LIght wave-particle-duality

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Light can act as either a particle or as a wave.
Does anyone know when it is a particle and when it is a wave:?:

It is always a particle. It is always a wave. Its behavior depends on the method used to examine it.

Physics is fun!

We don't know what light IS. We do know how to predict the outcome of measurements made on light to a high degree of precision. The terms "wave" and "particle" are mathematical metaphors. We know how to measure the energy in light as a function of its temporal frequency. energy = frequency X Planck's Constant. When we send a beam of light particles (aka photons) thru a multiple slit apparatus we detect interference fringes characteristic of waves. Strangely enough when we send a beam of electrons through a similar apparatus we detect the spatial distribution of the electrons on the other side of the apparatus and they produce a similar interference pattern.

Richard Feynman's answer to the Question was Both and Neither.

ruveyn

Wavicle.

Light travels away from a source as an electromagnetic wave. When it encounters the slits of an inteferometer, it passes through and divides into two wave fronts. These wave fronts overlap and approach a screen. At the moment of impact, however, the entire wave field disappears and a photon appears. Quantum physicists often describe this by saying the spread-out wave "collapses" into a small point - a particle.

Light waves travel at the speed of light (c = 299,792,458 meters per second in a perfect vacuum). Changing the energy content of a wavefront will not alter its velocity, only its amplitude (brightness) or frequency (hue).

I used to explain the doppler effect used by police radar to people by likening it to throwing a "perfect" tennis ball at a moving object and measuring the speed of its return.

It went like this:

If the tennis ball comes back faster, the object is moving toward you. The faster it is moving toward you, the faster the ball comes back. If the tennis ball comes back slower, than it is moving away from you. The slower it is when it comes back, the faster the object is moving away from you. If the tennis ball returns with the same velocity as when it returns, the object is stationary. In all cases, when you are measuring the velocity of the tennis ball, you are measuring the kinetic energy of the ball.

With electromagnetic waves, the speed doesn't change. Rather, the energy is reflected in the wavelength/frequency of the wave. When you aim the radar at the vehicle, the returned beam has the same wavelength and frequency if the vehicle is stationary -- that is, the energy of the beam didn't change. If you aim the radar at a vehicle traveling away from you, the returned electromagnetic wave has a longer wavelength and lower frequency and thus a lower energy. The faster the vehicle is traveling away from you, the longer the wavelength and lower the frequency. If you aim the radar at a vehicle traveling toward you, the returned electromagnetic wave has a shorter wavelength and higher frequency and thus a higher energy. The faster the vehicle is traveling toward you, the shorter the wavelength and higher the frequency you will measure.

Whether or not you are throwing tennis balls at the car or electromagnetic waves, it is the change in energy of the reflected wave that indicates its speed. The difference is in how the energy manifests itself and the way it is measured.

Isn't this a problem with all subatomic matter? I'm pretty sure if memory serves they found this to be true with electrons, protons, neutrons, whole atoms, all the way up to bucky balls.

THe photoelectric effect shows that energy is a function of frequency. Intensity is determined by how many photons strike a target per unit of time.

ruveyn

Imagine you're standing on a pier in Boston harbor. Imagine that you drop an empty beer bottle into the water.

When the bottle hits the water it causes a circular wave to eminate from it in the harbor.

This wave then spreads out beyond the harbor out into the North Atlantic.

The wave crosses the ocean and by the time it reaches the shores of europe its a wide front stretching thousands of miles.

The waves simultaneously reaches Liverpool, Brest, and Lisbon.

Then suddenly -the wave vanishes- and just as suddenly -an identical empty beer bottle floating in Lisbon harbor leaps up out of the water exactly ten feet-and lands in the hand of
someone standing on the pier in Lisbon.


Thats how water waves would act if they were like light waves- and could collapse from waves into 'photons'.

Is the wave collapse time asymmetric?
Can electromagnetic waves be made from photons in their particle state?

I like to think that light is a wave when it is free but collapses into a particle when it interacts with something.

What is the wave waving in. The famous Maxwell Experiment shows that aether does not exist. So how does a wave get from Here to There?

ruveyn

I believed it was fluctuations in an electromagnetic field.

The answer is that light is a quantum mechanical object that isn't a wave or particle - i.e. something much more mathematically complicated that has some wave-like and some particle-like properties.

In spite of the description of photons "striking" or hitting other objects, photons don't have quantum mechanical wave-functions... oddly enough, they don't even have positions... even in that hazy cloud that quantum mechanics considers positions (RE: protons, neutrons and electrons)...

Feynman and his colleagues developed the quantum electrodynamic description (QED) of light that describes its nature and behavior in much more detail, BUT that is just for now. Some new observation may change this in the future. For now, however, QED remains the most accurately tested theory in physics with no observational disagreements to date.

Light as a wave pretty much explains all of the macro properties of light. Light as a particle pretty much explains all the micro properties of light. Keep in mind, when ever anyone speaks about light as a wave or particle... nature is much more subtle.

A+

What really then turns reality inside out is adding to this that light is fully time-dilated.

I'd love to really push that one of these days to see if it can be articulated (using that as a starting block) what that would mean regarding everything from the real 'shape' of the stuff we experience when we orient reality to light's perspective rather than our own as well as the implications on how cause and effect operate when both time and space become nothing more than extrapolation of data that all exist at the exact same time and possibly even the same place.