Connecting the last mile and a couple of light years more
Introduction
The messy cables connecting your home computer to the internetwork follow roughly the same core ideas as when you'd like to talk to some extraterrestrials somewhere in a universe: it doesn't really work out particularly well.
In the majority of the countries on earth the provided service by ISPs is a rather pathetic dial-up modem and mostly unreliable (not "getting through", being kicked out within 5 minutes, and slooooww). Then again, everybody knows that there's technology available that allows you to surf and download faster and more reliable, so why doesn't everybody has that? (For convenience here, I prefer to ignore the cost and other social/environmental factors that prevent to make it happen). Secondly, if all sorts of fancy fast speed communication is possible, can't we just use something similar in space, to explore if somebody is out there?
Hopefully this page will give you some insight in the theory behind the communication channels we have gotten used to, hurdles that are (partially) overcome and some challenges to face.
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The last mile
In every day life, people talk about the speed of their connection to the Internet in the way of "I have a dull 56 Kb/s modem". However, in reality those (kilo)bits travel through your telephone cable via an electromagnetic wave. Figure 1.1 shows an example of such a wave as a function of time, which is also called an oscillation.
Source: http://id.mind.net/~zona/mstm/physics/waves/partsOfAWave/waveParts.htm#frequency (contains a fancy dynamic animation where you can adjust the frequency and see the effect of moving waves as a function of that)
The amount of oscillations per seconds is called the frequency and notated as the amount of Hertz, or Hz (after the German physicist Heinrich Hertz). A higher frequency means a shorter wave in time, i.e. more coded data can be transmitted in the same amount of time. Here is where the so-called bandwidth comes into play: bandwidth defines a certain frequency range where lower and higher frequencies are ignored (filtered out) except the defined one. Voice over the telephone line travels at 3 kHz, but the telephone line also can accept a higher bandwidth like 500 MHz, which would equal data transport in the Gb/s range! So why don't we all just use that? Because there are some not-negligible hurdles to overcome that are related to the characteristics of any physical layer: noise, attenuation and cross-talk. First, you're not the only one with the cable. Other wires, stray electrical signals from lightning or e.g. your neighbour's ham radio interfere with the smooth wave, changing the shape of the signals unpredictably in that the receiving end may not be able to interpret the signal correctly anymore (see Figure 1.2). A better shield around the cable can reduce this interference, e.g. a thick coaxial cable instead of a flimsy telephone (twisted pair) cable, or go into the UV spectrum with fibre optics.
Source: http://www.cs.washington.edu/homes/tressel/adsl/adsl_body.html
Secondly, there is so-called cross-talk (actually also noise), where the radiated signals leak into adjacent wires due to induction of the copper lines. Technologies such as ISDN and DSL do their very best to reduce just that: an ISDN modem is just a bit more complex, whereas DSL is supposed be able to go up to 8 Mbit/s over the very same telephone line. DSL in the implementation of Asymmetric DSL is on the rise: the cross-talk can be significantly reduced by providing different connection "speeds" upstream and downstream (respectively 256kbit/s and 512kbit - 2 Mbit/s). Fibre-optic cables are different in that respect: light (i.e. photons) is not leaking out of the cable in the way like the electrons of a copper cable do, "lost" photons outside the cable do not influence the ones on the inside, and the photons don't act upon each other like electrically charged particles do.
Third, there is the problem of attenuation: the loss of energy when the electrical signal is moving forward is on a logarithmic scale with the distance, and depends on the frequency used. And guess what? A higher frequency is more prone to attenuation. To overcome this problem, repeaters are placed down the line to give the signal an extra boost. The repeaters amplify the signal by adding power to the signal equal or more than it lost due to travelling from the source to the repeater (also called amplifier). More power to the start signal may sound like a quick fix, but it radiates more energy, thus more noise. Long distance telephone lines need a repeater about every 5 km, whereas fibre-optic cables won't need one if the distance is less than 30 km (new technologies allow for 100 km). That fibre-optic cables need repeaters as well may come as a surprise, but there can be impurities in the glass core and bends in the cable can cause some loss of the beam as well.
Summarizing, bandwidth is a range of frequencies where the electromagnetic waves go through copper cables like the telephone network, or photons through fibre-optic cables. Higher bandwidths result in higher data transmission rates but have the drawback of more noise, cross-talk and attenuation distorting the signal in the copper cables. Technologies using these plain old twisted pair cables of the telephone network like ISDN, DSL and ADSL are attempts to address these problems, especially the cross-talk part of it.
