Journey of Physics — Particles to String theory and Black Holes.
I wrote this piece on Quora long time back, and I thought it’s time when I should share it with larger Medium audience.
It all started with some questions like — What is string theory? How is it related to black holes? What is happening at the LHC? What are sub atomic particles? How is their discovery contributing to string theory (in layman’s terms)?
So, I tried to talk in as common terms as possible, and yet try to engage the reader who may have just middle school knowledge of Physics.
Before we talk about String theory or black holes or LHC, let us set some ground for it.
In our schools it was told to us that, everything in this world is made up of atoms. What ever you see around, the air you feel, the water you drink, the mobile phone in your hand, the watch on your wrist, your own body, this earth, the moon, the sun — all are made up of atoms. Just like a house is constructed brick by brick, the brick becomes the smallest unit of that house. Similarly whatever is present in this world, if you try to break it into the smallest unit, that will be ATOMS.
We were able to find out all those distinct smallest units or atoms, like Hydrogen, Oxygen, Iron, Gold, Uranium etc. They are all tabulated in “ Periodic Table”. There are around 98 such distinct atoms which exists naturally.
Newton and other scientists have already formulated the laws of physics knows as classical mechanics or Newtonian mechanics. Those laws were able to explain the movements of planets, motion of objects, trajectories. Basically those laws were followed by bigger objects. Objects like apples, stones or even as big as planets, stars or even galaxies. But when those laws were applied to predict the behavior of smallest units of universe or better call atoms, then those laws failed miserably. It was sure that for smallest units of universe the new theories or laws are required.
Later on, in first 3–4 decades of 20th century, the Superheroes of science like Einstein, Max Planck, Neil Bohr, Erwin Schrödinger, Richard Feynman, Lorentz, Heisenberg, Pauli, de Brogli, Dirac, Langmuir etc started deriving laws for those smallest units. They were astonished by the calculations and results based by this new theory that they were not believing even themselves. They gave the birth to one of the most interesting field of physics — Quantum Mechanics. Quantum mechanics deals with the laws and theories which are followed by those smallest indivisible units of universe.
But then there was a twist in the story, as believed the smallest units are not the atoms. Atom itself is made up of more smaller particles like protons, electrons and neutrons. Those are called as sub-atomic particles. With the advent of quantum mechanics, scientists were able to find approximately 90 different types of sub-atomic particles theoretically. Though electron, proton, neutron and few others were observed and confirmed experimentally. Suddenly there was an ocean of sub atomic particles in the world of quantum mechanics.
It was necessary to maintain this particle flood. It was observed that many sub-atomic particles can be categorized in some kind of logical groups, just like there are atoms classified in periodic table.
The sub-atomic particles were categorized and an interesting model in physics came into existence — The standard model. In layman’s term, Standard Model is kind of like periodic table for sub atomic particles, where sub atomic particles are categorized. Now it is more fair to say that each and everything in this world is made up of sub-atomic particles. Even the natural forces which governs the universe can also be explained by standard model. Below is a graphical representation of Standard Model.
Now imagine you want to get your driving license from traffic office. The examiner who examines your driving tests decides to fail you. In order to get the license and manipulate the examiner, you might offer him some money/bribe. The examiner might accept your bribe, take your money and pass you in the driving exam. What you actually did technically is, you “forced” examiner to pass you in “exchange” of money. If you wouldn’t have given money to him, he would not have passed you.
Same way, the four fundamental force which is observed in nature works.
Electromagnetic, gravitational, strong nuclear force and weak nuclear force.
There is actually the exchange of some sort of specific force carrying particles when this forces are in action.
1. Electromagnetic Force: Carrier particle — Photon.
When a negative electron gets attracted towards a proton, how do you define that attraction. There is no physical thing which is connected to +ve and -ve entities, still they apply and feel the force, how? By exchange of photons — the force carrying particle of electromagnetic force. Just like you exchanged money with examiner and forced him.
2. Strong nuclear force — Carrier particle — Gluons
Imagine you have to keep Batman and Joker without fight at one place. Both are so powerful and repellent to each other, that they cannot be together at one place, they will fight and try to get away from each other. They both are powerful. Only way you can keep them at one place without fighting, is if you will tie both of them in an unbreakable rope. They are now tied and bound so they cannot fight and get away, but the rope has to be unbreakable and stronger then both of them.
An atom consist of nucleus, where protons and neutrons are found. Nucleus is very tightly packed with protons and neutrons. Proton consist of positive charge, and because of that all proton try to repel each other and thus try to get away from each other. That way a nucleus can never form. So, the strong nuclear force keeps the protons and neutrons tied in the nucleus.
