Photons are elementary particles that make up electromagnetic radiation and carry the electromagnetic force. They have no mass, no electric charge, and travel at the speed of light. They can behave like both particles and waves, and interact with matter in various ways. The concept of photons is linked to the development of quantum theory, and their energy is related to their frequency. Quantum electrodynamics explains how photons carry the electromagnetic force. Photons always travel at the speed of light and do not experience time.
A photon is a type of elementary particle that forms the basic unit of electromagnetic radiation, which includes radio waves, infrared, visible light, ultraviolet, X-rays and gamma rays. Photons have no mass, no electric charge, and travel at the speed of light. Unlike some particles, such as protons and neutrons, they are not thought to be made up of smaller components. They belong to a class of particles that are responsible for the fundamental forces of nature and carry the electromagnetic force. According to the theory of quantum electrodynamics, the way electrically charged particles behave towards each other can be described in terms of photons.
Experiments conducted in the 19th century seemed to demonstrate that light is made up of waves. In the early 20th century, however, other experiments indicated that it was made up of particles. Although it seems contradictory, light and other forms of electromagnetic radiation actually behave like both forms. Photons are particles of light, but they also have wave properties, such as wavelength and frequency.
Photons and matter
Matter can interact with light particles in several ways. An electron in an atom, for example, can absorb a photon, causing it to jump to a higher energy level. Over time, the electron can drop back to a lower energy level, emitting the extra energy as a photon. The eye is able to detect light because certain molecules in the retina absorb energy from photons within the frequency range of visible light. This energy is converted into electrical impulses which travel along the optic nerve in the brain.
In some cases, the electrons can absorb particles of relatively high-energy ultraviolet light, then emit the energy as photons of longer-wavelength visible light, a phenomenon known as fluorescence. Molecules can absorb energy at infrared frequencies, which makes them move more, resulting in an increase in temperature; for this objects can be heated by sunlight or by an electric heater. Very high energy photons, such as X-rays and gamma rays, can have a destructive effect on matter. They have enough energy to remove electrons from atoms, forming positively charged ions, and to break chemical bonds. These effects cause chemical changes that can be very harmful to living organisms.
Discovery
The concept and discovery of the photon are closely linked to the development of quantum theory. Around 1900, theoretical physicist Max Planck found a solution to a problem that had long preoccupied scientists, involving the frequencies of electromagnetic radiation emitted by an object at various temperatures. He proposed that energy comes in small indivisible units, which he called quanta. Albert Einstein’s work on the photoelectric effect in 1905 provided strong experimental evidence that quanta were real. It wasn’t until 1926, however, that the term “photon” was first used – by chemist Gilbert N. Lewis – to describe quanta of light.
Energy and Frequency
Planck showed how the energy of a light quantum is related to its frequency. He defined a constant, known as Planck’s constant, which, multiplied by the frequency of a light quantum, gives its energy. High-frequency photons, such as those in X-rays, therefore have more energy than low-frequency ones, such as radio waves. Planck’s constant is extremely small; however, most light sources produce huge numbers of these particles, so the total energy can be considerable.
Quantum electrodynamics
With the development of quantum theory, it became apparent that the forces of nature must somehow be carried by agents that cannot travel faster than light, and that these agents must be “quantized”: they can only exist as multiples of indivisible units. The relationship between light, electricity and magnetism had already been clarified in the 19th century. At the time, however, light and other forms of electromagnetic radiation were assumed to consist of waves. Following the discovery of photons, a new theory called quantum electrodynamics was developed, which explained how photons carry the electromagnetic force.
The speed of light
Photons always travel at the speed of light in a vacuum, which is approximately 186,000 miles (300,000 kilometers) per second. According to Einstein’s Special Theory of Relativity, it is not possible for any material object to reach this speed, since mass increases with speed, so more and more energy is required to increase speed. Photons travel at the speed of light because they have no mass.
Light can slow down, for example when it passes through glass, but individual particles of light are not slowed down. They are absorbed by the atoms, which temporarily gain energy, quickly releasing it again as another photon with the same frequency. This happens many times when light passes through glass (or some other substance) and the slight delay between the absorption and release of energy means that the particles take longer to pass than they would take to pass through the air or the void. Each photon, however, always travels at the speed of light.
Special relativity shows that traveling near the speed of light has some strange consequences. For example, time slows down relative to objects that aren’t moving, an effect known as time dilation. If an astronaut accelerates away from Earth to just below the speed of light and then returns a year later, according to his calendar, he may find that ten years have passed on Earth. It’s not possible for an astronaut to reach the speed of light, but many people have speculated what time dilation means for photons. According to special relativity, time must stop completely.
A human looking at the Andromeda Galaxy, which is 2.2 million light years away, sees photons that, from his point of view, traveled 2.2 million light years and took 2.2 million years to do so. It can be said, however, that from the point of view of the photons the journey did not last at all and that the distance traveled is actually zero. Since each particle of light is “born” in a star and exists until it hits the astronomer’s retina, one could also say that from his point of view a photon exists at zero time, and therefore does not exist at all. The consensus among scientists, however, is that it simply doesn’t make sense to think that particles of light have a point of view or “experience” anything.
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