But that didn't happen. The substance we label matter was, somehow, produced in greater abundance than antimatter. In the beginnings of the Universe, antimatter was eliminated, leaving only matter.

A closer look at the Standard Model reveals that some imbalance is expected. But it also predicts a Universe with much less matter than we observe. And, experimentally, we've only observed the relevant matter/antimatter asymmetry for a particular class of particles, called mesons. That notably leaves out the particles that make up the Universe, called baryons. Luckily, baryon asymmetry is exactly what one of the LHC detectors, called LHCb, is designed to investigate.

Read 13 remaining paragraphs | Comments

23 Jan 19:12


My life goal is to launch a barge into the air and have it land on one of Elon Musk's rockets.
18 Jan 19:06

Japan just tried to launch the world’s smallest orbital rocket

by Eric Berger


This weekend Japan tried to launch a 3kg cubesat into orbit aboard its multi-stage, SS-520 rocket. Were it to have succeeded, the SS-520 would have become the smallest rocket to ever deliver a payload into orbit. Alas, the rocket did not make it.

According to the Japanese Exploration Agency, or JAXA, the sounding rocket launched on Sunday morning from the Uchinoura Space Center on the country's southernmost main island, Kyushu. Although the first stage fired normally, a preplanned check between first-stage separation and the second ignition did not show consistent telemetry data. This prevented the firing of the second stage, and the rocket fell into the Pacific Ocean, southeast of the spaceport.

Read 3 remaining paragraphs | Comments

11 Jan 20:33

Bad Map Projection: Liquid Resize

This map preserves the shapes of Tissot's indicatrices pretty well, as long as you draw them in before running the resize.
27 Nov 02:29

In Fiji, ants have learned to grow plants to house their massive colonies

by Annalee Newitz

High in the trees on the Fiji islands, ants in the species Philidris nagasau are doing something extraordinary. They've brought in seeds from several species of a large, lumpy fruit from a plant known as Squamellaria and carefully planted them in the nooks and crannies of the tree bark. Once the plant takes root in the tree and begins to grow, the ants climb inside its young stalks and fertilize it. But then the real action starts. As the fruit swells, the ants move inside, carving tunnels and rooms into the fleshy interior. When the colony expands, it may include dozens of these fruits, which look like strange tumors sprouting from tree branches.

Though researchers have known for a while that ant colonies can live inside fruits, a new study in Nature Plants reveals that this housing arrangement is far more complex and ancient than we knew. University of Munich biologists Guillaume Chomicki and Susanne S. Renner went to Fiji to observe the ants and found that they inhabited six different species of Squamellaria. Each of these species evolved to grow in tree bark using a specialized root system called a foot. When the plants are still young, the ants enter a small cavity in the stalk called a domatium to fertilize it. Though the researchers never directly observed how the ants did the fertilizing, they speculate that basically the ants are pooping in there.

Read 4 remaining paragraphs | Comments

15 Nov 22:14

Global carbon emissions flatline continues

by Scott K. Johnson

Enlarge (credit: Frank Friedrichs)

Even before the international Paris Agreement was completed last December, we had some encouraging news about greenhouse gas emissions. Despite an increase in global GDP, emissions were basically unchanged from 2014 to 2015. Previously, only recessions interrupted the relentless year-over-year rise of emissions. Now, an early projection for 2016 looks just about as good.

Keeping the books on global emissions and the rising concentration of atmospheric CO2 is an incredibly complex undertaking—it’s not as easy as checking your electric utility statement. The overall carbon budget for our atmosphere involves all the carbon exchanged with land ecosystems and the oceans, and there are lots of human emissions beyond energy. In the end, you can figure out how much human activities released and how much ended up in the atmosphere.

In a new paper, a huge team of researchers led by the University of East Anglia’s Corinne Le Quéré has published the latest update to the state of Earth’s carbon budget. This effort involves many sources of information, including tracking fossil fuel use for power, industry, and in homes, as well as items like the production of cement (which releases CO2 directly) and global trade. On top of that, there are data and models used to monitor the world’s ecosystems.

Read 5 remaining paragraphs | Comments

22 Oct 20:17

Spider Paleontology

Whenever you see a video of birds doing something weird, remember: Birds are a small subset of dinosaurs, so the weirdness of birds is a small subset of the weirdness of dinosaurs.
14 Oct 21:59

Saturday Morning Breakfast Cereal - The Mammal Conspiracy


Well then, let's just construct prosthetic thumbs, and... shit, no thumbs.

