Student No:
Name:
2020-05607-MN-0 Lou Aidan M. De Jesus
Course/Year/Section:
BSPHY 3-1
Classical to Modern Physics
For a long time in the history of Physics, it was universally believed the universe, in every scale
obeyed Newtonian mechanics. It was just the most logical stance to have at the time and observations of
physical phenomena was also, for the longest time limited to macroscopic scales. These days, it’s generally
believed that Newtonian/classical mechanics are what govern the macroscopic world. The mission of
modern physicists is to create a unified theory that combines both classical and quantum mechanics. The
following timeline illustrates the shift between the time where classical mechanics was the unified theory
of the universe and the modern era.
1887 (Discovery of the Photoelectric effect) – The photoelectric effect was first documented by Heinrich
Rudolf Hertz while observing the change in voltage between two electrodes after shining an ultraviolet
light on them. These observations would later cause a discrepancy with classical mechanics that would
eventually be explained by Albert Einstein. In general terms, the Photoelectric effect states that metal
surfaces release charged particles when exposed to radiant energy, typically in the form of light or any
electromagnetic radiation.
1900 (Blackbody radiation and the ultraviolet catastrophe) – Blackbody radiation refers to the light
emitted by a theoretical “blackbody”. In theoretical mechanics, a blackbody refers to any object incapable
of reflecting light from its surface. Therefore, the only electromagnetic radiation it is capable of emitting
is from its own thermal radiation, thus referred to as its “blackbody radiation”. Any object above absolute
zero emits some level of thermal radiation but for any blackbody below the Draper point (798 K) would
just appear to us as a black mass.
Classical mechanics makes the assumption that energy is continuous and that there is essentially no limit
to the amount of energy produced by electromagnetic waves with increasing frequency. This is the basis
of Rayleigh-Jeans’ Equation of Blackbody Radiation and thus the origin of the ultraviolet catastrophe.
Though varying in distribution with respect to temperature, a blackbody would emit wavelengths along
the entirety of the electromagnetic spectrum, meaning that a reasonably warm object would be expected
Student No:
Name:
2020-05607-MN-0 Lou Aidan M. De Jesus
Course/Year/Section:
BSPHY 3-1
to emit some level of deadly gamma radiation and radiation of much shorter wavelengths whose energy
tends toward infinity relative to frequency, which is obviously not the case; this is known as the ultraviolet
catastrophe. Max Planck formulated a theory that stated energy is not continuous but can be described
as collection individual “quanta”, or discrete amounts of indivisible energy. Meaning light must also come
in the form of these discrete amounts, so relatively low temperatures are incapable of producing many of
those high energy electromagnetic waves if any at all.
1905 (The quantization of light) – As a piece of historical hindsight, it’s very surprising that Albert Einstein
did not win a Nobel Prize for standardizing both special and general relativity as staples in modern physics
but was instead awarded for his work on the Photoelectric effect. The effect was observed and had
created contradictions with classical mechanics that would be explained by Einstein over a decade after
it was initially discovered.
Classical mechanics dictates that all energy is continuous, which led physicists at the time to make the
following assumption: Given a dim light, it would take longer for the electrons to be ejected since more
energy needs to build up before the electron is energized enough to leave its orbital. However, this theory
is not consistent with observations. Electrons have been observed being ejected in very dim light without
any delay in time; conversely, there were instances where harsh, bright lights failed to eject any electrons
at all. The only correlating factor Einstein observed was the frequency of the light, not the intensity.
Einstein built upon Max Planck’s theory of quantized energy and came to the conclusion that light behaved
as a particle hence referred to as “photons”, holding a discrete amount of energy. Each electron only
interacts with a single photon and thus the dots were beginning to connect. The light’s intensity is
correlated with the number of photons, while frequency was what indicated the amount of energy carried
by a single photon. For an electron to be ejected, a single photon of a minimum energy (frequency) must
strike it. It doesn’t matter how many photons strike the metal plate if none of the photons striking it carry
the sufficient energy to knock out an electron out of its orbital. All of this contradicting the notion of
energy being continuous as stated by classical mechanics. This was the birth of the concept of “waveparticle duality” as electromagnetic waves seemed to posses qualities of both.
1913 (Bohr model of the atom) – We know that when a charge particle accelerates, it emits some
electromagnetic radiation, by virtue of the law of conservation of energy, a charge particle would also be
expected to lose some energy upon emitting electromagnetic radiation. Before the Bohr model of the
atom, we assumed that the electron orbited the nucleus not unlike how a planet would revolve around
the star. The electron would therefore by continuously emitting electromagnetic radiation by virtue of its
own centripetal acceleration causing its velocity to be constantly changing. The difference from the
planetary analogy would be that the unlike the planets, the electron would be constantly losing kinetic
energy and would just plummet into the nucleus. Physicists have calculated that if that were how an atom
is actually modeled, a hydrogen atom would have a lifespan of about 10-12 seconds. Obviously, we know
that’s not the case.
Building upon the concept of quantized energy, Niels Bohr formulated a model of the atom where
electrons exist in stable orbits around a fixed distance and energy level from the nucleus. Therefore, the
electron can only move in between each energy level when absorbing or emitting a photon of a specific
frequency, since frequency is directly proportional to the energy carried by the photon. Once again, these
theories were proven consistent with observations and even explained phenomena such as the hydrogen
emission spectra, which showed that the electromagnetic radiation emitted by a hydrogen atom can only
Student No:
Name:
2020-05607-MN-0 Lou Aidan M. De Jesus
Course/Year/Section:
BSPHY 3-1
exist in a few known frequencies. This corroborates with the theory that an electron within an atom can
only absorb and release energy in discrete amounts.
1924, 1927 (Particles with wavelike properties) – The concept of wave-particle duality has existed since
Einstein’s theory of photons in his explanation to the photoelectric effect. We know from Einstein’s work
that electromagnetic radiation, that was once thought of as just a wave also had properties of particles,
hence “wave-particle duality”. Louis de Broglie surmised that it worked in the opposite direction. In 1924,
he wrote a thesis that explained the movement of electrons around the nucleus as having a wavelike
motion. However, this did not come with experimental data that definitively proved that electrons
behaved similarly to radiant energy. 3 years later, Clinton Davisson and Lester Germer definitively proved
the wave nature of electrons by shooting a beam of electrons through a double slit and creating a
diffraction pattern, exactly how one would expect a wave to behave by virtue of constructive and
destructive interference. From this point forward, quantum mechanics have become a staple in the study
of physics. Newtonian mechanics were simply incapable of describing the rules of the universe within the
quantum scale.