Modern Physics

Modern physics started to develop at the beginning of the 20th century. In the late 19th century, many physicists thought that almost everything was known and that all that was left to do was to calculate the physical constants to more decimal places. This is illustrated by the famous quote by Albert Michelson at around this time:

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    While it is never safe to affirm that the future of Physical Science has no marvels in store even more astonishing than those of the past, it seems probable that most of the grand underlying principles have been firmly established … the future truths of physical science are to be looked for in the sixth place of decimals.

    Little did he know that brand new branches of physics would be established in the coming years that would drastically change the way we look at the world.

    Concepts of modern physics

    Modern physics attempts to understand the interaction of matter on the most extreme scales, where it has been found that the world behaves very differently from what had been predicted by classical physics and from what is witnessed in everyday life.

    ‘Modern’ is not used in this case in the normal sense of the word because many current topics in classical physics would then also be considered modern physics. ‘Modern’ actually refers to physics that includes parts of quantum mechanics or relativity, which are theories that came into fruition at the start of the 20th century. They were used to explain experimental results that classical physics could not explain. These theories led to many counterintuitive and seemingly ridiculous predictions, which, however, were slowly proven again and again as technology advanced. They completely revolutionized science and made Michelson’s statement look absurd.

    Examples of modern physics

    Notable fields of modern physics include special relativity, general relativity, and quantum mechanics. These are the main three theories that sparked so much scientific work in many different areas throughout the 20th century.

    Special relativity

    Special relativity is a theory credited mainly to Albert Einstein, who published his ideas in 1905, although there was also lots of great work done on the subject by physicist Hendrik Lorentz and by Henri Poincaré shortly before him.

    Special relativity deals with situations when objects move at extremely high velocities that are not experienced in normal life. When these velocities reach close to the speed of light, the equations of classical mechanics become invalid, and new ones have to be used. At high velocities, phenomena called time dilation and length contraction become significant. As the speed of an object increases, the time taken for it to move a certain distance will increase for an outside observer. On the other hand, an object moving close to the speed of light will become shorter in the direction of motion – its length will be contracted. .Modern Physics A diagram of a rocket launching into space StudySmarterFigure 1: An image showing how the length of an object will decrease from the perspective of an external observer as its speed approaches the speed of light.

    An example that illustrates time dilation is found in cosmic rays, which consist of high-energy particles traveling through outer space. When they travel through the atmosphere, they can cause particles called muons to be created, which are sometimes sent down towards the Earth.

    A muon has a short lifetime, so it should not be able to reach the Earths surface and be detected. However, these particles travel at speeds close to that of light and thus experience relativistic effects. Their lifetime appears to be much longer from our perspective, and they can reach our detectors on Earth.

    General relativity

    General relativity was an extension of Einstein’s special theory that he presented to the physics community in 1915. Special relativity applies to objects moving at constant velocity. After he finished this theory, Einstein worked furiously to extend it to incidents with acceleration, which is why it is called ‘general’ as it applies to all situations. This required a much more advanced and in-depth theory.

    At the time, there was a well-known problem with the orbit of the planet Mercury around the sun. It had been observed that the perihelion (the point of the orbit closest to the sun) shifted each orbit, and this shift was not correctly predicted by the use of Newton’s law of gravitation. Some suggestions were made as to why there was a deviation from the law, such as another planet even closer to the sun, but none of them solved the issue.

    To the amazement of physicists around the world, Einstein found the correct results by showing that Newton’s law was just an approximation and that general relativity effects from the sun needed to be taken into account to get the actual perihelion shift.

    Modern Physics Diagram of the Mercury perihelion shift StudySmarterFigure 2: A diagram showing how the perihelion of Mercury shifts by a small amount each century.

    Einstein worked on general relativity largely on his own, and he was very much ahead of his time. The theory was not used much for scientific work until it came into the mainstream of physics at the start of the 1960s, when it was used to arrive at some of the concepts that are still active topics of debate in the physics community today, such as black holes and singularities. These ideas were thought to be crazy at the time, but they simply came from following the ideas and mathematics of general relativity through to their logical conclusions.

    Quantum mechanics

    Quantum physics is the study of matter at the smallest of scales, the atomic and sub-atomic levels. Quantum mechanics is the mathematics describing the movement and interaction of particles at this scale. The theory was extremely revolutionary when it was first introduced because it seemed to go completely against ideas of classical physics and, at times, even against logic.

    As it became more developed, it was found that the theory led to some very counterintuitive phenomena, such as tunneling (where particles can jump through a barrier) and the idea of particles being in two places at once. These seemed to completely contradict everything we see in normal life, but they simply come from the mathematics of quantum mechanics and were later proven.

    Max Planck is known as the father of quantum mechanics. He did lots of great work in thermodynamics around the turn of the 20th century. At that time, there was a problem in thermodynamics called the ‘ultraviolet catastrophe,’ which described how classical physics predicted black bodies to emit an infinite amount of radiation at shorter wavelengths (towards the ultraviolet range), which did not happen.

    Planck realized that if energy only came in discrete quantities, in units of a certain constant h (which is now called the Planck constant), he could solve the problems and his equations worked. He was very reluctant to take his hypothesis further and did not truly believe that energy was quantized, just that it was a mathematical trick that worked in this instance.

