Hi, welcome to you for the year of light and welcome to you for this interesting lecture.
Yes, we are talking about light, the light which is responsible for the diverse life on Earth and spreads in the unimaginable universe. In the last lecture we spoke about the first light from the Big Bang called CMBR. But in this lecture I don’t directly talk about light. Instead I talk about atoms!!
Atoms are the building blocks of matter. We have more number of atoms in our eye than the total number of stars in a galaxy. This is true with any object which is of the size of an eye. In principle we humans are all collection of atoms that have different names! The matter which surrounds us is also a collection of atoms. Suppose if I am drinking water from a glass. If you shoot this in a video-camera which can only detect atoms regardless of their physical size, shape, and features, you can see that some arrangement of atoms drinking the same atoms which are in different arrangement. This means that, you can form infinite number of structures by arranging atoms. That is what the nature is doing from all these billions of billions years and it continuous do it in future also. So, if you look at animals, birds, plants, humans, mountains, earth, stars and galaxies all these are just arrangement of atoms.
So it is important to study about atom. Until the beginning of 20th century, scientist thought that atom is an indivisible particle of the universe and the fundamental particle of all the objects. Later many observations and sensitive measurements by the 20th century scientist proved that atom is divisible. Today we know that atom is made up of proton, neutron and electrons. Protons and neutrons make up the nucleus which concentrates most of the atoms mass while the electrons revolve around the nucleus. If I have to give you the feel of an atom or how big an atom is; imagine like this, consider this room (Auditorium) is of the size of an atom and this tennis ball (author is holding tennis ball) is a nucleus, than the smallest dust grain that you can find in this room is an electron! This means atom mostly contains an empty space! All the mass is concentrated in the nucleus. This is a distinctive sketch of an atom. But you might ask me a question… Do all atoms show the same characteristics? The answer is no. Atom with some fixed number of protons, neutrons and electrons have some physical and chemical characters. But if you change the number of constituent particles of an atom, the characteristics also changes. This means that if you change the number of electrons in an atom it exhibit different characteristics than the previous one. Depending on the number of electrons in an atom a specific name is given to them, like hydrogen which has one electron, helium which has 2 electrons and so on… These are called elements. These are arranged according to the ascending order of the number of electrons in their atoms. This is called Periodic Table. This table makes us to easily identify the element which has a specific character depending on its electron number. This is about atom, and its constituent particles and their arrangement and how there characteristics changes. But why am I talking about this, instead of light!
Yes, I will talk about light, but for a while let us go back to 1930. Wolfgang Pauli an Astrian physicist was trying solve the mysterious violation of law of conservation of energy in process called Beta Decay. I know all of you know about law of conservation of energy, in simple words the law says, energy can neither be created nor be destroyed. It can only transfer from one form to another form. And also the energy of the entire universe is constant. In a given process the energy before the process and after the process should be equal. No room for violation of this law in nature. While doing math’s we may do errors in calculating the energies, but nature do not do any error. It never violets the law of conservation of energy! But something strange was happening with beta decay. To know this let us learn about beta decay. Since we know about atom, it is very easy to understand this process. In certain elements the nucleus which contains protons and neutrons becomes unstable due to the more number of neutrons than the protons. If something is unstable it has to give out or it has to accept something in order to attain stable position. Here we have only protons and neutrons in the nucleus. So, what happens to this unstable nucleus? Yes, an extra neutron in the nucleus decays into proton and in this process an electron is emitted out of the nucleus as shown by the below nuclear reaction. (The elements which shows this behavior is called Radioactive Elements and the process is called Radioactivity). This process is called beta decay. (Beta particle = Electron). This is an observable process and the experiments have confirmed this nuclear reaction. But what is the strangeness about this nuclear reaction? You might be surprised to hear this; this nuclear reaction is actually violating the law of conservation of energy! But the question is HOW?
n -> p + e–
If you equate the energies in beta decay process, you will see that the total energy of the emitted particle is less than the energy of reactants (for now consider it as reactants). Let me explain this… In beta decay process an extra neutron in the nucleus decays into proton and stays inside the nucleus. But an electron ejects out from the nucleus and this electron does not belong to orbital electrons, because it is created in the nucleus and emitted out. But the amount of energy that electron is taking out from the nucleus is less compared to the energy used in creation of the electron. This tells us some energy is missing in this process. When we do the experiment, there is no signature of any other particle taking the missing energy. Then where is the missing energy? This is the question that haunted many scientists during the twentieth century. So what do you conclude based on our experiment? Is nature violating the law of conservation of energy or our calculations are wrong?
Pauli worked extensively on this problem of missing energy in beta decay and finally arrived at one conclusion. He believed that the universe is governed by the order of natural laws. And he had firm believes that nature does not violates these laws. More importantly he also believed that his calculations are correct. He said nature is not violating law of conservation of energy. The missing energy in the beta decay is transmitted through a particle called ‘Neutrino’ which has zero mass but has the energy which corresponds to the missing energy! He wrote down all the mathematical calculation to prove the existence of this particle which he called Neutrino and published the paper. But many scientists did not believe Pauli and went on claiming that, this kind of particle does not exist. But Pauli who was theoretical physicist, I mean the one who do only math and calculation to give the theory of nature had solid-believe on his theory. I should say he trusted his calculations so much that, he always said this particle; I mean the neutrino should exist in our nature. This is the beauty of understanding the nature. This is where we should appreciate our language called maths and equations. The numbers and equations hide the unseen exquisiteness of nature.
n -> p + e– +
Finally the beauty of Pauli’s calculation unfolded in the year 1956, when two physicist Frederick Reines and Clyde Cowan announced that they have indeed detected neutrino’s in beta decay process predicted by Pauli. It nearly took 16 years to find the existence of the particle predicted by the great physicist Pauli. But you know what, nearly trillions of trillions of trillions neutrinos have passed through your body while listening to this lecture. For every minute billions of neutrino is passing your body. The earth is in the shower of neutrinos from the sun, stars and galaxies endlessly.
