View Full Version : How Does Light Travel?

2016-May-19, 02:10 AM
Ever since Democritus - a Greek philosopher who lived between the 5th and 4th century's BCE - argued that all of existence was made up of tiny indivisible atoms, scientists have been speculating as to the true nature of light. Whereas most scientists until the modern era believed that light some sort of wave that required a medium to propagate, the 20th century has led to breakthroughs in our understanding of how light works.
These included the discovery of the electron, the development of quantum theory, and Einstein's Theory of Relativity (http://www.universetoday.com/45484/einsteins-theory-of-relativity/). However, there remains many fascinating and unanswered questions when it comes to light, many of which arise from its dual nature. For instance, how is it that light can be apparently without mass, but still behave as a particle. And how can it behave like a wave and pass through a vacuum, when all other waves require a medium to propagate?
Theory of Light to the 19th Century:During the Scientific Revolution, scientists began moving away from Aristotelian theories in the sciences and began to adopt a mechanistic view of the Universe. This included rejecting Aristotle's theory of light, which viewed it as being a outdistance in the air (one of his four "elements" that composed matter), and embracing the idea of light being made up of atoms.
In many ways, this theory had been previewed by atomists such as Democritus and Lucretius, both of whom argued that light must be a particle given off by the sun. By the 17th century, In the seventeenth century, several scientists emerged who accepted this view, which states that light was made up of discrete particles (or "corpuscles"). This included Pierre Gassendi, a contemporary of Descartes, Thomas Hobbes, Robert Boyle, and most famously, Sir Isaac Newton.
Newton's corpuscular theory was an elaboration of his view of reality as interactions of material points through forces. This theory would remain the accepted scientific view for more than 100 years, the principles of which were explained in his 1704 treatise "Opticks, or, a Treatise of the Reflections, Refractions, Inflections, and Colours of Light (http://www.gutenberg.org/ebooks/33504)". As Newton stated:

Every source of light emits large numbers of tiny particles known as corpuscles in a medium surrounding the source.
These corpuscles are perfectly elastic, rigid, and weightless.

