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Author Topic: SETI: Fundamentally Flawed?  (Read 19159 times)

lyner

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SETI: Fundamentally Flawed?
« Reply #25 on: 23/12/2007 00:36:46 »
Quote
next step further out is zero
No, it's not zero, it's just one electron per hour instead of one electron per minute or one per second. If you take long enough to do your energy gathering you can  look further and further away and at weaker and weaker signals. There is no fundamental limit, as long as you take long enough over it and do come statistical analysis of the signals your receive.
There would be, of course, practical limits, but it's a matter of money, in the end.
 

Offline Pumblechook

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SETI: Fundamentally Flawed?
« Reply #26 on: 23/12/2007 03:52:19 »
One electron per hour? 

No.  Drift velocity of electron is fairly slow anyway and doesn't vary much and an individual electron doesn't travel far.  The impulse to get them moving travels very fast.  Anyway we are talking about a high frequency signal. 

There must be a point where there simply isn't enough energy being received to stimulate any electrons at all. 

 

lyner

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SETI: Fundamentally Flawed?
« Reply #27 on: 23/12/2007 11:00:42 »
No. The energy of a photon, if it gets there, is the same as when it started (less a tiny bit for the red shift). All it takes, in principle, is one photon to get one electron excited.
At very low signal levels and with low energy photons, associated with radio waves things aren't as straightforward as with optical image intensifiers (in that case, you get one 'flash' per photon). Nonetheless,signal to noise ratio never actually bottoms out - it just gets harder and harder and takes longer.
 

Offline Pumblechook

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SETI: Fundamentally Flawed?
« Reply #28 on: 23/12/2007 13:47:17 »
Can't see it.  My watch is a tiny tiny fleapower transmitter at 32.768 KHz barely detectable with a receiver right next to it.  Is it going to be stimulating electrons in a bit of copper 1000 lt years away?  Can't see it somehow.  What if I put it 100 metres under water (assuming it survives.  Would it still be stimulating electrons at a great distance.  Of course not.  There MUST be a limit. 

If want you to talk photons then they are speading out covering a greater and greater area the further they travel so at some point the density beacome so low that they miss the bit of copper by millions of km. 
 

lyner

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SETI: Fundamentally Flawed?
« Reply #29 on: 23/12/2007 23:27:28 »
If one photon gets there it will have an effect.
Your idea of a 'limit'  implies that NO  photon will EVER get to your remote detector. In fact, there is a finite (but extremely small) probability that a photon will get there. There is a huge difference in these two  views.
There will be a practical limit, of course - you will only have a finite amount of time to look for photons from your submerged watch so it probably won't be detected. But it's not a complete 'NO'.
I don't think we really have any disagreement here as long as we acknowledge it is a matter of degree - not an absolute. It depends on the amount of time you are prepared to wait for an answer from your detecting equipment; that's, effectively using a narrower and narrower  bandwidth to reject the effect of noise.
 

manicgeek

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SETI: Fundamentally Flawed?
« Reply #30 on: 24/12/2007 10:49:11 »
SETI is flawed because it's pointless. It's the biggest waste of processing power in the world today.

Even if we detected something it could be long dead. If it wasn't long dead, we have no way of establishing contact. Even if it had it's own search for intelligent life going on, it might well not see anything we send.

The only purpose it could possibly serve for us to know that there is intelligent life somewhere else is so that we can all look up and say "We are not alone"... What a waste of time. Oh...oh... and think of the carbon footprint :lol:

The processing power being used would be far better used to process things that advance or change our understanding of things that we are technically capable of, try folding at home... that's at least working on a field that we can do something with.

 

Offline lightarrow

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SETI: Fundamentally Flawed?
« Reply #31 on: 24/12/2007 19:26:19 »
Can't see it.  My watch is a tiny tiny fleapower transmitter at 32.768 KHz barely detectable with a receiver right next to it.  Is it going to be stimulating electrons in a bit of copper 1000 lt years away?  Can't see it somehow.  What if I put it 100 metres under water (assuming it survives.  Would it still be stimulating electrons at a great distance.  Of course not.  There MUST be a limit. 

If want you to talk photons then they are speading out covering a greater and greater area the further they travel so at some point the density beacome so low that they miss the bit of copper by millions of km. 

