Is Space Black or Blue?

Space is black.

While the idea of a blue-tinged space might seem intuitive due to the blue appearance of our sky during the day, the reality is quite different when we look beyond Earth's atmosphere. The perceived blueness of our sky comes from the scattering of sunlight by the air molecules; a phenomenon called Rayleigh scattering, and it doesn't apply in the vacuum of space.

Here’s a more detailed breakdown:

Why Space Appears Black

• Lack of Atmosphere: Space is a near-perfect vacuum, meaning there are very few particles to scatter light. This contrasts sharply with Earth's atmosphere, which is filled with gases and particles that scatter sunlight.
• Olbers' Paradox: As explained in the reference, Olbers' Paradox highlights the apparent contradiction between our expectation of a bright night sky—given the vast number of stars—and the fact that it is dark. The paradox emphasizes that if the universe were infinite and static with stars spread evenly everywhere, the night sky would indeed be bright. The solution to this paradox, however, is beyond the scope of this response. The key takeaway is, the paradox underscores why the space we observe is dark, as stated: "Yet we know from experience that space is black!".
• Light Absorption and Redshift: Light from distant stars can be absorbed by dust and gas or redshifted, which decreases its intensity. This makes a lot of the light less visible and contributes to the overall darkness of space.

Why Our Sky is Blue

• Rayleigh Scattering: This phenomenon occurs when sunlight interacts with the gases and particles in Earth's atmosphere. Blue light, with its shorter wavelength, is scattered much more than other colors like red and yellow.
• Sunlight is filtered: This scattering effect is why we see a blue sky during the day. When the sun sets, the path of the sunlight through the atmosphere is longer, and the blue light gets scattered away, allowing more of the red and orange light to pass through, resulting in the beautiful sunsets and sunrises we see.

Summarizing the Difference

Therefore, while our sky is often blue because of atmospheric interactions, space itself is definitively black due to the absence of an atmosphere to scatter light and other factors such as light absorption and redshift.