Would You Notice If An Atom Of Anti-Hydrogen Annihilated In Your Room?

If an atom of anti-hydrogen came into existence in the room you are in, would you notice?
One of the first lead-lead collisions at the Large Hadron Collider, recorded by the ALICE detector in November 2010. In this collision of lead nuclei at a small impact parameter (central collision), 1209 positively-charged (darker tracks) and 1197 negatively-charged (lighter tracks) particles are produced. Image credit:  CERN, for the benefit of the ALICE Collaboration (License: CC-BY-4.0)

One of the first lead-lead collisions at the Large Hadron Collider, recorded by the ALICE detector in November 2010. In this collision of lead nuclei at a small impact parameter (central collision), 1209 positively-charged (darker tracks) and 1197 negatively-charged (lighter tracks) particles are produced. Image credit:  CERN, for the benefit of the ALICE Collaboration (License: CC-BY-4.0)

Originally posted at Forbes!

Anti-hydrogen is the antimatter equivalent of the hydrogen atom. The simplest atom in our Universe, hydrogen is usually made of a single proton and a single electron. Hydrogen is also one of the most abundant elements in our Universe by a large margin, but its antimatter counterpart has been rather difficult to study. Anti-hydrogen was recently in the news, as the folks at CERN have only recently succeeded in measuring the spectrum of light that a positron (anti-electron) produces when it is bound to an antiproton. So far as we can tell, anti-hydrogen makes exactly the same spectrum as regular old hydrogen which is good news for the current model of particle physics.

Anti-matter’s main feature is that it will rather catastrophically annihilate if it comes into contact with regular matter. These matter/antimatter annihilations are cases for Einstein’s E=mc^2 equivalence, as the energy that is produced by the destruction of the antiparticle and the particle itself is equivalent to the mass of the two particles, multiplied by c, the speed of light, squared. If an anti-hydrogen came into being in the room that you’re standing in, the first thing we would expect it to do is annihilate as soon as it runs into an air molecule. (I must note that an atom of anti-hydrogen spontaneously coming into being in your living room is extremely unlikely, or the ATLAS team would have a much easier time trying to coax anti-hydrogen atoms together, instead of doing the painstaking work to collect atoms together.)

A positron, our antimatter equivalent of the electron, when it annihilates with an electron, creates gamma radiation. This is about as clean an annihilation as you’re going to get -- the two particles convert their mass directly into light, with no intermediary cascade of other particles. Gamma radiation is generally bad for humans in large doses, but a single positron/electron annihilation event only produces two gamma ray photons. That’s it. And the photons go in opposite directions, so you’re only ever going to get hit by one of them.

This track is an example of simulated data modelled for the CMS detector on the Large Hadron Collider (LHC) at CERN, which will begin taking data in 2008. The Higgs boson is produced in the collision of two protons at 14 TeV and quickly decays into four muons, a type of heavy electron which is not absorbed by the detector. The tracks of the other products of the collision are shown by lines and the energy deposited in the detector is shown in blue. Image credit: Lucas Taylor/CERN (License: CC-BY-SA-4.0)

This track is an example of simulated data modelled for the CMS detector on the Large Hadron Collider (LHC) at CERN, which will begin taking data in 2008. The Higgs boson is produced in the collision of two protons at 14 TeV and quickly decays into four muons, a type of heavy electron which is not absorbed by the detector. The tracks of the other products of the collision are shown by lines and the energy deposited in the detector is shown in blue. Image credit: Lucas Taylor/CERN (License: CC-BY-SA-4.0)

Proton/antiproton annihilation tends to be a bit messier, with more of a cascade of intermediary, unstable particles. In the end, these events also produce high energy light particles, though they’re not usually gamma rays.

To get a handle on how much of a radiation dose you could potentially have from a single atom of antihydrogen, I’m assuming that the entire atom annihilates into light, with no leftover particles like neutrinos (which are a common byproduct of proton/antiproton collisions). This means that we’d be left with the maximal amount of energy as gamma radiation. If you convert the entirety of a hydrogen atom into energy, you get 1.5 x 10^-10 Joules. That looks like a very small number, and it is; but gamma radiation is usually bad for you, right? Let’s calculate the dose.