Look for recent threads about that subject, for example
"Direction of Radiation Emitted from Atoms":
http://www.thenakedscientists.com/forum/index.php?topic=11872.0
and you'll find interesting things. You can, for example, have a look to jpetruccelli's 6th post:

<<Classically light is a wave.  If you put a detector in the way, it will catch all the energy from the wave that would pass through the space it occupies.  Two people can see the light bulb from different distances because they are both absorbing different parts of the wave.  If I stood right behind someone else, they would absorb all the light coming to me, and I wouldn't see the light bulb.
If you go to quantum mechanics, the light comes in packets which move about according to probability waves.  for each photon that gets emitted, each detector has a probability of seeing it that's proportional to the amount of probability wave it intercepts.  If we both look at source that emits a single photon, only one of us will see it.  However, as lots of photons are emitted, we'll each see some in proportion to the probability we expect to see. >>

By the way: welcome in Quantum Physics! (Your could be a question from a 1900 physicist!).
« Last Edit: 24/12/2007 19:32:14 by lightarrow »
 

Offline Pumblechook

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« Reply #32 on: 25/12/2007 12:59:16 »
Some figures.....


A broadcast FM receiver might work down to 10E-15 Watts (including a bit of aerial gain).. A tiny signal.

A transmitter might be 100 kW.ERP (Effective power with transmit aerial gain).. 10E5 Watts.

So the signal path loss maximum is 10E20 times (diff between power transmitted and that received) . A very big number.


There are on-line calculators to work at the path loss at different frequencies.. At 100 MHz 10E20 (200 dB) occurs at a distance of 2.5 Million Km..


http://wireless.per.nl/reference/chaptr03/fsl.htm

Speed of light is 300,000 Km /sec so the signal will travel for about 8 seconds before becoming too weak.

2.5 Million km is 1/4,000,000 of a light year. So not very far at all.

Narrow band communications will travel much further but even then I think the record distance for communication with a space craft is only 1/600 of a light year.


As someone said signals get a bit garbled over such long distances. They have probs communicating with spacecraft.. There is a 'group delay' problem maybe partly due to the ionosphere but over such large distances very slight differences in the speed of propagation at the different frequencies in a wideband signal cause different bits of the signal to arrive before others.
 

Offline lightarrow

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« Reply #33 on: 25/12/2007 16:33:31 »
Some figures.....


A broadcast FM receiver might work down to 10E-15 Watts (including a bit of aerial gain).. A tiny signal.

A transmitter might be 100 kW.ERP (Effective power with transmit aerial gain).. 10E5 Watts.

So the signal path loss maximum is 10E20 times (diff between power transmitted and that received) . A very big number.


There are on-line calculators to work at the path loss at different frequencies.. At 100 MHz 10E20 (200 dB) occurs at a distance of 2.5 Million Km..

So, if an extraterrestrial civilization 100 light years away from us would send all the signal's power towards our solar system, we would have plenty of power to perceive it...
 

Offline Dick1038

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« Reply #34 on: 25/12/2007 18:31:02 »
Another thing to consider is that the aliens may be afraid to contact us. After all, we're aliens to them. They might worry that we would be dangerous. We humans are so peace-loving, you know :)
 

Offline Pumblechook

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« Reply #35 on: 25/12/2007 22:31:19 »
===So, if an extraterrestrial civilization 100 light years away from us would send all the signal's power towards our solar system, we would have plenty of power to perceive it...===

Eh?   Only if they had transmitter powers of many Suns.
 

Offline lightarrow

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« Reply #36 on: 26/12/2007 15:48:43 »
===So, if an extraterrestrial civilization 100 light years away from us would send all the signal's power towards our solar system, we would have plenty of power to perceive it...===

Eh?   Only if they had transmitter powers of many Suns.
In your previous computation which gave 2.5 millions km as the max distance of the source, I assumed that the signal was not collimated but emitted in all the 4π solid angle.
 

Offline Pumblechook

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« Reply #37 on: 26/12/2007 17:46:33 »
It is purely based on spreading out of the RF....Square law basically.

So if 100 kW will reach 2.5 km... to reach 1 ltr year which is 4,000,000 times further then you need 4 million squared times more power.   16 x 10E12 x 10E5 Watts..  = 16 x 10E17 Watts. 

or   1,600,000,000.   GW.    I think the max ouput of all the UKs power stations is about 78 GW.