Today I am here to talk about neutrinos. When Pauli predicted this particle, he said it has energy but rest mass of the particle is zero, which means these particles never come to rest!! So, how do we detect this particle?
Sir, I have question.
Go Ahead (Author)
Sir, Why do we need to detect neutrinos? Should we detect them only to explain beta decay or is there any other use from them?
Very good question. Beta decay problem is over, it is now confirmed that neutrinos are emitted from the nucleus and law of conservation of energy is successfully explained with this particle. I will reframe your question like this; why do we need to detect neutrinos on a large scale? Does it give any information to us? The answer is yes and let me give you one such example. Let us talk about Sun for a while; the thermonuclear reaction happening at the core is powering sun’s total energy. During this reaction photon (particles of light), neutrinos and other particles are emitted. The light that warms the surface of the earth as well as you and me has come from the photosphere of the sun. Photosphere is the visible disc of the sun. As we all know that light contains photons. These photons are generated at the core of the Sun. It takes billions of years for a photon to reach to the surface of the photosphere; from there the photon takes only 8 minutes to travel to the earth. So the photon which is warming you right now has left the sun’s interiors billions of years ago. Yes….. Absolutely amazing right! If you study this light you will be actually studying the sun’s interior which is present some billions of years ago. So you cannot study real time sun’s core in the visible light. But on the other hand the neutrinos which are created at the sun’s core travel to the earth in just 8 minutes. This is because neutrinos do not interact (very very less interaction) with matter. They just pass through it. But photon suffers trillions of collisions with the matter spending billions of years inside the sun before emerging out of the photosphere. So detecting these solar neutrinos is very helpful in monitoring real time sun’s core. Not only Sun in all the high energetic explosions (Supernovae and other) that happens in our universe emits neutrinos and detecting them will give us clear picture of these events as well as our universe.
How do we detect them? As I said neutrinos interaction with matter is very less. They can pass through humans, buildings and even mountains!! That is why they are called as most elusive particles of the universe. But if we detect them we get a lot of information about our universe. So here it is an astronomical observatory built half a mile down the earth. This is Super-Kamikonde Neutrino Observatory built under Mt Ikenoyama at Japan. This is a human eye to watch supernovae in our milky way galaxy.
The observatory is built underneath the mountain. This is because the instrument which detects the neutrinos is very sensitive. Apart from neutrinos there are other elementary particles reaching earth. If we have not shielded our instruments then the detector will pick up the signs of unwanted particles. How to shield these particles? Yes, go underneath the mountain; let nature itself shield these particles (Most other particles can interact with matter). Most of the neutrino observatories are built in the abanded mining places. Since neutrinos don’t interact with matter, they just pass through the mountain and to the detector. And because of zero interaction with matter for billions of years, they have the information of past! If you detect them you can actually get oldest of old information regarding our universe. Detecting even a single neutrino requires lot of big observatories like this and lot of patience because of its elusive nature.
In this observatory, 50000 tones of distilled water are stored and it is surrounded with scintillating tubes which detects the minute flash of light. But how do you get this flash of light in distilled water? In a minute trillions of trillions of neutrinos are passing through this huge detector. Suppose if one neutrino interacts with the nucleus of water molecule, an electron is emitted. This electron is detected by the very sensitive scintillating tubes by producing a flash of light. Each flash of light can signifies the presence of neutron in the detector. If the activity is more in outer space, I mean if there was a supernova in our galaxy, then flash of light increases indicating the presence of more neutrinos. This has been observed during the Supernovae-1987A in the year 1987.
A flash of light is used to detect one of the most elusive particles of the universe which hides rich information about our home. It is again the light that is doing our work!
Happy International Year of Light.
Thanks a lot for listening to me, that’s it for today.
Have a great evening.
First Image: Google Images
Second, Third and Fourth Images: Screen grab from the documentary Cosmos hosted by Astrophysicist Neil deGrasse Tyson
Year 2015 is celebrated as International Year of Light and Light Based Technology by UN as a global event. This year mark the 150th anniversary of Maxwell’s Equations by James Clerk Maxwell, the man who unfolded the secret of nature and answered what light is made up of.
More Information about IYL 2015
IYL – 2015 Home Page
IYL – 2015 Blog
Upcoming Topics for IYL 2015
- Biological Industry – Photosynthesis
- Principe, Africa – 1919 TSE
- Copy of Earth – Extraterrestrial Planet
- ದೀಪವು ನಿನ್ನದೆ ಗಾಳಿಯು ನಿನ್ನದೆ ಆರದಿರಲಿ ಬೆಳಕು
- Let there be light – Newton, Maxwell, Hertz and Einstein
Viswa Keerthy S