Photon Discovery Timeline

The photon, often referred to as the quantum of electromagnetic radiation, holds a pivotal position in the realm of physics due to its unique characteristics and fundamental role in understanding the nature of light.

In physics, a photon is defined as the smallest discrete quantity of electromagnetic radiation, possessing both wave-like and particle-like properties. Its significance lies in its role as the carrier of electromagnetic force and its involvement in various phenomena, from the photoelectric effect to the transmission of light.

Albert Einstein's 1905 paper on the photoelectric effect marked a significant milestone in the study of photons. The photoelectric effect, observed when light strikes a material surface, involves the ejection of electrons. Einstein proposed that light consists of discrete packets of energy, later termed photons, which interact with matter as individual particles. This concept challenged the prevailing notion of light as a continuous wave and laid the groundwork for quantum mechanics.

Einstein further elaborated on the concept of energy quantization in electromagnetic radiation, contrasting it with Maxwell's theory of classical electromagnetism. While Maxwell's theory described light as a continuous wave, Einstein suggested that light energy could be localized into distinct, quantized units—photons. This localization of energy into point-like quanta provided a novel perspective on the behavior of light and its interactions with matter.

Building upon Einstein's initial insights, further developments in photon theory emerged. Einstein's work demonstrated the relationship between photons and Planck's law of black-body radiation, revealing the quantized nature of energy emission and absorption. Additionally, Einstein proposed that photons possess momentum, contributing to their characterization as full-fledged particles with both energy and momentum.

Experimental validation of photon properties played a crucial role in solidifying the concept of photons. Robert Millikan's studies of the photoelectric effect from 1914 to 1916 provided empirical evidence supporting Einstein's theories, confirming the discrete nature of light energy. Arthur Holly Compton's experiments in 1923 demonstrated the phenomenon of photon scattering, providing direct proof of photon momentum and further bolstering the particle-like behavior of photons.

The recognition of Einstein's contributions to physics culminated in the awarding of the Nobel Prize in Physics in 1921. While Einstein was most renowned for his theory of relativity, his discovery of photons was specifically acknowledged by the Swedish Academy, highlighting the significance of this breakthrough in the scientific community. Similarly, Arthur Holly Compton's Nobel Prize in 1927 underscored the experimental validation of photon momentum, affirming the importance of his work in advancing our understanding of light.

In conclusion, the discovery of photons revolutionized our understanding of light and its interactions with matter. From Einstein's theoretical insights to experimental confirmation by scientists like Compton, the study of photons has played a pivotal role in shaping modern physics. The recognition of these contributions underscores the enduring impact of photon theory on scientific progress and innovation.
Photon Discovery Timeline

Gravitational theory by Einstein

Gravitation is commonly understood as the force of attraction between objects by virtue of their mass. In 1687, Newton published The Philosophiae Naturalist Principia Mathematica in which he prop0soed his law of gravitation.

Richer Cassini and Piccard had found evidence in 1672 that the earth had an equatorial bulge. Newton was able to use his new gravitational theory to calculate a theoretical value for this oblateness of 1/230. He then considered the gravitation attraction of the moon and sun on the oblate earth and calculated that the earth’s spin axis should process at baa rate do about 50’’.0 per annum.

According to this law the gravitational force of attraction between two bodies is always proportional to their masses and inversely proportional to the square of their distance apart, and it acts instantaneously through infinite distance.

A more widely applicable theory of gravitation is Albert Einstein’s general theory of relativity. According to Einstein’s general theory of relativity published in 1913, gravitation is not a forced of attraction but rather the force required to prevent the natural motion of matter, which is to follow a geodesic in space time.

Einstein introduced into gravitational theory a type of mathematics that was then unfamiliar to most physicists thus presenting an initial impression of incomprehension. In his theory Einstein used the space-time continuum introduced by Hermann Minskowski in 1907, the non-Eucladian geometry developed by Bernhard Riemann in 1854 and the tensor calculus published by Gregorio Ricci in 1887.

