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Brother Oni
2011-04-07, 07:11 AM
Please excuse any mis-understandings I have about light - I'm a biochemist, not a physicist. :smallredface:

As I understand it, an object is visible because it reflects light, or rather, it absorbs light then emits it at a visible wavelength dependent on the material in question when the excited electrons return to their resting state.

This emitted light is then received by your eye, which your brain interpets.

So what happens if a theoretical substance absorbs all light that hits it? It's not invisible as light can't pass through it, but also you can't see it directly as it doesn't emit any light for you to see.

Would it appear just as a shadowy material or just a eye-aching blackness? Or something else entirely?

Eloel
2011-04-07, 07:12 AM
That's what black is, actually, as far as I know.

Tarinaky
2011-04-07, 07:13 AM
What you describe is called, in layman's terms... 'black'.

Ranger Mattos
2011-04-07, 07:28 AM
As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

Seonor
2011-04-07, 08:02 AM
An object that doesn't reflect light is simply black to the eye. But nothing absorbs 100% of all light, it only seems that way because the human eye can't detect small rest of the light that is still reflected.

There is some research to create a "perfect black" which would absorb absolutely every light but until now there is no such thing, even if there are some promising developments (http://news.bbc.co.uk/2/hi/science/nature/7190107.stm) in that direction. It would still look like normal black to human eyes.

Eloel
2011-04-07, 09:13 AM
As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

For something to be invisible, light from the other side needs to come through it.

Eldan
2011-04-07, 09:25 AM
As far as I know, if a substance reflected absolutely no light, it would be effectively invisible, but would still cast a shadow. I'm probably wrong though.

Not invisible. Just absolute black. Which would really stand out against almost anything.

Normal black objects you see still have textures, reflections, dust particles and so on, so that gives them some shape. A perfectly black object would basically just be a black shape.

The Rose Dragon
2011-04-07, 09:25 AM
Well, you probably know that your eyes don't exactly see much - they can't even interpret the world as three-dimensional. That's what the brain does. Therefore, even if your eyes don't receive any visible light from an object, the brain will interpret the absence of light in an area of the retina as a black object. It should not ache your eyes any more than sensory deprivation tanks, though suddenly replacing the object with a light source after a while can hurt the eyes. However, if it is completely absorbent, you should not be able to see texture or shape except by contrast.

Form
2011-04-07, 09:46 AM
Relevant link if you want more detail:

http://en.wikipedia.org/wiki/Blackbody

ZombyWoof
2011-04-07, 09:49 AM
Would it appear just as a shadowy material or just a eye-aching blackness? Or something else entirely?
Close your eyes in a dark room.

There you go.

There's no functional difference between something that absorbs all visible light and something that reflects some/all of it when there is none to reflect, so just create an environment where there's not enough light for your eyes to detect and voila.

Asta Kask
2011-04-07, 09:52 AM
Doesn't a black hole do this? Sure there's Hawking radiation, but if you take a really supermassive black hole then that's negligible.

shawnhcorey
2011-04-07, 10:01 AM
So what happens if a theoretical substance absorbs all light that hits it?

It would be black. That's why black holes are called black. They adsorb almost all light. They to emit light as Hawking radiation.

Also, since heat is light at a low frequency, it's temperature would be absolute zero. For comparison, the background radiation of the universe is about 3K or -270C or --454F. Absolute zero is 0K or -273C or -459F.

valadil
2011-04-07, 10:27 AM
Doesn't a black hole do this? Sure there's Hawking radiation, but if you take a really supermassive black hole then that's negligible.

Effectively, yes. It's not absorbing the light in the same way a blue object absorbs all red and green. But its gravity is such that light can't get away.

A fully absorbent sphere would be invisible. It can't be seen. It would not be transparent, so it would still be detectable by vision even if you're seeing the absence of the object rather than the object.

grimbold
2011-04-07, 11:07 AM
there are things called metamaterials that can actually refract light around themselves rendering them essentially invisible

Ranger Mattos
2011-04-07, 10:30 PM
For something to be invisible, light from the other side needs to come through it.


Not invisible. Just absolute black. Which would really stand out against almost anything.

Normal black objects you see still have textures, reflections, dust particles and so on, so that gives them some shape. A perfectly black object would basically just be a black shape.