Double-Slit Experiment:By the early 19th century, scientists began to break with corpuscular theory. This was due in part to the fact that corpuscular theory failed to adequately explain the diffraction (https://en.wikipedia.org/wiki/Diffraction), interference (https://en.wikipedia.org/wiki/Interference_%28wave_propagation%29) and polarization (https://en.wikipedia.org/wiki/Polarization_%28waves%29) of light, but was also because of various experiments that seemed to confirm competing theories that claimed light behaved as a wave.
The most famous of these was arguably the Double-Slit Experiment (http://www.universetoday.com/83380/double-slit-experiment/), which was originally conducted by English polymath Thomas Young in 1801 (though Sir Isaac Newton is believed to have conducted something similar in his own time). In Young's version of the experiment, he used a slip of paper with slits cut into it.
According on classical (i.e. Newtonian) particle theory, the results of the experiment should have corresponded to the slits, the impacts on the screen appearing in two vertical lines. However, this was not the case. The results showed in many circumstances a pattern of interference, something which could only occur if wave patterns had been involved.
This contradicted classical particle theory, in which particles do not interfere with each other, but merely collide. As such, the experiment should have resulted in two discreet lines on the screen that corresponded to the slits. However, the experiment demonstrated that the coherent beams of light were interfering, creating a pattern of bright and dark bands on the screen.
This experiment dispelled the notion that light consisted of corpuscles and played a vital part in the acceptance of the wave theory of light. However subsequent research, including updated version of the Double-Slit Experiment, would show that it also behaved as a particle, giving rise to wave-particle duality theory.
Einstein and the Photon:In 1905, Einstein helped to resolve a great deal of confusion surrounding the behavior of electrons when he proposed that they are emitted from atoms when they absorb energy from light. Known as the photoelectric effect (http://physics.info/photoelectric/), Einstein based his idea on Planck's earlier work with "black bodies" - materials that absorb electromagnetic energy instead of reflecting it (i.e. what a white body does).
At the time, Einstein's photoelectric effect was attempt to explain the "black body problem", in which a black body emits electromagnetic radiation due to the object's heat. This was a persistent problem in the world of physics, which was still struggling to address the discover the electron, which had only been discovered eight years previous.
At the time, scientists still believed that electromagnetic energy behaved as a wave, and were therefore hoping to be able to explain it in terms of classical physics. Einstein's explanation represented a break with this, asserting that electromagnetic radiation behaved in ways that were consistent with a particle - a quantized form of light which he named "photons". For this discovery, Einstein was awarded the Nobel Prize in 1921.
Wave-Particle Duality:Subsequent theories on the behavior of light would further refine this idea, which included French physicist Louis-Victor de Broglie calculating the wavelength at which light functioned. This was followed by Heisenberg's "uncertainty principle" (which stated that measuring position accurately would disturb momentum and vice versa), and Schrodinger's paradox that claimed that all particles have a "wave function".
In accordance with quantum mechanical explanation, all the information about a particle (in this case, a photon) is encoded in its wave function, a complex-valued function roughly analogous to the amplitude of a wave at each point in space. At some location, the measurement of the wave function will randomly "collapse", or rather "decohere", to a sharply peaked function.
This function evolves according to a differential equation (aka. the Schrödinger equation (http://scienceworld.wolfram.com/physics/SchroedingerEquation.html)). For particles with mass, this equation has solutions, but particles without mass have none. Further experiments involving the Double-Slit Experiment confirmed the dual nature of photons. where measuring devices were incorporated to observe the photons as they passed through the slits.
When this was done, the photons appeared in the form of particles and their impacts on the screen corresponded to the slits - tiny particle-sized spots distributed in straight vertical lines. By placing an observation device in place, the wave function of the photons collapsed and the light behaved as classical particles once more.As predicted by Schrödinger, this could only be resolved by claiming that light has a wave function, and that observing it causes the range of behavioral possibilities to collapse to the point where its behavior becomes predictable.
The development of Quantum Field Theory (QFT) was devised in the following decades to resolve much of the ambiguity around wave-particle duality. And in time, this theory was shown to apply to other particles and fundamental forces of interaction (such as weak and strong nuclear forces). Today, photons are part of the Standard Model of particle physics, where they are classified as boson - a class of subatomic particles that are force carriers and have no mass.
What we have learned about light and electromagnetism has been intrinsic to the revolution which took place in physics in the arly 20th cenutry, a revolution we have been grappling with ever since. Thanks to the efforts of scientists like Maxwell, Planck, Einstein, Heisenberg and Schrodinger, we now know that light behaves both as a wave (electromagnetic radiation) and a particle (a photon).
So what have we learned about how light travel? Basically, we have learned that it behaves as both a wave and a particle, traveling at incredible speeds (299*792*458 m/s) and at different wavelengths, depending on its energy. It has no mass, but can still be absorbed, reflected, or refracted if it comes in contact with a medium. And in the end, the only thing that can truly slow down or stop or slow down the speed of light is gravity (i.e. a black hole).
Despite knowing all this, there are still many unanswered questions when it comes to light and how it behaves. For instance, its interaction with gravity (along with weak and strong nuclear forces) remains a mystery. Unlocking this, and thus discovering a Theory of Everything (ToE) is something astronomers and physicists are busy working towards. Someday, we just might have it all figured out!
We have written many articles about light here at Universe Today. For example, here's How Fast is the Speed of Light? (http://www.universetoday.com/43806/how-fast-is-the-speed-of-light/), How Far is a Light Year? (http://www.universetoday.com/65644/how-far-is-a-lightyear-in-miles/), What is Einstein's Theory of Relativity? (http://www.universetoday.com/45484/einsteins-theory-of-relativity/)
If you'd like more info on light, check out these articles from The Physics Hypertextbook (http://physics.info/light/) and NASA's Mission Science (http://missionscience.nasa.gov/ems/09_visiblelight.html) page.
We've also recorded an entire episode of Astronomy Cast all about Interstellar Travel. Listen here, Episode 145: Interstellar Travel (http://www.astronomycast.com/2009/07/ep-145-interstellar-travel/).
The post How Does Light Travel? (http://www.universetoday.com/83832/how-does-light-travel/) appeared first on Universe Today (http://www.universetoday.com).

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2016-May-19, 02:34 AM
Answer: ...... just fine. :)

2016-May-20, 12:37 PM
I've read the article a couple times. Where does it answer the question?