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Properties of Light Question

Started by carterfox, 01/09/2018 02:25AM
Posted 01/09/2018 02:25AM | Edited 01/09/2018 02:25AM Opening Post
Hello friends,

I am a freshman in college studying astrophysics and have been learning about electrostatics and electromagnetic radiation. I have a question that I cannot find the answer to.

What does the position of a photon look like over time? I understand the wave-particle duality behavior and see how the wave feature derives from propagating electric and magnetic fields. What I do not understand is how the photon itself travels. Is it in a straight line? Is it like a wave, literally moving up and down at specific wavelengths? Or do we simply not know?

Send your thoughts if you think you have an answer or other ideas. I'd love to understand this some day.
Posted 01/09/2018 04:59AM | Edited 01/09/2018 05:05AM #1
Hi, Carter; Cool! Even Feynman confessed that he didn't Really understand that (and that no one did, does or will?) But, regarding the ~duality~ (and, hence, implicitly, your "where is/was/will be the photon, as it propagates thru spacetime" query). Here's my take on it, which sorta eliminates the ~paradox~ and has other coolish aspects: NOTICE that the ONLY TWO times the photon manifests its ~light particle~ aspect is when it is Emitted and when it is Absorbed. More generally, only when it Manifests interaction with other particles. For example, when we see the little bitsy flash ~Right There~ on the phosphor screen... a (bunch of) photons were born There and Then, in space-time. And, ever so slightly later, some of them found their way to your retina There' and Then' in space-time. Now HERE's a cool extended note regarding those two events: Unto Themselves, Because they were [identically, in the rigerous sense] traveling from there to here ~at the speed of light/information~ Unto themselves, it took Zero Time to make the trip. They were born and died instantaneously/simultaneously. So far, so good. NOW... during the trip, it is ONLY the EM field that ~exists~ [Here, note that the EM field is nothing more or less than the space-time Probability Density Distribution Function associated with the Photon(s)] Feynman, most notably, noticed that one can and should sum ALL POSSIBLE contributors over ALL space-time. That's his ~sum over ALL possible histories~ (see where the word probability and possible mesh?!) So... where "The Photon" was as it made the trip is, best stated... as "Everywhere and Everywhen" as given by Feynman's (infinite number of) possible histories. This is Most Simply manifested in a Twyman Green Interferometer. Each and every photon ~travels both arms~. That is not an analogy... it is exactly what happens! And, this is so easily demonstrated by blocking one, other, both or neither arms. ALL four cases instantly obey The inclusion and exclusion of possible histories. It's as simple as that! Sorry for being so long-winded. I hope that helps. I, and friends, have "argued" over this stuff for decades. PS I was an aerospace scientist, specializing in optical metrology. We did a lot of complex wavefront shaping stuff for Uncle Sam, so were heavily immersed in this stuff. The EM field is complex (amplitude and phase) throughout space-time. You might say it happily carries the essence of (all) photons as they interact throughout the universe. Ummm... if this is for some homework question, just say you discussed with others. Your teacher might be looking for a much simpler answer, or one that disagrees with mine! Tom

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Posted 01/09/2018 07:44AM #2
Hello Carter, Tom & All,

Carter, you ask a good question: A Deep, Thoughtful question. Tom's answer was good. Here, I'll try to also give you a good, thoughtful answer from my experience as well.

I am an Electro-Magnetic Compatability (EMC) Engineer who works in a Test Laboratory where measurements are made to determine if a computer accessory makes too much interference to be sold. My lab measures frequencies between 30 MHz and 2500 MHz in a room that is sealed off (think Faraday cage) from the outside world where Television and Radio signals from legitimate broadcasting stations would interfere with the sensitive measurements of the computer accessory we are tasked to measure. Additionally, we want only signals to get from the product under test to the measurement antenna either directly or via a reflection off of the reflective floor, but not from a reflection off the ceiling or the four walls of the Faraday cage so the walls and ceiling are treated with expensive RF absorbent material. What we have is effectively a Lloyd's Mirror Interferometer (but usually called a "Semi-Anechoic Room" or "Absorber-Lined Chamber") and this Interferometer is large enough to walk around in because the separation from product to antenna is 10 meters !!! Additionally, we have two antennas that are separated from each other by 36 Degrees, so the interior of the chamber is wider than eight meters and taller than 6 meters.

Now, when I measure the interference (which is measured in decibells relative to a Micro-Volt-per-meter), I have to search to receive antenna from 1 to 4 meters in height above the metal, conductive floor (a mirror) to find where the direct and reflected waves add (constructively) and not cancel (destructively). The signals that are low frequency (ie. low energy photons) are spread out and cover all of that 1-4 meters. The higher frequency signals look like they are more "localized" because they have more distinct peaks and nulls, as I raise and lower the mast which holds and positions the receive antenna from 1 to 4 meters.

What I would point out to you is that the spatial orientation of the E-M wave is frequency dependent (ie. Wavelength dependent), which implies that the photons involved have a certain "color" or energy content (in my case Micro-electron Volt). If I were working with light frequencies (Tera Hertz) like optical scientists and engineers work with, the wavelengths would be Nano-Meters or Angstroms and the energies would be higher Electron-Volt levels, but the concept of wavelength and frequency implying energy of the photons is really important.

I also struggle to wrap my brain around the concept of the photon and its "path" from source to destination. I agree with Tom: Richard Feynman was a very smart guy who essentially said that "Photons like to be every-where and every-when" in space-time. I have a four year degree in Physics and about 40 years in High-Tech Engineering. I don't completely understand the answer to your question either. I do know that the energy that a particular photon carries is part of the answer you seek: Low energy photons spread out much more than higher energy photons. I can see this in my measurements, trust me.

Best regards,

Posted 01/09/2018 09:15PM #3
Hi Carter,

As a first year astrophysics student, you’re probably studying classical electromagnetism as summarized by Maxwell’s equations while the behavior of photons is more accurately described by Quantum Electrodynamics (QED) as devised by Feynman, Schwinger, and Tomonaga for which they won the Nobel Prize in 1965. Feynman’s book QED: The Strange Theory of Light and Matter is a good introduction to this topic.

Photons travel like a wave but interact like a particle. This behavior can be observed in Young’s double-slit experiment and in interferometers.

In Young’s experiment, photons are emitted (by a single emitter) on one side of a screen that contains 2 slits and detected (by one of multiple detectors) on the other side of the screen. Photons are emitted and detected one-at-a-time like particles. Photons take every possible path between the emitter and detector but the screen (with 2 slits) reduces the possible paths to just 2.

Even though the photons are emitted and detected one-at-a-time like a particle; an interference pattern is produced as the detections build-up at the detectors that indicate each photon travels through both slits and interferes with itself like a wave. If a detector is placed at either slit to determine which slit the photon went through, the interference pattern will not be produced.