Science Questions

[Master Gunner]

They have essentially nothing to do with each other.

[AdmiralMemo]

The only relation is that virtual particle-antiparticle pairs that pop into existence near the event horizon can have the particle swallowed up and the anti-particle go free off into space. So, a black hole can "generate" anti-particles. However, there's just as much probability that the anti-particle gets swallowed, so it generates just as many particles.

[Hepheastus]

Ahh sorry Memo. Your question falls outside my knowledge but I had a chat with my sister, an Astrophysicist last night so I think I have an answer for you.
Firstly the Alpha Magnetic Spectrometer is searching for antimatter by collecting cosmic rays and subjects them to a magnetic field and measuring the deflection of the particle. Because Antiparticles have the same mass as regular particles but the opposite charge their deflection will the the opposite of what you would expect. The Alpha Magnetic Spectrometer experiment is actually trying to prove the existence of Dark Matter as it is theorized that Dark Matter decay creates antimatter. The results are promising but not definitive.
Now onto detection in general. We know our solar system is matter for obvious reasons. We know that the interstellar medium is almost entirely matter or antimatter because it still exists and if it were a mixture it would disappear in the space of a few hundred years. We also know that the interstellar medium is matter because it interacts with our solar system without annihilation. A single particle pair gives of something in the region of 100 MeV so it is easy to detect. Because we cant detect this energy we can be very confident that our own Galaxy doesn't contain much antimatter. Because our own galaxy interacts with the intracluster gas between us and nearby galaxies we know that that doesn't contain antimatter and by extension that the nearby Galaxies do not as well.
So the TL;DR is we detect antimatter by detecting the energy of annihilation. which can reach GeV and even TeV (annihilation is incredibly efficient, nearing 100%, at converting rest energy.) and even though Space is big it is not empty, a large antimatter construct would interact with matter somewhere which would create detectable energy emissions.
Hope this is helpfull

[Lord Chrusher]

We can really only talk about whether there is significant antimatter in the observable universe. If the universe was sufficiently big and the laws of physics changed in the right way on the largest scales (much larger than the scale of the observable universe) their could be antimatter dominated regions of the universe. We would have no way of knowing that they are there though.

The Alpha Magnetic Spectrometer has seen an excess of high energy positrons (anti-electrons). While these positrons being the result of dark matter decays is exciting, plenty of other things produce high energy positrons such as pulsars and supernovae.

Antimatter annihilation would produce a fairly distinctive gamma ray spectrum. Electrons are the lightest electromagnetic interacting particles so electron positron annihilation almost always produces a pair of photons. Since electrons have a rest mass of 511 keV, a strong 511 keV gamma ray flux would indicate that a lot of antimatter was annihilating. Protons are much more massive (rest mass 938 MeV) and are composite particles - they are made up of two up quarks and a down quark. When a proton annihilates with an anti-proton, the reaction produces mesons - particles made up of a quark and an anti-quark. These mesons are unstable and quickly decay into gamma rays, (anti-)muons, (positrons)electrons and neutrinos. Although more stable than mesons, muons, heavier versions of electrons (rest mass 106 MeV) also quickly decay into electrons (or positrons for anti-muons) and neutrinos. Since the mesons and their decay products produced by proton - anti-proton annihilation are quite energetic, the gamma rays eventually produced show a range of energies. However, the shape of the gamma ray distribution from proton decay can be predicted. Heavier element matter-antimatter interactions would be even messier than the proton - anti-proton interaction.

Another argument for there not being areas of antimatter in the observable universe is the properties of the cosmic microwave background. The cosmic microwave background is the radiation left over from the big bang. Although the universe was insanely hot early in the big bang, the universe has expanded so much that the radiation has cooled to 2.7 K where it emits mostly at microwave frequencies. The temperature of the cosmic microwave background is not perfectly smooth. Since temperature of the cosmic microwave background is dependant on the matter density when the universe became transparent (about 400 000 years after the big bang), the spatial distribution of cosmic microwave temperatures tell us about the spatial matter density distribution.

One of the things that the spatial matter distribution depends on is the ratio of photons to baryons (protons and other nuclei). If the original ratio of matter to antimatter varied across the observable universe, then the ratio of photons to baryons would vary as would the baryon density. If you went from a region dominated by matter to a region dominated by antimatter, you would see the photon to (anti-)baryon ratio rise then fall and the (anti-)baryon density drop then rise again (For the purpose of creating the cosmic microwave background, antimatter would produce the same signal as matter). This would all mean the at cosmic microwave background would look nothing like it would in the case of a uniform matter-antimatter ratio.



There is another way besides Hawking radiation for black holes to produce (a tiny amount of) antimatter. A small amount of matter that falls onto a black hole is ejected in a pair of jets. For the massive black holes at the centres of galaxies, these jets are expelled at a significant fraction of the speed of light. When the particles ejected by these jets collide with other matter some antimatter is produced by pair production.

(This post took way too long to write but I had fun researching it and writing it.)

[mariomario42]

I read an article a long time ago that antimatter was the most expensive price/kg material in the universe. Is that still the case?

I did see a xkcd about a rare stamp that came out to $30 billion/kilogram, so wondering if that holds.

[Master Gunner]

Estimates on the cost of producing antimatter vary, and depends on what type of antimatter you want to make (making antiprotons is more difficult than making positrons). Not to mention the difficulty of producing a "meaningful astimate" - they can only be (artificially) produced in particle accelerators, which have many other demands on their time, and only a handful of particles are produced at a time.

Nonetheless, positrons are estimated at around $25 billion per gram, while NASA has estimated $62.5 trillion to produce a gram of antihydrogen (going by the wikipedia page on Antimatter, at least).

