10241

Geek Speak / Re: When to stop decoding

« on: 13/07/2012 13:16:44 »

Letters in encrypted text are random, while letters and words in a natural language have a characteristic distribution (somewhat different for different languages): http://en.wikipedia.org/wiki/Zipf%27s_law#Motivation

At Bletchley Park, apart from having some very bright mathematicians (like Alan Turing):

• The early Enigma machines had a weakness that they could never encrypt a letter as itself. If you had a large block of text encrypted with a single key, you could rule out any encryption keys that decrypted any letter as itself: http://en.wikipedia.org/wiki/Cryptanalysis_of_the_Enigma#The_Enigma_machines
• Sometimes a marine buoy would be destroyed intentionally, which would be reported by a standard, short message. This "known plaintext" attack often allowed the cryptanalysts to work out the encryption keys for the day.

Note: Modern encryption codes will happily encrypt a byte as itself, so that method won't work any more.
 

10242

Geek Speak / Re: What are femtobarns?

« on: 13/07/2012 12:55:00 »

Inverse Femtobarns are a measure of how many collisions a particle collider has generated over its lifetime.

http://en.wikipedia.org/wiki/Barn_(unit)#Inverse_femtobarn

10243

Geek Speak / Re: software derived radio

« on: 12/07/2012 22:15:49 »

SDR is a big new thing - an attempt to cross the Universal Turing Machine with a Crystal Set!

This will impact most consumers by having a smart phone that can access several frequency bands for faster downloads, and be able to dynamically switch between different modulation formats between home and on the road, or when entering the service area of a different carrier.

This will impact manufacturers by allowing one hardware product to be sold in multiple countries, despite their having different TV/Wireless modulation formats and different transmission bands.

The overall impact for society will probably come through cognitive radio - dynamically reusing the scarce spectrum resources, most of which is unused, in most places, most of the time.

The challenge is to get a copy of the software to decode a given format (it's usually proprietary) or write it yourself (it is large and complex) - and then to pick up a signal in the right format (the right antenna and preamplifier), and even then it will often be encrypted.

So for most people, it is easier to go to the NOAA website to get images of the weather.

SDR is not quite as universal as the computer: With the Universal Turing Machine, if it is not quite as fast as you want, you just wait a bit longer to calculate Pi to 1000 decimal places.
But decoding Wireless signals is a Real-Time process - if your SDR is not quite fast enough, it just gets further and further behind.
So there is a certain range of modulation formats that can be decoded by a given SDR chip; but the next generation of modulation formats will require even more processing power, for which you need an SDR hardware upgrade.

Perhaps there will be more interest within groups like IEEE, eg http://www.ewh.ieee.org/r6/scv/ssc/Abidi.pdf

10244

Chemistry / Re: What happens when hydrogen atoms collide?

« on: 12/07/2012 12:46:28 »

Under high-pressure conditions (eg the center of Jupiter), it is thought that hydrogen atoms will turn into a metal, which may be a superconductor. There is some progress towards demonstrating this in the laboratory: http://en.wikipedia.org/wiki/Metallic_hydrogen

Under more normal-pressure conditions (or even low-pressure conditions), two colliding hydrogen atoms will form a covalent bond, to make hydrogen gas, H₂.

Under higher-energy conditions, the H₂ gas will immediately break up again into individual Hydrogen atoms.

Even higher-energy conditions (like the surface of the Sun) will even rip the electrons off the nuclei, to make a plasma.

Under extremely high-energy conditions and high pressures (like the center of the Sun), the electrostatic repulsion of the nuclei may be overcome; they will mostly bounce off again, but they sometimes fuse together to make deuterium+positron+neutrino: http://en.wikipedia.org/wiki/Proton%E2%80%93proton_chain_reaction#The_proton.E2.80.93proton_chain_reaction

Under extraordinarily high energy conditions (like the LHC), colliding hydrogen nuclei can make almost any particle which has an energy less than one colliding nucleus (including some that might be like a Higgs Boson).
A video showing the quarks inside a hydrogen nuclei colliding is shown here:

The LHC is trying to probe the types of events that may have happened soon after the Big Bang, as shown here:
http://www.youtube.com/watch?NR=1&feature=endscreen&v=io8rQrQUxEc

10245

Chemistry / Re: How do electrons behave in covalent bonds?

« on: 12/07/2012 12:24:34 »

In the covalent bond analogy, each electron is viewed like a little bar magnet (electrons spin on their axis, and an electric current spinning around creates a magnetic field).

Magnets tend to stick to another magnet with the opposite orientation, and so electrons tend to stick to another electron with the opposite spin.

If one atom has a lone electron spinning in one direction, and a nearby atom has a lone electron spinning in the opposite orientation, they will tend to stick the atoms together, with a covalent bond. (If one atom attracts the electron much more strongly than the other, they will form an ionic bond, like common salt NaCl, rather than a covalent bond).

Depending on how many electrons each atom has, and what their energy is, the electron will reside (or orbit) in a different electron orbital, which has a particular shape. This shape influences the final shape of the molecule.
There is a table of orbital shapes at: http://en.wikipedia.org/wiki/Atomic_orbital#Orbitals_table

10246

Chemistry / Re: What are the forces that can break atoms apart from each other?

