# I am very new to quantum physics. Want to know how photons move.

• anantchowdhary

#### anantchowdhary

I am very new to quantum physics.Now i would like to know how a photon moves.
Is it in a straight line,or is it in a wave like pattern?

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Like all quanta, photons/light have both particle and wave properties and exhibit wave-particle dualism. Light has a magnetic and an electric component, both these vectors are perpendicular to each other and are in phase. The direction of these two vectors is the mutually perpendicular direction to both these vectors.

Reshma, it sounds like you are describing EM waves, not photons. As for "how a photon moves", no one knows! In order to observe a photon, it has to interact (that is, be absorbed by) some detector. So the only way to register the presence of a photon is to destroy it. That's why you can't know what happens inbetween points A and B. If you try to detect it at some intermediate point C to see what is happening between A and B, you'll destroy it sooner and get nothing at B.

Thanks for correcting me (I was unsure of the exact nature of the OP's question :shy:).

Yes, the conventional wisdom seems to assume that light exhibits simultaneous properties of waves and particles. This creates some ambiguities that could be solved if only we take a new view and assume that light travels as a wave, but only "becomes" a particle when it is stopped by a measuring device such as a retina or a screen. Keeping the Uncertainty Principle in mind, neither are electrons, protons or photons "things". A photon is rather an artifact of the measuring device; it is an interaction between a light wave {or a wave function} that “collapses” when absorbed by the measuring device.

Hope I make better sense now.

Resh, how is the Uncertanty Principle involved in this? I thought that HUP was merely a mechanism used iin measuring. I did not realize it made any statement about electrons, protons, and photons not being "things".

Baryon,
HUP will be relevant here. Particles like electrons, protons etc. cannot be pin-pointed like macroscopic objects. If we are able to precisely point out an electron/photon i.e. its "position" nothing can be said about its wavefunction and vice-versa.

I am very new to quantum physics.Now i would like to know how a photon moves.
Is it in a straight line,or is it in a wave like pattern?

According to R.P. Feynman Photons have a probability amplitude to move from a source to a detector. It appears to move in a straight line only because that is where the Highest Area of probability for the amplitude of an event is. Interestingly enough for any given photon moving between Source A and Detector B in order to calculate the Probability Amplitude correctly you must determine how many ways there are for a Photon to get from A to B. Most of the "paths" that a photon may take from Source A to Detector B are very small percentages of probability and each of those paths can and must have an opposite path so that if we define a path from A to C to B then there must be a path from A to -C to B and of course these paths cancel each other out it is only in the area of least energy where the paths become "straighter" that the Probability Amplitudes begin to reinforce rather than cancel i.e. where there is less "turning of the arrows", see QED the strange theory of light and matter for a full discussion of this, and thus photons have a "tendency" to travel in linear directions it is only the Area of the Probability Amplitude for an event that gives a wave like motion to a photon.

Btw I am on my 5th reread of QED and I still have trouble with much of it :uhh:
So I may have buggered this response up quite badly so those with more learning and knowledge please feel free to correct me

hth

According to R.P. Feynman Photons have a probability amplitude to move from a source to a detector. It appears to move in a straight line only because that is where the Highest Area of probability for the amplitude of an event is. Interestingly enough for any given photon moving between Source A and Detector B in order to calculate the Probability Amplitude correctly you must determine how many ways there are for a Photon to get from A to B. Most of the "paths" that a photon may take from Source A to Detector B are very small percentages of probability and each of those paths can and must have an opposite path so that if we define a path from A to C to B then there must be a path from A to -C to B and of course these paths cancel each other out it is only in the area of least energy where the paths become "straighter" that the Probability Amplitudes begin to reinforce rather than cancel i.e. where there is less "turning of the arrows", see QED the strange theory of light and matter for a full discussion of this, and thus photons have a "tendency" to travel in linear directions it is only the Area of the Probability Amplitude for an event that gives a wave like motion to a photon.

Btw I am on my 5th reread of QED and I still have trouble with much of it :uhh:
So I may have buggered this response up quite badly so those with more learning and knowledge please feel free to correct me

hth
How timely. I too am re-reading Feynman's QED book that you mention. I was just going to start a new thread to ask a question about the path integral for photons when I caught this post. Thanks.

Does the path integral for a photon also include paths that go backwards in time from detector to source.

How timely. I too am re-reading Feynman's QED book that you mention. I was just going to start a new thread to ask a question about the path integral for photons when I caught this post. Thanks.

Does the path integral for a photon also include paths that go backwards in time from detector to source.

Not in Feynmans work. There may be such a treatment in others but the photon (and electron) as treated in Feynmans work is a "spin 0" particle. It is and Ideal particle and not a "real" particle. For his treatment of a "real" electron/photon he terms the particle electron as a spin 1/2 with a coupling j of -1 and the photon as a spin 1/2 with no j component.

Reading further into Lecture 4 "Loose Ends" we find that the photon is made up of a quark/antiquark pair with either red-antired green-antigreen or blue-antiblue coupling depending on the original particles Mass. As the photon has a rest mass of 0 and a j of 0 and no other couplings except the electron and proton I will go waaaaaay out on a limb and hypothesize that because of it being a "mediator" particle ( it gets emitted or absorbed ) that it has no path integral that goes backwards in time i.e. there is no anti particle to the photon. If there is such a beast someone please feel free to enlighten me as to it and its properties and correction is always welcomed, else how am I possibly to learn

AFAIK photons are their own anti particle, thus anti-photons do exist they are merely indistinguishable from photons

Not in Feynmans work. There may be such a treatment in others but the photon (and electron) as treated in Feynmans work is a "spin 0" particle. It is and Ideal particle and not a "real" particle. For his treatment of a "real" electron/photon he terms the particle electron as a spin 1/2 with a coupling j of -1 and the photon as a spin 1/2 with no j component.

Just found it: on page 98,
"Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle... Photons look exactly the same in all respects when they travel backwards in time - as we saw earlier - so they are their own anti-particle."

So my next question would be is there a path backwards in time corresponding to every path forwards in time? thanks.

Just found it: on page 98,
"Every particle in Nature has an amplitude to move backwards in time, and therefore has an anti-particle... Photons look exactly the same in all respects when they travel backwards in time - as we saw earlier - so they are their own anti-particle."

So my next question would be is there a path backwards in time corresponding to every path forwards in time? thanks.

Just looked back at page 98 again: "This phenomenon is general. Every particle in nature has an ampitude to move backwards in time, and therefore has an anti-particle...Photons look exactly the same in all respects when they travel backwards in time-as we saw earlier-so they are their own anti-particle..."

I stand corrected they do have an anti-particle themselves and as they can move backward or forward or indeed in any direction then on its face there must seem that there is a path backward for every forward path but as my knowledge is very limited and very subjective I am not 100% sure that this Must be the rule. for all of me there could exist a path forward without a backward path and a path backward without a forward path. I just don't know. Would very much like to find out tho