The Universe Is All History

It took 300 years of experiment and calculation to pin down the speed at which light travels in a vacuum: 186,282 miles per second.

Light will travel slightly slower than this through air, and some wild experiments have actually slowed light to a crawl and seemingly made it go backward, but at the scales encountered in our everyday lives, light is so fast that we perceive our surroundings in real time.

Look up into the night sky and this illusion begins to falter. Because light takes time to get here from there, the farther away 'there' is the further in the past light left there and so we see all objects at some time in the past.

We see the relatively close moon as it was 1.2 seconds ago and the more distant sun as it was about 8 minutes ago. The measurements — 1.2 light-seconds and 8 light-minutes — can be thought to describe both time and distance.

The distance to more remote objects such as other stars is so great it is measured in light-years—the distance light will travel in a year, or about 6 trillion miles (10 trillion kilometers). Even the nearest star system, Proxima Centauri, lies more than four light-years away, so it appears to us on Earth as it was just over four years ago when the light began its journey.
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In this way, light's finite speed gives us a valuable view into the past, and as we strain our gaze deeper into the universe we look further back in time. In the case of distant galaxies, we see them as they were billions of years ago when the universe was relatively young.

Glittering star cluster is galactic heavyweight This cluster of thousands of stars lies 20,000 light years from Earth in the Carina spiral arm of our galaxy. It is embedded in a star-forming nebula called NGC 3603, a cloud of gas and dust with enough material to form 400,000 stars like the Sun. Most of the bright stars in the image are very hot and massive. Their radiation and stellar winds have blown out a large cavity in the nebula around them.

Some galaxies are so remote that their light hasn't had sufficient time to reach us yet, despite about 13.7 billion years of travel.

There could also be more distant objects that will forever remain unknown to us. Because the universe may be expanding and the expansion appears to be accelerating, there may be distant galaxies which if we can't see them now because their light has not had time to reach us, we will never see.

So we can never see the universe as it is, only as it was at various stages of its development. To interact with remote parts of the universe — to see them as they are now — would require some exotic means of travel, such as to travel faster than light which, according to Einstein's special theory of relativity, is impossible as it would require an infinite amount of energy.

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The fantastic skies of Orphan Stars from NASA Science
Hubble finds Youthful-looking galaxy conceals ancient stars
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