Note to self: Don't attempt to do physics late at night. I knew I was wrong this morning before math, but didn't have any opportunity to get on and correct myself until now.

Zexion
2011-04-07, 10:42 PM
Well, perfect black would look absolutely, completely black, and any three-dimensional perfect black object would look like a silhouette, I believe.

thubby
2011-04-08, 01:08 AM
a perfectly black object would be difficult to even interact with. you could never know how close you are to the nearest face.

golentan
2011-04-08, 03:35 AM
It would look like my soul (read, as everyone else has sad, black outline filled with black without texture or detail to mitigate the all pervasive blackness).

Thubby: I think you could still get a fairly good idea comparing outlines with parallax. Though I'm not sure how close you'd need to get to get an accurate fix on it. It... might be pretty close.

Edit: Also, how hot would this object get leaving it in the sun. Ouch!

Ted_Stryker
2011-04-08, 04:21 AM
Relevant link if you want more detail:

http://en.wikipedia.org/wiki/Blackbody
Quoting an excerpt for emphasis:


A black body is an idealized physical body that absorbs all incident electromagnetic radiation. Because of this perfect absorptivity at all wavelengths, a black body is also the best possible emitter of thermal radiation, which it radiates incandescently in a characteristic, continuous spectrum that depends on the body's temperature. At Earth-ambient temperatures this emission is in the infrared region of the electromagnetic spectrum and is not visible. The object appears black, since it does not reflect or emit any visible light.

The thermal radiation from a black body is energy converted electrodynamically from the body's pool of internal thermal energy at any temperature greater than absolute zero. It is called blackbody radiation and has a frequency distribution with a characteristic frequency of maximum radiative power that shifts to higher frequencies with increasing temperature. As the temperature increases past a few hundred degrees Celsius, black bodies start to emit visible wavelengths, appearing red, orange, yellow, white, and blue with increasing temperature. When an object is visually white, it is emitting a substantial fraction as ultraviolet radiation.
In fact, what would happen with a perfect absorber is that it would change temperature until the amount of radiation it emits is equal to the amount of radiation it absorbs. However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

Phishfood
2011-04-08, 06:05 AM
Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

Thats what she said.

This whole thing reminds me of a similar but not quite question I asked once in high school. What would happen if you got a perfect one way mirror and made a glass sphere out of it? Pretty much the same result I suppose, but in my head I imagined you could throw it at someone, it would crack and all the light would escape - laser grenade :D

I am now older and wiser. Well, older anyway.

Brother Oni
2011-04-08, 07:10 AM
Thanks for all the answers everybody. :smallbiggrin:


However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

I assume that if this theoretical material absorbed all visible light instead, rather than all incidental EM radiation, it would still have the same visible appearance to people, but would be a temperature based off the ambient temperature/material difference (its heat transfer coefficient?) rather than equal to the amount of radiation it absorbs?

douglas
2011-04-08, 09:51 AM
Thats what she said.

This whole thing reminds me of a similar but not quite question I asked once in high school. What would happen if you got a perfect one way mirror and made a glass sphere out of it? Pretty much the same result I suppose, but in my head I imagined you could throw it at someone, it would crack and all the light would escape - laser grenade :D

I am now older and wiser. Well, older anyway.
Sadly, the real answer to this is that it's a problem strictly for philosophers because perfect one way mirrors are not possible.

Weezer
2011-04-08, 10:03 AM
Sadly, the real answer to this is that it's a problem strictly for philosophers because perfect one way mirrors are not possible.

Or 1st/2nd year physics students still living in their ideal frictionless vacuum world.

EDIT: Also assuming this materials perfect absorbtion extends to the frequencies used for radar , it would be the perfect stealth fighter, completely indetectable by radar.

shawnhcorey
2011-04-08, 10:26 AM
EDIT: Also assuming this materials perfect absorbtion extends to the frequencies used for radar , it would be the perfect stealth fighter, completely indetectable by radar.

