How do photons work?

[Sky Pilot]

Can someone point me to one or more good resources that explain (in laymans' terms) how photons work? I'm curious about such questions as...

1) How do photons travel through space to make an object visible from all directions over great distances?

2) Do photons age or diminish when traveling over millions of light years?

3) How do all the zillions of photons from a distant galaxy converge in a 10" aperture without becoming a jumbled up mess?

[MKruer]

Can someone point me to one or more good resources that explain (in laymans' terms) how photons work? I'm curious about such questions as...

You try here http://en.wikipedia.org/wiki/Photons

1) How do photons travel through space to make an object visible from all directions over great distances?

This has more to do with the reflective properties of the objects in question. If you had a mirror that could perfectly reflect light you would not be able to see that object unless you were standing in the reelection path. Good thing noting is perfectly reflective. When a photon hits an object it ?€?bounce?€

[Sky Pilot]

They don?€™t converge (the optics within the telescope focuse light into a more condensed image). What you are seeing is a collection of photons that have nearly identical paths though space. This is because that light travels in a strait line unless affected by gravity or scattered by another object. The reason why it is so faint is because the further away an object is, the less and less the photon density becomes.

So does that mean that photons are spreading at an infinite density, so that every infinitesimally small point in space receives photons from every single light source in the universe? (Unless the path is blocked by gravity or some mass.) So that even a billion billion light years away from the light source, in every direction, photons from that distant object are passing through every point on the celestial sphere around that object?

If the photons have nearly identical paths through space, then they would travel in nearly parallel paths? So, let's say I'm observing a galaxy that's 100 thousand light years in diameter, but appears to be only 2 arcseconds across when observed from Earth, and that we call the left side "A" and the right side "B". Photons from the left side of the galaxy ("A") arrive in my aperture in a path that's perfectly parallel to my Dobsonian OTA. The photons from the other side of the galaxy ("B") must arrive in my aperture at a very slight angle to the photons from side "A". When those photons, from two paths, strike my primary mirror at the same point, why don't they mess each other up?

I hope I'm not being a dim wit here, but I really want to understand how this works. I have a good handle on software engineering, but I can use some help with the physics.

[julesstoop]

Regarding your first question:
Yes... But. The energy in a photon can not be devided for ever. There is a minimum amount of energy per photon because of quantum mechanics.

Regarding the 'not messing up'
When you're thinking photons, think waves. Wavefronts caused by throwing several pebbles in a pond will cross each other without hindrance. Much the same counts for photons.

[Sky Pilot]

Regarding your first question:
Yes... But. The energy in a photon can not be devided for ever. There is a minimum amount of energy per photon because of quantum mechanics.

So, if the energy in a photon can't be divided forever, then the waves eventually spread over a large enough area as to be virtually undetectable at some point? I know I've read that photons go on for infinity, but as they propogate, they have to spread over an ever widening area, thus they get dimmer at any single point of observation as you get farther away from the source. Is that right?

[selden]

but as they propogate, they have to spread over an ever widening area, thus they get dimmer at any single point of observation as you get farther away from the source. Is that right?

Yes and no

A photon is both a wave and a particle. The electrical and magnetic waves of an individual photon spread out in all directions, but the individual photon can be detected in only one location. One way of describing it is to say that when a photon is detected (by knocking an electron out of an atom) its probability wave has "collapsed" into that particular location. It's one of the primary examples of the quantum nature of the universe. As you get further from a light source, it's dimmer because fewer photons are detected in a given area.

One of the experiments often performed in undergraduate physics labs is to turn down the intensity of a light source until individual photons are detected, and then watch the interference pattern develop as their waves interfere with themselves after they pass through a double-slit interferometer.

[Sky Pilot]

Yes and no

A photon is both a wave and a particle. The electrical and magnetic waves of an individual photon spread out in all directions, but the individual photon can be detected in only one location.

Thanks, Selden, but you're confusing me (not that I doubt you!). If a photon can only be detected in one location, then how can photons be detected in EVERY location? IOW, if a photon occupies a finite and indescribably tiny space, then how can an object be observed from EVERY point in the universe? If I can see a galaxy from 100 million light years away while standing at point A, then how can there also be identical photons at every other point B that's at the same distance as point A? Unless there are an infinite number of photons leaving a light source at every instant in time? But if that was the case, the object shouldn't get dimmer with distance.

