A Question on Light and Temperature

[Navigator]

Greetings

I was told this today:

When light approaches the coldest temperature, which is not zero degress but negative 273 degrees celsius or negative 459 degress fahrenheit (termed "absolute zero"), it ceases to move. Nothing can ever be colder than absolute zero since it is the point at which particles stop moving completely. This stopping of light has not experimentally been verified, but scientists have succeeded in slowing light down quite a bit by approaching absolute zero.

I am informed that Light behaves differently sometimes wave and sometimes particle and that there is Light which can be seen and Light which is invisible.

Is it all still the same light even though different?
Can light be stopped from traveling?


Thank You

 

[neutrino_cannon]

I would be willing to posit that the temperature has no effect on the speed of light.

(credis hunc cum grano salis, I'm not infallable, and furthermore have not yet taken a high school physics course)

The experiments slowing light down were done using a medium that the light traveled through, not variations of temperature, and furthermore, the photons actually moving through the medium were still @ C. They just got re-emmited and refracted enough that the net speed was around 35 MPH.

As for invisable light, there's all kinds. Radio signals are light, after a fashion. Their "color" is so radically different though, that they have different properties, among those being that humans eyes don't pick up on them.

Usually when "light" is refered to, the speaker means visable light. Go look at a complete electromagnetic spectrum sometime, and you will see that there's plenty of kinds of electromagnetic radiation we're blind to. Radio, gamma, x-rays, infra-red, ultraviolet etc. The range of light that our eyes are responsive to is quite small, and I'm told, the band that our sun emits the most of. The difference between red and blue is a minute distinction in wavelength, whereas "invisable light" is distinguished by being radically higher or lower frequency energy than our eyes are receptive too.

Off topic, insects and birds that pollenate can see in ultraviolet, a range of color of higher frequency than the violet we can see. This is because flowers frequently have markings that can only be seen in ultraviolet. Goldfish can see in low red, another color beyond human visual range. Low red is also the frequency that your TV remote works in, so goldfish can see TV remote signals.

Light can be stopped when it absorbed by an opaque surface. Some surfaces, however, will only absorb certain frequencies, whilst allowing others to pass through.

Of course, the real, in-depth truth is likely way more complicated than that. I think I've got a (mostly) true basis for you to find out more though!

As a fina note, there's people on this forum who know this stuff way better than me, see if you can round some up.

 

[Navigator]

Greetings neutrino_cannon

Thank you for that information.

So on the certain opaque surface. light is absorbed. Stopped.

We are speaking of the visible light we see. Would the opaque surface feel warm/hot to touch?

Meaning, does the energy which was light, transform itself within the surface it cannot beam through?

Or has it been absorbed at all, or redirected throughout the opaque surface as in - scattered and reflected away?

Once again, thank you for you thoughful efforts to enlighten me with your reply.

 

[Matabiri]

You can't have a single photon with a temperature of absolute zero, so the postulate in this case is meaningless. As neutrino_cannon says, though, you can have light passing through a medium at absolute zero. Light is propogated by continual absorption and emittance - and in the general case for things like this, the chance of something happening within a set period of time is proportional to

exp(-E/kT)

where E is the "energy barrier" to the event, k is Boltzmann's constant, and T is the (absolute) temperature. So as T drops to zero, the chances of the photon being re-emitted after being absorbed fall as well - hence the apparent slow-down in the speed of light.

On a different point, because there are (usually) several ways an atom can get rid of energy it has absorbed, an absorbed photon can be emitted as several lower energy photons. Infra-red photons - radiative heat - are lower energy than photons of visible light. Alternatively "excited" atoms can dissipate energy through collisions - conduction of heat. While the opaque surface may not warm up enough to feel it by touch*, that is where the energy has gone.

* To give an example though - dark materials left in the sun on a sunny day will feel hot. If a car is left in the sun, visible light streams in through the glass, is "knocked down" into infra-red by interactions with surfaces inside the car, and cannot escape (glass transmits infra-red less efficiently than visible wavelengths). This is the greenhouse effect, basically.

