h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

I don't understand the SETI thing. If the most powerful telescopes can't see the albedo of a planet because it's lost in the overwhelming glare of the parent sun, then how do you expect to see a radio transmission that is orders of magnitude less energetic.

I've posted this query on another thread but there were no takers. Maybe you can help.

I think, basically, the star's emissions are not same at all wavelengths, and most good detectors can filter out the wavelengths they want to see. Your eyes can see the sun's visible radiation, but are completely blind to the vast majority of the spectrum.

A star is very hot, so produces a lot of IR, visible light and UV. Presumably it doesn't produce so much down at radio frequencies. Radio communication works very well on Earth, even using tiny transmitters like mobile phones, and we're pretty close to our own star.

SETI is also looking for signals that indicate intelligence which it is postulated would be coherent and artificial. Most of the detected signals from space are noise from all natural sources - stars, planets in the solar system, the galaxy etc. Radio waves produced by an intelligence would be in one of three forms and clearly have a pattern. Think of the way we broadcast radio waves - AM (amplitude modulation), FM (frequency modulation) and digital, which as far as I know would be a square wave.

t.

It's true that we can hear tiny transmitters such as cell phones but that's because we're much, much closer to them than we are to the sun.

Even listening to the FM radio I can't be more than a few miles from the transmitter if I want a clear signal. Shorter wavelengths carry farther but get chewed up by all kinds of interference, some of it coming from our sun.

If I were listening to a transmitter on a planet near a star light years away, a star that's just a point source from here, how much energy could the transmission still have at this distance?

The night sky looks black to me because only the very nearest stars still have enough energy density to trip the rods on my retina. The light started out with, to me, unimaginable amounts of energy. If you divided the original strength of an alien transmission by the same factor, how big a reciever would you need to detect what was left of the signal? Would anything be left of the signal considering the amount of interstellar stuff out there scattering it?

the news is always talking about mars rovers and the like and they always refer to the transmitter being no more powerfull than a mobile phone signal

Thanks for that.

Taff, the transmitter on the Mars rover is 1) many many times closer to us than an alien star 2) not being listened to in the glare of a nearby star. I would guess that when Mars gets close to passing behind the sun we have to wait till it comes round the other side before we can hear it again. I wonder what the angle would be? The sun with its energetic corona is very broad. Compare that to the angle between an orbiting planet around even a nearby star.

The reason that I'm asking is that I want to believe in aliens so I'm trying to think of a rational excuse as to why we haven't heard from them.

time and distance!!!!

the only one who could detect us are in a sphere with a 200 light year diamiter. a tiny part of the galaxy

anything we might find, might be long dead in reality

The problem with looking for a planet around a nearby star is that the planet is illuminated by the star so the light from the planet is exactly the same as the light from the star. The only thing that tells them apart is their separation, which at the distance we are talking about is tiny.

If an intelligent species on a planet around a nearby star is broadcasting radio waves, then there are two things we can watch out for.

1. They are sending a type of wave which is not given off by the star. Most stars do not transmit a huge amount of radio waves, for example.

2. There is information in the signal. This means that it varies in a way that we can detect. The variation may be only 1,000th part of the total signal but we'll still be able to spot them.

Note that in neither case do we care how far the planet is from the star.

Tumsup - your post interested me in the detection abilities and sensitivity of the radio telescopes used for SETI.
The SETI@Home uses the Arecibo dish so I concentrated on that.

I can't pretend this is 'my' knowledge I am presenting - I asked elsewhere about this and was directed to:
http://www.faqs.org/faqs/astronomy/faq/part6/section-12.html

lots of maths and workings out which is all very interesting but if what you want is a bottom line then about two thirds down is a table which shows, for the Arecibo dish, that with three different detectos it can detect a narrow beam signal:
22TW @ 720 ly
1TW @ 150 ly
1GW @ 5 ly

there's further discussion and tables for smaller dishes etc on that site.

The important thing to notice though is that these are high power focused narrowbeam transmissions - not media broadcasts. According to that page, the best that could be managed would be detecting a 5MW FM transmission somewhere around the orbit of Jupiter.

So for long range - interstellar or intergalactic signals - what we'd look for would be these high power narrowbeam transmissions would (if our tech is anything to go by) would be radar or somesuch.