UGC 1382 appeared to be a simple elliptical galaxy, based on optical data from the Sloan Digital Sky Survey (SDSS). But spiral arms emerged when astronomers incorporated ultraviolet data from the Galaxy Evolution Explorer (GALEX). Combining that with a view of low-density hydrogen gas (shown in green), detected at radio wavelengths by the Very Large Array, scientists discovered that UGC 1382 is a giant. Image credit: NASA/JPL/Caltech/SDSS/NRAO

UGC 1382 appeared to be a simple elliptical galaxy, based on optical data from the Sloan Digital Sky Survey (SDSS). But spiral arms emerged when astronomers incorporated ultraviolet data from the Galaxy Evolution Explorer (GALEX). Combining that with a view of low-density hydrogen gas (shown in green), detected at radio wavelengths by the Very Large Array, scientists discovered that UGC 1382 is a giant. Image credit: NASA/JPL/Caltech/SDSS/NRAO

Sieverts are used to calculate radiation doses, and they’re measured in Joules/kilogram. If you are a person, on average, you are made of 80 kg of material. If you divide our tiny amount of joules by 80 kilograms, you get a radiation dose of 1.87 x 10^-12 Sievert. A small number here is good, because a single Sievert of radiation exposure is usually considered to be not something you should do every day. It's actually the career limit for cumulative exposure for a female astronaut at age 25. A milliSievert (0.001 Sievert) per year is the recommended threshold to stay underneath if you’re a member of the general public.

However, our number is so low that it is below the Banana Equivalent Dose, which is the amount of extra radiation your body endures in the time immediately after eating a banana, and is a comically small amount of radiation. The Banana Equivalent Dose is 9.8 x 10^-8 Sieverts; our atom of antihydrogen is 1,800 times less of a dose than eating a banana.

To give you a sense of how little banana we’re working with, I weighed a banana I had: 137 grams. Dividing that by 1800 gives us 0.07 grams of banana. My kitchen scale doesn’t work on precision that small, so here’s my analogy; eating a piece of banana about the size of the last digit of your little finger is 26 times more of a radiation dose that you’d be exposed to from a single atom of antihydrogen annihilating in your room.

You wouldn’t notice.

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How Can We Tell When Something Hits The Moon?

How can we tell when something hits the moon if we can’t hear it?
Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Artist’s conception of the March 17, 2013 lunar impact as seen from Earth. Image credit: NASA's Scientific Visualization Studio

Originally posted on Forbes!

Sound notoriously is poorly transmitted in space; it’s a pressure wave, and there’s just not enough material floating around in space in order for that pressure to survive any distance in space. So anything that happens outside the confines of our little atmosphere is soundless to us. There are, of course, ways to reconstruct sounds from other information, but it’s usually not a reconstruction of what you would hear if you were there and had heard the noise through an atmosphere like our Earth’s.

But we know that the Moon should be constantly getting bombarded by small pieces of debris, because our own Earth gets hit by a considerable amount of small debris, and any dirty patches of the Earth’s orbit (such as those which are responsible for the meteor showers) are also going to be dirty patches for the Moon - it’s not really that far away from us, after all.

Before and after images taken by LRO show the location of a new 60-foot in diameter crater (right) that formed on March 17, 2013. Image credit: NASA's Goddard Space Flight Center

Before and after images taken by LRO show the location of a new 60-foot in diameter crater (right) that formed on March 17, 2013. Image credit: NASA's Goddard Space Flight Center

The main difference between a meteor shower on Earth and that same meteor shower on the Moon is that the Moon has no atmosphere. The atmosphere on Earth makes these meteors much easier to spot, because they leave a luminous trail across the sky. On the Moon, you’d expect these meteorites to make it all the way down to the surface of the Moon before there was any real observable trace of them.

Even then, they don’t make much of an announcement as to their arrival! Most impacts on the surface of the Moon are from relatively tiny pieces of grit, and so even though they hit the surface at incredible speeds, it can be hard to spot the aftermath on the surface. And even if you do spot a fresh crater, you won't know exactly how long it's been there, unless you can spot the moment of impact itself. There are a few observatories which do precisely this.

Any high speed impact is doing a lot of shifting around of energy, and even if a small fraction of that energy is converted into visible light, we can observe it. There are a few of these observatories, which look at the portion of the moon which falls in shadow, because the little blip of light will be more obvious there. NASA runs the Automated Lunar and Meteor Observatory (ALAMO) from Alabama, which is a multi-telescope setup and observes the shadowed part of the moon for small flashes of light. The multi-telescope nature of the facility means that any blip of light seen by all the telescopes isn’t very likely to be random noise.

