The mirror in a box thread got me thinking. Say I am in my room with my lights turned off. I can grab my flashlight turn it on and shine it on the wall. That part of the wall lights up, i can even see the actual beam itself. Now when i turn it off poof there goes all the light. Why is this? Do photons have a half life of like nano second? Where do they go? If i see the beam while the light is on why cant i capture it using one way mirros? So the beam is still there even after the flash light has been turned off?
Photons can be created and destroyed, unlike most other particles. The photons that were in the beam of light are absorbed by the wall. The energy is transfered to the atoms in the wall and the photon goes poof.
In a manner of speaking. The light is absorbed by the other matter in the room and is dissipated as heat. This will happen faster than you can perceive it.
In the case of a beam of light shining out into empty space, the light doesn’t disappear, but it is receding from you at quite a clip. Fast enough that it will seem to vanish instantaneously.
The photons don’t have a halflife at all. You can go outside at night and catch photons thousands of years old with your eye. Look at the Great Galaxy in Andromeda, which is visible to the naked eye. Those are 2.5 million years old.
But photons face a death threat of something like 10% to 90% each time they bounce off of typical surfaces.
Others have dealt with your other questions, so I’ll just answer these.
If a perfect mirror existed, you could. But the best mirrors there are absorb a little bit of light. Some light would be lost on each bounce, and the light would soon be gone.
Ok, so in theory could you make a box that would trap light in a detectable way (some form of small photocell) for say 10 seconds before the imperfections of the reflective surface/presence of the photocell dissipated it.
>trap light in a detectable way
The detection process of photocells and other real detectors involves absorbing the photons. If you made the box big enough, so that photons traveling for 10 seconds don’t hit a wall or, more practically, only have a small number of mirror bounces, then you’d have your 10 seconds. But you can’t sample the light many times without attenuating it. If you sample 1% of it, then the time over which that happens the amount of light becomes 0.99 of what it was before. If that time is a nanosecond, because your box is a breadbox, then you have 0.99^1e10 in ten seconds. That’s too close to zero for the calculator I have here. If I’m reasoning it out right, you’d have 1/(10^30,000,000) left of your original light.
Here’s something I’d never considered before, but:
Why don’t photons just pass thru solid objects?
Since an atom is mostly empty space, why doesn’t a photon just keep on passing thru the empty spaces. Or does it, and just sooner or later, it finally hits something “solid?”
If you can see the beam itself, that means something in the air is reflecting/refracting some of it towards your eyes. Any method of detecting light is going to destroy some of it. So there’s no way to keep photons around forever and constantly be aware of their existance. Plus like was said, there’s no such thing as a perfect mirror. I think telescope mirrors are around 95% efficient.
But they do! Ever look through a window before?
Sorry if that sounded a little snarky. Your question is a good one. We just have to delete the word “solid”, and insert “opaque”.
We now have a whole new question: What characteristic is it, which makes some materials transparent, and other materials opaque?
We can expand that question, because actually, varied materials are transparent/opaque to varied wavelengths. Infrared will pass easily through materials which are opaque to visible light. A piece of red cellophane is transparent to red, but opaque to green. And so on…
>Why don’t photons just pass thru solid objects?
A photon represents a wave in electromagnetic field strength that travels. Solid objects have local distributions of charge that will move a little if the field strength changes, and in moving they impart mechanical movement to the structure of the solid. This couples the electromagnetic field and the mechanical solid object and means the object will interact with the photon.
In a simple but instructive case, the movement of molecular groups resonates at frequencies that are characteristic of their mass and stiffness, which is the basis of infrared spectroscopy, and the molecular groups are already wiggling around and so can emit photons too, which is the basis of thermal radiation.
And when you do catch them in you eye, you kill them.
Sad, sad, sad.
mangeorge
And from a photon’s perspective its death happens instantaneously after its birth. Even the ones from Andromeda.
Don’t forget the Light Sucker Theory.
This is a little like what happens to sound. The photons hit atoms, send the electrons to higher energy levels, the other photons are emitted as the electrons return to a lower energy level, and that new photon is often emitted in the infrared range, causing energy to be radiated off as heat. Sound is different since there is no particle model for sound, but it still adds kinetic energy that is radiated off as heat.
Indeed, for certain crystal lattices there is an elementary unit of sound vibration called a phonon, which has quasi-particle properties. In particular, they are bosons with integer spin just like photons.
And people have, in fact, proposed building “Phasers” that are analogues to lasers, but use these quantized vibrational energy packets in the same way lasers use photons.
The verdict: Hard to implement because of scattering in the lattices due to inevitable imperfections. Won’t be building phasers soon.
Rats, and I was so looking forward to setting my phaser on stun.
[sub]Okay Mr. Bad Guy, if you could just step inside this large crystal lattice chamber for me, I’ll aim my weapon at you menacingly.[/sub]
An atom is not mostly empty space. The space in an atom is jam packed clear full of various fields. Of most interest to us here are the electromagnetic fields, produced by the charges of the electrons and nucleus. Light itself is an electromagnetic field of sorts, so it’s no surprise that it interacts with the fields in the atom.
Now, you may try to nitpick that even with the fields, the atom is still empty, other than the protons, neutrons, and electrons. But the truth is, the protons, neutrons, and electrons are themselves just manifestations of fields, too, no more nor less real than the electromagnetic field.
There’s an old thread that goes into more detail on this. Ah, here it is: [thread=299054]Why can’t my hand go through my desk?[/thread] In short, just as Chronos says, (both here and in that thread) atoms are all “empty” in terms of any bits of actual stuff being there, and they’re completelly filled with fields.
Stranger