« on: 12/07/2012 12:00:52 »

As you say, there are several kinds of bonds in molecules and crystals - covalent, ionic and metallic.

When these bonds form, they release energy.
All of these bonds can be broken by supplying sufficient energy, eg in the form of heat, strong chemicals like acids or alkalis, a spark or an electric current (which works better on ionic bonds).

Unless you maintain the material as a plasma at a very high temperate (like a spark, or the surface of the Sun), the bonds will quickly reform into new molecules, as electrons "prefer" to hang around in pairs, and atoms "prefer" to have a full shell of electrons around them. For most elements, this means hanging around in molecules or crystals (the main exception is the noble gases: Helium, Neon, etc).

Even if you can't supply enough energy to separate every atom from every other atom:

• it takes a relatively small amount of energy to separate molecules from each other (eg heat makes water boil or sugar melt)
• Depending on the molecule, a small amount of energy may be enough to break some of the bonds, causing it to form other molecules (eg heat makes meat cook)

10247

Chemistry / Re: What is going on with Oxygen and Gasoline that makes it combust with heat?

« on: 12/07/2012 11:35:14 »

Gasoline = C₈H₁₈ (approximately - it's a mixture of many chemicals containing Carbon and Hydrogen in various ratios)
Oxygen = O₂
Gasoline + Oxygen = liquid sloshing in a petrol tank
Gasoline + Oxygen + spark = CO₂ + H₂O + Energy (more spark) = Explosion

The purpose of the electric spark is to disrupt some of the gasoline & oxygen molecules, which then recombine to form carbon dioxide & water, releasing energy which can be harnessed to drive your vehicle.

To ensure that it works well, you need:

• The right ratio of gasoline & oxygen
• With the gasoline completely vaporised and completely mixed with the oxygen
• At the right temperature and pressure
• With the spark applied at just the right time to ensure complete combustion
• All of these depend on the type of fuel that you use (gasoline for cars is a different hydrocarbon mixture than Diesel for trucks or Biofuel)
  

If you don't get it exactly right, you can get:

• Pollution: carbon soot, carbon monoxide, nitrous oxide
• Backfire: Uncombusted gas released into the exhaust pipe, where it explodes
• Poor fuel consumption
• An engine that overheats, or wears out too quickly
• An engine that may actually run backwards(!)
• An engine that doesn't start at all
  

10248

Technology / Re: Is it possible to create a Star Trek-style Sick bay?

« on: 11/07/2012 11:19:34 »

If in future everyone were fitted with a biochip which continually analysed the blood, and did a DNA analysis on anything odd it saw floating around....  then the tricorder would only need a wireless receiver to pick up a patient's vital signs (and many not-so-vital signs).

A non-contact pathology lab!

10249

Question of the Week / Re: QotW - 12.07.15 - Can you get sick from your own intestinal bacteria?

« on: 09/07/2012 11:31:34 »

On the question "Will ... the person whose bacteria got into the food, be sick as well?", the answer is: Not always.

In most cases, a person will develop an immunity to a disease, and clear the pathogen from their system (or they develop a chronic infection, and perhaps die).
However, some people are not badly affected by the pathogen, do not develop an immunity and do not clear the microorganism from their system; these "carriers" can continue to shed the pathogen for their whole life.
One of the most infamous cases was "Typhoid Mary", who had no symptoms of Typhoid fever, but managed to infect many people in her occupation as a cook, and some died. See http://en.wikipedia.org/wiki/Typhoid_Mary

The same can happen for many microorganisms; gut bacteria in particular are somewhat protected from the immune system. Helicobacter Pylori causes stomach ulcers in some people, but most infected people are symptom-free.

10250

Question of the Week / Re: QotW - 12.07.08 - Does choice of cultery alter the taste of food?

« on: 07/07/2012 22:28:01 »

It is said that a souffle mixed in a copper bowl will rise better.
This is because during the short period of mixing, the copper reacts with conalbumin from the eggs, to produce a compound which sets at a higher temperature, so it can rise higher before it sets.

10251

Question of the Week / Re: QotW - 12.07.08 - Does choice of cultery alter the taste of food?

« on: 07/07/2012 22:24:35 »

Update: The Neurogastronomy expert was Professor Charles Spence, of Oxford University.
The interview with Dr Karl can be found here: http://www.abc.net.au/triplej/media/s3494891.htm
The segment on airplane food begins at 27:20 minutes, and runs for 2 minutes.
In the same interview, he suggests that "metallic" is an actual taste sensation, but a minor one, unlike the usual 5 flavours.

10252

Physics, Astronomy & Cosmology / Re: Electron's energy

« on: 07/07/2012 14:13:20 »

We often imagine an atom as a little solar system, with the electrons circling the nucleus like planets circling the sun. In this analogy, if the planet is initially outside the sun, it will keep circling the sun, ie the initial separation (gravitational potential energy) provides the initial kick (kinetic energy).