New comic!
Today's News:
14 Oct 21:58

Taxes, Healthcare and Culture

by Scandinavia and the World
Taxes, Healthcare and Culture

Taxes, Healthcare and Culture

View Comic!

14 Sep 08:00

Evolution: The Movie, courtesy of a really big Petri dish

by John Timmer

Enlarge / Evolution in action, as distinct lineages leave traces while they expand into new habitats. (credit: Harvard Medical School)

For most species we see, evolution is a slow process, requiring generations to show its effects. But the species we can't see—bacteria and other microbes—can go through dozens of generations in a single day. For them, evolution can be a rapid process, as antibiotic resistance has made us painfully aware.

That's why researchers often use bacteria to study evolutionary processes. In perhaps the most famous experiment, a single lab has now sent E. coli through tens of thousands of generations of competing with each other for limited resources and has tracked the resulting changes on the DNA level.

But evolution isn't always a constant competition of all against all, as takes place in these experiments. Instead, you get migrations and exploitation of new habitats, allowing rare founders to spawn entire populations. Now, a research team has figured out a nice way to study founder dynamics in a bacterial culture and has consequently allowed the branching of evolutionary lineages to be watched like a movie.

Read 10 remaining paragraphs | Comments

12 Sep 19:47

Earth Temperature Timeline

[After setting your car on fire] Listen, your car's temperature has changed before.
07 Sep 18:42

Saturday Morning Breakfast Cereal - Fossils


I'm just saying, it's what WE would do.

New comic!
Today's News:
06 Sep 21:24

Bizarre ant colony discovered in an abandoned Polish nuclear weapons bunker

by Annalee Newitz
  • Taken in 2014, this picture shows the partly blocked entrance to the Soviet-era bunker system in Poland. In the background, pine-spruce forest overgrowing the hillock was built to camouflage the structure.
    Wojciech Stephan

For the past several years, a group of researchers has been observing a seemingly impossible wood ant colony living in an abandoned nuclear weapons bunker in Templewo, Poland, near the German border. Completely isolated from the outside world, these members of the species Formica polyctena have created an ant society unlike anything we've seen before.

The Soviets built the bunker during the Cold War to store nuclear weapons, sinking it below ground and planting trees on top as camouflage. Eventually a massive colony of wood ants took up residence in the soil over the bunker. There was just one problem: the ants built their nest directly over a vertical ventilation pipe. When the metal covering on the pipe finally rusted away, it left a dangerous, open hole. Every year when the nest expands, thousands of worker ants fall down the pipe and cannot climb back out. The survivors have nevertheless carried on for years underground, building a nest from soil and maintaining it in typical wood ant fashion. Except, of course, that this situation is far from normal.

Polish Academy of Sciences zoologist Wojciech Czechowski and his colleagues discovered the nest after a group of other zoologists found that bats were living in the bunker. Though it was technically not legal to go inside, the bat researchers figured out a way to squeeze into the small, confined space and observe the animals inside. Czechowski's team followed suit when they heard that the place was swarming with ants. What they found, over two seasons of observation, was a group of almost a million worker ants whose lives are so strange that they hesitate to call them a "colony" in the observations they just published in The Journal of Hymenoptera. Because conditions in the bunker are so harsh, constantly cold, and mostly barren, the ants seem to live in a state of near-starvation. They produce no queens, no males, and no offspring. The massive group tending the nest is entirely composed of non-reproductive female workers, supplemented every year by a new rain of unfortunate ants falling down the ventilation shaft.

Read 6 remaining paragraphs | Comments

17 Aug 20:09


This car has 240% of a horse's decision-making ability and produces only 30% as much poop.
11 Aug 18:50

Facebook tests full-scale solar-powered Internet drone

by Sean Gallagher

Facebook's Aquila drone takes off from its launch dolly. (credit: Facebook)

Facebook's Connectivity Lab announced today that the company has for the first time test-flown a full-scale version of Aquila, the solar-powered high-altitude drone that Facebook hopes to use to deliver Internet connectivity to the remotest populated corners of the Earth. The test flight took place June 28 but was only announced today by Facebook.

The low-altitude test flight was originally intended only as a 30-minute “functional check” flight. "It was so successful that we ended up flying Aquila for more than 90 minutes—three times longer than originally planned," wrote Jay Parikh, Facebook's vice president of infrastructure engineering, in a post to Facebook's Newsroom blog published today.

