Visible light is just a small part of the spectrum of electromagnetic radiation that the human eye responds to. Besides visible light, the spectrum includes the most energetic gamma rays followed by X-rays, ultraviolet rays, infrared rays, microwave and radio waves. But the miniscule range (less than one percent) of the spectrum to which our eyes are sensitive provides an awe-inspiring view of the Universe.
Our eyes with an average vertical diameter of 23.5 millimeter and axial length of 24 millimeter are tuned to the visible part of the spectrum. We cannot see infrared light because they don’t carry enough energy to excite the photo-receptors in our eyes. However, the nerves in our skin can sense the thermal energy carried by the infrared radiation.
What if our eyes were sensitive to infrared radiation? Some snakes, vampire bats and beetles have evolved infrared vision. For us to see infrared rays, the dimensions of our eyes have to be larger – about 5 to 10 times. Since most objects around us emit infrared radiation, infrared sight will add a totally new dimension to what we see now.
Infrared-sensitive eyes will allow us to observe a person’s emotional state. Someone who is excited or angry often has more blood near the skin, and thus gives off relatively hotter infrared radiation. Conversely, someone who is scared emits cooler infrared radiation because there is less blood near the skin. Also, we will be able to see objects in the darkness of night, just like soldiers can detect enemy movements with infrared “night vision” goggles.
With infrared-sensitive eyes, the night sky will be simply breathtaking. The transparency of the Earth’s atmosphere to most of the infrared radiation will enable us to see with naked eyes distant stars whose wavelength got red-shifted to the infrared region due to expansion of the Universe.
(Increase in the observed wavelength of electromagnetic radiation from a receding source is called the red-shift.) The highly red-shifted galaxies from the edge of the Universe will also become visible to us. Our concept of stars will change dramatically, too. The coldest bodies in the heavens, ranging from distant planets to dead stars, boldly announce their presence by emitting infrared radiation.
Hence, we will be able to see some of the chilliest objects in the Universe. We will also have a clear view of the regions of the cosmos which are surrounded by cocoons of gas and interstellar dust. They prevent visible light from reaching the Earth and hence cannot be seen with normal eyes. For example, young stars emit mostly visible and ultraviolet rays, but the dust surrounding them absorbs the emissions, radiating only infrared rays. We will, therefore, be able to penetrate the veils of dust that shroud the heart of the galaxies and hide young and newborn stars from our view.
With heat-seeking infrared eyes, we will be able to see nebulae with rapidly growing embryonic stars tucked deep inside the stellar “incubators.” We will also get a rare glimpse of massive stars that are about to burst out of the stellar “womb.”
Our new eyes will unmask a myriad of activities taking place at the core of the galaxies where black holes and quasars lurk. The interstellar matter will glow as diaphanous wisps throughout the nighttime sky. What we will miss though is the white glow of the Milky Way. It will appear dimmer because the gas and dust clouds surrounding the galaxy do not scatter infrared light as much as they do visible light.
Does this mean we won’t be able to see the stars and constellations in the night sky that we see with our normal eyes? Absolutely not, because almost all heavenly bodies that emit visible light are also infrared ray emitters.
With infrared eyes, the entire sky will be surrounded by wild strokes of color. It will resemble an impressionist artist’s work, as can be seen in the photograph of the Trifid nebula taken by NASA’s space-based infrared telescope Spitzer.
The writer is a Professor of Physics at Fordham University, New York.