Help me, I'm sinking! Am I in too deep for my tiny brain?

[selden]

An individual photon can be detected only at one location.

A 100 Watt light bulb gives off about 10^20 photons per second. (1 followed by 20 zeros.)

A star like our Sun emits about 10^47 photons per second in all directions. That's 1 followed by 47 zeros. A galaxy like our Milky Way has about 100 billion stars. In exponential notation, that's 10^11 stars. So it would emit about 10^58 photons. (That's 1 followed by 58 zeros.)

If you're far away, only a tiny fraction of those photons are coming toward you. You (or your eyes) are occupying only a tiny fraction of the spherical area that those photons have expanded to cover. From a galaxy about 50 million light years away, you'll see only about 150,000 photons per second. That's the same number of photons that someone standing next to you would see, although they wouldn't be seeing the same photons that you see.

If the galaxy is further away, even fewer get to you. The Hubble telescope had to stare in one direction for weeks to build up the image of the "Ultra Deep Field". Its CCDs were seeing maybe one photon per minute or fewer from some of the dimmest and most distant galaxies.

Does this help at all?

I think the description at http://astro.neutral.org/articles/tea/tea.html may help to clarify some of this. It fills in some of the steps that I left out.

[t00fri]

Yes and no

A photon is both a wave and a particle. The electrical and magnetic waves of an individual photon spread out in all directions, but the individual photon can be detected in only one location.

Thanks, Selden, but you're confusing me (not that I doubt you!). If a photon can only be detected in one location, then how can photons be detected in EVERY location? IOW, if a photon occupies a finite and indescribably tiny space, then how can an object be observed from EVERY point in the universe? If I can see a galaxy from 100 million light years away while standing at point A, then how can there also be identical photons at every other point B that's at the same distance as point A? Unless there are an infinite number of photons leaving a light source at every instant in time? But if that was the case, the object shouldn't get dimmer with distance.

Help me, I'm sinking! Am I in too deep for my tiny brain?

Sky Pilot,

the way how photons appear depends crucially on the /resolution/ at which you look at them! In order to talk about a resolution you need a characteristic length scale as a reference.

You should imagine a light source as a continuous (time-independent, stationary...) emitter of very many photons.
Since --according to the particle-wave duality in quantum mechanics-- each photon corresponds to a (/spherically/ expanding) wave, the traces of photons may be seen equally on spherical shells in space, propagating with the /universal/ speed c of light away from their source...

Since the emission of photons is continuous, the flux of light e.g. from a star is indeed to be viewed as a superposition of very many photons into the /full/ solid angle.

The confusion often arises about when light is behaving like a "particle" = photon and when like a wave. This depends on your resolution, as I emphasized above. If your reference length scale is much /larger/ than the light wave length (bad resolution) then we may talk about geometrical "light rays", i.e. the wave nature of light becomes hardly visible. Once the reference scale becomes of the order of or smaller than the wave length, things change dramatically (good resolution): the wave nature of light becomes apparent, diffraction & interference phenomena appear etc.

An extreme example concerns the transmission of light quanta through a slit/hole, the dimensions of which are much smaller than the light wave length...Then you indeed can chop light into individual photons due to the excellent resolution that you have enforced in your setup...

Analogous phenomena occur also in case of radio waves. You may have noticed already that FM (ultra short wave radio) reception may get very bad if there is some obstacle like a mountain or even a house between the transmitter and the receiver (you). The wavelength of FM radio is less than 1 meter and thus such radio "waves" rather behave like /staight rays/ on the much bigger reference scales of mountains or houses! The straight line propagation may thus easily be blocked by such obstacles. Long wave radio waves in contrast behave very differently. On the size scale of mountains their wave nature becomes apparent such that these /long/ radio waves may easily /creep over them/ like a snake without being significantly hindered!

I hope this helps a bit...

Bye Fridger

[Sky Pilot]

Thanks guys. Yes, you both have helped.

If it was possible for a star to blink on only long enough to send out one spherical shell of photons in all directions, where all of the propogating photons are always equal distance from the source, I would expect that at a billion light years away I may or may not be able to detect a photon, depending on whether my detector is in a photon's direct path. But it might be possible that the photons have spread apart enough that my detector lies between the propogating paths of the photons and they'll pass me by without a single one colliding with my detector??? I must be thinking of photon = particle. Is that right?

[selden]

Yes to both.