The initial test goals were simply to ensure that the huge Aquila drone—with a wingspan comparable to a Boeing 737 and mass more like an automobile—could even get airborne. To minimize its weight, Aquila doesn't have "traditional landing gear," according to Martin Gomez and Andy Cox of the Aquila team. "We attached the airplane to a dolly structure using four straps, then accelerated the dolly to takeoff speed. Once the autopilot sensed that the plane had reached the right speed, the straps were cut simultaneously by pyrotechnic cable cutters known as 'squibs.'"

Read 7 remaining paragraphs | Comments

09 Aug 19:19

Sun Bug

by xkcd

Sun Bug

How many fireflies would it take to match the brightness of the Sun?

Luke Doty

Not that many! I mean, it's definitely one of those gigantic numbers with lots of zeroes, but in the grand scheme of things, there aren't as many zeroes as you might expect.

Our first question: Where does firefly light even come from?

Fireflies may look like they're full of glow-in-the-dark goo, but the light they give off actually comes from a thin layer on their surface.[1]You can see some diagrams of the organs here and here. Lots of insects have glowing surface patches, and some of those patches have been studied carefully to calculate their brightness. A 1928 paper on beetles called "headlight bugs"[2]Such a great name. found that their glowing patches, which were a little over a square millimeter in area, emitted about 0.0006 lumens of light. Fireflies have luminous organs (bright patches) that are about the same size as those of headlight bugs,[3]See this paper on some common American fireflies. and their organs tend to have a similar peak brightness per area, so this figure is a good guess for the brightness of a firefly's lantern.

Firefly lights aren't "always-on." They blink on and off, with patterns that vary from species to species and situation to situation. These flashes carry information, some of which you can decode using this delightful chart.[4]You can also use LEDs to mess with firefly patterns, which feels strangely invasive.

To get the brightest light, let's assume we're using a species with a mostly-on duty cycle—like a headlight bug. How does its 0.0006-lumen light output compare to the Sun?

The Sun's brightness is \( 3.8\times10^{28} \) lumens, so by simple division, it would take \( 3\times10^{31} \) of those fireflies to emit the same amount of light. That's a surprisingly small number; adult fireflies weigh about 20 milligrams, which means \( 3\times10^{31} \) fireflies would only weigh about a third as much as Jupiter and 1/3000th as much as the Sun.

In other words, per pound, fireflies are brighter than the Sun. Even though bioluminescence is millions of times less efficient than the Sun's fusion-powered glow, the Sun can't afford to be as bright because it has to last billions of times longer.[5]If you like Fermi problems—and silly equations—there's an interesting route you can take to this answer without doing any research on fireflies or the Sun at all. Instead, you can just plug this equation into Wolfram|Alpha: (5 billion years / (4 hours/day * 3 months)) / (1% * (speed of light)^2 / (3200 calories/pound)).

Let's walk through it: The first half—the numerator—is a guess for the ratio between how long the Sun has to keep glowing compared to how long a firefly does. I took a wild guess that fireflies have to light up for a few hours each night for one summer, while the Sun has to last another five billion years. The second half—the denominator—is a guess as to the ratio between the stored energy in a pound of firefly vs a pound of star. Nuclear fusion converts about 1% of the input matter to energy, so from E=mc2, the stored energy is c2 kg/kg, whereas animal matter (say, butter) is about 3,200 food calories per pound. The result should tell us the ratio between a firefly's brightness per pound and the Sun's. And the answer we get says that the fireflies are a few thousand times brighter—which is roughly what we got from working through it the other way!

It's true that we got lucky with some of our guesses, but since we made errors in both directions, they tended to cancel out. This kind of thing works more often than it seems like it should!

But wait! A mass of fireflies that big would run into problems. Besides the obvious problems with gathering that many animals in one place, the fireflies would block each others' light. The inner fireflies would be hidden behind the outer ones, and the total brightness would be limited.[6]But the light from the core fireflies wouldn't just vanish. After bouncing around a few times, it would be absorbed by neighboring fireflies, which would get warmer. This is sort of like how radiation makes its way out of the Sun's core—but in the case of the fireflies, they'd die from the heat before the process got very far.

Since the only light that matters is the light at the surface, we could imagine arranging the fireflies in a hollow sphere, with their lanterns pointing outward. Or, to make thing simpler, we could imagine a single giant firefly. How big would it need to be?

Since we know our firefly will need to give off about \( 3\times10^{31} \) times as much light as a normal firefly, it will need a glowing patch \( 3\times10^{31} \) times larger. Since surface area is proportional to length squared, our firefly will have a body length \( \sqrt{3\times10^{31}}=5\times10^{15} \) times longer than a normal firefly, which would make it about the size of the Solar System.