A similar setup exists in Spain, with five telescopes working together to observe the shadowy Moon, called the Moon Impacts Detection and Analysis System (MIDAS). (Astronomers love a good acronym.) This system found a particularly bright impact flash, which was suggested to have come from a reasonably large object (a few feet across), and crashed into the surface at a whipping 38,000 miles per hour. These observatories are great for pinpointing exactly when new craters should be appearing on the Moon, and with satellites which map the Moon's surface, we can tie these flashes of light to brand new craters, and work backwards more accurately to determine what kind of object must have ended up smashing into the Moon.

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How Dense Is Dark Matter?

Components of the galaxy cluster Abell 2744, also known as the Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). The image measures about half a degree across. The image is sprinkled with foreground stars belonging to our Galaxy, the Milky Way, which are visible as the roundish objects with diffraction spikes. Image credit: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter)

Components of the galaxy cluster Abell 2744, also known as the Pandora Cluster: galaxies (white), hot gas (red) and dark matter (blue). The image measures about half a degree across. The image is sprinkled with foreground stars belonging to our Galaxy, the Milky Way, which are visible as the roundish objects with diffraction spikes. Image credit: ESA/XMM-Newton (X-rays); ESO/WFI (optical); NASA/ESA & CFHT (dark matter)

Originally posted on Forbes!

It very much depends on where you are! Dark matter as we understand it must be some kind of particle, or at least act like some kind of particle. We’re not exactly clear on what the exact nature of that particle would be, or what its individual mass is, or what kind of interactions it ought to have either with itself or with the matter that makes up our planet and all the stars.

But it certainly does seem that dark matter isn’t spread evenly throughout the entire universe. It’s clustered in lumps, and those lumps become the homes to galaxies. Small gatherings of dark matter are generally assumed to be roundish, since that’s the easiest shape for a three dimensional object to form under the influence of gravity.

For galaxy clusters, we can actually map out the shape of the dark matter surrounding these thousands of galaxies by looking at the way that light bends around that part of the Universe. Not all clusters have particularly spherical dark matter surroundings, and we can see the irregularities because the light from galaxies behind the cluster is not bent in the same way along all of the cluster’s edges.

At first glance, this cosmic kaleidoscope of purple, blue and pink offers a strikingly beautiful — and serene — snapshot of the cosmos. However, this multi-coloured haze actually marks the site of two colliding galaxy clusters, forming a single object known as MACS J0416.1-2403 (or MACS J0416 for short). Image credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and G. Ogrean (Stanford University)

At first glance, this cosmic kaleidoscope of purple, blue and pink offers a strikingly beautiful — and serene — snapshot of the cosmos. However, this multi-coloured haze actually marks the site of two colliding galaxy clusters, forming a single object known as MACS J0416.1-2403 (or MACS J0416 for short). Image credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and G. Ogrean (Stanford University)

Within any of these collections of dark matter (technically called halos) surrounding a galaxy or a collection of galaxies, the dark matter is densest at the center, and becomes gradually more diffuse the further out you go. For our own Milky Way, that means that the dark matter density is the highest towards the very center of the galaxy, and out near our solar system, the dark matter density is significantly lower.

Most galaxies contain significantly more mass in dark matter than in luminous matter, but this isn’t because it’s more dense -- the dark matter halo is simply much larger. In the case of the dark matter surrounding our Milky Way, it’s also spherical and not effectively flat, like the bright part of the galaxy is. You can pack a lot more material in a sphere than you can in a circle, so the combination of the dark matter halo being physically larger and a sphere means you wind up with a lot more mass.

This is a mass map of galaxy cluster Cl0024+1654 derived from an extensive Hubble Space Telescope campaign. The colour image is made from two images: a dark-matter map (the blue part of the image) and a 'luminous-matter' map determined from the galaxies in the cluster (the red part of the image). Image credit: European Space Agency, NASA and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France/Caltech, USA)

This is a mass map of galaxy cluster Cl0024+1654 derived from an extensive Hubble Space Telescope campaign. The colour image is made from two images: a dark-matter map (the blue part of the image) and a 'luminous-matter' map determined from the galaxies in the cluster (the red part of the image). Image credit: European Space Agency, NASA and Jean-Paul Kneib (Observatoire Midi-Pyrénées, France/Caltech, USA)

