StarChild Question of the Month for December 2002
Question:
Why is space black?
Answer:
Your question, which seems simple, is actually very 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.
There are two things to think about here. 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 (which for light means more red.) 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. (Note: You hear the same effect when an ambulance passes you, and the pitch of the siren gets lower as the ambulance travels away from you; this effect is called the Doppler Effect).
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I'm a seasoned enthusiast in the field of astronomy and astrophysics, with a deep understanding of celestial phenomena and the underlying principles governing our universe. My expertise is rooted in both theoretical knowledge and practical applications, having closely followed the works of prominent scientists such as Johannes Kepler, Edmond Halley, and Wilhelm Olbers.
The article you provided, titled "StarChild Question of the Month for December 2002," delves into the intriguing question of why space appears black. Allow me to dissect the concepts presented in the article to shed light on this cosmic mystery:
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Daytime Sky Color:
- Explanation: The article begins by addressing the color of the daytime sky on Earth. It accurately states that the blue color results from sunlight scattering off molecules in the Earth's atmosphere.
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Nighttime Sky Darkness:
- Explanation: The article goes on to explain that during the night, when a specific part of Earth faces away from the Sun, space appears black. This is because there is no nearby bright source of light, like the Sun, to cause scattering.
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Lack of Sky Brightness Despite Many Stars:
- Olbers' Paradox: The article introduces Olbers' Paradox, which questions why, given the vast number of stars in the universe, the night sky isn't constantly bright. It points out the apparent contradiction between the expectation of a bright night sky and the observed darkness.
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Solution to Olbers' Paradox:
- Finite Universe Age: The article proposes that the resolution to Olbers' Paradox lies in the finite age of the universe, approximately 15 billion years. This means that light from stars beyond this distance hasn't had sufficient time to reach us, contributing to the darkness of the night sky.
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Doppler Effect and Redshift:
- Explanation: The article further suggests that the light from stars moving away from us may experience a shift towards the red end of the spectrum due to the Doppler Effect. This shift in wavelength could make the light invisible to the human eye, contributing to the darkness of space.
In conclusion, the article provides a comprehensive overview of the factors influencing the appearance of space, addressing both the color of the daytime sky on Earth and the enigma of the seemingly dark night sky despite the abundance of stars. The inclusion of Olbers' Paradox and the consideration of the finite age of the universe adds depth to the discussion, showcasing a nuanced understanding of cosmic phenomena.
