8621

Question of the Week / Re: QotW - 12.09.02 - How can I use one telephone line for multiple uses?

« on: 10/09/2012 11:15:11 »

Amplitude and Phase modulation of a Carrier Frequency works well in hardware, when there is a small amount of information in a fairly narrow frequency band (compared to the carrier frequency).

The ADSL transmission used on most telephone lines spans quite a wide band - from 25kHz to 125kHz, and so modulating a single carrier would not work very well. This wide range is broken up into sub-bands.

Modern Digital Signal Processors use a mathematical technique called the Fast Fourier Transform (FFT) to generate signals in all of the sub-bands simultaneously. The inverse FFT is used to decode them off the telephone line, while accounting for the variable frequency and phase response over this wide frequency band, and cancelling the interference from the adjacent sub-bands.

Like an increasing number of things today, this is a software-based technique, rather than a hardware-based technique. Most of the hardware effort goes into making sure the software can run as fast as possible, while consuming as little power as possible.

8622

Physics, Astronomy & Cosmology / Re: Any big changes going on in our solar system?

« on: 07/09/2012 22:24:50 »

There is a simple mathematical formula for the orbit of two objects (like one planet around a star), and this is stable.

However, as soon as you have 2 planets around a star, it gets a lot more complicated. Except for a few special solutions such as the ones discovered by Lagrange, the "3 body problem" has no stable solutions, so the orbits change chaotically (randomly) over time.

It is now known that a solar system like ours with a number of planets will be chaotic in the long term, even without external influences. http://en.wikipedia.org/wiki/Stability_of_the_Solar_System

Discoveries of exoplanets have shown a large number of "hot Jupiters", where a planet the size of Jupiter is orbiting closer to its star than Mercury. Theories of planetary formation suggested that Jupiter-sized planets should form much further out.

However, in a chaotic planetary system, a Jupiter-sized planet passing close to a smaller planet could fling the smaller planet further out, dropping the giant planet closer to the star. (In extreme cases, the smaller planet could be flung right out of the system.)

http://en.wikipedia.org/wiki/Hot_Jupiter#Terrestrial_planets_in_systems_with_hot_Jupiters

8623

Physics, Astronomy & Cosmology / Re: mass of a photon

« on: 07/09/2012 22:09:25 »

A clarification, please Emc2:

a Photon does not affect other particles gravitationally, cause it has zero mass....
... photons do not have mass, but they do have momentum.

Eddington showed that photons are diverted by the Sun's gravity.
This produces a change in the direction of their velocity, which represents a change in the direction of their momentum.
Because momentum is conserved in interactions, a change in the momentum of the photon must be balanced by a change in the momentum of the Sun (as small as that may be).
Or you could look at it in a Newtonian sense - the photon feels a gravitational force towards the Sun, so the Sun feels an equal and opposite gravitational force towards the photon...
This suggests that the photon does affect other objects gravitationally, does it not?

Some of the advantages quoted for the newer momentum-based relativistic notation is that momentum is conserved in all inertial reference frames, and applies equally to all particles regardless of whether they travel at the speed of light or not...

8624

Physics, Astronomy & Cosmology / Re: Any big changes going on in our solar system?

« on: 07/09/2012 10:24:39 »

If the solar system were disturbed into a chaotic state, that certainly would be a modern apocalyptic vision of astronomical proportions:

• On the one hand, the Earth could be robbed of its angular momentum, and plunge into the Sun
• Or on the other, the Earth could gain angular momentum, and be flung out into interstellar space

That certainly puts a different spin on a couple of earlier apocalyptic visions:

...they were thrown alive into the lake of fire... Revelations 19:20 (http://www.biblegateway.com/passage/?search=rev%2019:20&version=ESV)
...they will be thrown into the outer darkness...  Matthew 8:12 (http://www.biblegateway.com/passage/?search=Matt%208:12&version=ESV)

8625

Question of the Week / Re: QotW - 12.09.02 - Could we make an infinitely powerful laser with mirrors?

« on: 06/09/2012 10:56:24 »

The output power of a laser is always less than the power going into the laser (usually a lot less).
You can't have an infinitely powerful continuous laser beam without an infinite power source to drive it.
Pulsed lasers can reach very high power levels, but only for a small fraction of the time.

Lasers use mirrors facing each other at the ends to ensure that the photons are all "in-step" with each other, as they bounce repeatedly off the mirrors. When the laser material is "pumped" with energy, a photon (ie Light) passing by will Stimulate the Emission of another photon (ie Radiation) with the same direction, frequency and phase. Hence the acronym LASER = Light Amplification by Stimulated Emission of Radiation. http://en.wikipedia.org/wiki/Laser#Design

One end of the laser is a semi-transparent mirror, allowing some of the laser energy to be emitted, while bouncing most of it back through the laser.