Since mass is proportional to length cubed, our firefly would weigh \( \left( 3\times10^{31}\right)^{\tfrac{3}{2}}=1.6\times10^{47} \) times as much as a normal firefly, which works out to about half as much as the entire Milky Way galaxy.

Such a firefly would immediately collapse under its own weight and become a black hole. In fact, given the distribution of galaxies in our universe, there's an upper limit to how large black holes can grow, and this firefly would be bigger than that limit. That means our firefly would become the largest black hole in the universe. It would give off a lot of light as it devoured our galaxy, and then, eventually, it would give off none at all.

Black holes last a long time, but they eventually evaporate through Hawking radiation. When the black hole era of our universe comes to an end, black holes will evaporate one by one, with the smallest evaporating faster. Since our firefly's black hole would be the largest one in the universe, it would be the last to evaporate—a final outpost of irregularity in a universe fading toward heat death.

We should probably add that to the identification chart, just in case.

18 Jul 17:19

xkcd Phone 4

The SpaceX system carefully guides falling phones down to the surface, a process which the phones increasingly often survive without exploding.
14 Jul 16:46

Make Mars great again


Don't miss our next comic:

The post Make Mars great again appeared first on Fredo and Pidjin. The Webcomic..

01 Jul 21:36

Software faults raise questions about the validity of brain studies

by John Timmer

(credit: Walter Reed National Military Medical Center)

It's not an exaggeration to say that functional MRI has revolutionized the field of neuroscience. Neuroscientists use MRI machines to pick up changes in blood flow that occur when different areas of the brain become more or less active. This allows them to noninvasively figure out which areas of the brain get used when performing different tasks, from playing economic games to reading words.

But the approach and its users have had their share of critics, including some who worry about over-hyped claims about our ability to read minds. Others point out that improper analysis of fMRI data can produce misleading results, such as finding areas of brain activity in a dead salmon. While that was the result of poor statistical techniques, a new study in PNAS suggests that the problem runs significantly deeper, with some of the basic algorithms involved in fMRI analysis producing false positive "signals" with an alarming frequency.

The principle behind fMRI is pretty simple: neural activity takes energy, which then has to be replenished. This means increased blood flow to areas that have been recently active. That blood flow can be picked up using a high-resolution MRI machine, allowing researchers to identify structures in the brain that become active when certain tasks are performed.

Read 10 remaining paragraphs | Comments

29 Jun 18:48

TweetDeck, HipChat and Gmail Added to Franz Messaging Client

by Joey-Elijah Sneddon

franzThe Franz desktop messaging app has received a big update, adding a host of popular web services to its roster.

This post, TweetDeck, HipChat and Gmail Added to Franz Messaging Client, was written by Joey-Elijah Sneddon and first appeared on OMG! Ubuntu!.

05 May 20:39

First autonomous robot to operate on soft tissue outdoes human surgeons

by Beth Mole

(credit: Axel Krieger])

Step aside, Ben Carson. The once lauded ability to perform delicate operations with gifted hands may soon be replaced with the consistent precision of an autonomous robot. And—bonus—robots don’t get sleepy.

In a world’s first, researchers report using an autonomous robot to perform surgical operations on soft tissue and in living pigs, where the adroit droid stitched up broken bowels. The researchers published the robotic reveal in the journal Science Translational Medicine, and they noted the new machinery surpassed the consistency and precision of expert surgeons, laparoscopy, and robot-assisted (non-autonomous robotic) surgery.

The authors, led by Peter Kim at Children’s National Health System in Washington, DC, emphasized this feat is not intended to be a step toward completely replacing surgeons. Rather, they want the technology to provide new tools that help every operation go smoothly. “By having a tool like this and by making the procedures more intelligent, we can ensure better outcomes for patients,” Kim said.

Read 8 remaining paragraphs | Comments

11 Feb 01:53

The childhood of a coder: Can’t. Stop. Watching.

by CommitStrip


10 Feb 12:36

Fire From Moonlight

by xkcd

Fire From Moonlight

Can you use a magnifying glass and moonlight to light a fire?

—Rogier Spoor

At first, this sounds like a pretty easy question.

A magnifying glass concentrates light on a small spot. As many mischevious kids can tell you, a magnifying glass as small as a square inch in size can collect enough light to start a fire. A little Googling will tell you that the Sun is 400,000 times brighter than the Moon, so all we need is a 400,000-square-inch magnifying glass. Right?

