How does the eye see? –
Do we see things
as they are, or as they were?
In astronomy it is assumed that when we look at space and objects far away in space, they don’t appear to us how they currently are, but how they were in the past. Basically, this assumption means that
• We see he Sun as it was 8 minutes ago, because this is how long it takes for light to travel to earth.
• With telescopes we can see the furthest of objects how they were hundreds of millions and billions of years ago, because they are hundreds or even billions of light years away from us, and because it also takes this long for light to travel to earth.
The following quotation will further explain. It tells how we only see the past state of space, and never its current state:
When we look at the stars, we partake in time travel… That is due to extreme distances. Let’s look at Sirius, which is the brightest star on the night sky and is very visible. Sirius is 26 times as strong as the Sun, and it is 8,6 light years away, i.e. roughly 80 thousand billion kilometers away. It takes 8,6 years for its light to reach us, which is why when we look at it in 2007, we see it as it was back in 1999.
North Star Polaris, which is recognized by most people (at least by all orienteers), is approximately 400 light years from earth, according to the most recent calculations. The light we see coming from it at this moment, has actually left the North Star in 1606. Similarly, if there was an astronomer on the North Star with a good enough telescope to see the earth, they would see England as it was during the time of Shakespeare. (1)
But how are things really? Do we really see celestial bodies as they were in the past, or as they are now? We are going to investigate this question in the light of the following points:
• Firstly, we should note that light year measures distance, not time. It has been calculated that one light year equals to ca. 9,6 thousand billion kilometers. This is easily forgotten when talking about light years.
What about, when the light reaches the eye? Is our ability to see far away celestial bodies dependent on their light reaching our eyes, and is that why we are able to see them? Or, do we see them, because they are big, bright and close enough for the naked eye to detect them? Which one of these options is correct? We are going look into this.
• If seeing is dependent on light reaching the eye, we should be able to see all the objects, whose light has reached the earth. Distance should not be a factor. This means that, for example, if someone pointed a flashlight to earth from the moon, we should be able to see it, because it only takes a moment for light to travel to earth and to our eyes. The size of the flashlight should not matter, because we assume that seeing is only dependent on the light travelling to earth.
But what is the reality of things? Wouldn’t we be unable to see the flashlight, because it is too small? We can detect larger light sources, but not very small ones. This clearly indicates that seeing is dependent on the size of the object and its brightness, and not dependent on the light reaching our eyes.
• With a naked eye we can see approximately only 5000 stars, whereas with a powerful telescope one can see millions of stars. Why is that? Isn’t it evidence that seeing is dependent on the object used for detection, and not dependent on light reaching the eyes (or the earth)? If seeing was only dependent on light traveling from far away objects, we should also be able to see what we see with a telescope, with the naked eye too.
• It is the same with glasses. Why would we need them if our ability to see was only dependent on light reaching our eyes? No one would need glasses, and telescopes for detecting stars if we see them purely based on light travelling to earth and reaching our eyes. Two things are being confused here: light moving and seeing.
• The former quotation explained how Sirius is 26 times more powerful than the Sun. Yet, this bright star seems like a tiny dot compared to the Sun. Why is that? Because of distance, of course. The further we go from an object, the smaller it looks in the horizon. That doesn’t mean, however, that we would see the star Sirius as it was 8,6 years ago; we only see it smaller, because it’s so far away. For the same reason, the Sun can appear relatively large from the earth – even though it is much smaller than Sirius – but from Neptune it would seem rather small. All objects appear smaller, the further we travel from them. And at some point, we’ll reach the point after which we are unable to detect them, because they are too far away or too small.
• It has been said that when we look at the Sun, we see it like it was 8 minutes ago, because that is the time it takes for light to travel to earth.
However, people are confusing two different things here. Us looking at the Sun is an entirely different thing to it bringing us light and warmth. Even though, it would take 8 minutes for light to travel to earth, we are not looking at the light in our surroundings, we are looking at the Sun. They are two completely different things that should not be confused together.
• In a previous quotation we mentioned that if we look at the earth from the North Star, we would see it as it was 400 years ago, because it takes that long for light to travel between these two celestial bodies. According to this principal, if someone was to suddenly travel, e.g., 70 light years away from earth (Light years measure distance, not time. It has been calculated that one light year is approximately 9,6 thousand kilometers), they should be able to see events from the Second World War.
There is reason to doubt this theory, however. How could another location that is further, enable us to see things that no longer exist? If we cannot see it now, it won’t be much use if we move further to inspect it. We cannot see past events by moving further from our object. The only thing that will happen, when we get further, is that we won’t see our target as clearly, and if we do see it clearly, we see it in its current state and not how it looked decades ago.
• If we assume that seeing is dependent on light reaching our eyes, we shouldn’t be able to see light beams that are directed away from us. For example, if we place a lamp to face away from us in a dark space, according to the former assumption, we should not be able to see the light beam at all. Yet, we still see it, even though we would be standing behind the lamp or ten meters outside of the light beam. This again suggests that seeing and light beams are two completely different things.
• The fact that we must place our telescopes to face the stars and galaxies we wish to see, serves as another example of how astronomy is not about light reaching the telescope from a galaxy or a star. We can detect a remote star or a galaxy, because we point our telescope at it, not because the light reaches our telescope. If it was down to light reaching the telescope, how could we separate light coming from a single star or galaxy from all the other billions of light sources in space? That would probably be impossible.
1. Brian May, Patrick Moore, Chris Lintott: Bang! Maailmankaikkeuden historia (Bang! The Complete History of the Universe), p. 21
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