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### Elusive Dark Matter May Be Detected With GPS Satellites

Re:Explain it like I'm five (67 comments)

Sure, I'll give it a try. If you put two bar magnets next to each other, they tend to flip each other around so that they point in the same direction. Now try to picture an infinitely large universe, which is filled with an infinite number of tiny bar magnets. If all of these magnets pointed in the same direction, there wouldn't be much interesting going on; since all the tiny magnets are already aligned, they won't try to flip each other over, and the universe would be a stable place. (You could still have some fun by flipping a few magnets, and watching the ripples spread as a wave throughout the universe; but that's not what I'm gonna talk about now.)

But let us now consider a different scenario: in one end of the universe, all the magnets are pointing "up", while in the other end of the universe, all the magnets are pointing "down". By themselves, both these regions are stable, since there is nothing inherently "better" about pointing up than pointing down. However, somewhere in between these two far ends of the universe, there has to be a region where the magnets change from pointing up to pointing down; and this is a region of higher energy, since you have all these tiny magnets which are constantly fighting among themselves about which way to point, and constantly trying to flip each other over. This is called a "domain wall" in the case of magnetism, which is an example of a "topological defect". This domain wall can be moved and twisted by flipping a finite number of magnets in the vicinty of the domain wall; but you can't truly get rid of it without flipping an infinite number of magnets throughout the universe, which would end up requiring an infinite amount of energy.

In some quantum field theories, you get analogous situations where a theory has multiple stable "vacuum solutions". If the universe contains fields like that, we would then have two possible scenarios: (i) the entire universe has the same vacuum state (corresponding to all the magnets pointing in the same direction); or (ii) the universe could in principle consist of different stable regions with different vacuum states, with an unstable region called a "topological defect" inbetween, where the different vacua fight for dominance.

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### New Atomic Clock Reaches the Boundaries of Timekeeping

In general relativity, gravitation is not modeled as a direct force between massive objects. Instead, any form of energy density (mass according to E=mc^2, electromagnetic fields, and so on) causes spacetime to curve, and this curvature of spacetime then alters the motion of particles through spacetime. I've always liked the summary "energy tells spacetime how to bend, and spacetime tells matter how to move".

So yes, it is true that electromagnetic fields also act as a source of gravity. However, you'll need some really crazy field configurations before that effect starts becoming comparable to the gravitation from stars and planets though. For a very rough estimate of the sizes involved, you can try setting the volumetric energy density of an electric field (vacuum permittivity)*(electric field)^2/2 equal to the mean energy density (earth mass)c^2/(earth volume) of the earth, which leads to the result 10^16 V/m for the electric field. This is roughly 10^10 times the electric breakdown voltage of air at standard temperature and pressure...

about a month and a half ago
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### New Atomic Clock Reaches the Boundaries of Timekeeping

wouldn't that make the concept of time fundamentally flawed?

In any given reference frame, time is a well-defined quantity. The fundamentally flawed concept here is the idea of some kind of universal time that passes at the same rate everywhere in the universe, because relativity tells us that the observed passage of time is affected by things like velocity, acceleration, and gravitation, and therefore varies between different reference frames -- and we have no objective reason to say that any particular reference frame in the universe is inherently superior.

So while the atomic clock might measure the local passage of time with near perfect accuracy in the reference frame where we place it, the results will just be approximate in any other reference frame.

about a month and a half ago
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### New Framework For Programming Unreliable Chips

I can imagine a couple of applications of these transistors though...

Many numerical simulations require repeated random sampling of some process, and then combine the results in the end. If you're averaging some billion simulations, the result should be quite robust to fluctuations in the results of each simulation. Thus it might well be worth it to use 10 billion unreliable transistors instead of 1 billion reliable transistors, if they cost the same.

Another application could be to generate random numbers. Let's say that you have a pseudorandom number generator with periodicity N, and your unreliable transistors makes the algorithm do a random jump after an average of N/100 numbers. Wouldn't that be "random enough" for more applications than just the pseudorandom number generator itself?

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### DNA Sequence Withheld From New Botulism Paper

Its DNA sequence has been withheld, until an antidote has been found. [...] Is this the right move?

We arrive at the same question as with security and open source software: if the DNA sequence is withheld, doesn't that reduce the probability of an antidote being discovered?

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### Scientology's Fraud Conviction Upheld In France

France regards Scientology as a cult, not a religion

A second woman claimed she was forced by her Scientologist employer to undergo testing and enroll in courses, also in 1998. When she refused she was fired.

It shouldn't matter whether it's a cult or a religion; if someone got fired for not undergoing religious courses and testing, that should be treated the same way by the law.