Wrong. Here's the real answer: You can't start a fire with moonlight[1]Pretty sure this is a Bon Jovi song. no matter how big your magnifying glass is. The reason is kind of subtle. It involves a lot of arguments that sound wrong but aren't, and generally takes you down a rabbit hole of optics.

First, here's a general rule of thumb: You can't use lenses and mirrors to make something hotter than the surface of the light source itself. In other words, you can't use sunlight to make something hotter than the surface of the Sun.

There are lots of ways to show why this is true using optics, but a simpler—if perhaps less satisfying—argument comes from thermodynamics:

Lenses and mirrors work for free; they don't take any energy to operate.[2]And, more specifically, everything they do is fully reversible—which means you can add them in without increasing the entropy of the system. If you could use lenses and mirrors to make heat flow from the Sun to a spot on the ground that's hotter than the Sun, you'd be making heat flow from a colder place to a hotter place without expending energy. The second law of thermodynamics says you can't do that. If you could, you could make a perpetual motion machine.

The Sun is about 5,000°C, so our rule says you can't focus sunlight with lenses and mirrors to get something any hotter than 5,000°C. The Moon's sunlit surface is a little over 100°C, so you can't focus moonlight to make something hotter than about 100°C. That's too cold to set most things on fire.

"But wait," you might say. "The Moon's light isn't like the Sun's! The Sun is a blackbody—its light output is related to its high temperature. The Moon shines with reflected sunlight, which has a "temperature" of thousands of degrees—that argument doesn't work!"

It turns out it does work, for reasons we'll get to later. But first, hang on—is that rule even correct for the Sun? Sure, the thermodynamics argument seems hard to argue with,[3]Because it's correct. but to someone with a physics background who's used to thinking of energy flow, it may seem hard to swallow. Why can't you concentrate lots of sunlight onto a point to make it hot? Lenses can concentrate light down to a tiny point, right? Why can't you just concentrate more and more of the Sun's energy down onto the same point? With over 1026 watts available, you should be able to get a point as hot as you want, right?

Except lenses don't concentrate light down onto a point—not unless the light source is also a point. They concentrate light down onto an area—a tiny image of the Sun.[4]Or a big one! This difference turns out to be important. To see why, let's look at an example:

This lens directs all the light from point A to point C. If the lens were to concentrate light from the Sun down to a point, it would need to direct all the light from point B to point C, too:

But now we have a problem. What happens if light goes back from point C toward the lens? Optical systems are reversible, so the light should be able to go back to where it came from—but how does the lens know whether the light came from B or to A?

In general, there's no way to "overlay" light beams on each other, because the whole system has to be reversible. This keeps you from squeezing more light in from a given direction, which puts a limit on how much light you can direct from a source to a target.

Maybe you can't overlay light rays, but can't you, you know, sort of smoosh them closer together, so you can fit more of them side-by-side? Then you could gather lots of smooshed beams and aim them at a target from slightly different angles.

Nope, you can't do this.[5]We already know this, of course, since earlier we said that it would let you violate the second law of thermodynamics.

It turns out that any optical system follows a law called conservation of étendue. This law says that if you have light coming into a system from a bunch of different angles and over a large "input" area, then the input area times the input angle[6]Note to nitpickers: In 3D systems, this is technically the solid angle, the 2D equivalent of the regular angle, but whatever. equals the output area times the output angle. If your light is concentrated to a smaller output area, then it must be "spread out" over a larger output angle.

In other words, you can't smoosh light beams together without also making them less parallel, which means you can't aim them at a faraway spot.

There's another way to think about this property of lenses: They only make light sources take up more of the sky; they can't make the light from any single spot brighter,[7]A popular demonstration of this: Try holding up a magnifying glass to a wall. The magnifying glass collects light from many parts of the wall and sends them to your eye, but it doesn't make the wall look brighter. because it can be shown[8]This is left as an exercise for the reader. that making the light from a given direction brighter would violate the rules of étendue.[9]My résumé says étendue is my forté. In other words, all a lens system can do is make every line of sight end on the surface of a light source, which is equivalent to making the light source surround the target.

If you're "surrounded" by the Sun's surface material, then you're effectively floating within the Sun, and will quickly reach the temperature of your surroundings.[10](Very hot)

If you're surrounded by the bright surface of the Moon, what temperature will you reach? Well, rocks on the Moon's surface are nearly surrounded by the surface of the Moon, and they reach the temperature of the surface of the Moon (since they are the surface of the Moon.) So a lens system focusing moonlight can't really make something hotter than a well-placed rock sitting on the Moon's surface.

Which gives us one last way to prove that you can't start a fire with moonlight: Buzz Aldrin is still alive.