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Beaming into space
Taking into account all the previously mentioned small-scale problems, then what would it take to build e.g. a radio-transmitting system that would have even the slightest chance of being detected by a receiver tens or hundreds of light-years away? Or the other way around: is there even the slightest probability that we would be capable to detect communications sent out by others? The SETI@home project attempts the latter, but is that a reasonable goal when we're limited by our own knowledge of communication systems and its physical laws that constrain these communication possibilities?
First of all, you can't just lay a fibre optic cable into space and start transmitting. Without the cable, light photons suffer from scattering and adsorption by interstellar dust, therefore we'll have to stick with wireless: radio waves. The choice for frequencies between 1 and 3 GHz (see previous section for detail on frequencies) is not as arbitrarily as it might seem. In that range the absorption and obscuration of waves by the interstellar molecules and dust clouds and the background radiation aren't too bad. The radio waves move around through space by varying electric and magnetic fields and together can go up to the speed of light. But we're still stuck with the aforementioned problems like noise, power of the transmitter signal, quality of the receiver, and the size of the transmitters and receivers.
Lets do a calculation to put things in proportion. Information will flow at a meagre 5 bits per second (compare to a dial-up modem of realistically 49 kilobits), which will require a bandwidth of about 2.5 hertz. This will allow you to send "Hello" in 5 seconds. Now the power requirement to send out hello into space:
- Noise power is defined by the formula (Pn) = kTB, where k is the Bolzmann constant, T the noise temperature of the earth (15 kelvin) and B the bandwidth (2.5 Hz). Substituting all parameters, you'll need 5.2x10-22 Watt. The receiving end would need similar power for e.g. a dish of a square meter.
- Total power need depends on the distance we want to send the signal, and where to, i.e. ideally we'd send out the signal to everywhere in space (omnidirectionally) and not to specific locations to achieve a higher "hit-rate" with an equally advanced, or retarded, civilization. 100 light-years (9.47x1017 meters) sounds like a comfortable distance to send it out to, though it covers less than a millionth of 1 percent of the starts in the galaxy. This means a total power requirement of:
(5.2x10-22) x 4p x 9.47x1017 = 5.8x1015, or about 7000 times the total electricity-generating capacity in the US (...).
Ouch! Ok, next step is to adjust our wishful thinking. What if we beam towards more specific locations in space after all? Take a dish of a 1.5 m in diameter, operates at a wavelength of 20 cm, resulting in a reception beam of about 11 degrees. Reasonably, we could line up a square kilometre with those small dishes, and maybe the extraterrestrials as well, resulting in a gain one million times greater than the 1 m2 antenna. "Minor" drawback is that it would also have a beamwidth of only 11 thousandths of a degree. Nevertheless, this would greatly reduce the power consumption to a manageable 5700 watts. As if you're looking for the needle in a haystack; then again, that is possible (burn the haystack and move a magnet through the ashes).
It's like one or the other: either you have minimal antenna areas requiring excessive power to transmit the signals all around, or you have very large (or a lot of small) antennas with a modest power consumption but such a narrow beam into infinity that the hit-rate would reach very close to a zero chance. I'm not even taking into account more realistic distances of a couple of hundred or even thousand light years, or the interfering multipath effects caused by gas clouds and static-magnetic fields that divert the radio waves, change polarization and produce sporadic fluctuations. Even if there is life out there, and that life is capable of receiving our signals, and can send a response back as well, the time delay may very well be longer than a century.
Though it's nice to ponder about the (im)possibilities of communicating with potential "intelligent" life that may be out there somewhere in (one of) the universe(s), narrow-minded as I am I'd prefer to see communication(s technology) on and around earth improving first.
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References:
Anon. Extract 2 from T293: Transmitting the signals. OU UK. 1998. Ch3.
Dettmer, R. Broadband@home.com. IEE Review, nr 1, January 2001.
Leeuwen, H. van. Computerinfrastructuur 3 - Datacommunicatie. Syllabus HIO Enschede. 1994. p 13.
Swenson, G.W.. Intragalactically Speaking. The Frontiers of Space, Scientific American, 2001. pp 78-81.
Tanenbaum, A.S.. Computernetwerken. 2nd ed., 1999. pp 97, 99-101, 114.