Yeah, but don't forget where you parked it because you never find it again if you don't. :smallsmile:

RebelRogue
2011-04-08, 10:40 AM
It will look like this:
http://www.2001aspaceodyssey.org/BigImages/Monolith-Sun-Moon.png

The Extinguisher
2011-04-09, 03:50 AM
Quoting an excerpt for emphasis:


In fact, what would happen with a perfect absorber is that it would change temperature until the amount of radiation it emits is equal to the amount of radiation it absorbs. However, the emitted radiation will have a frequency distribution that depends solely on the temperature of the emitting object, so any knowledge of the frequency of the absorbed radiation is lost. In principle, if the blackbody is hot enough, it won't be invisible in the sense of falling outside the visible range of the EM spectrum, and in any case it will certainly be visible to a detector sensitive to the appropriate frequencies.

Also, don't confuse black holes with blackbodies. A black hole has an escape velocity equal to the speed of light at its event horizon. A blackbody is just a perfect absorber of radiation. While all black holes are blackbodies, not all blackbodies are black holes.

This.

An object that absorbs all light would be emitting light of a certain frequency distribution, so it would appear visible. Hell, a star is a great approximation of an object that absorbs all light, since the reflected radiation is negligible compared to the blackbody radiation.

Also, even if it emitted no visible light (lets say it wasn't hot enough) the brain would do something. The brain is a crazy thing, able to extrapolate and reinterpret optical signals. It's why an apple still appears to be the same colour at noon and at sundown, despite different frequencies of photons being reflected off of it more.

Actually, come to think of it, your brain my just treat it like your blind spot, and if it was small enough, may fill it in with whats in the surrounding area, so it would pretty much be invisible

Heliomance
2011-04-09, 04:51 AM
It's actually possible to see this in real life, though it does need to be deliberately set up. Basically, you can set up an opaque container with a hole in the lid, with internal dimensions such that light going in through the hole bounces around inside and is entirely absorbed before any of it manages to get out through the hole again. The hole will then look perfectly black.

Ted_Stryker
2011-04-10, 01:42 PM
I assume that if this theoretical material absorbed all visible light instead, rather than all incidental EM radiation, it would still have the same visible appearance to people, but would be a temperature based off the ambient temperature/material difference (its heat transfer coefficient?) rather than equal to the amount of radiation it absorbs?
I'm not sure if I understand your question. If we're talking about a room-temperature object that absorbs all visible radiation, it will appear black to human eyes.

Sacrieur
2011-04-10, 02:10 PM
An object that doesn't reflect light is simply black to the eye. But nothing absorbs 100% of all light, it only seems that way because the human eye can't detect small rest of the light that is still reflected.

There is some research to create a "perfect black" which would absorb absolutely every light but until now there is no such thing, even if there are some promising developments (http://news.bbc.co.uk/2/hi/science/nature/7190107.stm) in that direction. It would still look like normal black to human eyes.

May I interest you in a black hole (http://en.wikipedia.org/wiki/Black_hole)?

Erts
2011-04-10, 05:02 PM
Black holes are invisible.
(Source: recent National Geographic that I just read.)
So, I think this thing might be invisible, but also leave a shadow because light isn't getting by it.
Does it also absorb other forms of electromagnetic radiation? Such as radio, microwave, infared, ultraviolet, X-rays, and gamma rays? Because we can detect those without our eyes.
The spectrum. (http://upload.wikimedia.org/wikipedia/commons/c/cf/EM_Spectrum_Properties_edit.svg)

ForzaFiori
2011-04-10, 08:16 PM
Have you ever been spelunking or cave diving? You know that complete, total, terrifying darkness that comes when you turn off the light? The object would look like that. Not emitting light would give you the same visual cues as not having any light, except just in the one spot.

Brother Oni
2011-04-11, 01:24 PM
I'm not sure if I understand your question. If we're talking about a room-temperature object that absorbs all visible radiation, it will appear black to human eyes.

Don't worry about it - 'incidental radiation' doesn't actually mean what I thought it meant.

Yora
2011-04-11, 03:00 PM
This inspired me to think about another physics question, which I'm afraid could be much more complicated. :smallbiggrin:

Most people heard of Schrödingers Cat, but the full truth about it is that it's not meant to illustrate a model of quantum mechanics, but actually supposed to show that a certain model doesn't work at all.

