“Is it my imagination or is has it suddenly got wet?”
No you’re not imagining it, we’ve just hit the front of an eye ball. These are tears.
“We travelled quadrillions of miles through space, just to arrive as someone is crying? What is so sad?”
I don’t think they are sad. There is a thin layer of tears in your eyes all the time. Have you noticed we’ve changed direction again and slowed down?
“Tears can slow down light as well?”
Oh yes, and change our direction much more than the gravity of the sun managed according to the theory of special relativity. Remember the total eclipse of 1919 that we talked about before? Well, the sun changes the direction of starlight that just skims the surface by only 5 ten thousandth’s of a degree.
“But we’ve just slowed down by a quarter and turned about five degrees. Are you telling me that tears can bend light ten thousand times more than the sun?”
I am because it is true, in the words of Washington Irving tears ‘are not the mark of weakness, but of power. They speak more eloquently than ten thousand tongues.’ In this case tears are indeed ten thousand times more powerful than the theory of general relativity. These tears are sitting on the equivalent of the windscreen of the eye, a curved structure called the cornea that you have probably never seen.
“I can look at my eyes in a mirror so of course I can see my cornea even if I didn’t know what it was called.”
When you look in the mirror you can see your eye lashes, the colour of your eyes and the black pupil. You can be looking directly at the cornea but you can’t see it. The cornea is transparent when you try to look at it you literally see straight through it.
“I'm very glad that this cornea thing we're going through is transparent but how can anything solid be transparent. I can see how outer space is easy to fly through, but how do we get through this solid stuff?”
Anything can be transparent provided it doesn't reflect, scatter or absorb light. Even glass can be hard to see through if the surface is bumpy, like in a bathroom window. Bumpy glass scatters light because light is refracted in all sorts of directions by the bumps. Although light still gets through you can't see any details of what is on the other side of the window because all the rays of light are jumbled up.
“Clouds scatter light too. You told me that a little while back. That’s why they are white.”
Very good. But sometime even transparent materials can scatter light.
“How is that possible?”
It only works in materials where the atoms and molecules are arranged in a very regular pattern. When we talked about light as a type of wave, I told you about constructive and destructive interference.
“Remind me about that.”
In destructive interference the peak of one wave exactly meets the trough of another and they cancel each other out. In constructive interference both peaks meet making a new wave that is twice the size. If the spacing of the atoms or molecules is just right any scattered light is canceled out by other scattered light rays. So it looks like there is no scattering at all and the material seems transparent. That’s how the cornea manages to be see-through but there is one more thing a substance needs to be transparent.
It must not absorb light. Remember the dot of the “i" last time. Things look black if they absorb most of the light that hits them. To get to the bottom of absorption you need to go back to the nuts and bolts of how matter is made up. We talked about it a few million billion miles back, right at the start of our journey. Matter is made up of atoms and these atoms are made up of protons, neutrons and electrons.
“I just about remember that.”
Good, now in the middle of an atom is the nucleus where protons and neutrons are all packed together. The electrons are almost two thousand times smaller than protons or neutrons but are much more important for photons. As they fly around the nucleus in clouds they take up most of the space of an atom. So a photon is far more likely to hit the electron clouds than the nucleus. When light is absorbed it is electrons that do the absorbing.
“OK, so far so good.”
The way electrons absorb the energy of a photon explains how objects appear and even what colour they are. If an atom or molecule absorbs all colours equally it will look white, grey or black depending on how much of the light is absorbed. If different colours are absorbed to different degrees then the substance will be coloured.
"So grass is green because it absorbs all the green light."
Sorry, it’s just the opposite.
If it absorbed all the green light there wouldn't be any green light to see. If white light hits grass and green light comes off, then everything except the green must be absorbed.
"Oh I see. So why do some things absorb light of a certain colour?"
Electrons, like photons, follow the strange rules of quantum mechanics and are only allowed to have certain amounts of energy. If a photon is absorbed it has to be completely absorbed. An electron is only "allowed" to absorb a photon if it will end up with an acceptable amount of energy. This aspect of quantum mechanics of electrons always sounds like it was invented by a bureaucrat but it seems to be true. An electron can also lose energy in certain fixed amounts and create a photon out of thin air so to speak, that’s how things make light from light bulbs to TV’s.
“So, if an electron falls to a lower energy level a photon is released and that photon contains all the energy that the electron loses.”
Exactly and the amount of energy the photon has determines its colour. Short wavelength or blue photons have more energy than long wavelength or red photons. More importantly every blue photon has exactly the same amount of energy as any other blue photon, provided it is exactly the same blue. When light is absorbed by an atom the same rules apply. Only a photon of exactly the right energy can be absorbed. So what colour light is absorbed depends on what energy levels are allowable in a certain atom or a molecule. Remember, a molecule is simply a collection of atoms that are held together by sharing electrons. In terms of this quantum-electron-photon idea molecules are like atoms but just more complicated. If an atom or molecule has only a few energy levels for absorbing light, it will only absorb certain colours of light and so will look coloured itself. If it can absorb light at lots of energy levels it will be white or grey. How the electrons and atoms are arranged can make the difference between jet black or brilliantly transparent. Take carbon for example.
“As in carbon dioxide the gas?”
That’s right, but carbon dioxide is two different atoms stuck together, carbon and oxygen. Pure carbon is solid and it also comes in different forms. Charcoal for a barbecue is almost pure carbon and is jet black. Diamonds are also pure carbon and transparent. The atoms are the same in both, but the electrons and atoms are in a different pattern and the quantum rules for the electrons are different.
“So quantum mechanics can make the same stuff either jet black or transparent?”
That’s right, atomic physics and quantum mechanics in action in front of your eyes.
We spent about 2 trillionths of second negotiating the cornea which is only half a millimetre thick in the centre. With a slight increase in speed we slosh through a few millimetres of salty water towards another black hole.
“A real one this time?”
No, the type of black holes that we call pupils.
This is the story of a great journey that started with a great thought. One day in 1895 a boy looked into a mirror and wondered what the universe would look like if he could travel on a beam of light. That sixteen year old boy was Albert Einstein and that one thought started him on the road to discover his Theory of Relativity. The great man has been reinvented as Albert 2.0 to come back and blog about a journey through space on a beam of light and explain the science behind everything from atoms, blackholes to global warming. If you've just joined and want to start at the beginning use the index on the left. If you're bored try these links below just for fun.
UNSCRAMBLE EINSTEIN'S BRAIN
PRACTISE SAVING THE WORLD FROM ASTEROIDS
ALIEN CONTACT CALCULATOR
HEAR THE REAL EINSTEIN TALK ABOUT E=Mc2.