Light, light years and the universe

Light is the most important tool that we have used to know what we know about the universe. It is as much as a mystery as the entirety of the universe, with so much that we still don’t understand about it.

Light can be described as a wave and a particle. We understand that light travels at approximately 300,000,000 m/s, which is the theoretical maximum speed that any can go before it no longer follows the laws of physics. This means that light can travel 300,000,000 (300 million) meters every second. In one second, light travels 300,000,000 meters, in two seconds, 600,000,000 meters and so on and so forth. 

To give you an idea, the moon is located approximately 300,000,000 meters away, which means that light takes approximately 1 light second away.

• 1 light second – distance that light travels in 1 second
  • 300,000,000m/s • 1s = 300,000,000m
• 2 light seconds – distance that light travels in 2 seconds
  • 300,000,000m/s • 2s = 600,000,000m
• 1 light minute – distance that light travels in 60 seconds (1 minute)
  • 300,000,000m/s • 60s = 18,000,000,000m
• 1 light hour – distance that light travels in 3600 seconds (60 minutes or 1 hour)
  • 300,000,000m/s • 3600s = 1.08×10^12m
• 1 light year – distance that light travels in 1 year
  • 300,000,000m/s • 365 days/year • 24 hours/day • 60 minutes/hour • 60 seconds/minute = 9.4608×10^15m

Light years

As we noted above, a light year is a distance measurement and is equivalent to how far light travels if it traveled in a year. This distance is equal to 9.4608×10^15m.

As  it turns out, we now use the light year as a way of determining distances in our universe. For example, the nearest star is located 4.3 lya (light years away), which means that light had to travel for 4.3 years at 300,000,000m/s until it reaches here. This is far from instantaneous.

Time travel?

When two people are speaking to each other, face-to-face, it takes time for their sound to reach each other, and it takes time for light to reach each other too. In the case of the two people speaking, everything is happening almost simultaneously to the point that we can’t tell that it isn’t happening at that instant. Increase their distance from each other and you’ll see that the time it takes for light to get from one place to the other to be longer. Such is the case when we gaze at the stars. Since it is taking years for the light to reach us, we never know what those stars look like right now. We only know what they looked like in the past, when the light started traveling towards us.

So for the star that was 4.3 lya, we only know what that star looked like 4.3 years ago. And this is the closest star to us, called Proxima Centauri. There are other stars in the galaxy that are millions and billions of light years away. Which means that those stars are probably not there in the sky anymore (because they’ve either become white dwarfs and died off, become neutron stars and died off or become black holes).

When you’re looking at the sky, you are actually seeing remnants of stars that have once shown light towards you, but are now nothing but dissipated space dust!

The furthest object we can see in the sky is approximately 14,500,000,000 lya. Beyond that, what we see is total darkness, which leads us to believe nothing existed beyond that in space and in time, AKA to beginning of the universe is total darkness (before the big bang).

The Big Bang

Contrary to popular belief, the big bang was not necessarily a big explosion. Using data that physicist got from light, we are able to estimate that this event that initiated the the creation of the universe took place approximately 14,500,000,000 years ago. At the time of the big bang, we think that this is when all energy and all matter in the universe was created. At first, we think there was only hydrogen atoms. Through many millions of years, stars were then created burning the hydrogen, fusing them to obtain the more complex elements: first helium, and getting more and more complex. The hottest stars are able to fuse their atoms to create iron. Hence, the supergiant stars have solid iron cores.