Lasers always lose some of their input energy - the pump source is not turned into laser output with 100% efficiency. The laser material is never perfectly transparent, so more energy is lost the more times it bounces backwards & forwards. This lost energy is turned into heat - often causing the laser material to shatter or burn out if the power levels get too high. No mirror is perfectly reflective, so the mirror is likely to crack at high power levels.
 
By making the mirrors more reflective, the power levels in the laser will reach higher levels, but is likely to make the laser fail sooner.

The US National Ignition Facility is a pulsed laser with a peak optical power of 500 TeraWatts, for a few picoseconds - but it takes about 6 hours before it can cool down enough for another shot. The average output power is similar to a hand-held LED flashlight. See http://en.wikipedia.org/wiki/National_Ignition_Facility

8626

Physics, Astronomy & Cosmology / Re: mass of a photon

« on: 05/09/2012 13:56:41 »

Preface: I see that current practice prefers to talk in terms of a particle's momentum, rather than it having a relativistic mass which increases with its speed (http://en.wikipedia.org/wiki/Mass_in_special_relativity#Relativistic_mass). However, I am a bit old-school, so apologies in advance...

Let's ignore the photon for a while, and take a simpler example of the difference between Relativistic Mass and Rest Mass: the electron neutrino.

• Mass and the Energy are equivalent, due to E=mc²
• Particle physicists quote the Mass of a Particle by its Energy, in electron Volts (eV) or Mega-electron Volts (MeV).
• A neutrino has a Rest-Mass, thought to be around 0.04eV to 2.2eV (but it's very hard to measure: http://en.wikipedia.org/wiki/Neutrino#Mass)
• This means that if you could ever capture a neutrino, stop it moving, and turn it entirely into energy, you would get somewhere around 1 eV  (give or take a decimal place)
• However, neutrinos emitted from a nuclear fission reactor have energies of up to 10MeV. (http://en.wikipedia.org/wiki/Neutrino#Artificial)
• This means that if a neutrino from a nuclear reactor interacts with a hydrogen nucleus (eg in http://en.wikipedia.org/wiki/KamLAND), a neutrino from a nuclear reactor will produce particles with an energy millions of times larger than the energy of a stationary neutrino
• This increase in energy (equivalent to an increase in momentum, and an increased relativistic mass) is because they are traveling at very close to the speed of light; a rough calculation suggests that a 4MeV neutrino will be traveling somewhere around 99.999999999997% of the speed of light
• So their momentum and energy is much higher than you would expect from their velocity (effectively c) and their rest mass (1eV).
• No matter how much energy you give the classical neutrino (ie how much you increase its momentum), it cannot reach the speed of light, as this would imply infinite momentum (or infinite relativistic mass, in older terminology).

So, for a 4MeV neutrino:

• It has a small rest mass (= a small energy) of somewhere around 1 eV
• It has a non-zero and finite relativistic mass (which may be millions of times larger)
• It can never reach the speed of light in a vacuum
• Time is slowed down significantly, but it has not stopped, so we can still see phenomena like neutrino oscillations (http://en.wikipedia.org/wiki/Neutrino#Flavor_oscillations)
• The neutrino will have severe length contraction - but not to zero

Heading back to the original question: if we contrast this with a violet photon

• It has a small relativistic mass of around 0.5 eV (= a small energy), similar in magnitude to a stationary neutrino
• The photon always travels at exactly the speed of light in a vacuum.
• Time is stopped, so the photon does not change (unless it interacts with something)
• Length contraction will also apply to photons, but the wave nature of light as a transverse electromagnetic wave means that the size will be non-zero at right-angles to the direction of travel.
• If you could ever slow down a photon in a vacuum, and turn it entirely into energy, you would detect 0eV, ie the photon's rest-mass would be zero.

PS: If you find an error in my rough calculations, please feel free to point it out!

8627

Physics, Astronomy & Cosmology / Re: calories in an apple

« on: 04/09/2012 13:44:08 »

Another way to get a lot of energy out of an apple is to let it get swept up in the accretion disk of a black hole.

Before it disappears forever beyond the event horizon, it will radiate strongly in X-Rays, as it is shredded and spaghettified (a technical term).