3. Weak nuclear force — Carrier Particle — W and Z bosons.
Consider the same example of Batman and Joker tied together with an unbreakable rope. After some time passes by, Robin comes to rescue Batman, and loosens the rope tied around. Robin is kind of like weak force, enabling Batman unbound himself from rope and get away from Joker.
Weak force is the reason behind radioactivity, where nucleus of heavier atoms gets disintegrated into new smaller nucleus or atoms.
4. Gravitational Force — Carrier Particle — Graviton (Not observed till now)
Sir Newton cleverly detailed the laws of gravity. His equations are able too predict the behavior of objects due to gravitational force, but he was unable to explain what is gravitational force? How gravitational force works. Though gravity is responsible for shaping up the universe, holding planets around stars and stars around galaxies, but still gravity is the weakest force as compared to other three fundamental forces.
Einstein tried to explain the reason behind the gravitational force. He postulated something called as General theory of relativity. As per his theory, gravity is a bend in space time. What he meant was, the space is like fabric sheet, where the fabric is called as space-time, and the objects with mass actually distorts this fabric of space, and thus creates bend in it.
Very famous analogy is Trampoline. If you put a bowling ball at center of a trampoline then its fiber will stretch and bowling ball will bend the trampoline cloth. Same way, consider the space as a fabric sheet. This phenomena of bending of space-time is confirmed by an observed phenomena called as “Gravitational Lensing”, I don’t want to go in to the detail of gravitational lensing.
General theory of relativity is just a model, a model which explains and answers many things. But still gravity or gravitational force is ultimately a force, which is bending space time, and there should be a carrier particle to carry gravitational force. Those particle are named as Gravitons. Gravitons are not observed experimentally. They are still theoretical.
What is Theory of Everything?
Now we know that there are four fundamental forces, and everything in this universe is governed by these 4 forces. Everything in this universe can be explained by these 4 forces. Thus Einstein started a quest to unite these 4 forces in one single equation which can explain anything. A THEORY OF EVERYTHING. Stephen Hawking and other scientists are still on the quest. Quantum Mechanics cannot lead to Theory of Everything (TOE). So some other theory is required to continue and chase the TOE quest.
What is String Theory?
Have you ever played guitar or violin, I am sure you must have heard it at-least. When you press the string at different frets on guitar and play it, each fret will result in different sound with the same string. Same string can produce many sound at different frequency.
Remember sub-atomic particles, which are particles as per Quantum Mechanics. String theory says that sub-atomic particles are actually little tiny vibrating string made up of energy. The different frequencies at which the string vibrates result in different particle. Imagine if the size of particle is equal to earth, then the vibrating string which is actually responsible for that particle will be equal to the size of a tree on earth.
From 98 natural elements/atoms — — to — → few subatomic particles — — -to — → Vibrating String.
Physics has changed a lot, and so does our understanding.
Why String Theory is important?
String Theory successfully explains many things which are not explained by quantum mechanics. The only glitch with String Theory is, laws and mathematics of String Theory fails miserably with 3 dimensions of space.String Theory and its maths becomes stable only if we add more dimensions in addition to already known 3 dimensions. Addition of not 1 or 2 but 7 extra dimensions. Thus, String Theory predicts that the universe consists of total 11 dimensions, 3 (x, y, z) + 1 time dimension + 7 extra dimensions.
String theory in its initial phase was not able to attract many people, and only few countable physicists were pursuing the idea of strings. As the popularity grown over time, many people joined the quest of string theory. As a result of which there was not 1 but 5 different types of string theories were formulated.
In layman’s term, imagine as if 5 musicians are playing guitar, violin, violoncello, harp and Sitar.
Later on in 1995, very renowned physicist Edward Witten shocked the string theorists. He said the the 5 string theories are like reflection of a musician playing guitar in 5 mirrors. It is like looking at one thing in 5 different ways. Edward Witten unified the different string theories as one, called as M-Theory. What does M stands for, no one knows, its just a name given. Some people say M stands for mystery, magic or matrix or it is W from Witten upside down.
Originally string theories had only 10 dimensions, but M-Theory came up with a cost of adding one more 11th dimension (as I mentioned above).
Dimensions are independent directions in which a body can move, sometimes often referred as degrees of freedom.
What are extra dimensions and why I can’t see it?
Extra dimensions predicted by string theory cannot be observed or felt by us. Those dimensions are very small and limited to only subatomic particles. Let me explain it with an analogy. Take or imagine a drinking straw.