Quantum theory describes the electrons as a probability density which is "spread out" around the nucleus, representing the probability that an electron will be found at a particular point in space. If the electron starts out at "infinite" distance from the nucleus (electrostatic potential energy), it will release energy in the form of photons of specific energy as it comes closer to the nucleus, and the shape of the probability density will change depending on the electron's energy.

Of course, all analogies have their limitations.... Unlike planetary systems, there is a minimum energy that an electron can take around a nucleus, so ordinary Hydrogen is stable. With the Kepler telescope we have seen many "Hot Jupiters" which are very close to their star.

10253

Physics, Astronomy & Cosmology / Re: where does the energy go from a redshift?

« on: 07/07/2012 13:45:09 »

There are two different sources of red-shift in the above discussion:

• Increased wavelength from a non-massive source moving away from you: Doppler Shift http://en.wikipedia.org/wiki/Doppler_shift
• Increased wavelength from a massive source which is stationary relative to you: Einstein Shift http://en.wikipedia.org/wiki/Gravitational_redshift
• Of course, most bright astronomical objects moving away from us (eg stars or galaxies) also have a significant gravity field

Energy is being generated at the source at a faster rate than it is being received by us:

• Doppler shift: For an object moving away from us at (say) 1% of the speed of light, the light energy generated in 1 second will be received over 1.01 seconds. This reduction in power is reflected in a reduction of the photon energy, which is seen as a reduction in frequency = red-shift (or blue-shift, if the source is moving towards you).
• For objects traveling at closer to the speed of light, an additional relativistic correction must be made for time going slower on the moving object.
• Einstein Shift: For sources in a strong gravitational field, time goes more slowly in the gravitational field. The reduced rate of emission is revealed as a reduction in the frequency of the photons = red-shift (or blue-shift, if the observer is in the more intense gravitational field)
  

So it seems that energy is not gained or lost, but is being received over a longer (or shorter) time period than it was transmitted in the source's frame of reference.

10254

Physics, Astronomy & Cosmology / Re: Seismic study of Pulsars?

« on: 07/07/2012 13:14:49 »

Thanks, imatfaal.
It seems that with the fastest neutron star pulses occurring at intervals >1ms, many of the oscillation modes < 1ms would not be observable from the pulses.
However, some of the longer-period oscillations may be observable, someday.

10255

Physiology & Medicine / Re: Why is a delay between video and audio so annoying?

« on: 07/07/2012 13:02:07 »

Our brains unconsciously know that light is faster than sound. So our brains are moderately tolerant of sound lagging behind the picture (up to 45ms), but are less tolerant of the sound arriving before the picture (15ms).

http://en.wikipedia.org/wiki/Audio_to_video_synchronization#Recommendations

10256

Physics, Astronomy & Cosmology / Re: Could we construct a building that reached into space?

« on: 07/07/2012 12:53:36 »

Long, thin buildings under compression tend to crumple - they need lots of guy-wires.
The key to Arthur Clarke's hypothesised space elevator was to keep it in tension, by putting the center of mass at geostationary orbit. The building extends out well past 22000 miles/35000 km, and must be built out from geosynchronous orbit.

The tension is higher than we can sustain with today's materials, but it is mechanically more stable than a building which is under compression.

The reality check comes with launch costs, and powering the space elevator. 

10257

Question of the Week / Re: QotW - 12.07.08 - Does choice of cultery alter the taste of food?

« on: 07/07/2012 04:55:10 »

One psychologist suggested that the weight of the cutlery affected the perceived quality of the food.
Metal tends to be denser than plastic....

10258

Physics, Astronomy & Cosmology / Re: Forget gravity, how does andromeda know the milky way is attracted to it.

« on: 06/07/2012 23:30:24 »

Another approach: As well as the general motion of stars and galaxies, each star or galaxy has its own, random, "proper motion".

At this point in time, Andromeda and the Milky Way have proper motions that are towards each other, and this would continue even if gravity were not acting between them.

We know that gravity is acting, so their closing velocity will increase before they meet.

10259

Physics, Astronomy & Cosmology / Re: If the universe is expanding faster is more energy coming into it?

« on: 06/07/2012 23:18:08 »

To rephrase the question, if Dark Energy causes objects to increase their relative velocity, does this increase their kinetic energy, and can we turn this into "useful" energy?

Background: Normally, if you double the speed of an object, it's kinetic energy increases by a factor of 4: E=¹/₂Mv²
This energy can then be turned into "more useful" energy if moving air collides with something like a turbine blade (or into "less useful" energy, in the case of a car crash...)

Energy is neither created nor destroyed in this process, just changed from one form into another.

Dark Energy is still a new idea, but some of the challenges are:

• The increased velocity comes from a stretching of the space between the objects
• Is this transforming "Dark Energy" into "Kinetic Energy"?
• Because already-distant objects are moving apart even faster, it is not obvious how to make them interact with each other to produce "useful" energy.

10260

Physics, Astronomy & Cosmology / Re: Form where do neutrons come from?

« on: 04/07/2012 23:39:33 »

Correction: proton proton fusion is thought to be the dominant process in the sun. Wikipedia has some nice diagrams on this.