Open Questions in Physics

This is a seminar presented by me in Physics Department, KFUPM on 7, Jan. 2007
entitled "Open Questions in Physics:

Open Questions in Physics

Superposition in Physics ( another jewel or a mystery ?!)

Superposition plays a central role in physics both in classical and quantum mechanics.
In classical mechanics, two force fields (e.g. electric fields or gravitational fields) existing in the same point in space add together and they are equivalent in every since to a single force having the direction and magnitude of the mathematical vector sum.
Any model to describe how do electric forces add together to form a single electric force along the resultant vector?
What is the difference between the addition of two electric forces and the addition of an electric force and a gravitational force?

In quantum mechanics, the same particle can be in a superposition of two completely different states of energy or momentum or position !! This of course leads to the removal of the definiteness of such old classical concepts as energy and position. In my opinion this is the origin of many of the mysteries of quantum mechanics. How can a particle be in a superposition of two different positions ?? Is there any physical model of that ?? Not only the definiteness of position and momentum have been thrown out in quantum theory but also the definiteness of the concept of particle itself. Discarding definite position makes it easy to imagine the particle as a wave. But what is a wave ?? It's a disturbance in the motion of many particles that propagates through space with a finite velocity. The concept of wave was generalized in electromagnetics to include the abstract concept of a field. What is a field ? Originally It was a mathematical concept to describe the effect of a force. But the fact that we can generate electromagnetic waves from a satellite and receive it thousands of miles away suggests that the field (or the EM wave) is a real thing and not just a mathematical artifact.

Rotation - The jewel of physics !

Many of the non-intuitive phenomena in physics are associated with rotation.

• First, while linear motion is a relative motion which means that two observers with a relative velocity will measure the velocity of the same object differently, rotation seems to be an absolute motion since the two observers will measure the angular velocity of a rotating object to be the same.

• In the frame of a rotating object a new fictitious force appear such as the Coriolis force.

• If you mounted a wheel from a robe fixed in its axle, the wheel will fall down due to the force of gravity. While if you rotate this wheel, its angular momentum will interact with the torque due to the force of gravity and will make it acquire a new component of rotation. This is very non-intuitive and amazing as Professor Walter Lewin from MIT described it in his lecture about the conservation of angular momentum

• Need more ? There is much more...

The magnetic field associated with the elementary charge, its magnetic dipole moment, is associated with its intrinsic angular momentum, which is after all some strange kind of rotation !! ( The strangeness here stems from the counter-intuitive aspects of quantum mechanics. For example, the spin is no longer of a definite magnitude and direction, but can be in a superposition of a multitude of values and directions !)


An interesting question is: If the elementary charge had zero spin, would there ever exist any magnetic field in the world ??

consciousness and quantum mechanics !!

What is the relation between consciousness and quantum mechanics ??

when one is alert, he is interacting with his environment, or in the language of quantum mechanics entangled with it. This is a mixed state, where classical physics manifests itself.
While when one is unconscious, he is isolated from his environment. In the language of quantum mechanics, he is in a coherent state, or a pure state.
All non-classical phenomenon happen in such a state. One's soul can go to the past or travel to the future or travel around the world.

Questions in Quantum Optics

How does a single photon detector work?
Is it just a passive element that doesn't affect the field of the emitted photon from a two level atom? How does it interfere with the entangled field-atom system and cause it to collapse ??

The Density of States

The density of states in free can be calculated as in Loudon 1.1 by assuming standing waves inside the cavity and summing in the reciprocal space over an octant. An alternative is to assume running waves and hence periodic boundary conditions and summing over the whole shell in wave vector space. Both methods yield the same density of states in free space , namely w^2 /(pi^2 c^2) Why ? Which method is conceptually correct ?

New Kind of Mathematics ?!

Can our current structure of mathematics be an obstacle towards developing a rigorous complete physical theory? Most of our physical laws are built on differential equations, the branch of mathematics born with Newtonian gravity. Can we reach a better description of gravity if we developed a new kind of mathematics ?! Diffenrential equations describe the continous change in physical variables, maybe the sought new mathematics will describe this change in a smarter way.