There are some assumptions and exclusions from those calculations (as it mentions) so we could could double the range figures if certain values hold true. Also it assumes minimal signal processing which is not the case with SETI so we could again up the distance / reduce the transmission power if we include the effect of processing.

So we aren't going to see any alien TV Shows. But we might pick up on military or science transmissions.

(interesting little detour there, thankyou!)

>> light from the planet is exactly the same as the light from the star<<

Not so. Light is only truly reflected from a metallic surface. Ordinary 'reflection' is light absorbed at one wavelength and re-emitted at another. One way to see planets around a star is to look for absorbed lines from molecules such as water that would never occur on the star.

>>Most stars do not transmit a huge amount of radio waves<<

Then what are radio telescopes looking at?

>>They are sending a type of wave which is not given off by the star<<

You're right here. My original question has to do with this. According to quantum theory every photon a packet of energy of a discrete amount, higher frequencies, higher energy. The wavelength can't change with distance so the photon is either there or it isn't.
Think of the carrier wave of the radio broadcast as a string of photons evenly spaced. As they travel across vast distances the photons disappear randomly to the point where there is no enough regularity to function as a carrier wave.

I don't know enough physics to do the math. Hence my original question.

see my post for the math

"Light is only truly reflected from a metallic surface.
Ordinary 'reflection' is light absorbed at one wavelength and re-emitted at another."
Reflection on a metal is also due to absorbtion and release of photons. It isn't absorbed and re-emitted at another wavelength - but some wavelengths are not remitted due to the composition of the 'reflecting' material - which is why we can tell chemical composition by spectra absorbtion.

"One way to see planets around a star is to look for absorbed lines from molecules such as water that would never occur on the star"
Water does occur in stars. Lots of things do - depends on the star and where in its lifecycle it is.

"The wavelength can't change with distance"
well, implicitly it can hence red/blue shift. Which is how come we can tell how far away some stuff is by looking at how red-shifted it is. More red shift, greater velocity, further away.

remitted = re-emitted.
Nothing to do with galactic particle forgiveness

Thanks for the link Icotan, it was still mostly over my head but I learned alot from it.
I didn't know that water could be found in stars, I wasn't thinking of the cooler ones.

I was ignoring the redshift because it wasn't relevant to my argument. The link mentioned something about the broadening of the bandwidth over distance. Is that an absorbtion/emission thing or is something else going on?

Radio telescopes are not for looking at stars. They are for looking at "radio galaxies". The source of the radio waves tends to be a black hole at the centre.

Thanks Gnomon,

I've said it before, HooToo is better that university. Instead of having one prof and many students, it's like I'm one student with many profs.

bandwith broadening is a quantum effect but wether the fundemental limit is the dominant reason for it or reflection and/or absorption/emission is more important I'm not sure about. I imagine that refelection extra is negligble as the most noticeable affect will be to scatter the light so you wouldn't see the scattered light.

not quite true Gnomon - stars do emit radio frequencies. And of course things like quasars and pulsars and black holes emit a heck of a lot. It's what is used to work out what stars are made of and how come we know about the different generations of stars.

Tumsup
"I didn't know that water could be found in stars, I wasn't thinking of the cooler ones."
Stars are stars. A cool one now might well have been a hot one back in time. Depends on the star. Planets to be detected might be orbiting around any of them.

"I was ignoring the redshift because it wasn't relevant to my argument."
Well, except it allows for wavelength change. But OK.

"The link mentioned something about the broadening of the bandwidth over distance. Is that an absorbtion/emission thing or is something else going on?"
No, that's the effect of the interstellar medium affecting the signal.
In this case bandwidth is the range from lower to upper frequency. So the frequencies used by the carrier wave signal are broadened by 0.1hz through the milkyway.

Redshift is the Doppler effect and has intrinsically nothing to do with distance as far as I'm aware.

It has to do with velocity of the emitting source away from the observer (towards produces a blue-shift).

The reason we can use it for distance calculations is that - assuming the big bang happened in one spot - everything accelerated away from this one spot and the acceleration slows down as time goes on. Therefore one can model this and calibrate velocity (calculated from Red-shift data) with acceleration and distance from the center of the Big Bang.
Hence Red shift data tells us velocity relative to us and we can then use that to estimate distance from us indirectly