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### When Does the Universe Compute?

Re:I don't think encoding/decoding are fundamental (182 comments)

DIVIDED BY ZERO. No thank you. I'll stick with good old Newtonian physics until you idiots come up with something that accurately explains the laws of motion AND magnetism.

Sticking to Newtonian mechanics because the math looks prettier doesn't sound very scientific. Isn't the point of science to explain and predict the largest number of phenomena, with the smallest number of independent assertions?

Special relativity predicts and explains many phenomena that Newtonian physics doesn't, and even more so for general relativity. The same goes for quantum mechanics and quantum field theory, which can be used to derive all of chemistry and electromagnetism, and is the only theory so far that can predict what happens in particle accelerators. I'm not saying that any of these theories will never be superseded, but so far they explain a lot more than the theories we had in the 1800s.

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### When Does the Universe Compute?

Re:I don't think encoding/decoding are fundamental (182 comments)

Actually, he's right, and the analogy is quite good too. Newtonian physics is "wrong" in the sense that it doesn't hold for very massive, very fast or very small objects. However, for medium-sized objects moving at medium speeds, it holds very well.

Similarly, the second law of thermodynamics, that entropy always increases, can be derived in statistical mechanics by assuming that there are an infinite number of particles in your system. Thus, it holds for the entire universe, and it holds extremely well for any macroscopic system that I know of. However, for microscopic systems, it becomes quite probable that entropy decreases in small periods of time (the fluctuation theorem tells you the probability for this to happen.)

If you're interested in how this "makes sense": in statistical mechanics, it is shown that entropy is actually just a measure of microscopic disorder. There usually exists a lot more of possible disorderly states than orderly states for a system, so if no particular microstate is preferred (the probability of entering any microstate is equally probable), it's simply more probable that you will observe a transition from an ordered state to a disordered one, not the other way around. For a small system, the discrepancy is small, so you see transitions in both directions on small enough timescales. But as the number of particles in the system grows, the number of disordered states of the total system will grow far faster than the number of ordered states (the discrepancy is O(n!) for n particles in the system), so transitions from disordered to ordered states become extremely unlikely.

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Why is it that every time foreign people do something amazing, someone has to question them about the actions of their governments?

Someone in Isreal did science? But they oppress the Palestinians, so it doesn't matter.
Someone in Saudi Arabia did science? But they oppress their women, so it doesn't matter.
Someone in China did science? But they censor their internet, so it doesn't matter.
Someone in Russia did science? But Putin is a fascist, so it doesn't matter.

Assuming that you're american, how would you react if you published a scientific paper after making a great breakthrough, and people started asking why you killed all those people in Afghanistan and Iraq, and why you tortured them without due process at Guantanamo Bay?

Don't get me wrong, I do support human rights; but you shouldn't blame every single individual for the actions of their government.

about a year and a half ago
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### Physicists Create Quantum Link Between Photons That Don't Exist At the Same Time

Re:Getting so tired of this "instantaneous" BS (364 comments)

Special Relativity makes quite clear that if two particles are spacelike separated when measured, that the concept of "instantaneous" is devoid of meaning.

From TFA:

Eisenberg emphasizes that even though in relativity, time measured differently by observers traveling at different speeds, no observer would ever see the two photons as coexisting.

So the separation was timelike, not spacelike.

about a year and a half ago
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### Physicists Create Quantum Link Between Photons That Don't Exist At the Same Time

Re:Getting so tired of this "instantaneous" BS (364 comments)

Thanks for the link to the SR paper! It was a fascinating read :).

about a year and a half ago
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### Congress Wants Federal Government To Sell 1755-1780 MHz Spectrum Band

When observing the universe, you want radiation to pass through the atmosphere unaltered. When communicating on Earth, you want radiation to bounce off the atmosphere. So I would guess that the best frequencies for doing radio astronomy, are actually terrible for terrestrial communication :)

about a year and a half ago
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### Passthoughts, Not Passwords: Authentication Via Brainwaves

Since when is "works correctly 99% of the time" good enough for an authentication system?

It isn't. But it is an interesting proof-of-concept, which shows that using passthoughts as identification is actually possible.

One interesting thought would be to combine passthoughts with other authentication technologies. Imagine walking up to a door that first performs face recognition and retina scans to determine who you appear to be. The system then accesses a database of passphrases associated with your user, displays a random one on a screen, and asks you to read it out loud. The system then uses a combination of voice recognition and brainwave scans to check if you're really who you appear to be.