The first indirect experimental proof of gravitational waves was provided in 1984 by Weisberg and Taylor. By studying the pulsar 1913+16, they showed that the period of the pulsar around its companion star decreased exactly as predicted by the Einstein equation.

In 1969, Joseph Weber, a physicist at the University of Maryland, claimed to have detected gravitational waves using a six-foot-long aluminum cylinder as an antenna. In Feb 2016, a team of scientists at the Laser Interferometer Gravitational Wave Observatory (LIGO) announced they had detected agravitational waves resulting from the collision of two black holes some 1.3 billion years ago.

Scientists say that some 1.4 billion years ago two black holes - one the size of 36 suns and the other the size of 29- circled each other in a distant galaxy before finally colliding.

The collision of the two black holes—about 29 and 36 times more massive than the sun, respectively—produced a gigantic amount of energy in a fraction of a second, the equivalent of about 50 times the power of the entire visible universe. This energy, in the form of gravitational waves, is still spreading outwards today.
Gravitational theory by Einstein

Black Hole

Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a black hole. On the other hand, a black hole exerts the same force on something far away from it as any other object of the same mass would. For example, if our Sun was magically crushed until it was about 1 mile in size, it would become a black hole, but the Earth would remain in its same orbit.

John Michell, a British geologist and astronomer, designed the experiment made by Henry Cavendish to measure the mass of the earth. Cavendish published the results of the experiment in 1798.

In 1783 Michell published his work, that showed that a star, that has the same density of the sun, but 500 time as big, would have such a gravity, that "All light emitted from such a body would be made to return towards it". He said we wouldn't be able to see such a body, but we sure will feel its gravitational pull.

Pierre-Simon Laplace, got to the same conclusion in 1795, and explained it by saying that "It is therefore possible that the greatest luminous bodies in the universe are on this very account invisible". Michell took to account a body that has the density of the sun, which equals to the density of water, while Laplace took to account a body that has the density of the earth, which is 5.5 more dense that water. To such bodies, there was invented in 1967 the name "Black-Holes"- a black hole in Space-Time. But it seems that over the years Laplace thought of this as a crazy idea, and he didn't work on this subject any more (over the 19th century more and more people believed the wave theory, and not the particle theory).

The people who believed light was composed of only small particles, compared it to a cannon shell, and said that if a cannon shell was pulled after some time to the earth, so would the light. But this comparison isn't completely true, because a cannon shell was also slowed down, while the light's speed is stable.

The first really main theory that dealt with gravity's effect on light was Einstein's General Theory of Relativity in 1905. Even then, it took time until it was used to see the effect of big stars on light.

The Indian research-student, Subrahmanyan Chandrasekhar, based his calculations on the life cycle of a star, while sailing to study in Cambridge with Arthur Eddington, an expert for the General Theory of Relativity, as his professor. He tried to calculate how massive a star can be, and still be in a stable condition, in spite of its gravitational pull, after the star has cooled down.
Black Hole

Discovery of Photon

The photon is known as the quantum of electromagnetic radiation. In physics, a quantum is a basic indivisible unit or state that may be present or absent but never stronger or weaker.

In 1905, Albert Einstein published a paper describing his discovery of the photoelectric effect where a photon acts like a particle.

Einstein proposed that for some purposes light can be regarded as made up of photon particles.

In 1905, Einstein was the first to propose that energy quantization was a property of electromagnetic radiation itself. Although he accepted the validity of Maxwell's theory, Einstein pointed out that many anomalous experiments could be explained if the energy of a Maxwellian light wave were localized into point-like quanta that move independently of one another, even if the wave itself is spread continuously over space.

In 1909 and 1916, Einstein showed that, if Planck's law of black-body radiation is accepted, the energy quanta must also carry momentum , making them full-fledged particles.

The 1921 physics Nobel prize was awarded to Einstein in most famous for his theory of relativity, but it is his discovery of photons that is mentioned by the Swedish Academy.

This photon momentum was observed experimentally by Arthur Holly Compton, for which he received the Nobel Prize in 1927.

He verified experimentally by Milikan’s studies of the photoelectric effect in 1914 – 16.
Discovery of Photon