Memory in Classical and Quantum systems

In a classical system the trajectroy of a particle under an invere square law force is typically the result of solving the equation m r''(t) =const/r^2. By integrating this eqaution we can see that the r'(t) at any instant depends on all the history of that system till this instant. Can we consider this a memory effect? Then what is the difference between it and the evolution equation governing the a two level system interacting with an (infinte) bath like the field modes of vacuum? In studying this phenomenon in Weisskopf Wigner theory we had to solve a similar equation for Ca(t), the upper state probability amplitude, and neglect the effect of the history of the system on Ca'(t). (See for example eq. 14.102 in "Laser Physics" by scully et. al.) Even then we can say that the equation Ca'(t)=Gamma/2 Ca(t) depends on the history simply because we have to include initial conditions.So in general when can we say that a system is Markovian (doesn't have a memory ) and when are we able to neglect this memory ?

Why do objects move ??

In Newtonian Mechanics, objects change their state of motion by being affected by a force. In General Relativity, Objects move inertially but their motion is dictated by the curvature of space-time. Is there any other ( and better ) explanation or a conception of why objects move ??

The Quantization of Electromganetic Energy

Isn't it strange that there is no theoretical proof till now of the quantization of electromganetic energy ? The Planck hypothesis was accepted since it yielded the correct spectral distribution of the blackbody radiation energy. Poincare showed that quantization is a necessary and a sufficient condition to obtain the correct blackbody formula. In QED, the field is quantized in an analogous way to the quantum harmonic oscillator, it is not a still a theoretical proof for quantization. In quantum optics, many experiments verify the quantization hypothesis, but still no theoretical proof !!

A fluctauating Vacuum, is it a beautiful one?

The idea is clear from the title. Don't you feel from your heart that the universe is beautiful?

Does QED represent this beauty by saying that vacuum is full of random electromagnetic fluctuating field?? I don't think so...

On General Relativity

• The equivalence Principle, the cornerstone of General Relativity, says that acceleration is equivalent to force. Or as Einstein said: "we [...] assume the complete physical equivalence of a gravitational field and a corresponding acceleration of the reference system." Acceleration relative to what? Don't we need an external frame to relate this acceleration to it ??
• What about rotation? I think the equivalence of the centripetal acceleration to a gravitational field is what we call the centripetal force.
  
• How can General relativity account for the precession of a spinning wheel mounted from a rope or the precession of a gyroscope having a weight on one end of its axis??

Conservation of angular momentum in Quantum Mechanics

Einstein De-Hass effect shows that the angular momentum associated with spin is a real angular momentum of the same nature appearing in classical mechanics (i.e rotation).
Measuring the spin state of a state superposition between spin up and spin down can yield any of them. In addition if you measured the spin in a different axis (e.g. an orthogonal axis) of such a state you can get a non-zero value. So in what since is angular momentum conserved in QM ??

The answer for this question may be in decoherence. The pure state is an ideal case. The real state is entangled with the environment and gradually becomes a totally mixed state. The angular momentum of a totally mixed state, whose density matrix is diagonal, is conserved in the classical sense.

Spin Relaxation

In atoms, spontaneous emission is (claimed to be) due to the interaction of the continuum of vacuum modes with the atom that causes an irreversible decay of the atom( why irreversible ??). What about a system of spin half particles in a magnetic field. If initially in the higher energy state (anti-parallel to the field), the particles will eventually relax to the lower energy state. Why? What kind of vacuum does the particle need to interact with and relax ??
The only guess in my mind is a bath full of magnetic dipoles in all orientations and may be energies. Is this correct ??

Digging into the roots of Physics

Welcome to my Blog !

I am a masters student at King Fahd University of Petroleum and Minerals. I'm doing my thesis in Quantum Optics. The purpose of this blog is to share some of my thoughts in theoretical physics-especially the fundamentals- with you in addition to keeping a record of them for myself. Digging into the roots of Physics is not of course to destroy, but to examine and hopefully refine.

Tareq, KFUPM