The Microworld of Atoms

A study of the peculiarities in the world of atoms


presented by Subhashis Barad

Max Planck's Quantum Theory

Electromagnetic radiations are emitted out of a radiant hot body in discrete packets of energy called quanta

Einstein's Photoelectric Effect

Light is observed to displace electrons from the surface of metals. Light beam is viewed as a hail of discrete particles bolstering Planck's Quantum theory.

Unresolved problems in classical mechanics

• Unhinged revolution of electrons around the nucleus
  • Orbiting electrons should steadily radiate energy and eventually meet the nucleus following a spiral path according to EM Wave theory
• Electron-like wave-particle behavior of other subatomic particles

Bohr's Model

• The orbits in which electrons revolve are quantized.
• They are fixed energy levels where electrons, when revolving, do not loose energy.

Davisson and Debroglie

Electrons as well as photons can behave both as particles and waves.

Polarization of Light and Malu's Law

Light with axis parallel to the axis of polarization of the polarizer can only pass.

What if the intensity of light is reduced to such level that only one photon passes?

Polarization with no Light

If the axis of light is perpendicular to the plane of polarization then, clearly, no light passes.

Malu's Law

clearly, if the angle() is the intensity of light is zero.

The Puzzle

If we reduce the intensity such that only one photon is capable of passing, and since it is indivisible, it will either pass or be blocked

The results however are intriguing

It is observed, by experiment, that two equal configuration of the same setup yields different results in one case the photon passes through the same setup but in the other it doesn't.

Which breaks the reproducibility of classical mechanics under same conditions.

Young's double slit experiment

The interference, constructive and destructive, shows the wave-like nature of light.

A special case in Young's double slit experiment

If we were to reduce the intensity of the light source such that only one photon is emitted from the light source at a time.

Case 1: With both the slits open

• The patterns seem random at first
• Eventually they create the wave interference pattern

Case 2: With one slit closed

• The interference pattern disappears
• Superimposing the patterns created through either of the slits doesn't achieve the same result

Conclusions

• Somehow, the photon takes into account the state of both of the slits before passing through one of them even if it is indivisible.

Wheeler's delayed-choice experiment

Case 1: If the second beam splitter is absent

• no recombination of the reflected photons
• 50-50 chances of detection(clicks) on either of the two detectors
• shows particle nature

Case 2: If the second beam splitter is present

• causes interference
• one detector primarily clicks (constructive interference)
• the other one clicks less (destructive interference)
• shows wave nature

Hidden Variables

• The uncertainty or unpredictability of the things in our surroundings is the experimental clumsiness.
• Accurate predictions can be made taking unto account all the hidden details.
• All those underlying hidden details that we choose to ignore are called Hidden Variables.

Why can't we attribute Hidden Variables to the fuzziness and uncertainty to the Quantum Phenomena?

Einstein-Podolsky-Rosen Experiment

• Cheating Heisenberg's uncertainty principle
• Quantum Mechanics contradicts relativity
• Use of law of conservation of momentum to define a definite momentum or position
• Local realism exists
• Quantum Mechanics is incomplete

Bell's Theorem

• Bell investigated the correlations between the measurements(spin, position, momentum etc) carried out simultaneously on two separate particles and he put theoretical limits up to which the correlations could go.
• Any outcome of a measurement if is due to local realism obeys Bell's inequality if it is otherwise, however, it violates the inequality.

The chsh inequality

An example of Bell's Inequality

The Principle of Locality

• An object is influenced only by its immediate surroundings.
• For a cause to happen at one point and an effect for it to happen at another there is a need of transfer of information between cause and effect.
• For a particle at one point to be influenced by another particle at another point, when it is disturbed in any way, the transfer of information from the disturbed particle to the undisturbed should occur.

The Nature of Reality

• The world about us enjoys an independent existence i.e., it consists of objects like atoms, chairs, tables, planets, and stars which exists irrespective of our observation.
• The universe is a collection of such independent objects
• Any observation of the object involves some interaction with it and with interaction comes some sort of disturbance
• We can minimize the disturbance and compute with complete detail the object, after which we'll be able to deduce the exact state of the object after our observation.
• We should thus be able to ascribe features to the object like position, momentum, spin etc to the object and the only difference between bigger objects like the planet and the atomic particles will be that of the scale only.
• But the Copenhagen Interpretation contradicts it all

The Copenhagen interpretation

A quantum system will be in a state of containing a collection of quantum states superimposed. When a measurement is performed one of these quantum states materializes and at the same instant all the other states simply vanish.

An example: electron in a box

• Without performing a measurement or observation, we cannot ascribe any quality to a quantum object.

• As we saw in the example of the electron, the waves of existence of the electrons were uniformly distributed throughout the box at first. Then after it was divided into two impenetrable chambers the waves existed in both of them. When, however, we decided to see where it really is, and find the answer the wave in the other box, where it isn't found, vanishes entirely.

• This is also called a collapse of the wave function. Where the quantum object first existed in a superimposed state of possibilities and after observation one of them is realized and the other simply goes away.

Schrodinger's cat paradox

• A cat, a sealed box with radioactive(decays one atom per hour) substance, a geiger counter are placed in a box.
• If decay is detected in geiger counter, the flask gets broken and the cat gets killed.
• If it is otherwise however the cat remains alive.
• But, where is the dead/alive cat?

The Multi-universes interpretation

• To explain the peculiarity and absurdity of measurements performed on quantum phenomena, a series of interpretations were theorized, one of which the multi-universes interpretation is a popular one.
• It attempts to explain how and why when a measurement is performed the quantum system jumps abruptly to one quantum state.

Everett's Interpretation

• The transition, one of the quantum states realizing, occurs because the universe splits to realize all of the states. i.e., if a quantum system exists in the superposition of n different quantum states then the universe splits into n different universes to realize all of them.
• In the Schrodinger's cat experiment, the universe splits into two, one with a live cat and another with a dead cat and in each of them the observer who thinks he is unique
  

David's Modifications

• Most universes start out completely identical. When a measurement takes place, differentiation occurs so that in one universe the cat is alive whereas in the other it dies.

Possible explanation for the origin of the universe

• The multi-universes interpretation is a likely explanation of the universe's origin.
• On a large scale, the universe presents itself highly ordered, with matter and energy distributed in a highly improbable fashion.
• It is hard to explain, how a such a perfect arrangement emerges out of a random chaos of big bang.
• With the help of many-universes however it becomes easy to explain. As every possibility is a universe of its own. Which results in infinite number of universes where each arrangement exists.

Sources

• Wikipedia
• The Ghost in the Atom by PCW Davies and Julian Brown

Thank You!