Still, there's often talk about how in quantum mechanics object behave differently when someone looks at them than they would when nobody looks. This obviously makes no sense at all, but people claim "it's magic!" ... no, "It's quantum mechanics". There's even the stupid claim that scientists have started the process to make the universe implode, because they used some telescope to capture radiation from the big bang.
But I strongly suspect that those people are getting it all wrong.

Isn't it just the same effect as, when I stick my finger into a pot of water to measure the temperature, I no longer observe "a pot filled with water" but "a pot filled with water and a finger in it"?
To measure or observe something, you have to "catch" some of the effect, meaning you changed how the effect would normally behave if you didn't had put a sensor in place. Is it just that simple thing, or is there more to it in quantum mechanics?

factotum
2011-04-11, 03:31 PM
As I understand it, Schrodinger's Cat is a thought experiment intended to show the uncertain nature of quantum equations. Basically, if you sit down and use quantum mechanics to try and calculate if the cat is dead, you will *always* get two answers, one of which shows the cat is still alive and the other saying it's gone to meet its maker; the only way to ascertain which of those answers is actually correct is to open the box and find out!

shawnhcorey
2011-04-11, 04:25 PM
It is a thought experiment since the physics of the apparatus is such that whether the cat is still alive is determined long before anyone opens the box.

The thing about quantum physics is that you can't measure a quantum without influencing it is some way. This is because the quanta are indivisibly. If you stick your finger in a pot of hot water, your finger will have an influence on its temperature (your finger is cooler than the water so it will lower its temperature slightly) but the influence is insignificant. But when you measure an electron, you can't break of a tiny bit and examine that. An electron, being a quantum, is indivisible. Therefore your measurement has a profound effect on the electron.

Does that make sense?

Yora
2011-04-11, 05:42 PM
I think what Schrödinger started with was the claim, that a certain particle can appear to be in different states depending on the observer. Thinking this claim was rediculous, he propsed the following experiment.

You take a box with a cat and a machine that detects if a certain particle has a certain state or not. If it detects the state, it releases a poison that kills the cat. If the machine detects nothing, the poison is not released and the cat lives.
So logically, while the box is closed, the machine might have detected what it was looking for and killed the cat, or it did not and the cat is alive. Of course, the only way to make sure is to open the box and look inside.
But that theory he was criticising claimed that the particle can have both states, which results that the machine detected something and was set off, and also not detected something and did not go off. Which means the cat was killed, and not killed. But when to people look into the box, there are two observer who observe if the machine detect what it was looking for, and since multiple observers would get a different reading, one sees a dead cat and one a life cat.
Which is of course bogus, and that's Schrödingers reason why he wanted to debunk the whole theory of a particle having different states depending on the observer.

Though I think there's a good chance that Schrödinger himself did get the theory he wanted to debunk wrong. And then that story got told by people who don't know anything about physics and all we know today is that Schrödingers cat is a transdimensional Zombie-Cat. :smallbiggrin:

douglas
2011-04-11, 06:04 PM
Isn't it just the same effect as, when I stick my finger into a pot of water to measure the temperature, I no longer observe "a pot filled with water" but "a pot filled with water and a finger in it"?
To measure or observe something, you have to "catch" some of the effect, meaning you changed how the effect would normally behave if you didn't had put a sensor in place. Is it just that simple thing, or is there more to it in quantum mechanics?
There's a lot more to it than that. Scientists studying quantum mechanics have devised experiments to tell the difference between various explanations for weird quantum effects, and these experiments produced results conclusively favoring the "quantum information really is multivalued until we force it to not be" interpretation.

For example, if you send a beam of light through two thin parallel slits and look at the resulting light display on a panel on the other side, you'll see an interference pattern - the light waves going through the two slits are reinforcing each other in some spots and canceling in others, resulting in a pattern of many bright and dark lines alternating. Now, light does have a fundamental indivisible unit - so what happens if you do this experiment and send only a single photon at a time? It's only one photon, it has to go through just one slit even if we don't know which one, right? Turns out the answer is no, the single photon behaves as if it's going through both slits simultaneously if you only measure where it ends up - one photon at a time produces the full interference pattern for the distribution of where it ends up over repeated trials. But if you modify the setup in any way that allows you to know which slit the photon went through, the interference pattern disappears and you get the behavior you'd expect for photons going through just one slit at a time.

