Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: _Stefan_ on 19/05/2007 14:57:35

Title: How is information actually transmitted by particles and waves?
Post by: _Stefan_ on 19/05/2007 14:57:35

How do EM waves and particles actually transmit information? How is the information for this web page carried to my computer, for example? How does the data travel along with the wave/particle from one place to another?

Title: How is information actually transmitted by particles and waves?
Post by: _Stefan_ on 20/05/2007 06:43:35

Anyone have an answer? :)

Title: How is information actually transmitted by particles and waves?
Post by: daveshorts on 20/05/2007 10:55:17

There are two slightly different questions there. How is the information from this web page transmitted to your computer?

The web page is a whole lot of instructions encoded in HTML which your computer will use to make a picture on your screen. It starts off on our server (in Manchester as it happens). The instructions are encoded as a whole string of numbers and those numbers are in binary eg 5 = 00000101 or 9 = 00001001 etc

These numbers are then transmitted within the datacentre as voltages, so if the voltage is 5V it means a 1 or 0V it means a 0

They are then converted into light, so the light being on is a 1 and off is a 0, the light then gets shone down optical fibres, is occasionally converted back to electrical signals to route it around the place, and eventually it ends up at your telephone exchange where it is converted back to an electrical signal and sent down your telephone wire.

If you have a wireless router the signal will then be converted to a radio signal, approximately like you listen to music on and then back to your computer where it is reconstructed and a web page is drawn.

There is an immense amout of extra cunningness which goes into making sure that your data gets to you and not somone else on the other side of the planet, and getting back the bits which get lost etc. etc.

How is the information transferred by the actual particles?

In most stages you have a sensor of the kind of particles you are sending the data with, if lots of them turn up it is a 1 if none do then it is a 0. So you just affect whether the particles end up at the other end - turn the light on and off or turn the flow of electrons on and off.

Title: How is information actually transmitted by particles and waves?
Post by: lyner on 21/05/2007 15:53:59

I think the issue is about how the waves actually carry the  information.

The answer involves the idea  of 'modulation'.
If you flash a light and send a message using Morse code, you are  transferring the information by modulating the light beam on - off etc.
The simplest radio transmissions consist of bursts of a radio wave. The transmitter is  simply keyed on and off, using morse code or some other 'binary code' modulation that the distant receiver / operator can recognise. This was called 'wireless telegraphy', which was used in a similar way to 'telegraphy', which had to use wires on telegraph poles, between towns.
If you just connect the output from a microphone to an antenna, nothing will happen. The waves produced by the antenna just wouldn't propagate through the air / space because the frequencies involved are only a few kHz (1000 oscillations per second).
For decades, speech and music has been sent, using Amplitude Modulation. of a radio wave  (A.M.).
A carrier wave at a frequency that will  propagate through space - usually way above  1MHz (1,000,000 oscillations per second)  is transmitted. By modulating the size (or amplitude) of the carrier  wave in step with the electrical voltage from, for example, a microphone, the signal from the microphone is carried to the receiver. The receiver is 'tuned' to the wanted carrier frequency (rejecting all signals at other frequencies) and detects how the received wave is modulated. The detected voltage varies in the same way that the original microphone signal varied and it can be heard with an earphone or amplified  and fed to a loudspeaker.
This allows the information to be carried  from A to B and also, by using carrier waves of different frequencies, many signals can be transmitted and received at the same time, as long as they are separated in frequency (each channel needs 'bandwidth').
There are many other methods of modulating a carrier wave; altering its frequency, pulsing it with high speed digital signals or by changing its phase etc.. In all cases, the term 'carrier wave' describes just what happens - a low frequency signal is carried by modulating a high frequency, transmittable, wave.
What a bit of luck - without it we'd have no Radio 4.
Of course, transmitting TV pictures has a whole new layer of complication on top of this. . . .

Title: How is information actually transmitted by particles and waves?
Post by: _Stefan_ on 22/05/2007 04:52:59

Thanks for the replies :D Very informative.

So information is not actually 'carried' on particles or waves in the sense that I would pick up a book from a shelf, hold it, and put it on a table. It simply has to do with whether a signal is produced or not and what a receiver is programmed to do with that signal or lack thereof. Have I understood correctly?

Cheers :)

Title: How is information actually transmitted by particles and waves?
Post by: lyner on 22/05/2007 11:22:13

Right.
Of course, the information in your book is 'carried' in the pattern of the writing, in just the same way as in the pattern of the varying radio  wave.
The paper and ink aren't the information. are they?
In both cases, the sender and receiver of the information have to use a common 'code' so that it makes sense the other end.