The dark matter density near the solar system, from what I could find, sits at around 0.006 solar masses per cubic parsec, which is a set of units that’s not going to make much sense unless you’re a professional astrophysicist. This is extremely low density. Six-thousandths of a solar mass is approximately the same as six Jupiter mass planets, and a parsec is a 75% of the distance from the Sun to the nearest star. So this means if you wanted to reproduce the dark matter density with the luminous matter that planets are made of, you’d have to clear out a cube of space that’s three light years to a side of absolutely everything. No dust, no gas, no stars, no planets. You get six Jupiters in that box, and you’ll have to spread those Jupiters around, since we don’t have any indication that dark matter comes in chunks.

We can scale this metaphor down a bit; if you wanted to get the same kind of density but in a cubic kilometer, you’d have to evacuate that square kilometer of absolutely every single atom of material. A single grain of birch pollen floating in that cubic kilometer would contain 20 times more mass than there would be in dark matter in that same volume.

At the center of the galaxy, the dark matter should be more than 150 times more concentrated, but this is very difficult to measure within our own galaxy. So far, our observations seem to line up with the models we’ve developed, but there’s definitely room to improve. In any case, 150 times the density of the solar neighborhood is still not very dense! That gets us all of about eight grains of pollen.

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How Big Is The Biggest Galaxy, And How Small Is The Smallest?

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

This image from the NASA/ESA Hubble Space Telescope shows the rich galaxy cluster Abell 3827. The strange blue structures surrounding the central galaxies are gravitationally lensed views of a much more distant galaxy behind the cluster. Observations of the central four merging galaxies have provided hints that the dark matter around one of the galaxies is not moving with the galaxy itself, possibly implying dark matter-dark matter interactions of an unknown nature are occuring. Image credit: ESO

Originally posted on Forbes!

Of all the galaxies in our surveys of the sky, the biggest, most massive systems are always giant elliptical galaxies. These galaxies are thought to be the end product of repeated galactic collisions, and so the end result is an object, which, though full of stars, no longer resembles the galaxy it was, very early in the lifetime of the Universe.

After so many collisions, the stars within these galaxies no longer orbit in an orderly fashion, like those in our own Milky Way, but are more or less moving randomly with respect to each other. Each star follows its own particular elliptical orbit within the gravitational pull of the galaxy, but with no particular regard for what the other stars are doing around it. As a result, the galaxy has no particular directionality to it. If you could hold it in your hands, you could turn it over and around without finding any particular features to give it an orientation.

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

This NASA/ESA Hubble Space Telescope image shows an elliptical galaxy known as IC 2006. Massive elliptical galaxies like these are common in the modern Universe, but how they quenched their once furious rates of star formation is an astrophysical mystery.  The quenching of star formation seems to have started in the cores of the galaxies and then spread to the outer parts. Image credit: ESA/Hubble & NASA

The very largest of these ellipticals are found in areas of the Universe where many galaxies gather together; with lots of galaxies in the area, there’s more chance for the galaxy to grow into the size it is by devouring its neighbors. The biggest of all are found lurking at the center of galaxy clusters, which are huge associations of thousands of galaxies. The galaxies within the cluster go whirling around the center of mass of those thousands of galaxies, and like the stars within an elliptical, they do this with no particular order.

There’s a little bit of order; the most massive galaxies sink to the center. And so this is where we find the most massive galaxies in our observable Universe. The current heavyweight sits at the center of the cluster Abell 3827 (shown at the top of the page), and has the entirely unpronounceable name of ESO 146-IG 005. It’s in the process of consuming a number of other galaxies, rapidly growing its own mass in the process. This galaxy is currently measured to be 27 trillion times the mass of our sun, which puts it more then ten times the mass of the Milky Way - it is definitely a giant.

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

Dwarf Galaxy Pisces A. Image credit: NASA, ESA, and E. Tollerud (STScI

The least massive galaxy, on the other hand, is much harder to find, because it's not very bright. By definition, small galaxies have very few stars, so finding that faint light is an observational challenge. These faintest galaxies are extra hard to find because we’re limited to those which are nearby. A faint galaxy too distant from us will be doubly faint, and impossible to spot.

The other problem with defining the least massive galaxy is that the definition of a galaxy gets a little messy at the low mass end. However, if we use the definition that a galaxy has to have some amount of dark matter surrounding it, the current least massive galaxy seems to be Segue 2Segue 2 is about 1000 stars, held together by dark matter, orbiting our Milky Way, and is only about 800 times brighter than our Sun! 

