X-Message-Number: 19378 From: Date: Sun, 30 Jun 2002 10:21:32 EDT Subject: Intensity interferometer 3 --part1_d0.298156c2.2a506dec_boundary Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Intensity interferometer has been invented by Robert Hanbury Brown (who died January 16 2002). When presented with the theory of the instrument, many physicists said it can't work, that strong view continued unabated even as the Australian Narrabri interferometer worked for 20 years. Today, that viewpoint seens to have been restated as: It can't produce picture because it throw aways the phase information. That is plain nonsens as what allows an interferometer to build a picture is the spatial sampling of information, not the phase in a single detector. I know that if I continue on that subject, particularly if I build some hard wired systems, I'll be "informed" by knowing people that it can't work, can't produce pictures,... and so on. So I choose to put things in place at start: I know the story, I know too that someone gave a "science" conference in November 1959 about the "proof" that artificial satellites are impossible, two months after Sputink 1. So thank you, I know the Earth is flat, at least for some.:-) I have said before in part 2 that more we have beams in a 2 waves interferometer, the best the picture quality at the end. Optical systems with lenses and mirrors are limit cases of interferometers with nearly infinite entry beams. In the same way, insect composed eye are thousand beams intensity interferometers. Intensity interferometers have a strange property: they are wave lenght insensitive. To see that in a simple way, assume for example that we look at one micrometer radiation (near infra-red) and we sample it at 3 Ghz or 10 cm wavelength = 100 000 micrometers; If we used a 2 wave instrument, we would directly get information at incoming wavelength scale, that is one micrometer. If we sample information at a scale x time larger, we need x squared experiments to reduce the uncertainty to the initial value. There x = 100 000 and so we need 10 billions experiments. If we had started with 10 micrometer radiation (far infra-red) only 10 000 x 10 000 = 100 milions experiments would have bring us down to wavelength precision, but we would need 100 times more experiments to go down to the micrometer scale. And there we hit what make intensity interferometer bad looking: It can go beyond the wavelength: it can work as a near field instrument at long distance. So, that instrument sample a signal at a scale, use a radiation at a second scale and look at a third one. What define the number of experiments is the difference between sample and observed scales. With long wavelengths, experiments are done on short bunch tails, with short wavelengths, longuer tails are sampled and the energy efficiency drops. The Narrabri astronomical intensity interferometer had a sampling frequency near 100 Mhz or 3 m (9') and used starlight near half a micrometer. The big difference between these two values constrained it to look at long tails giving a poor energy efficiency. That limited it to look at only bright stars, and observing at an intermediate scale between these extremes. Going from 3 m to 30 micrometers asked for 10 billions experiments and 3 months of observing time. Some years ago, I had undertook a survey of high speed photometers, I could not found systems far better that the one used by Hanbury Brown fifty years ago. May be someone could tell me what is the state of the art today? Using balistic gate transistors, I assume 100 Ghz is a possibility. Moving from 100 Mhz to 100 Ghz would divide observing time on an intensity interferometer by one million. For low power light sources such stars, the benefit is even larger: Oberving shorter tails would enhence energy efficiency and so allows observation of dimer sources. Going back to brain reader, a 100 Ghz sampling frequency gives a sample scale of 3 millimeters, if the observed scale is 100 000 times smaller, it is 30 nanometers, what is requested to see molecular sized objects. The observing time would be 3 months, something workable for an experiment, not for business constrained device. Yvan Bozzonetti. --part1_d0.298156c2.2a506dec_boundary Content-Type: text/html; charset="US-ASCII" [ AUTOMATICALLY SKIPPING HTML ENCODING! ] Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=19378