Of course, it would take more energy than found in the mass of an apple to transport an apple to our nearest known black hole in a reasonable number of millennia. http://www.space.com/7678-black-hole-closer-earth-thought.html

8628

Physics, Astronomy & Cosmology / Re: calories in an apple

« on: 04/09/2012 12:38:21 »

E=mc² is iconic, and inspiring - but in practice it is exceedingly difficult to turn matter into energy!

Perhaps the easiest way to see this is to look at a periodic table (eg http://www.ptable.com/), and compare the mass of 4 hydrogen atoms (each 1.00794 grams/mole) with the mass of 1 helium atom (4.00260).

Together the 4 hydrogen atoms add up to 4.03176, which means that if you could fuse them together to make helium, you would release roughly 0.029 as energy, a mass to energy conversion yield of less than 0.1%. And it takes huge amounts of energy to create the right conditions where hydrogen fusion reactions could occur - and even then you can't capture all of this energy, because neutrinos are such slippery little suckers....

Hydrogen fusion, despite its incredibly small mass conversion ratio, produces enough energy to keep the Sun burning, and inspire several large research projects looking at controlled fusion power plants.

It's just as well you don't get all this energy from an apple - because those bazillion calories certainly exceeds your recommended daily intake of gamma rays!

8629

Physics, Astronomy & Cosmology / Re: mass of a photon

« on: 04/09/2012 12:17:51 »

A slight clarification about the difference between Mass and Rest Mass:

• The photon, traveling at the speed of light does have mass, and is deflected by a gravitational well like the Sun - in 1919 this was one demonstration of Special Relativity http://en.wikipedia.org/wiki/Tests_of_general_relativity#Deflection_of_light_by_the_Sun
• This Mass is derived from the photon's energy by the famous E=mc²
• In turn, the photon's energy is derived from it's frequency v by E=hv http://en.wikipedia.org/wiki/Photon#Physical_properties
• This is an example of the prediction of relativity that objects get infinitely more massive as they approach the speed of light. This effect is demonstrated daily in particle accelerators.
• Photons have Zero Rest Mass. This can be roughly described mathematically as: (Finite Mass)/Infinity = 0. If it were possible to slow photons down below the speed of light (in whatever medium they were traveling), they would have no energy, and would cease to be observable.
  

8630

Physics, Astronomy & Cosmology / Re: transport plasma via glas fiber

« on: 04/09/2012 12:01:00 »

A plasma is a gas of ionised atoms, with their outer electrons ripped off, usually by being heated to an extremely high temperature, or being subjected to an electrical discharge.

An optical fiber is traditionally a thin, solid rod of silica.

So a plasma (hot gas) cannot flow through an optical fiber (solid) - but a sufficiently intense laser pulse could turn an optical fiber into a plasma.

There was a company making laser cutters by using many semiconductor lasers, and using many optical fibers to combine these laser beams into a cutting beam that could turn the target into a plasma. Despite the very low attenuation of optical fibers, they needed to water-cool the optical fibers to stop them from self-destructing!

There is some research underway into advanced optical fibers with multiple touching cores and airspaces between them, which I guess could carry ionised gas a short distance - but not if you wanted it to continue to function as an optical fiber!

8631

Physics, Astronomy & Cosmology / Re: Does dark matter interact with the Higgs Field?

« on: 03/09/2012 12:37:28 »

CPT ArkAngel & yor_on: A short discussion about why Dark Matter composition should only be a  small fraction made up of neutrinos: http://en.wikipedia.org/wiki/Dark_matter#Hot_dark_matter

8632

Physics, Astronomy & Cosmology / Re: Two dead stars provide low-tech way to test Einstein

« on: 03/09/2012 12:02:44 »

In 1993, Russell Hulse and Joe Taylor were awarded the Nobel Prize in Physics for indirectly detecting gravitational waves from a pulsar in a binary system. http://en.wikipedia.org/wiki/Gravitational_wave#Radiation_from_other_sources

Certainly, finding a system which is closer to us, and in a closer orbit, makes it much easier to study.

8633

The Environment / Re: What can we do to reduce the size ofour nitrogen footprints?

« on: 03/09/2012 11:52:38 »

Plants that host Rhizobacteria in their root nodules are well placed to capture the nitrogen straight into their circulation, and into the soil. This should produce less runoff than spraying fertiliser on the soil.
http://en.wikipedia.org/wiki/Nitrogen_fixation#Root_nodule_symbioses

8634

Physiology & Medicine / Re: What’s The Equation For Quenching Your Thirst ?

« on: 01/09/2012 04:19:16 »

It also matters what you drink.

