Question:
What propels light particles travelling through space endlessly with the same speed?
Shahid
2006-10-16 07:17:41 UTC
I thought light particles contain limitless energy, and if is brought to a complete halt, they would explode, which obviously is not very correct, as nothing can contain limitless energy in the universe. Besides, light particles, or waves whatever the version of interpretation it is, travelling horrendously fast get absorbed by material things in their way soon as they come to touch with them. This happens way too calmly and smoothly causing just a little rise in temperature. Where does then all that energy go?
Ten answers:
Jens F
2006-10-16 07:40:32 UTC
Even though photons is very much a relativistic particle, one can use a Newtonian argument to argue for the first.



As long as a particle moves along at constant speed, no force is acting on it. A photon doesn't need a "power source" to propel it through the universe.



And as for the second question, then a single photon doesn't have all that much energy. The energy is given by Einsteins E=h*f, where h is Planck's-constant (= 6.6*10^-37 J*s) and f is the frequency. For a typical photon coming from the sun, this means that it has the energy: 3,6*10^-22 J. It is only because of the huge number of photons hitting us, that we can feel the warmth of the sun on a bright summer day.



As for the absorption of photons in intergalactic space, then yes, it is true that photons will get absorbed when it hits something. Something to think about when you hear that astronomers have found a new star billions of light-years away - that means that the photons from that object have traveled across most of the known universe, only to be stopped by our telescopes - that is fascinating.
The Cheminator
2006-10-16 07:35:29 UTC
Light doesn't have limitless energy. Here are some energy values for light.

Energy (J)

Radio > 2 x 10-24

Microwave 2 x 10-24- 2 x 10-22

Infrared 2 x 10-22 - 3 x 10-19

Optical 3 x 10-19 - 5 x 10-19

UV 5 x 10-19 - 2 x 10-17

X-ray 2 x 10-17 - 2 x 10-14

Gamma-ray > 2 x 10-14



Also...light can either be reflected or absorbed....

When absorbed only a small part if the total energy is absorbed..ie microwaves to make things hot...while colors are reflected to be able to see the object
Stuart T
2006-10-16 07:23:42 UTC
Most of the light energy is reflected, but visible light does not contain all the much energy. Things which are 'black' to certain wavelengths of light absorb the maximum amount of energy, but the light will only cause a slight increase in temperature as the heat is spread via conduction through the object it hits.



Light particles do not contain limitless energy, but travelling through space has zero resistance and anything will travel forever unless it is resisted in some way.



So nothing propels light particles, they simply travel as an an electromagnetic wave.
2006-10-16 12:54:45 UTC
None of the above answers is really correct. By calling light a "wave" or "particle" only shows how little we really understand the phenomena. It is a bit like saying now we have the word "god" we totally understand what that would be. A name can never be an explanation can it?



All we can do is try to create rules that predict events but that unfortunately is a world away from any true understanding of what's really happening. We have theory and guesses producing models of what we think is going on.



I agree light is not propelled as that assumes a mechanism powering it. It's speed stays constant due to our rulers and clocks being distorted. As you say high speed should equal great damage to the target but as the mass is sooooo tiny the impact is tiny also (an electrons mass is just under 1/2000th of that of a proton. However it can be measured look up some of JJ Thompson's experiments on this.
?
2006-10-16 07:54:26 UTC
Actually there is no such thing as light particles. Light is just energy which travels in waves. Energy affects particles which get excited, causing light, sound etc... Energy can travel through space, but it is useless without particles. In other words, an astronaut (theoretically) can only hear in space because of the air molecules in his spacesuit. Same goes for light. He will only be able to see the light because it passes through his mini atmosphere. My answer, though is that "all that energy" is what keeps the earth's temperature above.... wait for it....

somewhere in the region of -255 degrees celsius. If you think winter's cold, think again. We live on a warm planet.
?
2016-05-22 10:15:26 UTC
The only way to convert proton, neutron and electrons (constituent of atomic matter) is to annihilate them with their respective antiparticles. Once the space ship has been converted to light, it will be going at the speed of light (although sending all the photons in only the intended direction may be a bit challenging). But then, what do you do with the light? I am pretty sure the occupant of the ship that was completely disintegrated may have some issues with the process, as it cannot be reverted.
?
2017-01-14 19:56:04 UTC
What Propels Light
iadorelara
2006-10-16 13:31:28 UTC
dear Mr S., it does not matter. what matters is your well being. it seems you lost your focus on the road to well being, or even better, to enlightenment. promise me you will be good to yourself, so that love can find the way to you again. don't close yourself, shahid, you are such a rich person.

if i can give you any power, ask and i will.

Mr. iadorelara, the ultimate liver wishing you all the best.
2006-10-16 07:50:32 UTC
Good God! If I'd wanted to read a book I'd have gone to the library!
2006-10-16 07:35:46 UTC
Light is electromagnetic radiation with a wavelength that is visible to the eye (visible light) or, in a technical or scientific context, electromagnetic radiation of any wavelength. The elementary particle that defines light is the photon. The three basic dimensions of light (i.e., all electromagnetic radiation) are:

Intensity (or amplitude), which is related to the human perception of brightness of the light,

Frequency (or wavelength), perceived by humans as the colour of the light, and

Polarization (or angle of vibration), which is only weakly perceptible by humans under ordinary circumstances.