    It was Einstein again who took Planck’s ideas seriously. He explained the photoelectric effect (the emission of electrons from a metal when light is shone on it) by using Planck’s constant and suggesting that energy was discrete. Although his ideas were met with skepticism at first, more physicists came to realize that quantum mechanics described the true nature of matter, and huge amounts of work were done on it in the first half of the century. In the end, it would completely revolutionize physics.

    Types of modern physics

    Modern physics can be categorized into two main types:

    • Theoretical physics: Theoretical physicists spend their time analyzing experimental data and observations and formulating consistent mathematical models which to explain the data and observations. The aim of theoretical physics is to provide comprehensive frameworks which we can use to understand the world around us.
    • Experimental physics: Experimental physicists spend their time designing and performing experiments and collecting and analyzing the results. Experimental physics is an essential component of the scientific method because science relies on observation and experimental data in order to draw conclusions about the world.

    Applications of modern physics

    Applications of modern physics can be seen in lots of different areas in scientific research and also in daily life. One example of an application of relativity and one of quantum mechanics are explained below.

    Black holes

    The effects of general relativity are only noticeable on the largest of scales when masses are enormous and gravitational fields have incredible strength. One of these situations is when two black holes collide. Much of what we know about the universe is through studying the electromagnetic waves that reach the Earth. However, theoretically, general relativity suggests another way of studying the universe: gravitational waves.

    Gravitational waves are ripples that travel through space-time, the fabric that pervades all of space. They are normally extremely small and impossible to detect on Earth, but when there is a cataclysmic event in the universe, such as two black holes colliding, the gravitational waves may be sent toward us, and extremely sensitive equipment can be used to detect them.

    There is a large-scale experiment called LIGO (laser interferometer gravitational-wave observatory) that detects gravitational waves through the use of an interferometer. This is a special piece of equipment that can detect distortions of space-time. The masses of the black holes and the location of the collision can be found from the experimental readings.

    Modern Physics Photograph of the LIGO interferometer StudySmarterFigure 3: The LIGO interferometer used to measure gravitational waves. The arms are 4 km in length. Image credit: LIGO Laboratory


    A superconductor is a material that has no resistance at low temperatures, meaning that an extremely high current can run through it. The phenomenon of superconductivity can be understood through quantum mechanics. The charge carriers pair together in what are called Cooper pairs.

    These pairs formed are bosons, which can all fall into the same quantum state. This means that they all travel around the material as a coherent wave and pass through irregularities without construction. Superconductors have many uses. For instance, they can create very large magnetic fields, which are needed in applications such as particle accelerators and MRI (magnetic resonance imaging) scans.

    Modern Physics Photograph of the Fermilab main ring main injector StudySmarterFigure 4: The Fermilab main ring and main injector seen from above.

    Importance of modern physics

    Modern physics helps us understand the true nature of the universe. It allows us to probe right to the extremes: from the edges of black holes down to the inner workings of nuclei. Modern physics has led to many technological advances on top of the examples mentioned above. But even these statements don't fully point out the importance of modern physics.

    In order to see the importance of modern physics, we should also remember to mention that quantum mechanics is the basis for many electrical components such as diodes and transistors. On the other hand, relativity needs to be taken into account for satellite-based measurements as the satellites are moving with respect to Earth. In addition, the different branches of modern physics often contain the underlying principles for other subjects. For instance, the chemistry of atoms can be understood through the mathematics behind quantum mechanics.

    Modern Physics - Key takeaways

    • The main theories behind modern physics were introduced at the start of the 20th century.
    • Modern physics refers to branches of physics that include quantum mechanics or relativity.
    • Modern physics deals with the extremes of nature.
    • Quantum mechanics is the study of matter on the smallest of scales.
    • Special relativity must be taken into account when the velocity is very high, and general relativity is significant when the masses are very great and the gravitational fields strong.
    • Quantum mechanics and the theories of relativity completely revolutionized physics and the way that physicists looked at the world.
    • Einstein is credited with both the special and the general theories of relativity.
    • Planck was the father of quantum mechanics, but Einstein was the physicist who took his ideas forward and sparked vast amounts of work on the theory.
    • Gravitational waves were predicted by the general theory of relativity and are frequently detected nowadays.
    • Quantum mechanics has had many useful applications since its birth. Notable examples are superconductors and components in electronic circuits, such as diodes and thermistors.


    1. Figure 1: Length-contraction ( by MikeRun ( is licensed by CC BY-SA 4.0 (
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    Frequently Asked Questions about Modern Physics

    What is modern physics?

    Modern physics attempts to understand the interaction of matter on the most extreme scales, where it has been found that the world behaves very differently to what had been predicted by classical physics.

    What is the difference between classical physics and modern physics?

    Classical physics applies to everyday conditions whereas modern physics is used for extreme physical situations.

    Who is the father of modern physics?

    Albert Einstein is the father of modern physics.

    What is modern physics used for?

    Modern physics has many useful applications. Quantum mechanics is behind the functioning of superconductors. Special relativity needs to be taken into account for satellite-based measurements.

    What are the two pillars of modern physics?

    The two pillars of modern physics are quantum mechanics and relativity.

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