Today's Lecture: Why is Space Black and the sky Blue?

A simple enough question but it is actually quite difficult to answer! It is a question that many scientists pondered for many centuries - including Johannes Kepler, Edmond Halley , and German physician-astronomer Wilhelm Olbers.

Let's take the easy one first and ask "why is the daytime sky blue here on Earth?"

That is a question we can answer. The daytime sky is blue because light from the nearby Sun hits molecules in the Earth's atmosphere and scatters off in all directions. The blue color of the sky is a result of this scattering process.

At night, when that part of Earth is facing away from the Sun, space looks black because there is no nearby bright source of light, like the Sun, to be scattered. If you were on the Moon, which has no atmosphere, the sky would be black both night and day. You can see this in photographs taken during the Apollo Moon landings.

So, now on to the harder part - if the Universe is full of stars, why doesn't the light from all of them add up to make the whole sky bright all the time?

It turns out that if the Universe was infinitely large and infinitely old, then we would expect the night sky to be bright from the light of all those stars. Every direction you looked in space you would be looking at a star. Yet we know from experience that space is black!

This paradox is known as Olbers' Paradox. It is a paradox because of the apparent contradiction between our expectation that the night sky be bright and our experience that it is black.

Many different explanations have been put forward to resolve Olbers' Paradox. The best solution at present is that the Universe is not infinitely old; it is somewhere around 15 billion years old. That means we can only see objects as far away as the distance light can travel in 15 billion years. The light from stars farther away than that has not yet had time to reach us and so can't contribute to making the sky bright.

Another reason that the sky may not be bright with the visible light of all the stars is because when a source of light is moving away from you, the wavelength of that light is made longer. This lengthening of the wavelength makes the light appear more red than blue.

This means that the light from stars that are moving away from us will become shifted towards red, and may shift so far that it is no longer visible at all. This is called the Doppler Effect or Dopler Shift.

The Human Eye

All living creatures on our planet have a limited range of light that they can detect and tolerate. There is an upper and lower threshold. If you look at it from the simple Goldilocks selectiveness perspective; some frequencies are too low, some are too high and some are just right.

Many birds, animals and insects can see very well in low light because they spend most of their time out of the sun. Therefore there ability to detect a fixed range of light is limited to the light that they most often hunt, live and work in, low light. This range selection will be part of their evolutionary development, allowing the animal to thrive, breed and prosper.

Equally, we humans exist mainly in the bright light of day, therefore the range or intensity of light that our eyes can detect are limited to the brighter end of the spectrum. We cannot and need not, detect low light ranges, we have devised other ways of penetrating the darkness. Our eyes have developed in such a way as to attenuate and normalise the intensity of light that we can detect, allowing us to function at our best, during the appropriate time of day.

What we cannot do, is to detect all light ranges and neither can other living creatures on the planet. There is no evolutionary 'advantage' to be gain from this. Therefore if the sky appears black to us, it is because there is simply not a high enough intensity or frequency of light entering the eye receptors, to stimulate a detectable reaction. It is not because there is no light present.

Artificial detection

We believe from science, astrophysics and our academic studies that the full range of light frequencies can be detected artificially by the use of intervening filters, attenuators and sensitive materials. We have enhanced our human vision by the use of these 'systems' and can penetrate the darkness and extend our perception of the universe and the night sky.

The night sky is not black it is simply bathed in a low frequency light that is difficult for us to detect, through the clouds and polluted atmosphere of our only planet.