What is Modern Physics?
- kieronconway
- Mar 9
- 6 min read
© 2026 Kieron Conway - All rights reserved.
The Development of Quantum Physics & Relativity
Modern physics developed in response to phenomena that classical physics could not explain. Through necessity, the evolution of modern physics started to explain the unexplainable and in so doing led to a fascinating picture of reality that has endured decades of testing.
Predictions have been made from the theories of quantum physics and Einstein's relativity that have been proved right time-after-time, with remarkable precision.
Reality, defined by modern physics is bizarre and not in the least intuitive, but absolutely fascinating.
Modern physics shows us that the universe is far stranger and far more interesting than anyone could have imagined a hundred years ago.
When did modern physics begin
The development of quantum physics predominantly concerned events at the atomic and sub-atomic scale, objects moving close to the speed of light, and phenomena that appear to behave both as waves and as particles.
Gradual understanding of these puzzling observations led to the development of quantum theory.
At the same time, a new theory of gravity emerged that resolved long-standing problems in classical gravity.
All very mysterious - this was how things were at the beginning of the twentieth century, over 100 years ago - ironically not long after the general belief in the science community that there was little more to explain before everything was understood!
What were the hints that all was not well?
Two major anomalies triggered the development of modern physics at the atomic scale. Classical physics could not explain the photoelectric effect and what became known as the ultra-violet catastrophe.
In the world of the very large, Newton's theory of gravity couldn't explain how changes in gravity were predicted to act instantaneously across enormous distances, something even Newton himself found difficult to accept.
Let's take each these of these trigger points in turn.
What was the ultra-violet catastrophe?
This concerned what happens when a substance is heated up.
To begin with, it gives off infra-red radiation, then it starts to glow red, then orange, then white and finally, it gives off ultra-violet light.
Classical physics claimed that as things got really hot, they should just keep on producing unlimited amounts of harmful ultra-violet light.
There were experiments that showed that this dangerous, runaway effect did not occur. The amount of radiation produced eventually falls off at the higher energy frequencies and classical physics could not explain why.
What was the photoelectric effect?
By 1900, it was clear that light was a wave. There was an experiment that showed that light shone onto a barrier with two narrow slits created interference patterns as light passed through and beyond the slits. Interference was a clear indication that light was a wave.
Just before 1900, James Clerk Maxwell produced the mathematics describing electromagnetic waves, of which light is just one set of frequencies.
At the start of the 20th century, light was definitely considered to be a wave.
Then it was discovered that certain frequencies of light could knock electrons out of atoms in some metals in the photoelectric effect. To do this, the frequency was critical. Classical physics was clear: only the intensity of light waves should do this, not the frequency. However, it was found that intensity only increases the number of electrons produced, it's the energy or frequency of the light that is critical. Light appeared to be behaving like little particles!
This led to the mind-blowing concept of the wave/particle duality of light, which was then extended to particles.
What was wrong with Newtonian Gravity?
Classical physics produced equations that indicated how gravity worked and did this very successfully, predicting the motions of planets and comets.
However, the theory implied that gravity acted instantaneously leading to a prediction that if the sun suddenly moved position, the earth would feel the effect immediately.
Even Newton was concerned that this implied a mysterious mechanism acting across empty space.
What started the development of quantum mechanics?
Max Planck cracked the mystery of the ultra-violet catastrophe by introducing the idea that energy is quantised - meaning light is emitted in tiny, indivisible, multiple energy-packets rather than in continuous frequencies.
Each frequency emitted by a body as it heats up, has its own quantum of energy. This radical idea fully explained how radiation was released by heated objects and the ultra-violet catastrophe was no more!
Planck's discovery led to the birth of quantum mechanics, one of the most mind-blowing fields in physics today! Quantum mechanics explained how light is produced by a substance when it is heated, classical mechanics failed to get it right!
The world of the atom began to open up through quantum mechanics.
The universe follows some pretty wild rules down at the atomic scale and below. Nevertheless, although great strides were made using quantum mechanics, interactions involving the transfer of energy from light to particles, was still not well understood.
How did quantum field theory evolve?
The electromagnetic field was well understood in the early part of the 20th century, light consisted of excitations in this electromagnetic field, but quantum mechanics failed to explain how energy was transferred from light to particles and phenomena associated with speeds close to that of light.
So, quantum field theory was developed to combine quantum mechanics and relativity, where all particles are considered to be excitations in their own fields, just like photons in the electromagnetic field.
Particle interactions occur when energy is exchanged between the relevant fields.
This is currently our best theory that explains the standard model of particle physics.
Uncertainty rules the quantum world
Classical physics gave us a world of predictability and certainty. If you knew the initial state of a system and the laws of physics that resulted in its changes over time, you could predict how the system would evolve.
Quantum physics showed us a world where instead of a single outcome to a problem, many are predicted, each with a measure of its probability of occurrence.
The most probable outcomes happen frequently, but even the least probable can occur.
What were the new theories of relativity?
Albert Einstein produced two theories that revolutionised our understanding of space, time, mass, energy and gravity.
First came special relativity, that explained how particles behave at close to light speed as space and time become interconnected. At close to light speed, time slows down and distances shorten – all revolutionary stuff.
Then came general relativity, which provided an alternative to Newtonian gravity. This alternative theory solved the issue of the force being instantaneous. In general relativity, gravity is described as the curvature of space-time caused by mass and energy.
Changes in gravity are not instantaneous, they propagate at the speed of light.
Summing up Modern Physics
Through necessity, the evolution of modern physics started to explain the unexplainable and in so doing led to a fascinating picture of reality that has endured decades of testing.
Reality, defined by modern physics is bizarre and not in the least intuitive, but interesting beyond measure.
Gone are the predicable outcomes of classical physics and in came a world of uncertainty and probability. The evolution of a system may involve a multitude of possible solutions, each governed by a measure of its likelihood.
The most probable solutions occur most often, but even the least probable can occur from time to time.
Modern physics shows us that the universe is far stranger and far more fascinating than anyone could have imagined a hundred years ago.
Why does Modern Physics Matter?
Modern physics is not just about theories and experiments to prove the theories, it underpins many of the technologies of today, including;
lasers
semiconductors
TVs
computers
mobile phones
GPS tracking
Medical imaging
Nuclear energy
Artificial Inteligence
Robotics
and so much more!
Without quantum mechanics and relativity, much of our modern technology would not work.
Further Reading
If you enjoyed this brief introduction to modern physics, you will enjoy the Journey into Modern Physics series of three books, by Kieron Conway, which explains complex ideas in clear, simple language.
These are not text books, but create an exciting journey, ideal for young people contemplating further education in science, technology, engineering or mathematics, or anyone, of any age, who is simply fascinated by the workings of the universe.
There are two series to choose from. The main series is suitable for anyone and the second LIGHT-series is more suitable for young minds,
Find out more about the two series from this blog's web-site;
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