Due to the wave-particle duality of matter, light simultaneously exhibits properties of both waves and particles. The precise nature of light is one of the key questions of modern physics.



The Speed Of Light:

The speed of light in a vacuum is exactly 299,792,458 meters per second (fixed by definition). Although some people speak of the "velocity of light", the word velocity is usually reserved for vector quantities, which have a direction.



The speed of light has been measured many times, by many physicists. The best early measurement in Europe is by Ole Rømer, a Danish physicist, in 1676. By observing the motions of Jupiter and one of its moons, Io, with a telescope, and noting discrepancies in the apparent period of Io's orbit, Rømer calculated that light takes about 18 minutes to traverse the diameter of Earth's orbit. If he had known the diameter of the orbit in kilometers (which he didn't) he would have deduced a speed of 227,000 kilometres per second (approximately 141,050 miles per second).



The first successful measurement of the speed of light in Europe using an earthbound apparatus was carried out by Hippolyte Fizeau in 1849. Fizeau directed a beam of light at a mirror several thousand metres away, and placed a rotating cog wheel in the path of the beam from the source to the mirror and back again. At a certain rate of rotation, the beam could pass through one gap in the wheel on the way out and the next gap on the way back. Knowing the distance to the mirror, the number of teeth on the wheel, and the rate of rotation, Fizeau measured the speed of light as 313,000 kilometres per second.



Léon Foucault used rotating mirrors to obtain a value of 298,000 km/s (about 185,000 miles/s) in 1862. Albert A. Michelson conducted experiments on the speed of light from 1877 until his death in 1931. He refined Foucault's results in 1926 using improved rotating mirrors to measure the time it took light to make a round trip from Mt. Wilson to Mt. San Antonio in California. The precise measurements yielded a speed of 186,285 mi/s (299,796 km/s [1,079,265,600 km/h]). In daily use, the figures are rounded off to 300,000 km/s and 186,000 miles/



Theories about light

Wave theory

In the 1660s, Robert Hooke published a wave theory of light. Christian Huygens worked out his own wave theory of light in 1678, and published it in his Treatise on light in 1690. He proposed that light was emitted in all directions as a series of waves in a medium called the Luminiferous aether. As waves are not affected by gravity, it was assumed that they slowed down upon entering a denser medium.





The wave theory predicted that light waves could interfere with each other like sound waves (as noted in the 18th century by Thomas Young), and that light could be polarized. Young showed by means of a diffraction experiment that light behaved as waves. He also proposed that different colours were caused by different wavelengths of light, and explained colour vision in terms of three-coloured receptors in the eye.



Another supporter of the wave theory was Leonhard Euler. He argued in Nova theoria lucis et colorum (1746) that diffraction could more easily be explained by a wave theory.



Later, Augustin-Jean Fresnel independently worked out his own wave theory of light, and presented it to the Académie des Sciences in 1817. Simeon Denis Poisson added to Fresnel's mathematical work to produce a convincing argument in favour of the wave theory, helping to overturn Newton's corpuscular theory.



The weakness of the wave theory was that light waves, like sound waves, would need a medium for transmission. A hypothetical substance called the luminiferous aether was proposed, but its existence was cast into strong doubt in the late nineteenth century by the Michelson-Morley experiment.



Newton's corpuscular theory implied that light would travel faster in a denser medium, while the wave theory of Huygens and others implied the opposite. At that time, the speed of light could not be measured accurately enough to decide which theory was correct. The first to make a sufficiently accurate measurement was Léon Foucault, in 1850. His result supported the wave theory, and the classical particle theory was finally abandoned.





Quantum theory

A third anomaly that arose in the late nineteenth century involved a contradiction between the wave theory of light and measurements of the electromagnetic spectrum emitted by thermal radiators, or so-called black bodies. Physicists struggled with this problem, which later became known as the ultraviolet catastrophe, unsuccessfully for many years. In 1900, Max Planck developed a new theory of black body radiation that explained the observed spectrum correctly. Planck's theory was based on the idea that black bodies emit light (and other electromagnetic radiation) only as discrete bundles or packets of energy. These packets were called quanta, and the particle of light was given the name photon, to correspond with other particles being described around this time, such as the electron and proton. A photon has an energy, E, proportional to its frequency, f, by

E = hf = \frac{hc}{\lambda} \,\!



where h is Planck's constant, λ is the wavelength and c is the speed of light. Likewise, the momentum p of a photon is also proportional to its frequency and inversely proportional to its wavelength:

p = { E \over c } = { hf \over c } = { h \over \lambda }.



As it originally stood, this theory did not explain the simultaneous wave- and particle-like natures of light, though Planck would later work on theories that did. In 1918, Planck received the Nobel Prize in Physics for his part in the founding of quantum theory.



Wave-particle duality

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.



Quantum electrodynamics

The quantum mechanical theory of light and electromagnetic radiation continued to evolve through the 1920's and 1930's, and culminated with the development during the 1940's of the theory of quantum electrodynamics, or QED. This so-called quantum field theory is among the most comprehensive and experimentally successful theories ever formulated to explain a set of natural phenomena. QED was developed primarily by physicists Richard Feynman, Freeman Dyson, Julian Schwinger, and Sin-Itiro Tomonaga. Feynman, Schwinger, and Tomonaga shared the 1965 Nobel Prize in Physics for their contributions.


This content was originally posted on Y! Answers, a Q&A website that shut down in 2021.
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