All in all many times more than all the World's power stsions.   
 

lyner

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SETI: Fundamentally Flawed?
« Reply #38 on: 26/12/2007 22:48:54 »
Don't knock Seti; they're doing less harm than the Government is doing with all my personal details. The screen saver looks nice too.
 

manicgeek

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SETI: Fundamentally Flawed?
« Reply #39 on: 27/12/2007 01:58:14 »
Do your personal details have a higher carbon footprint than a computer that's wasting it's time processing random static, in the vain hope that one day someone will be able to look skyward and say "We are not alone" and know that he's right... you do realise that's probably all we will be able to do with any discovery that SETI makes don't you ?

FAH do a screen saver version too  ;)
 

Offline lightarrow

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« Reply #40 on: 27/12/2007 08:19:39 »
It is purely based on spreading out of the RF....Square law basically.

So if 100 kW will reach 2.5 km... to reach 1 ltr year which is 4,000,000 times further then you need 4 million squared times more power.   16 x 10E12 x 10E5 Watts..  = 16 x 10E17 Watts. 

or   1,600,000,000.   GW.    I think the max ouput of all the UKs power stations is about 78 GW.

All in all many times more than all the World's power stsions.   
If the signal is more collimated you don't need more power at all.
 

Offline Pumblechook

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« Reply #41 on: 27/12/2007 08:24:58 »
collimated

??

We are talking about low frequencies here...very long wavelengths...not light.  It will always be a diverging beam even if you had parabolic dishes of enormous size.   If it is diverging then square law applies.   
 

lyner

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SETI: Fundamentally Flawed?
« Reply #42 on: 27/12/2007 15:36:08 »
Every 'beam' diverges. The term 'collimated' is not absolute. The inverse square always applies but, sometimes, the origin of the transmission is a' virtual' one; waves diverge as if from this point and follow the ISL. The principles of optics and waves  apply to every transmission. Have you heard of the concept of Antenna Gain?
As far as I can see, there are plenty of potential sources  within the range of the Seti receiving system. It's just a matter of bandwidth and the rate at which you want to receive the information.
The recent discovery of  many stars with their own solar systems is increasing the probability that we will get something from someone.
Home computers will always be used inefficiently and left idle for, at least, minutes on end, even if they are not left on overnight. Productive screensavers will always be justified for use in such circumstances.
In addition to the 'long shot' of discovering alien intelligence, there is the other possibility of discovering other regular signals from, as yet, unknown and unexplained, natural phenomena. Seti can reveal these, too.

There are many other things to have a more justifiable rant about.
Give nerds a break.
 

Offline lightarrow

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« Reply #43 on: 27/12/2007 18:59:05 »
collimated
??

We are talking about low frequencies here...very long wavelengths...not light.  It will always be a diverging beam even if you had parabolic dishes of enormous size.   If it is diverging then square law applies.   

Yes, but sending 1W of power in all directions  as a spherical wave (solid angle = 4π) it's different than sending the same power in a very small solid angle; you say that it will always be a diverging beam but this phrase is meaningless, as sophiecentaur wrote; what counts is how much you can make it collimated, that is, how small you can make the solid angle of the emission. Yes, it's a radio signal and not light, but as we are able to make MASERs (Microwave Amplification by Stimulated Emission of Radiation):
http://en.wikipedia.org/wiki/Maser
a technologically advanced civilization could make "RASERs" (Radiowave Amplification...).
« Last Edit: 27/12/2007 19:01:27 by lightarrow »
 

Offline Pumblechook

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« Reply #44 on: 27/12/2007 19:57:16 »
Doesn't matter how small you make the beam (usually defined as the angle between 3dB points) square law still applies. Even with a very tiny angle the beam will be very wide in km at the distances we are talking about.  Double the distance and it will be twice as wide and the RF has to cover 4 times the area.   We talk about ERP..Effective Radiated Power.  ERP = Transmitter Power x Antenna Gain.  The distant receiving point (in the beam) doesn't know whether the transmitter is high power with a small antenna or a lower power transmitter and high gain antenna (more focussing). 

I can't see a maser generating a very narrow beam without a large parabolic dish. They don't seem to be used for transmission. They have been used as low noise amplifiers but probably modern Gallium Arsenide devices are much simpler and better. 

 Long distance lasers need large lens and even then there is  divergence... 

