You're outside on a dark and starry night wondering about those stars. Is someone out there? You take your flashlight out and point it at the sky. Your message in a bottle tossed into the cosmos. I'm guessing most of you did it when you were a kid. The problem is it doesn't work. The light you send skyward is scattered or absorbed before it gets out of the atmosphere. At least your imagination was fired - what would it take, There are people who point lasers beams skyward and that leads to one of the most unusual job openings I've ever seen. But more on that later, there's another interesting question to ask first.
Our eyes are amazing at what they do. We get reasonably detailed color images along with less detailed, but much more sensitive night vision. We usually think about owls as having some of the best night vision eyes. They're pretty good, but some frogs can detect a single photon of light. So image this. You're a frog watching a light that glows with an unwavering brightness. If you were a space frog it might be the Sun.
You measure the brightness of the light and then start a journey outwards. Twice as far from the source the light appears a quarter as bright. Three times as far and it's a ninth as bright. One hundred times and it's one ten thousandth it's original brightness. The measured brightness at your distance goes as 1/r2 where r is the distance. The inverse square law.1 You continue on and and the light gets dimmer and dimmer. How did can it get and what would it look like? I posed the question to a class of premed students I was teaching physics to..
The light gets dimmer to a point - the single photon point. Rather than diminishing further it begins to flash at that brightness. . At first are flashes most of the time, but travel further and there an increasing number fleshless regions. Add up the energy you receive - the number of photons over some unit of time. The frog is bored and haven't croaked yet, so it plots the energy over some amount of time .. just the number of photons (flashes) it sees in some length of time. The curve is still the inverse square law.
Light, as it happens, comes in chucks. In physicsy terms it's quantized and that chunk is a photon.2
When first encountered the paper on the sensitivity of frog's eyes I was blown away. For perspective a 60 watt equivalent light bulb emit's about ten billion photons a second. The back of the envelope suggests the Sun would start to look like a blinking single photon about 1000 light years out - the range of our deep space frog.
If you're sending information you have to worry about how quickly you can flash the light to begin with. If your receiver is seeing a bright light or radio signal (radio is just longer wavelength photons), you can send information at a fast clip. If it's very dim things get slow. Even with huge exotic antenna arrays the science from the New Horizons spacecraft that flew by Pluto had to be recorded and sent back over a period of months, NASA and JPL have been experimenting with deep space laser communication systems. The idea here is a laser beam can be made to not spread out much and will appear bright, HDTV from Mars orbit can be done. As long as you don't have to go through the atmosphere.
We've almost circled back to the beginning - the people who shoot laser beams skyward. But the atmosphere absorbs and scatters so much. What's going on?
You can point a very bright, highly culminated beam of laser light to the sky. Some small lasers are bright enough to travel through a few miles of atmosphere and still be focused and bright enough to temporally blind a pilot. In fact it's a federal crime to shine a laser at an airplane.
There are people who shine extremely powerful lasers skyward legally. The atmosphere can be thought of as a sea of small regions about the size of your hand. They have slightly different optical characteristics than their neighbors. Their turbulent movement is what causes stars to twinkle. They represent an enormous limiting factor to the resolution of telescopes with mirrors larger than a half meter in diameter.
The trick is to look at a reference star near the object you're studying. You know where it is so you image it, look at it's new position, do a bit of math and change the geometry of your optics in real time to adapt. Simple in principle, but difficult in practice, it has revolutionized optical and infrared astronomy at a half dozen of the largest observatories in the world.
One of the problems is you need a fairly bright reference star and for about 99.99% of interesting astronomy there aren't any suitable stars that are close enough to use.
Astronomers get around that by making their own reference stars. The standard technique is to use a very powerful pumped dye laser to create a yellow beam with a wavelength of 589nm..
The laser is pulsed on for a short time - microseconds - and the beam soars skyward. For the first 30 kilometers there's enough atmosphere to scatter most of the light so it's a visible yellow line. At about 95 km above the Earth it encounters a layer of sodium atoms - leftovers from constant bombardment of our atmosphere with micrometeorites. 589nm is a resonant frequency of atomic sodium so they get excited and then spontaneously de-excite emitting yellow light in all directions. A little artificial guide star shines during the pulse. Some of it's light makes it back to the telescope through nearly the same path light from the astronomy target has to travel. Everything you need to compute your mirror changes. All of this happens about a thousand times a second. The process works so well that the largest 'scopes on Earth now resolve better than the Hubble Space Telescope, although it still has other advantages.
That upward beam is really bright. It's only on for about a tenth of a percent of the time, but it could blind anyone on an airplane who happened to be in the way. You get around this with one of the most exotic jobs I know of - a night watch for the 21st century.
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1 The way to think about it is to recognize light travels in a straight line. Image two imaginary spheres - one is one unit of length away from from a glowing sphere and the other two units. If you pick a region with any area on the first sphere (say 10 square inches) it covers a fraction of the first sphere that is four times larger than the fraction covered by the same area on the second sphere because the area of the spheres goes as the square of their radius 4*Π*r2
2 In your physics classes you probably learned light can be a particle and a wave. You can think of the photon as the particle of light. Well.. and this would take too much to get into ... the particle/wave duality model is an artifact about how people saw quantum mechanics in 20s and 30s. I'll just state what we now know ... everything's just a wave. But unless you are deep into physics it's probably safest to think about particle wave duality.
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Recipe Corner
Really easy and just right for the cooler weather
White Bean Toast
Ingredients
° extra virgin olive oil
° 6 slices of crusty whole grain bread (or whatever constitutes excellent toast for you)
° 1 clove garlic, minced
° 1 tsp chopped fresh rosemary
° a 15oz can of no salt cannellini beans drained and rinsed
° 2 tbl lemon juice
° salt
° 2 tsp chopped parsley
° bit of red pepper flakes
Technique
° oven to 425°F. Use 1 tbl of EVOO and brush one side of each piece of toast. Arrange face up on a sheet and toast on top rack until brown and crisp.
° heat 2 tbl of EVOO in a skillet over medium heat. Add garlic and rosemary when the oil is hot and cook for a minute and remove from heat.
° add beans, lemon juice and salt and mash some of the beans a bit to give a variety of textures and some whole beans.
° spread over the toast, drizzle on some good EVOO, sprinkle on parsley and red pepper flakes.