Although all these technologies currently have suboptimal success rates, they might yield good security if you combine them.

about a year and a half ago
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### Physicists Discover 13 New Solutions To Three-Body Problem

Depends on what you mean with completely wrong. There is a class of numerical algorithms called Symplectic Integrators, which make sure that energy is conserved. You can also choose algorithms with an adaptive stepsize, which means that the simulation should converge to within a given error tolerance. (The simulation can still suffer from e.g. accumulated floating point errors, though.)

The classic example of a simulation gone completely wrong, is the Flying ice cube problem...

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### Ask Slashdot: Is the Bar Being Lowered At Universities?

Slashdot readership is an older crowd; they are not college students.

That statement is not entirely true. I'm a 22-year old university student, and I've been reading Slashdot since I was 13. I also know other students that have read Slashdot for many years, so I'm not alone.

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### 71 Percent of U.S. See Humans On Mars By 2033

After each of those events, Earth was still more hospitable to life than any other known planet.

Life will probably survive, but I doubt that it will be human life. For instance, what survived the event 65 million years ago was not the dinosaurs that at the time dominated the Earth, but small creatures like cockroaches and rodents.

We might have the technology to travel to Mars (barely), but we have no precedent for creating a sustainable environment for supporting life.

At present, no. But if we don't try, we will never know if it's possible.

Given the choice between staying on Earth and riding out a cataclysmic event or trying to survive on another planet without outside supplies, the rational choice is to stay put.

There are more than enough people that want to stay on Earth, but projects like Mars One have already gotten over 1000 volunteers for a one-way mission to Mars. I think the rational thing is to do both.

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### 71 Percent of U.S. See Humans On Mars By 2033

If that happens [a supernova goes off] the people on mars will have even less of a chance than everyone on earth because of the thin atmosphere.

That's true... To be safe from gamma ray bursts or nearby supernovas, we'll need to colonize other stellar systems. But the first step towards leaving our solar system, will be to leave our planet. And there are still lots of calamities that could affect Earth, but not Mars; supervolcanoes, pandemics, wars, asteroid impacts, etc.

How will a colony that depends on earth for survival be any sort of backup?

I agree, a backup should not depend on Earth for survival. That's why our long-term goal should be to create a self-sufficient colony with local mines, power plants, factories (the new 3D printers seem promising), greenhouses, etc. The colonial population needs to exceed around 200 residents to prevent long-term inbreeding; but if Mars One succeeds, we can reach that limit in about 100 years.

You talk about terraforming but that will literally take thousands of years if it can be made to work at all.

Are you sure about that? There are already polyextremophile bacteria and microorganisms on Earth that can withstand cold, drought, low pressure and radiation; see this and this for examples. Wouldn't it be possible to gene-manipulate these to perform photosynthesis, and breed them in laboratories to survive in a martian climate? In that case, we can ship them off to Mars, and let evolution do the rest. Sure, it'll take a while, but it's a good start.

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### 71 Percent of U.S. See Humans On Mars By 2033

One-way mission to Mars, you say? Then this is relevant.

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### 71 Percent of U.S. See Humans On Mars By 2033

During the past 540 million years, there have been five major events which killed over 50% of life on Earth. Do you think humanity will survive the next mass extinction? What if a supernova goes off some hundred light years away, an asteroid heads towards Earth, a global pandemic breaks out, or a third world war erupts? If we make it our long-term goal to establish a permanent colony on Mars, at least we'll have a backup of humanity in case disaster should strike.

I also believe that we'll benefit from developing the technology to settle on another planet. For instance, you mentioned faster space drives; if we don't continue to explore space, where will the motivation and funding for heavy propulsion research come from? If we settle on Mars, can't we use similar technology to populate more arid regions on Earth? If we eventually manage to terraform Mars, wouldn't that revolutionize agriculture on Earth too? And I bet such an expedition will be accompanied by thousands of minor breakthroughs in materials technology, medicine, etc. that we don't yet know how will benefit us.

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### Updated Model Puts Earth On the Edge of the Habitable Zone

Interesting. The orbit of Mars is at 1.38-1.67 au, which is at the other end of the habitable zone. So our solar system actually has two planets in the habitable zone!

# Submissions

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### Heating nanoparticles to kill tumor cells

jouassou (1854178) writes "Magnetic fluid hyperthermia (MFH) is a promising new cancer treatment that essentially "fries" cells inside tumors. The procedure has been used successfully in prostate, liver, and breast tumors. Magnetic nanoparticles (each billionths of a meter in size) are injected into the body intravenously and diffuse selectively into cancerous tissues. Add a high-frequency magnetic field, and the particles heat up, raising the temperature of the tumor cells."

# Slashdot: News for Nerds

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