If you look this straw from some significant distance, you will just see a line. You won’t be able to see those circles at the neck of straw. You will say that the straw is just a 2 dimensional line. At the same time, imagine an ant over those circles at the straw neck. When the straw is appearing a line for you, at the same time an ant can walk 360 degree on those circles, an ant can experience the width of straw.
The sub-atomic particles are like those ants, which can experience those extra dimensions which are out of reach from our bigger observable 3 dimensions.
I have also asked a question on Quantum Entanglement and extra dimensions here.
You see, just like if you open an image in your computer, and try to zoom it, after a certain amount of zoom, the image will pixelate. Our universe is just like a picture, which pixelates as you go deep into it’s fabric.
Why Gravitons (carrier particle of Gravitational force) are not observed and why gravity is the weakest force?
Gravity is the weakest force, you don’t believe me, then just lift your mobile phone in your hand from the table. The entire gravity of earth is not enough to hold your mobile just against your hand muscle. On the other hand, try separating two very strong magnets from each other. You have to really put some hard work to separate two strong magnets. Electromagnetic force is actually billions of zillions of times stronger than gravity.
String theory does provides an answer for this question as well. String Theory says that the strings which vibrates and results in particles (standard model)are open ended strings. Means the string is linear, and those open ends of the string is tied to the fabric of our universe.
Where as the String which results in Graviton is a closed loop of string. A closed loop, means there are no open ends, and if there are no open ends then that string cannot get tied to the fabric of our universe.
That is the reason why we are not able to detect any Graviton till now. Graviton is not tied to the fabric of our universe so it gets leaked from our universe to some other parallel dimensions very easily. That is the reason why gravitational force is so weak, because it gets leaked in other dimensions. Gravity is leaky.
In layman’s term, consider that you have a bread and then apply butter on it. Now no matter however you turn your bread the butter will not leave the surface of bread. On the other hand take the second bread and put some sugar crystals on it, as soon you will turn the bread, the sugar crystal will fall, they will not stick to the surface of bread.
That bread is kind of like our universe, where butter is all the particles which we observe and sugar is like graviton.
Before understanding how string theory is associated with black holes, we should some important things about black holes.
What is a black hole?
As we all know that a black hole is very small object with a comparatively huge amount of mass. Nothing can escape black holes, not even light. At the center of each black hole as we go deep down inside it, there will be a point knows as singularity. All of us know about this things. Now some less known and interesting things.
If black hole really absorb every thing even light, and do not emit any thing, then black hole is a perfect black body. If it is perfect black body, then it will have zero temperature and no entropy as per thermodynamics.
But, Hawking has considered quantum fluctuations and quantum corrections for black hole, and he theorized that the black hole is not perfect black body, it does emit something known as Hawking Radiation. Jacon Bekenstein showed the thermodynamics of black holes, and showed that the black does have some finite temperature and entropy. The entropy of a black hole is proportional to the surface area of black hole and not the volume of it. As the black hole emits Hawking radiation, it means that the black hole loses both mass and energy over time, and slowly black hole will evaporate over time. Hawking’s solution is that when the black hole evaporates, then at its last moment black hole will emit a final burst of random energy. This solution give rise to the so-called black hole information paradox, because quantum mechanics doesn’t allow information to be lost, but the energy from the evaporation doesn’t seem to carry the information about the matter that originally went into the black hole.
What is information paradox?
Imagine this, you are preparing a milk shake in a blender. While blending a piece of paper from your pocked falls inside blender and the paper gets ripped off and gets mixed with the shake. As per quantum mechanics the information written on your paper is still inside the blender, though you cannot recognize it in normal way, but it is there. Information is not lost, it is in some other form inside blender. Quantum mechanics says the information cannot be lost.
String theory and black holes:
String theory predicts many different types of black holes, one of which is called as super-symmetric black holes. String explains the Bekenstein thermodynamics of black hole in view of super-symmetric black hole, though the approach is bit difficult to understand, where they try to measure the various quantum or micro states. Anyways the important point is black hole wouldn’t really be black hole anymore, but some strange conglomerate of strings. Read about Fuzzball (string theory)
Fortunately, however, we got used to the fact that certain quantities, like the number of states available to a super-symmetric object, does not change as we change from one regime into another. Thus, we expect the entropy we measure in another regime still to be valid where general relativity is a good approximation. And indeed, the entropy calculated from the statistical mechanical explanation that string theory gives to general relativity agrees with the entropy as predicted by Bekenstein. That is a major advance in understanding black holes from string theory perspective.
