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How far do Earth radio/TV transmissions reach?
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Mr. Kevvy Send message Joined: 15 May 99 Posts: 3776 Credit: 1,114,826,392 RAC: 3,319 |
Whoever wrote that article got the whole thing completely backwards: Wang and colleagues begin by using a third continuous-wave laser to confirm that there are two peaks in the gain spectrum and that the refractive index does indeed change rapidly with wavelength in between. Next they send a 3.7-microsecond long laser pulse into the caesium cell, which is 6 centimetres long, and show that, at the correct wavelength, it emerges from the cell 62 nanoseconds sooner than would be expected if it had travelled at the speed of light. 62 nanoseconds might not sound like much, but since it should only take 0.2 nanoseconds for the pulse to pass through the cell, this means that the pulse has been travelling at 310 times the speed of light. Moreover, unlike previous superluminal experiments, the input and output pulse shapes are essentially the same. Light in a vacuum does indeed cover 6cm in 0.2 nanoseconds; since it took 62 nanoseconds, the pulse is traveling 310 times slower, not faster. In other news, snail smashes land speed record travelling 310 times slower than walking. :^) Edit: Article is also 17 years old. No faster-than-light communications after all this time so I think I am correct. |
Gordon Lowe Send message Joined: 5 Nov 00 Posts: 12094 Credit: 6,317,865 RAC: 0 |
I haven't got the knowledge to explain it further, perhaps others have. Way over my head. Where is Dr. Hawking? The mind is a weird and mysterious place |
janneseti Send message Joined: 14 Oct 09 Posts: 14106 Credit: 655,366 RAC: 0 |
Faster-than-light travel remains a fantasy at the moment. http://www.bbc.com/earth/story/20160429-the-real-reasons-nothing-can-ever-go-faster-than-light |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
Thursday October 12 2017, 7:37 AM I’m having a hard time reconciling these sentences: River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
rob smith Send message Joined: 7 Mar 03 Posts: 22186 Credit: 416,307,556 RAC: 380 |
The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light. Too many people fail to recognise the difference between "detecting" a signal and "decoding" a signal. One can detect signals that are far weaker than those that can be decoded. Thus one may be able to detect radio waves from earth at a distance of say 100 light years, but one can almost certainly not decode the same signal. I don't know what the noise floor of the radio telescopes that provide us with our data is, but both of the use cryogenic cooled receivers. Bob Smith Member of Seti PIPPS (Pluto is a Planet Protest Society) Somewhere in the (un)known Universe? |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light. Thursday October 12 2017, 9:35 AM Hi Bob, Very interesting.... Yes, transmission, earth to probe, or to an alien her/him or 'it' are limited to light speed. However, as you know, they are attenuated greatly when radiating outward, and subject to the radar range equation. Sadly so. :( When I first learned of our ability to 'see' signals far below the thermal noise floor I was truly anamzed. To me, prior to that, you could NEVER 'see' a signal unless it was at least 3 dB stronger than -174 dbm/Hz. I gave no thought, back then, to noise being random, and transmissions, sent repetitivly, and using a CRC code, as being viable. :) River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
The "one hundred light years" is based on the fact that we've been using radio transmissions for somewhat over 100 years and radio waves travel at the speed of light. Friday October 13 2017, 6:18 AM The "noise floor' of a radio telescope goes hand in hand with the "noise figure" of the receiving apparatus. The noise figure of any really GOOD earth-bound receiver LNA, (Low Noise Amplifier), is usually in the range of 1.5 to 2 dB or so. Cooling of critical parts of the receiver input certainly will lower the noise figure, and thus the detection noise floor, but not below "0 db." So, it's reasonable to assume that the noise figure of a radio telescope receiver is surely lower than anything in "normal" commercial use, but not lower than zero. If you do a search on "noise figure of a radio telescope" you will find many references. One that I saw at the top of a search using Bing on Mozilla Firefox indicated that the noise figure of one was 0.2 dB. This falls in line with my assertion that a noise figure less then zero is not possible. :) See https://www.rtl-sdr.com/radio-astronomy-0-2db-noise-figure-lna/ One source I found says that "lowering the noise figure of a satellite receiver by 1 dB is equivalent to increasing the size of the large receiving antenna by about 40%." Given the SIZE of a typical radio telescope dish, (they are humongous), a 40% increase is dramatic and very significant. We are in total agreement that there is a BIG difference between "detection" and "decryption." :) THAT makes GOOD sense indeed! You can DETECT noise but to DECRYPT a signal below -174 dBM/Hz requires the signal to be transmitted