Short version: If you set up Schroedinger's Cat and managed to genuinely isolate the box from yourself so that the cat's activity would not affect you even the slightest bit until you open the box, the cat really would be both alive and dead at the same time until you open the box. Getting the required degree of isolation would be practically impossible - even the slightest tiniest imaginable butterfly effect would break it - but the cat really would be actually both alive and dead simultaneously, not just unknown to you.

Yora
2011-04-11, 06:22 PM
The first point I understand. Apparently at the quantum level subartomic particles behave in ways that are impossible on the atomic level. As strange as it sounds, a particle passing through multiple slits does not get rejected by my mind. :smallbiggrin:
But what about the observer? When I have that electron beam behaving strangly and turn my back to it, does it suddenly bhave completely different until I look back at it? What if I make a photo of the pattern without looking at the pattern? Will the photo miraculously show a different image to everyone who looks at it?
Most certainly not!

The last point doesn't make any sense though. If the cat is in the closed system, the cat is also an observer. Or alternatively, you can put the cat-box into another sealed box with yourself and then you could check on the cat, but for the outside world you would have checked and not checked?

I'm not debating about the slit-problem, that I can live with. What bothers me is the idea that a human who looks at a quantum experiment miraculously changes everything. That just seems like total bogus.

The Extinguisher
2011-04-11, 07:10 PM
The first point I understand. Apparently at the quantum level subartomic particles behave in ways that are impossible on the atomic level. As strange as it sounds, a particle passing through multiple slits does not get rejected by my mind. :smallbiggrin:
But what about the observer? When I have that electron beam behaving strangly and turn my back to it, does it suddenly bhave completely different until I look back at it? What if I make a photo of the pattern without looking at the pattern? Will the photo miraculously show a different image to everyone who looks at it?
Most certainly not!

The last point doesn't make any sense though. If the cat is in the closed system, the cat is also an observer. Or alternatively, you can put the cat-box into another sealed box with yourself and then you could check on the cat, but for the outside world you would have checked and not checked?

I'm not debating about the slit-problem, that I can live with. What bothers me is the idea that a human who looks at a quantum experiment miraculously changes everything. That just seems like total bogus.

Except that taking a photo is measuring it.

The key here is that particles are also waves. The idea is that the only physical thing about the wavefunction of the particle is that it's modulus squared is related to the probability of finding that particle in a particular state. Observing the particle doesn't somehow change anything about it. Instead of the particle being in any state (the probability of which is determined by the wavefunction), instead it is in an Eigen-state of the measurement operator you take on it.

Imagine that you have a whole bunch of different experiments, each set up exactly the same. If you make the same measurement operator on each one, you won't get the same result each time.

Yora
2011-04-11, 07:39 PM
I don't have any problem with the measuring, that part makes sense. Where it gets silly is where the conscious observer is needed to force the particle into a defined state.

After reading a lot about the subject, it turns out that even among the first Quantum scientists, there were some who did introduce asian religion into the whole thing. It wasn't just some New Age guy picking up books about things they didn't understand and buidling their own theories on it.
But that was also heavily criticized by other of the early quantum physicists, who considerd the whole as ridiculous as I did.
While I do believe that some hindu and buddhist concept about how the world is more than what we percieve actually works very well when combined with those fields of physics, in this case it seems quite silly.

From my limited understanding, the whole thing doesn't seem like an actual problem. So our electron exist at the quantum level in multiple possible states and locations at the same time. It's free to do that for as long as it wants until something happens at the atomic level that depends on what state the electron has. At that point the electron has to fall into one defined state and something or another happens on the atomic level. For as long as it doesn't matter on the atomic level what state the electron has, is has all of them at the same time. No big deal?
As I understand it, when sending a single particle through the slits, it will still hit at only one single spot. We only get that pattern that looks like an inference pattern once we have send millions of electrons through the slits. That still leaves the question how an electron interacts with itself (read that up if you're not following, I don't want to type that all up here :smallwink: ), but so long as it still makes only one dot, it works for me.

Brother Oni
2011-04-11, 07:54 PM
Imagine that you have a whole bunch of different experiments, each set up exactly the same. If you make the same measurement operator on each one, you won't get the same result each time.