• If you drink sea-water (much saltier than blood), then you end up thirstier afterwards.
• If you drink alcohol, your kidneys flush it out with lots of water, so you also end up thirstier afterwards.
• But pure water is not the best, either! Adding a small amount of salt & sugar helps with absorption in people with diarrhea: http://en.wikipedia.org/wiki/Oral_rehydration_therapy#Basic_solution

8635

Physics, Astronomy & Cosmology / Re: How would the Earth be affected by a supernova really close by, say 10 ly?

« on: 31/08/2012 12:04:41 »

If a star is large enough, after it has finished fusion of most of its hydrogen to helium, it can continue fusing heavier elements, until the core is made up of elements like iron and nickel (found in certain certain classes of meteorites).

However, nuclear processes consume energy to build a nucleus beyond 56 nucleons. It is thought that the elements beyond Iron-56 are built in an intense burst of neutrons that accompany a supernova explosion.

So the Iron-60 could have built up by Iron-56 being bombarded by neutrons, or by decay of even larger nuclei formed in a supernova.

It is thought that much of the momentum imparted to the supernova debris comes from neutrinos. The density of the atoms collapsing towards the center of the star to form a neutron star is sufficient to interact with the normally elusive neutrinos, blasting the surface into space.
http://en.wikipedia.org/wiki/Supernova_nucleosynthesis#Supernova

8636

Question of the Week / Re: What would be the societal impact of a major computer failure?

« on: 31/08/2012 11:43:07 »

To some extent, the internet was originally conceived as a resilient network which can route around broken links and routers.

However, a widespread impact from events such as you describe could disconnect large sections of the population from communications. Our city-based society is rather fragile, and working on very lean margins - the average supermarket only stocks a few days of food. Without effective transport, communications, electricity, water/sewage and banking systems, things could get very bad, very quickly.

Geomagnetic storms can damage the electricity grid and cause widespread blackouts: http://en.wikipedia.org/wiki/Geomagnetic_storm#Electric_grid
Military actions could cause EMP: http://en.wikipedia.org/wiki/Electromagnetic_pulse
...and terrorists are always looking for vulnerable spots in the fabric of society.

Fortunately, the military has equipment designed to be used off-grid in electrically hostile environments, so they could do some coordination in many of these conceivable scenarios.

8637

Technology / Re: Hacking your brain: how close are we to mind-reading?

« on: 31/08/2012 11:23:40 »

You may not get a strong recognition signal from just a single random digit.

However, if you presented a 4-digit PIN which matched the person's PIN, it might trigger a stronger P300 signal.

Of course someone might get suspicious (or at least, very bored) if someone attached something to their heads, and then started displaying thousands of 4-digit numbers.

8638

Chemistry / Re: fusion deuterium and tritium

« on: 31/08/2012 11:12:47 »

Here is a diagram: http://en.wikipedia.org/wiki/Nuclear_fusion#Overview

A table of possible hydrogen fusion reactions is shown here: http://en.wikipedia.org/wiki/Nuclear_fusion#Criteria_and_candidates_for_terrestrial_reactions

The same article also talks about muon-catalysed fusion, but muons are an expensive catalyst which decays rapidly.

8639

Plant Sciences, Zoology & Evolution / Re: Why Do Hens Lay Unfertilised Eggs ?

« on: 31/08/2012 10:00:36 »

In a "normal" population of free-range hens & roosters, I would imagine that most of the eggs would be fertilised, so there is no advantage in not laying them.

It is only in our artificial environment of single-sex egg factories that it seems a bit odd.

I have heard of rodents which live as solitary animals in a desert environment, which ovulate only after sex. Closely-related rodent species living in groups ovulate regularly, even without sex. However, a mammal egg is much smaller than a bird egg, so it may require less of a "production line".
http://www.thenakedscientists.com/HTML/content/news-archive/news/2643/

8640

Physics, Astronomy & Cosmology / Re: How would the Earth be affected by a supernova really close by, say 10 ly?

« on: 30/08/2012 12:21:45 »

I heard a suggestion that a "nearby" supernova could impact us in an unexpected way: via neutrinos.

Neutrinos don't often interact with matter - it is said that about a billion neutrinos from the Sun pass through an area of a fingernail, every second, and nearly all of them pass right through the Earth.

However, in 1987, a Supernova in the Magellanic satellite galaxy produced a burst of neutrinos, 24 of which were detected by some large detectors on Earth (http://en.wikipedia.org/wiki/SN_1987A#Neutrino_emissions). This is 168,000 light-years away.

At 10 light years away, a supernova would be about 300 million times more intense. This could cause an increase in cancers - and the thickness of the Earth would not provide any useful shielding.