From a site....   Like all electromagnetic beams, lasers are subject to divergence, which is measured in milliradians (mrad) or degrees.
« Last Edit: 27/12/2007 20:30:58 by Pumblechook »
 

lyner

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SETI: Fundamentally Flawed?
« Reply #45 on: 28/12/2007 11:55:16 »
Diffraction will always occur as waves go through an aperture. For a Maser, Laser  or whatever, the 'divergence' of the beam will be affected by how wide it is. To get a Maser with low divergence, you would need a large diameter, in wavelengths.
I'm not sure that a maser is of any benefit for a transmitting system; it is easy to produce high powers levels of coherent microwaves with other means, these days and Masers are hideously inefficient - as are lasers.
Maser amplification may still be good value for low  noise receivers (with noise temperatures of 10K).
A large dish - many km across would give gain / directivity but we all (?) agree that there is divergence, whatever you do.
Signal to noise ratio is the issue.  You can get some information out of any signal as long as you are prepared to wait, using a narrow enough bandwidth. The receiver noise performance is also very relevant.
 

manicgeek

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SETI: Fundamentally Flawed?
« Reply #46 on: 28/12/2007 15:11:18 »
Home computers will always be used inefficiently and left idle for, at least, minutes on end, even if they are not left on overnight. Productive screensavers will always be justified for use in such circumstances.
In addition to the 'long shot' of discovering alien intelligence, there is the other possibility of discovering other regular signals from, as yet, unknown and unexplained, natural phenomena. Seti can reveal these, too.

There are many other things to have a more justifiable rant about.
Give nerds a break.
Rant? Who has been ranting ?

It's still a waste of processing power! You could be using it to much better effect, helping to advance something we can work on, instead of dreaming of ET.

As a geek I resemble that statement LOL
 

Offline lightarrow

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« Reply #47 on: 28/12/2007 16:58:10 »
Diffraction will always occur as waves go through an aperture. For a Maser, Laser  or whatever, the 'divergence' of the beam will be affected by how wide it is. To get a Maser with low divergence, you would need a large diameter, in wavelengths.
I'm not sure that a maser is of any benefit for a transmitting system; it is easy to produce high powers levels of coherent microwaves with other means, these days and Masers are hideously inefficient - as are lasers.
Maser amplification may still be good value for low  noise receivers (with noise temperatures of 10K).
A large dish - many km across would give gain / directivity but we all (?) agree that there is divergence, whatever you do.
Signal to noise ratio is the issue.  You can get some information out of any signal as long as you are prepared to wait, using a narrow enough bandwidth. The receiver noise performance is also very relevant.
Ok. What if they exploited a massive object's focusing power?
 

Offline Pumblechook

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SETI: Fundamentally Flawed?
« Reply #48 on: 28/12/2007 17:55:36 »
If you compare light with a 3 metre wavelength I have been on about then a 30mm (quite small) reflector for light would become a 180km reflector..rather large.

The problem is with a very narrow angle beam is the difficult of keeping it on track with the movement of Earth and the target 'world'....that is if you know where to point it in the first place. 

Some SETI people think the only chance is using a very low carrier frequency (0.1 Hz) and consequently an extremely low data rate.  As there is a wavelength term in path loss caluclations due to lower frequencies having physically larger antennas... For the same field in volts per metre then a very long 0.1 Hz diople antenna would produce a billion  times  the voltage of a 100 MHz antenna.... A billion billion times the power.   And dipoles produce a very broad beam.  However the only place you could build such antennas would be in space and they would be 3 milllion km in length! 
« Last Edit: 28/12/2007 18:03:09 by Pumblechook »
 

Offline Pumblechook

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SETI: Fundamentally Flawed?
« Reply #49 on: 28/12/2007 17:57:51 »
Quote.....
 
 ------100 How far away could we detect radio transmissions? -----

-------It should be apparent then from these results that the detection of AM
radio, FM radio, or TV pictures much beyond the orbit of Pluto will be
extremely difficult even for an Arecibo-like 305 meter diameter radio
telescope! Even a 3000 meter diameter radio telescope could not
detect the "I Love Lucy" TV show (re-runs) at a distance of 0.01
Light-Years!------

http://stason.org/TULARC/science-engineering/astronomy/100-How-far-away-could-we-detect-radio-transmissions.html

3000 metres is 30 times bigger than any dish we have on Earth.

They seem to have ignored that very little of the AM transmissions would get through the various layers of the ionosphere.





« Last Edit: 28/12/2007 18:00:44 by Pumblechook »
 

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