Introduction of Modern Physics Notes for Engineering Physics

The term modern physics refers to the post-Newtonian conception of physics. In simpler form, modern physics deals with the underlying structure of the smallest particles in nature (quantum mechanics), as well as a rigorous understanding of the fundamental interaction of particles, understood as forces.

The Triumphs of Newtonian Physics:

Here we will discuss quickly the statements and achievements of Newtonian physics. Beginning with , which united the actions of the heavens and those of the earth, we will review the achievement of the mechanical view of nature. We will also review electricity and magnetism, including the revelation that time-varying magnetic fields produce electric fields and time varying electric fields produce magnetic fields. This representing the uniting of electricity and magnetism into a single electromagnetic idea. Newtonian physics refers to the laws of nature developed by Newton and extending forward 200 years through the 1800s. This is a large body of work that stemmed from Newton’s original laws of motions, themselves based on observations of the natural world. The recognition that force and acceleration are directly related to one another through inertial mass (F=ma) was a fundamental statement that the behavior of objects on earth and the behavior of the stars, the sun, and all other bodies all stem from the same principals. Newton’s ideas united the heavens and the earth, and represented the first logical and fundamental unification between seemingly disparate phenomena.

There were many successes that stemmed from this work:

• Kepler and others derived predictions of the motions of heavenly bodies based on Newton’s laws of motion. This predictions was stunningly accurate, with only a few exceptions (the perihelion of Mercury).
• Energy was recognized as a fundamental quantity that was neither created nor destroyed, but transferred from one body to the other. Any losses incurred along the way were recognized as energy going into processes (e.g. heat) other than the intended one (e.g. motion).
• Mass was recognized as a body’s tendency to resist motion (inertia), and was something that could be quantified by the laws of motion. Light was identified as a wave phenomenon, owing to the behavior of light when it (a) scattered around small objects (interference) (b) . . . .
• Electricity and Magnetism were revealed as two faces of the same underlying phenomenon – Electromagnetism. All electric and magnetic phenomena can be described as waves; in fact, here light was identified as alternating electric and magnetic fields propagating at the speed of light ( ). It was assumed that, as for all other wave phenomena, light propagated in a medium (the ether) and was affected by motion of that substance, or motion relative to that substance.

A coherent, mechanical picture of the universe began to emerge – the universe as a perfect machine. Thus is was assumed that measurements were limited only by the instrumentation, and that with perfect measurements of initial conditions the exact outcome of any situation can be determined before it occurs. The behavior of the physical world was perfectly deterministic, governed by the mechanical laws of nature (nature as a ticking clock). All the universe plays out on the stage of space and time, which are absolute concepts unchanged by the state of the observer.

The Failures of Newtonian Physics:

Here we will discuss a few examples of where the triumphs necessarily led to failures. The mechanical view of the universe suggested that space and time are the same for all observers, regardless of their state of motion (consider the case of a person dropping a ball while standing still, vs. doing so while walking and being observed by a person at rest). Energy (motion) is different from mass, and both come in continuous units. All things could be predicted, and this extended to the energy of a black body whose spectrum was INCORRECTLY predicted by these notions. The laws of electromagnetism predicted that EM disturbances all travel at the same speed, the speed of light, independent of any propagating medium. Worse, the classical need to invoke such a medium led to the observation that the speed of light is the same for all observers, regardless of their state of motion (Michelson-Morley).

1. Light appears to be unaffected by the motion of the observer: the experiments of Michelson and Morley. This made no sense, yet experiments repeatedly failed to prove the existence of an ether and our motion through it. Worse, the laws of Electromagnetism (“Maxwell’s Equations”) appeared “special”, in that their form changed depending on the state of motion of the observer. Using a Galilean transformation, where and , all the mechanical laws of nature remain unchanged but Maxwell’s Equations change radically and made untenable predictions.
2. The theory of heat failed to predict the energy emitted by a black body. If heat is described as thermal motion of atoms in a body, and we consider heat trapped in a cavity (the perfect black body), the emitted spectrum of energy becomes infinite as the energy of the emitted radiation increases. This ultraviolet catastrophe would result in the destruction of the universe if even one black body were in existence, yet the measured spectrum of energy emitted from real black bodies showed that it increased and then cutoff.

Building Comfort with Reference Frames and the Speed of Light Review a few examples of reference frames and do some exercises with the speed of light. These will be used to get the minds flowing and focused on meaning and interpretation. Some useful examples would be:

1. Everyday speeds: walking, cars, baseballs, bullets, planes.
2. Compare the speed of sound and the speed of light and apply those differences to real problems

• Lightning and thunder – estimate the distance to the storm 
• Synchronizing the symphony – why you need a conductor 

3. Then think about time and space

• discuss the speed of light over astronomical distances
• what does it mean to “see” a distant star? Use the big dipper as an example, since that is ~70 ly away and thus has relevance to the scale of human existence.

Reference :
http://www.physics.smu.edu/
https://en.wikipedia.org/wiki/Modern_physics

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