Light Explained: Peering Back into the Past

In Part 1 of our Light Series, we discuss how light is a form of energy. In Part 2 we discuss the speed it travels as it radiates through space.In Part 3 we explain why you are looking back in the past when you look up at space.

Looking back 13.2 billion years via Hubble eXtreme Deep Field (XDF): image credit NASA/Hubble

 

Stop staring at your screen and look around you right now. You may see a desk or a table, maybe walls and ceilings. If you are near a window or outside, you may see cars, buildings, trees – maybe even the sun or moon depending on the time of day. But what if I told you that you are not seeing those objects as they are right now, but as they were in the past?

 

Light Gives us Information About Our Environment
We define the world around us by our observation of light. Light zips around at its constant speed of 299,792,458 meters per second, continually banging into objects in our environment. When light hits a surface, some of it gets absorbed by these object while some of it gets reflected back toward us.

Light Absorption and Reflection
A black object absorbs all wavelengths of visible light, a white object reflects them all back to our eyes. A red object will absorb all wavelengths except red which is reflected back to us. (Credit: Dorling Kindersley Ltd.

This constant process of absorption and reflection is what gives objects their color and brightness. Without this process of light moving from an object to the receptors in our eyes, we would not be able to see.

Some objects don’t reflect and absorb light but rather generate light on their own. They create light by converting other forms of energy into visible light.

The sun’s creates light by burning its fuel. The sun’s nuclear fusion engine breaks down hydrogen and helium atoms into all sorts of energy, including the visible electromagnetic radiation that lights up the world around us. An incandescent light bulb uses electricity as energy, which is then converted to heat, which causes a wire to glow and converts the heat energy to light.

We have learned that energy travels at a constant speed limit bound by the laws of the Universe. Since light has a speed limit, it takes time to go from its source to our eyes. Light does not travel instantaneous between two points.

 

The Further we Go, the Further Back we See
For objects close to us, that time is so minuscule that we can’t perceive it; remember that light travels at 299,792,458 meters per second. We need to have more space between us and an object before we can start appreciating the time light takes to travel to our eyes.

Speed of Light Distances
Light takes time to travel to our eyes on Earth. When we view the sun, we’re seeing it as it was over 8 minutes ago.

Let’s hop in our spaceship and head off to the moon. If we take a powerful laser and shine it from our landing spot on the moon to an observer on Earth, the light takes about one and one-third of a second to go from the Earth to the moon. That’s still awfully short.

But if we hop back in our ship and go further to Venus, our nearest planetary neighbor. When Venus is at its closest to Earth the laser light we shine would take just over two minutes to get to Earth. For those two minutes, our observer has no idea that we have even turned on our light. Taking our ship even further out to a position near our sun, that time between turning on our laser and it being seen on Earth stretches to just over eight minutes.

Likewise if I’m on my spaceship orbiting closely to the sun and I speak into a radio linked back to earth, my radio waves (which travel at the speed of light) will take over eight minutes to get to mission control on Earth.

Let’s now pretend that mission control on Earth has a very powerful telescope that can see me in the windows of my spaceship. When I speak into the radio while looking out the window, it takes eight minutes for all electromagnetic radiation to move across space to reach my observers. These frequencies of radiation include my radio waves and the visible light of my spaceship, my window and me.

Four Light Years from Proxima Centaurii
Light takes over four years to get from our closest star Proxima Centauri to Earth. Mission Control would not receive my message sent in 2020 until the year 2024.

So if it takes this same constant time for visible light from the Sun to reach Earth, my mission control always views me with an eight-minute delay. They are not seeing me in their telescopes as I am now; they are always seeing me as I was eight minutes ago.

Now I will fly my spaceship to Proxima Centauri, the nearest star outside our solar system. This puts me about 40 trillion kilometers away from Earth. Yes, our nearest star is that far from the sun!

When I announce my arrival to the star in my radio link to Earth, I will have to wait a while for my congratulations from mission control. You see, it will take my message just over four years to travel from Proxima Centauri to Earth. When those on Earth receive my message, it will take another four years for their reply to come back to me. Such is the vast distance between us and our nearest neighboring star.

 

Old Friends and Familiar Sights

If mission control viewed me through their extremely powerful telescope and could see my spaceship orbiting the star, they would constantly be observing me as I was four years ago. They would always be looking at my past from their location on Earth. Light is the carrier of information, it shows your eyes how your environment is defined. At these great distances and with light’s constant universal speed, it takes many years for any new information carried by light to reach an observer on Earth.

445 Light Years to Polaris
The North Star Polaris is 445 light years from Earth. It is so far away that if a ‘stellar” flare erupted on Polaris, we would not see it for many centuries.

Now let’s move from Proxima Centauri to explore some of the other 200 billion to 400 billion stars in our galaxy. Most of you in the northern half of the world are familiar with the Big Dipper (or the Plough) constellation in Ursa Major. The light from these stars takes between 60 to 125 years to reach Earth. We are not seeing the Big Dipper as it is now; we are seeing it as it was a century ago.

For those living in the south, you are seeing Canopus the bright star in the Southern Hemisphere as it was 313 years ago. In fact the light from the stars of the Southern Cross takes up to 364 years to get to our eyes. You are seeing the Southern Cross with light that started its trip around the year 1650.

Moving further out from Earth we see Polaris, the North Star with its light taking 445 years to reach us. You are also seeing Rigel, Betelgeuse and Deneb as they were 1400 years ago. Moving further and further away from Earth, the years you are seeing back in the past quickly escalate. If we move to the edge of the galaxy we see stars from Earth as they were 750,000 to 900,000 years ago.

Now let’s leave our galaxy and travel to our nearest neighboring Galaxy – Andromeda. This galaxy can be seen under dark skies with just our naked eyes, but we see it as it was 2.5 million years ago!

The light from the closest galaxy to ours takes millions of years to get to us. Pretend you live on alien planet located within Andromeda right now and you had an extremely-advanced telescope that could see fine details on Earth. Peering through our telescope we would see the earliest ancestors of man, before humans had even learned to tame fire. The light that is hitting systems in Andromeda now carries the information that started its trip from Earth 2.5 million years ago.

2.5 Million Light Years to Andromeda
If you were on a planet in the Andromeda Galaxy today and could see Earth with a very powerful telescope, you would be seeing the earliest ancestors of man at a time before humans had even learned to tame fire!

Andromeda is just one of the billions of galaxies that are spread out across our Universe. In fact our Universe is so large that light from its edge takes over 13 billions of years to reach Earth. With our powerful Hubble and Spitzer telescopes and their sensitive detectors we are able to collect light and other electromagnetic radiation from space that started its trip across the Universe billions of years ago. We can then image this light to form pictures of what the Universe looked like far back in its past.

 

So the next time you look up at the stars and galaxies, realize that you are looking back in time. You are not seeing objects as they are in space now, but as they were hundreds, thousands or even millions of years ago.

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