Quantum Mechanics and CDs

Syth

I read somewhere that if quantum mechanics wasn't true, CD wouldn't work. Is this true? Does any other day to day thig rely on strange physics? I've heard that GPS has to use time-dilation to calculate where you are.

simu

Yeah, quantum mechanics underlies the functioning of lasers (and, as a result CDs), transistors as well.

Quantum cryptography (based on QM properties) is also being developed to provide secure ways to transmit data...

Syth

Yeah I heard about the quantum encription, imagine a cipher so secure that the laws of physics prevent it working. Now that'd be privacy. (Good ol' Code book by Simon Singh).

Couldn't you use quantum entanglment as a cipher? Since information is transmitted in zero time, then there wouldn't be any information actually moving, right? So noone could intercept it, then noone would be able to break it if they don't have it. Right?

Nietzschean

Originally posted by Syth
I read somewhere that if quantum mechanics wasn't true, CD wouldn't work.

Well not nessairly, it just means the properties of this function we use don't violate the laws of QM, it doesn't per say guarantee they are a universal law.

Capt'n Midnight

Any update ?
AFAIAA
CD's work cos the pits are a quarter wavelength deep.
So it's a half wavelength by the time the light is reflected back up again. It's the half wavelength causes destructive interference and so less light is seen to bounce back.
Not sure if coherence is needed for this to work.

All GPS satellites have synchronised atomic clocks, all are in stable 12 hour orbits. If you can see several GPS satellites then from the differences in time you can work out how much further away one is than another. A bit of trig later. you can work out where you are. Since the statellites are moving so fast the clocks have to be slowed down...

secret_squirrel

GPS has nothing to do with time dilation - time dilation only becomes a factor when you are moving at a significant percentage of the speed of light.

See here for a full description of how gps works.

And like any theory QM cannot be said to be 'true'...it can only be said that it hasnt been proved wrong yet.

Basically all of physics just consists of approximate models we use to explain how we think the universe works - none of them ever offer a way to truly encompass complexity of the universe.

Capt'n Midnight

What the Global Positioning System Tells Us About Relativity
http://www.metaresearch.org/cosmology/gps-relativity.asp

GuanYin

I'm gonna stick this in Physics/Chemistry, seeing as its back in discussion.

Over the next while, if threads end up in General Science or resurface, I'll move them to the appropriate forum i the science catagory.

Hope noone minds.

dudara

The actual CD itself works because of light reflection and interference. But we wouldn't have CD players without the laser diode, which is definitely a QM device.

Capt'n Midnight

re:\But we wouldn't have CD players without the laser diode,

Does this mean coherence is needed for this to work ?
or just a very bright light ?

dudara

all those are things that define a laser. It it has the characteristics of extremely coherent light, centred around a single, or narrow band of wavelengths, etc then it is a laser.

How you achieve the population inversion to cause lasing is another matter. In gas lasers, etc, it is achieved by pumping the gas with other light, or electricity, causing stimulated emission from the excited atoms. Either way you're causing transitions from excited atomic states to ground states and vice versa. This was actually one of the first signs of QM, when Bohr studied electron energy levels in Hydrogen, and predicted transitions could take place from level to level, with the gain or loss of energy, which had to be of a finite amout (or quantum)

In a solid-state laser diode, you're talking transitions from conduction bands to valence bands and vice versa, all of which is governed by the laws of QM