Have you been sitting in on my method precision experiments or something? :smallsigh:

The Extinguisher
2011-04-12, 04:51 PM
I don't have any problem with the measuring, that part makes sense. Where it gets silly is where the conscious observer is needed to force the particle into a defined state.

After reading a lot about the subject, it turns out that even among the first Quantum scientists, there were some who did introduce asian religion into the whole thing. It wasn't just some New Age guy picking up books about things they didn't understand and buidling their own theories on it.
But that was also heavily criticized by other of the early quantum physicists, who considerd the whole as ridiculous as I did.
While I do believe that some hindu and buddhist concept about how the world is more than what we percieve actually works very well when combined with those fields of physics, in this case it seems quite silly.

From my limited understanding, the whole thing doesn't seem like an actual problem. So our electron exist at the quantum level in multiple possible states and locations at the same time. It's free to do that for as long as it wants until something happens at the atomic level that depends on what state the electron has. At that point the electron has to fall into one defined state and something or another happens on the atomic level. For as long as it doesn't matter on the atomic level what state the electron has, is has all of them at the same time. No big deal?
As I understand it, when sending a single particle through the slits, it will still hit at only one single spot. We only get that pattern that looks like an inference pattern once we have send millions of electrons through the slits. That still leaves the question how an electron interacts with itself (read that up if you're not following, I don't want to type that all up here :smallwink: ), but so long as it still makes only one dot, it works for me.

It's not a conscious observer. Just a measurement. Acting an operator on the system (like a position or momentum operator) puts the system into an eigen-state of the operator. So the system is in an undefined state, and you make a measurement of position. You get a result with a probability based on the wavefunction, and then you put the system in an measurement eigen-, so any further position measurements (without making another measurement) gives you the same result.

Also, even if you send a million electrons through the slit one at a time, you still get the interference pattern. The electron interferes with itself. It's weird like that.



Have you been sitting in on my method precision experiments or something?

Just blame quantum mechanics for all your lab problems. That's what I do.

Brother Oni
2011-04-12, 05:05 PM
Just blame quantum mechanics for all your lab problems. That's what I do.

In my industry (pharmaceutics), we've got a similar catch-all term for unexplainable weird results that happens - contamination.

Of course as soon as the contamination fairy has been invoked, you then have to justify why you can use your data and not repeat the work and also give a reasonable source and mechanism for how it got into your experiment in the first place, all of which has to satisfy the byzantine reasoning and lovecraftian thought processes of the arbitrators of legislation (aka the Quality Department).

shawnhcorey
2011-04-12, 05:09 PM
In my industry (pharmaceutics), we've got a similar catch-all term for unexplainable weird results that happens - contamination.

We have a similar one where I work: It's a hardware problem. :smallsmile:

PirateMonk
2011-04-12, 05:13 PM
From my limited understanding, the whole thing doesn't seem like an actual problem. So our electron exist at the quantum level in multiple possible states and locations at the same time. It's free to do that for as long as it wants until something happens at the atomic level that depends on what state the electron has. At that point the electron has to fall into one defined state and something or another happens on the atomic level. For as long as it doesn't matter on the atomic level what state the electron has, is has all of them at the same time. No big deal?

Why would interacting with something atom-sized or larger cause the electron to suddenly lose all but one of its states?

Mando Knight
2011-04-12, 05:59 PM
This whole thing reminds me of a similar but not quite question I asked once in high school. What would happen if you got a perfect one way mirror and made a glass sphere out of it? Pretty much the same result I suppose, but in my head I imagined you could throw it at someone, it would crack and all the light would escape - laser grenade :D

It would melt or crack from the increasing temperature and pressure of the air within. You didn't specify that the sphere was a vacuum. :smallwink:

douglas
2011-04-12, 09:09 PM
It would melt or crack from the increasing temperature and pressure of the air within. You didn't specify that the sphere was a vacuum. :smallwink:
Wait long enough, and that would happen even with a vacuum interior. Photons in large numbers exert pressure all by themselves.

thubby
2011-04-12, 10:09 PM
Why would interacting with something atom-sized or larger cause the electron to suddenly lose all but one of its states?

because if it were in those other states your observation would be different.