String theory and Super Symmetry (SUSY). What is SUSY, and why we need it?
Standard model talks about two classes of particles i.e.
Fermions(matter particle) and Bosons (Force carrying particles)
Standard models first predicts that all particles are mass less, but we know that particles do have some mass. How the particles are getting mass? So, to give mass to particles, a very special type of boson was hypothesized Higgs Boson (which is confirmed now). Higgs was having a special place in standard model, though it is a boson but it is not a force carrying particle. Hypothesis and confirmed CERN experiment also says one more thing that Higgs should be a light particle.
Now that is weird. The particle which is giving mass to other particles, how can it itself be a light particle, now this is some serious problem. As always, physicist solved this problem by hypothesizing a new range of particles called as Super-partners, and the association of these hypothesized super-partners with their real verified partners in standard model is called as Super-symmetry (SUSY).
So, if we account for super-partners, then the super-partner particle will cancel out the massive Higgs and thus making a light Higgs actually possible, which we have confirmed experimentally. So, then the quest to find super-partner experimentally started, which is the next goal, and it will be the next big success for LHC.
There will be new super-symmetric super-partners for each boson and fermions, but the catch is, each fermion will have a super-partner as boson, and each boson will have a super partner as fermion.
For example-
The bosonic super-partner of electron(fermion) will be selectron, for quark the super-partner will be squark and so on like stau, smuon, sneutrino.
In short we can say that every fundamental matter particle(fermions) should have a massive “shadow” force carrier particle (super-partner bosons), and every force carrier(bosons) should have a massive “shadow” matter particle (super-partner fermions).
The fermionic super-partner of bosons like photon will be Photino, graviton — Gravitino, gluon — Gluino, W boson — Wino, Z boson — Zino and finally Higgs will have Higgsino.
What makes the strong force of small range, or weak force even of more smaller range, and also electromagnetic is a short range force. Ofcourse the resistance faced by force carrying particle of these respective forces, because of higgs makes these forces short range.
But if super-symmetric particles gets confirmed, then the math show that these three fundamental forces have had the exact same strength at very high energies, as in the early universe. Thus uniting the 3 fundamental forces as Grand Unified Theory (GUT), which is a dream come true for physicists and a step closer to TOE.
What is happening at LHC — Large Hadron Collider?
SUSY and Dark Matter:
We know that the galaxies are circling at very fast speeds, and the total gravity of all the stars and planets in the galaxies is not enough to hold the galaxy in its shape. It means some extra matter is keeping the galaxy intact by providing some extra gravity. Those extra gravity is provided by dark matters. They do not absorb or emit light, hence dark. But they have to be present there, otherwise from where the extra gravity is coming.
It is believed that the lightest SUSY particle would be a good candidate for dark matter. So, finding the SUSY partners by LHC or Fermi lab will solve many mysteries.
Do you remember the colorful marbles from your childhood.
What would you do, if you want those colorful glass from inside of those marbles. You would probable take two marbles, and smash them each other with so much force that those marbles will break, and then you will be able to take out that colorful part from inside very easily.
Principle at LHC is simple. Break the cover and see what is hidden inside.
LHC kind of does the same thing with particles. LHC takes the protons and enables those protons to travel inside LHC with incredibly fast speed, and they collide with each other. In one collision millions of such protons takes part. Very few protons will have complete head to head collision, and because of full head to head collision those protons gets disintegrated in various other smaller particles.
In 2012, Scientist at CERN (LHC is at CERN) were able to experimentally verify a very important and theoretical particle knows as Higgs Bosons, also murmured as God Particle. It took more than 40 years to experimentally confirm the Higgs Boson and Nobel prize to Peter Higgs, since it’s theoretical prediction/discovery.
My another answer to know more about Higgs bosons is here.
After that LHC was closed down for up gradation and enhancements. Recently LHC was again started again, and this time it will try to uncover the mystery behind super-symmetry to support string theory, dark energy and gravitons. ( Vivek Keshore’s answer to What is dark energy? Why does it matter?)
How LHC will detect Graviton if it is not tied to the fabric or our universe?
Sometimes the absence of something can be verified or observed by the presence of some other thing.
For example, lets say your TV remote works on two batteries and one battery is missing. you opened the battery cover from the backside of your TV remote to replace the battery, and at the same time power went off, and it became dark, you cannot see any thing. Now how you will make sure out of two battery slots which slot is missing the battery. You will touch with your fingers and feel the empty space of slot. Thus, the absence of battery can be observed by the presence of empty space inside the slot.
