by Prasad Ganti

The word “telescope” conjures an image of a long scope with lenses or mirrors and an eyepiece to peep in to see the light collected by the lenses and mirrors. Well, this is the traditional optical telescope, the kind of device in existence since the days of Galileo and Newton. We have used these telescopes to learn about planets and stars in the sky and to get a picture of what our universe looks like. More powerful and bigger lenses and mirrors took us almost to the “edge” of the universe. There are some drawbacks because everything in the universe is not visible through optical telescopes.

First, by looking further and further, we look backwards in time. The further an object is away, the older is the picture we are viewing because light takes time to travel towards us, sometimes years, sometimes centuries, sometimes millions and billions of years. Up to a few hundred thousand years after the universe was born in the Big Bang, light was trapped in a cosmic soup of particles that did not resemble anything we have today. This means that even if we have the most powerful telescope we can design and build, we cannot see beyond this wall because there is no light to see from that era.

Second, all objects in the universe do not emit light, some emit x-rays, some gamma rays, some radio waves. All these waves belong to the family called electromagnetic radiation. They just differ in wavelength and frequency. The spectrum in the order of increasing wavelengths is: gamma rays, x-rays, ultra-violet rays, violet light, blue light, red light, infrared rays, microwaves and radio waves. The first rays detected in the invisible region were infrared rays. William Herschel, the discoverer of the planet Uranus, put a prism in front of the sun light to breaks up the light into different colors. He took a thermometer and measured the temperature of the different colors and found that temperature varied from color to color. What he did next was serendipitous. He put the thermometer beyond the red color and it showed a different temperature! This meant that there was some radiation there. Then came James Clerk Maxwell who tied all the radiation into a common family through his set of mathematical equations.

We see only a small portion of the electromagnetic spectrum, which we call visible light, presumably because our sun emits most of its radiation in this range, and life on earth evolved to naturally detect only this range. Aliens in some other planetary system may have x-ray vision, but not optical vision, depending on what their sun emits! Those alien’s “eyes” tuned to x-rays may look different.

Third, the universe is not empty. There is cosmic dust, clouds of gases and massive objects all across it. These block or cloud our vision. Gases and dust absorb light and send us distortions. Massive objects bend light and give us a different picture of the objects behind them. This phenomenon is known as gravitational lensing.

Due to these and other advances, astronomy has grown beyond just peeping into an optical telescope. We now have telescopes which operate in other parts of the electromagnetic spectrum: infrared, ultraviolet, radio, x-ray and gamma ray telescopes. Some of these telescopes operate in space beyond the atmosphere of the earth which blocks the radiation coming in from outer space. These telescopes show us different aspects of the universe than the optical ones do.

Astronomers do not peep into a telescope anymore. Most observations involves capturing the incoming radiation on a sensor like the ones we have in our digital cameras and analyzing it through a spectrograph, which is a high tech version of a prism. It not only splits the radiation into different wavelengths by showing it as a series of lines, it also shows the absence of wavelengths by showing dark lines (technically known as Fraunhofer lines). The presence of lines indicates emission of radiation by different objects.

Conversely, the absence of lines indicates absorption of radiation. Cosmic dust was discovered via absorption lines. Cosmic dust consists of minute particles of different elements spewed by dying stars. This dust and gas clouds form the inputs for the birth of next set of stars. It is not just the living beings on earth, but even the stars take birth and die all the time in our wonderful universe!

The radio telescope was invented by Karl Jansky of AT&T, after he accidentally discovered the radiation when he was working on a radio antenna. He detected noise which he could not reduce and finally traced it to extraterrestrial sources. This sounds similar to the serendipitous discovery of the cosmic background radiation by Penzias and Wilson. Two great cosmological discoveries in the quest for perfecting electronic communications on earth!

Peeping Toms can only go so far!