How does the eye see? –

Do we see things

as they are, or as they were?

We are told that we see from space and stars only past, not the present. However, this view is easy to question

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 the Sun as it was 8 minutes ago, because this is how long it takes for light to travel to earth.

• With telescopes, we see the most distant objects as they were hundreds of millions or billions of years ago, because they are hundreds or billions of light years away from us and because light takes the same time to travel to Earth.

The next quote continues on the same topic. It explains that we always see the past from space, not the present:

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 matter. 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 how is the matter? Isn't it the case that there is no way we can detect the light from the flashlight because it is too small? We may be able to detect a larger light, but not a small one. This shows even more clearly that seeing depends on the size and brightness of the object, and not on the arrival of light in our eyes.

• One observation is that we can only see about 5000 stars with the naked eye, while with a powerful telescope millions of them can be seen. What is the reason for this? Doesn't it prove that seeing is not dependent on the arrival of light in our eyes (or on the earth) but on the instrument of perception? If seeing depended only on the arrival of light from distant objects, they should also be seen with the naked eye and not just with an instrument of perception.

• Glasses are a similar thing. Why are they needed if seeing depends on the fact that light has reached our eyes? No one would need glasses, nor would they need telescopes to observe stars if seeing requires light to reach our eyes or the Earth. Two things are messed up here: the movement of light 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.

• When looking at the sun, it has been explained that we see the sun as it was 8 minutes ago because it takes this time 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 the previous quote, it was stated that if the Earth is observed from the North Star, we see the Earth as it was 400 years ago, because it takes this time for light to travel between these celestial bodies. According to the same principle, if someone suddenly travels at the speed of thought to a distance of e.g. 70 light years (A light year measures distance, not time. It has been calculated that one light year is approx. 9.6 thousand billion kilometers), he should see from this distance the events of World War II.

However, this view can be questioned. How could looking from another place, i.e. further away, help us to see things that are no longer there in reality? If you can't see them at that moment, it certainly won't help if you move to another place to observe. By looking further away, we cannot bring old things back into view. The only thing that will happen, when we go further, is that we won’t see our target as clearly, or if we do, we see it in its current state and not how it looked decades ago.

• If it is assumed that that seeing depends on whether a beam of light has entered our eyes, then we should not see those rays of light that are directed from ourselves in the opposite direction. If e.g. In a completely dark large hall, the lamp is directed in one direction and away from ourselves, according to the previous view, we should not see a wedge of light at all. However, we can see it even if we are behind the lamp itself or ten meters away outside the light wedge. This again suggests that seeing and rays of light are two 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.

REFERENCES:

1. Brian May, Patrick Moore, Chris Lintott: Bang! Maailmankaikkeuden historia (Bang! The Complete History of the Universe), p. 21