PirateMonk
2011-04-12, 10:10 PM
It would melt or crack from the increasing temperature and pressure of the air within. You didn't specify that the sphere was a vacuum. :smallwink:

If you make the sphere strong enough, you could probably get it to explode. So, you'd have a bomb that detonates when it gets exposed to a certain total amount of light, if our assumptions are correct.


because if it were in those other states your observation would be different.

Probably, but that doesn't mean it makes sense for most of the states to magically disappear just because an electron happens to bump into to something big.

The Extinguisher
2011-04-13, 02:54 AM
Probably, but that doesn't mean it makes sense for most of the states to magically disappear just because an electron happens to bump into to something big.

It's not that the states disappear, it's just that the electron isn't in those states. The operator that you're using has a set of eigen-values associated with it, and the act of making a measurement puts the electron in that eigen-state.

Autolykos
2011-04-13, 06:45 AM
It has absolutely nothing to do with a human (or cat) observing the quantum state. It's just that a measurement is required for the human (or cat) observing the system. And once you measure a quantum state, you force it to assume one of its possible values (with probabilities according to its former state) - You can't get the whole probability distribution from one measurement. What you do with your measurement afterwards doesn't matter - nobody needs to see it (you don't even have to record it), it just has to be done.

Eldan
2011-04-13, 07:13 AM
It is a thought experiment since the physics of the apparatus is such that whether the cat is still alive is determined long before anyone opens the box.

The thing about quantum physics is that you can't measure a quantum without influencing it is some way. This is because the quanta are indivisibly. If you stick your finger in a pot of hot water, your finger will have an influence on its temperature (your finger is cooler than the water so it will lower its temperature slightly) but the influence is insignificant. But when you measure an electron, you can't break of a tiny bit and examine that. An electron, being a quantum, is indivisible. Therefore your measurement has a profound effect on the electron.

Does that make sense?

I remember when our professor once explained electron microscopy (I think it was that) to us like this:

"Imagine that you know that behind that hill there is a stack of cannonballs. You want to know the size and shape of that stack. The only tool you have is a cannon and a near infinite amount of gunpowder and balls. How do you measure the stack? You shoot it at different angles and listen to the sound of impact, then triangulate from that."
:smalltongue:

Mando Knight
2011-04-13, 01:34 PM
If you make the sphere strong enough, you could probably get it to explode.

Only if it fails explosively. It could just crack, leak, and then melt as material around the crack heats up.

shawnhcorey
2011-04-13, 02:28 PM
Only if it fails explosively. It could just crack, leak, and then melt as material around the crack heats up.

If it's insides are a perfect reflector, then it won't heat up. All the light would be reflected back and none of it would heat the sphere.

And if it's inside were a perfect vacuum (might as well add another impossibility), then it would never explode. It would just keep sucking up the light until the universe was perfectly dark.

PirateMonk
2011-04-13, 02:56 PM
And if it's inside were a perfect vacuum (might as well add another impossibility), then it would never explode. It would just keep sucking up the light until the universe was perfectly dark.

Photons have momentum, so they should exert pressure when they're deflected.

douglas
2011-04-13, 03:13 PM
Photons have momentum, so they should exert pressure when they're deflected.
Yes (http://en.wikipedia.org/wiki/Photon_gas), they do (http://en.wikipedia.org/wiki/Radiation_pressure).

Even with a perfect vacuum, the photons themselves would eventually exert enough pressure to break the sphere - unless it's strong enough to hold enough photons to reach black hole energy density. Yes, photons produce gravity too, you just need an utterly ridiculously huge amount of them to produce enough gravity to be noticeable. Of course, if it does hold to that point then either the black hole's Hawking radiation will increase the pressure enough to break it (I don't actually know if the pressure would increase at all) or it'll keep absorbing light until the black hole's event horizon reaches the sphere. In any case, that sphere is not going to last forever. It will either crack or get absorbed from inside by a black hole.

Mando Knight
2011-04-13, 03:49 PM
Before the gravitic effects come into play, it would be more likely that the intense energy and pressure would force a mass-energy conversion, which would then be heated normally, and if the sphere were perfect in reflecting electromagnetic waves, cause it to rise to a temperature and pressure which the vessel cannot withstand.

Though a theoretically perfect one-way mirror-sphere would be a thermodynamically impossible situation, anyway. It would have to allow for energy to pass through with absolutely no energy loss or heat exchange, which isn't possible.