It's worth noting that both of these objects are likely to be holding temporary titles. As observational methods improve, we may find a galaxy was more massive than we had thought, or find dark matter surrounding stars that we had thought had none. In any case, these two objects are the far ends of the galaxy population - from a thousand stars, loosely held together, in the thrall of the Milky Way’s gravitational pull, to something ten times more massive than our entire Milky Way, itself doing the devouring of other galaxies.

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When You Turn Off A Light, Where Does The Light Go?

In space, light will go on, and on, and on… In a windowless room, where does the light go when we switch it off?

Originally posted on Forbes!

Light is a pretty simple beast. In lieu of any interference, it will go on, and on, and on, as we see it doing in the vast, often empty, realms of interstellar and intergalactic space.

Space is a rather unique case, because in between massive objects, light is traveling through something very close to a pure vacuum. This vacuum environment it's traveling through means that there’s very little chance for the light to run into any kind of interference - it's relatively easy for the light to travel enormous distances without anything changing its path or blocking its way.

So what are the options if there is something in the way? Well, light functionally has two options: reflection or absorption. Reflection we’re quite familiar with, as it’s the physics behind seeing yourself in a mirror. This can happen anytime light hits a surface which is smooth, to its perspective. (The smoothness required depends on the wavelength of the light - optical light needs a smoother surface to reflect cleanly off of than radio waves do, which are much longer in wavelength.)

The other option is absorption. This is the process which makes rocks warm in the sun. The rocks are absorbing sunlight over time, and over that time, the energy collected into the rock will warm the surface. Any light can be absorbed, not just the infrared (heat) portion of sunlight. A terrible mirror could absorb enough light that your reflection is only a faint ghost of an image. Unless you’ve got an old filament bulb left in your lamps, you won’t notice an appreciable warming to any of your possessions, because most light bulbs nowadays are designed very specifically not to produce much heat. We can’t see it, and it’s a waste of energy to produce heat which doesn’t help us see the room.

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These one-light-year-tall pillars of cold hydrogen and dust, imaged by the Hubble Space Telescope, are located in the Carina Nebula. This image of dust pillars in the Carina Nebula is a composite of 2005 observations taken of the region in hydrogen light (light emitted by hydrogen atoms) along with 2010 observations taken in oxygen light (light emitted by oxygen atoms), both times with Hubble's Advanced Camera for Surveys. The immense Carina Nebula is an estimated 7,500 light-years away in the southern constellation Carina. NASA, ESA, and the Hubble Heritage Project (STScI/AURA); Acknowledgment: M. Livio (STScI) and N. Smith (University of California, Berkeley)

These two options, absorption and reflection, work in tandem with each other, and most materials will do a little of both. Even your standard bathroom mirror absorbs a little of the light that hits it (typically about 10%), and few naturally occurring materials on Earth are perfect light absorbers. Some ultra-black materials are getting very close, but the only truly perfectly absorbing objects around so far are black body objects; an object heated until it glows. (An old filament light bulb would count.)

So, when considering what happens to the light from your light bulb when you switch it off, let’s consider what’s happening when we have the light on. Light is being continually produced by the bulb, which is streaming outwards through the air, mostly unperturbed by having to go through air instead of a vacuum. It will then hit every surface which faces the bulb, and some fraction of it will reflect in the direction of your eyeballs, which will absorb the light, and tell you how bright the room is, along with some information about the objects within the room.

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

NGC 1999 is an example of a reflection nebula. Like fog around a street lamp, a reflection nebula shines only because the light from an embedded source illuminates its dust; the nebula does not emit any visible light of its own. NGC 1999 lies close to the famous Orion Nebula, about 1,500 light-years from Earth, in a region of our Milky Way galaxy where new stars are being formed actively.  Image Credit: NASA and The Hubble Heritage Team (STScI)

The difference between this situation and switching the light off is simply that you’re no longer replacing the absorbed photons of light with new ones. The last rays of light that the light bulb produced will behave exactly as the rest of the light did: either absorbing into or reflecting off of the various surfaces in your room. The reflected light will bounce until it’s absorbed, but considering how fast the photon can traverse the room, and how few bounces it takes to absorb light, this loss of light is functionally instantaneous.

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