repetitively by the probe, and CRC coded to assure error-free decryption. There is another aspect to low-noise receiver design that is not obvious to many, and that is the LNA input VSWR. The noise figure calculations assume a "matched load." Simply stated, the noise source and the LNA are assumed matched impedance-wise. There is NO commercially available single-stage LNA that has an input VSWR of 1.0 . It is not at all unusual for a low-cost LNA to have a VSWR = 2, 3, or higher. So, the "dynamic," or "working" noise figure is different than the "static," or measured noise figure. To compensate for this, more expensive LNA's use TWO identical LNA's coupled together with a low-loss hybrid power combiner. The net effect of such a combiner is to present a matched load to the noise source. But, the combiner used has an "insertion loss," typically 0.3 dB, and THIS loss ADDS to the final input noise figure. You never get "something for nothing." A sad thing. :( A 2nd point rarely mentioned is "transmitter noise." It is entirely possible for a transmitter to add noise to the intelligence being amplified and sent to its antenna. This noise has to add to the difficulty of decryption below the thermal noise floor. A deep space probe transmitter HAS to be very clean. I've never been involved in the design of an LNA for a radio telescope BUT I have to assume that they MUST take into consideration input VSWR mismatch. I'm curious how they deal with this? Perhaps they gave found a "work around" for LNA input VSWR correction that doesn't involve a hybrid combiner? I doubt it tho. Hmmmm, an interesting topic for another day? River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
Friday October 13 2017, 9:33 AM Oops, sorry! I forgot to ADD the following to my previous post, and remembered it too late to do an 'Edit.' Deal with it, OK? Here is something of possible interest in regard commercially available LNA's. I feel that all LNA's have two different noise figures. One, and to me, the REAL one, is what you measure with a noise figure meter, which takes mismatch VSWR into account, and an IDEAL noise figure which is what you would measure with a noise figure meter IF, and only IF, you use two LNA's with a hybrid combiner. :) As an example of this idea, say you have two identical LNA's, each with a 2.5 dB noise figure, as measured in the normal way. OK, now you use a hybrid combiner with a 0.3 dB insertion loss. I would think the noise figure at the combiner input, measured with a noise figure meter, would then be 2.2 dB because there is no longer mismatch VSWR to influence the measurement, only the combiner insertion loss. I wonder, when commercial entities advertise LNA noise figure, if they use the 'single-ended' NF of one LNA, or if they advertise the somewhat lower number of a hybrid combined pair? mmmmmmmmmmm, huh? Sorry to rattle on, but it is an interesting topic to some, I hope? Question: if a tree falls in the forest, and there is no one there to hear it, does it make a sound? I wonder? River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
If anyone from far far away was that interested in looking for life in our part of the galaxy, they would likely deploy probes, which themselves would have the capability to transmit findings back home. Wednesday October 18 2017, 5:36 AM Makes good sense. :) But, depending on the method of transmission, it may take many years for the messages to reach back to the senders. Stay here on Earth. It's the only planet with DARK CHOCOLATE !! River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
River Song Send message Joined: 30 Jul 15 Posts: 268 Credit: 1,735,966 RAC: 0 |
This is a reply to my Message 1895062 dated Friday, October 13, 2017, 9:33 AM [quote]Friday October 13 2017, 9:33 AM (in part) As an example of this idea, say you have two identical LNA's, each with a 2.5 dB noise figure, as measured in the normal way. OK, now you use a hybrid combiner with a 0.3 dB insertion loss. I would think the noise figure at the combiner input, measured with a noise figure meter, would then be 2.2 dB because there is no longer mismatch VSWR to influence the measurement, only the combiner insertion loss. I wonder, when commercial entities advertise LNA noise figure, if they use the 'single-ended' NF of one LNA, or if they advertise the somewhat lower number of a hybrid combined pair? mmmmmmmmmmm, huh? ============================== Wednesday October 18 2017, 8:02 AM I made a technical ERROR in my previous post, and I wish to CORRECT it. I postulated that, by combining 2 identical LNA's, (Low Noise Amplifiers), with a hybrid combiner having an insertion loss of 0.3 dB that, as a result of improving the impedance match to the source, that the combined noise figure would be DECREASED by the 0.3 dB. This is WRONG, you NEVER "get something for nothing." You WILL achieve a low input VSWR,, BUT to GET it you must suffer a worsening in the combined noise figure. The noise figure of the pair would RISE to 2.8 dB from 2.5 dB. Tuff noogies, huh? So much for freebies. :) River Song (aka Linda Latte on planet Earth) "Happy I-Phone girl on the GO GO GO" |
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