The costs of storing antimatter, much less the "market value", are other matters entirely.

[Lord Chrusher]

Antimatter is far and away more expensive.

[Tycherin]

Really? Then what about... rare antimatter stamps?!?!?!

*Mind blown*

[Hepheastus]

It would have to be made entirely out of AntiHydrogen and AntiHelium, as these are the most advanced antimatter particles ever made or discovered in the universe

[Smeghead]

Ok so in the anime Planets (I haven't actually seen it) there was a girl who had grown up on the moon and because of the low gravity; grew very tall (tall enough to be mistaken for un adult in her pre-teens), now while I understand that part; I am curious of what would happen to a person that grew up in an environment with lower gravity and would it perhaps cause health issues if that person was then to go to earth? (as in the heart would have a hard time handling it etc.)

[Hepheastus]

Yes. Really Yes.
Gravitational acceleration on earth, depends on where you are on the planet but averages out to around 9.8m/s^-2. The moon has a gravity of just 1.626 m/s^-2. That is about a 6th Earths gravity. The force of Gravity governs not just how tall we are but also how strong our muscles are as well as our bone strength and density. Someone who had grown up on the moon would find it extremely difficult to even stand up, and there would be a danger of broken bones from minor falls. Their weakened hearts would struggle to pump blood. It would be the same as if someone from Earth moved to a planet which had 6g's of gravity, every single bit of you would weigh 6 times as much.

[AdmiralMemo]

For a vaguely realistic take on what it would be like, check out the Star Trek: Deep Space Nine episode "Melora"
The new stellar cartographer comes from a low-gravity planet and needs to go around on a wheelchair when she's on the station, or a specially-designed anti-gravity suit. Otherwise, she would just crumple onto the floor helplessly.

[Hepheastus]

Good shout Memo, I'd forgotten about that episode, mostly because It's a Bashir episode before Bashir became good

[AdmiralMemo]

I mainly remembered her because she's a character in the "Titan" Star Trek novel series.

[Smeghead]

Question; Does lighting storms cause interference with radio signals?

[Dutch guy]

Question; Does lighting storms cause interference with radio signals?

Potentially yes. Depending on the frequency of said radio signals.

[romangoro]

In general, yes. All electrical activity causes radio signals, in fact, turning on or of a lightbulb can cause interference and I remember being 100% unable to listen to AM radio when I put it too close to a CRT monitor. For some reason I don't know, FM radio is less afected by random sparks than AM.

Since I'm new to this thread, I'll ask a follow up to an old question: Heph, All that

Yes. Really Yes.
[...] The force of Gravity governs not just how tall we are but also how strong our muscles are as well as our bone strength and density. [...] It would be the same as if someone from Earth moved to a planet which had 6g's of gravity, every single bit of you would weigh 6 times as much.

Isn't all that 100% true only for someone who also evolved on the Moon? Wouldn't a human i.e. someone evolved to live on Earth be basically overengineered for the Moon? They would have a hard time surviving on Earth, maybe they won't be able, but is it the same as someone Earth-born moving to a 6g planet?

[Master Gunner]

One of the main goals of the International Space Station is to research the long-term effects of microgravity on people. Despite a rigorous exercise regime, they come back to earth with atrophied muscles and a loss of bone density, and it can be several weeks before their body adjusts back to Earth gravity and they can operate normally. Someone who spent years in low-gravity, especially during their childhood as their body is still growing and changing, would experience even more dramatic effects (I'm sure they've done experiments with small animals growing up in space, don't recall the results off-hand though).

Now, how would that compare to a human moving to a 6g planet, or an organism that evolved in lower gravity moving to earth? Well, obviously the research on that is limited. Physiology develops based around the environment, and changing environments can have catastrophic effects. Bring deep-sea life near the surface, or surface sea life to the bottom of the ocean, and most will be completely unable to survive (excepting those creatures, such as whales, that have evolved to handle that transition). Obviously humans can survive in microgravity for relatively short periods, but many of the long-term effects are still unknown.

[Smeghead]

So the effects of high altitudes and low air pressure are pretty well known. But what about the opposite? What effects would being exposed to higher pressure have on the human body? (lets just say maybe up to twice surface pressure?) and could a suite protect against it? (I don't mean like the robot diving suits they use for deep sea diving).

[Metcarfre]

Those effects are pretty well known - just ask a diver. Twice surface pressure isn't even that much, as I understand it. Deep-sea oil rig divers supposedly work up to 33 atmospheres.

Problems at such pressures - assuming you're already using the engineered solutions of pressure suits and high-pressure breathing gas mixes (helium-oxygen mix) problems arise with bone necrosis - basically lack of ability of fine capillaries to supply oxygen as the blood begins to be too thick.

Most problems really arise when trying to change pressures.

[plummeting_sloth]

Hey, anybody here familiar with Character Tables for Point Groups? I understand how to use them well enough and that's all I really need to do, but I've always wondered exactly what all the left-hand letters mean in terms of how an object looks

[Metcarfre]

Ugh, O-chem was way too long ago/not long ago enough for that.

[plummeting_sloth]

heh... I'm using them for inorganic chem, so even more perturbations.

[Darkflame]

Estimates on the cost of producing antimatter vary, and depends on what type of antimatter you want to make (making antiprotons is more difficult than making positrons)

Positrons are actually generated in nature above lightening strikes;
http://www.nasa.gov/mission_pages/GLAST ... torms.html

[/lurking about then pouncing with a random factoid]

I thus now propose the science-fiction idea for spaceships to "refuel" by positioning themselves above storms below.

[Tycherin]

That seems a bit like trying to inflate a hot air balloon by positioning it above an incandescent lightbulb. It might work in theory, but in practice... less so.