Einstein has told and proved that the mass and energy is same thing. Everything is energy, and energy can never be created nor be destroyed, it can only change its form. That is known as conservation of energy.
When we collide the particles or protons inside the LHC, we know the energy at which the protons are getting collided. After collision, we measure the pure emitted energy, other particles and its associated energy. As per the law of conservation energy cannot be destroyed or created, so the total measured energy should be equal to the energy of protons before collision. And if that is not equal, that it means some energy is missing. That missing energy could be the Graviton. Of-course it is not simple as it sounds. It involves massive computing and lots of calculations. But see the idea, the absence of missing energy, could lead us to the presence of Graviton.
Many places I have not gone into deep details to keep it simple and tried to answer in Layman’s terms. Criticism, mistakes, improvements and suggestions are welcome in comments section.
I want to quote a line said by a gentleman known as Richard Feynman.
How Z&W bosons are responsible for weak nuclear forces? Is the function of weak nuclear forces only to weaken strong nuclear force or does it separately drives the protons outside the nucleus?
I didn’t mention much about how Z and W bosons are responsible for weak force, as I didn’t want to make it complicated. But, let me try explaining it in detail and simple form.
We know that nucleus of atoms contains protons and neutrons. We also know that during radioactivity a bigger nuclei gets disintegrated in two smaller nucleus.
We know that strong force is a very short range force, it is so short range that for bigger nuclei, it may not cover the entire nucleus. If the strong force is not enough to hold the bigger nucleus then the electrostatic repulsion between protons (same charges repels) should be able to break the nucleus. Seems reasonable. In simple terms we can say 4 has become 2 + 2.
Proton — 2 up quarks + 1 down quark
Neutron — 2 down quarks + 1 up quark Imagine if you could change one of the down quark in neutron to up quark, then it will become a proton, isn’t it. Vice versa if you could change one of the up quark from proton to a down quark, it will become a neutron.
Now the problematic part. When Marie Curie was studying radi0active materials, she noticed that the bigger nuclei is getting disintegrated in small nuclei, but it was also emitting a high energy radiation (beta decay). Where does that radiation come from, radiation is form of energy, and energy cannot be created, it is against law of conservation of energy. In general terms you can say, 4 has become 2 + 2 + 1, which is not possible.
Enrico Fermi was also studying beta decay. He studied that the single neutron was decayed into proton and electron, but many times the energy of the electron was different whereas it should be same. Then Fermi proposed the neutrino particle, and suggested if neutron can emit both neutrino and electron, then neutrino will take some part of energy. (My other answer to neutrino here).
Initially it was thought that just like photons are responsible for electromagnetism, neutrinos are responsible for beta decay. But, when the quarks were discovered and the quark model of proton and neutron was found, the whole story suddenly took a twist.
Remember Standard Model, do you see proton and neutron in that model? No, right. Because protons and neutrons are itself made up of other fundamental particles known as quarks. Both neutron and proton are composed of 3 quarks.
Up quark and down quark also possess electric charges. Up quark : +2/3, down quark -1/3
Proton: 2 x (2/3) + (-1/3) = +1 charge of (Proton)
Neutron: 2 x (-1/3) + (2/3) = 0 charge of Neutron (neutral)
Somehow the same kind of interchanging of quarks must be happening in nature. But how? It was thought that some kind of force (later named as weak force) must be responsible for this phenomena. Each force must be carried by some particles. There is a change in mass of particles during radioactive decay, which means that the carrier particle of weak force must also contain some mass.
Later on it was discovered that, indeed the weak force carrier particle contains some mass, and charge as well. The discovery of Z and W bosons as weak force carrying particles, where Z is neutral but W came in two flavors, W+ and W- (electrically charged).
Remember quarks are also electrically charged. Now we have something, which can interact with up and down quarks of neutrons and protons, and thus interchange up and down quarks, based on W boson charge.
The charged W bosons interacts with quarks and thus allow them to change their mass and charge. During this interaction between quarks and W bosons, some of the mass gets converted to energy, which answers the mystery behind the high energy radiation during beta decay.
We would not have existed without weak force.
In our sun, when two protons come together, they would be together for a brief moment and then go away, because the strong force might not be enough to hold those two protons together. But, during that brief moment when two protons are together, the weak force interacts with the protons and one proton decays into a neutron, positron and neutrino. The strong force can hold together a deuterium, and collisions with deuterium eventually leads to helium. That is how we get energy from the sun, and also the shower of neutrinos.