** I also haven't done any of this stuff since I was an undergraduate, and may be wrong.

 

[wollery]

So on the certain opaque surface. light is absorbed. Stopped.

We are speaking of the visible light we see. Would the opaque surface feel warm/hot to touch?

Try it for yourself. Any surface which isn't a mirror absorbs light of some wavelengths.

Meaning, does the energy which was light, transform itself within the surface it cannot beam through?

Or has it been absorbed at all, or redirected throughout the opaque surface as in - scattered and reflected away?

The energy is absorbed by electrons in the surface of the material, pushing them up to higher energy levels within their atomic orbits. These electrons then drop back down to lower energy levels, usually in more than one hop. These smaller energy level changes give out light radiation at a lower energy (longer wavelength) than the light that pushed the electron up in the first place. This energy is therefore usually emitted in the infra-red part of the spectrum.

Of course, the amount of light hitting a surface under normal circumstances is quite small and the amount of energy absorbed is small and can be re-emitted at the same rate, so the surface does not heat up. On particularly sunny days dark surfaces will heat up as they are absorbing more energy than they can emit, although as they get hotter their emission efficiency goes up and the temperature levels out.

Edit to add Darn Matabiri beat me to it!

 

[Navigator]

Edit to add Darn Matabiri beat me to it!

Greetings wollery , Matabiri.

Thanks for both replies. You may have said the same thing differently, but this adds to the whole equation in other ways.

So it's seems that light is never stopped in its tracks, but transforms?

This would suggest that all things are made of light?

Just different aspects of the same thing, bounching around off itself.

There are different types of light, visible and nvisible, all doing what light does...what is it that light does?

Obviously, it does not just illuminate, although that is the common understanding of light, as has been mentioned.

But invisible light does,'t illuminate does it?

Maybe it does illuminate something which visible light doesn't?

Can invisible light penetrate the reflective properties of mirrors?
(I am thinking that they could.)

Is everything really one aspect of light or another, o mixtures of some or many?

It could be that the whole universe if filled with light, only we can't see this....

Thank You for your answers.

 

[MRC_Hans]

*snip*

So it's seems that light is never stopped in its tracks, but transforms?

Light can be transformed to other forms of energy; electrical energy, heat energy, chemical energy, even kinetetic energy, and, and of course, different wavelengths of EM energy.

This would suggest that all things are made of light?

No, not even for the broadest definition of the term light.

Just different aspects of the same thing, bounching around off itself.

No. Light is a form of energy. "Things" are matter. Matter can be converted to energy and vice versa, but it does not make sense to say that they are the same.

There are different types of light, visible and nvisible, all doing what light does...what is it that light does?

The term light denotes a small part of the spectrum of electromagnetic radiation. The difference is primarily in the wavelength, but also in the energy of the photon involved. The term "invisible light" covers the wavelengts adjacent to the visual parts of the spectrum. We call this light because it behaves very much like visible light (and indeed some creatures are able to sense this light).

Obviously, it does not just illuminate, although that is the common understanding of light, as has been mentioned.

Light moves in straight lines and will be reflected, deflected, and/or refracted by objectd in its path. Our visual sense uses this light that has been modified by an object to get information about the object, to see.

But invisible light doesn't illuminate does it?

It does, but our visual sense cannot percieve it. Other creatures and instruments made for the purpose can, however. An extreme example is RADAR; it "illuminates" an object with microwaves and "sees" it that way.

Maybe it does illuminate something which visible light doesn't?

Yes, you could say so. The way light in influenced by an object is very much dependent on the wavelength. For instance a piece of green glass is opaque to othere wavelengths than green.

Can invisible light penetrate the reflective properties of mirrors?
(I am thinking that they could.)

The reflective property of most mirrors comes from a thin layer of aluminum. Possibly this layer is transparant to some wavelengths, in which case your statement is correct.

Is everything really one aspect of light or another, o mixtures of some or many?

No.

It could be that the whole universe if filled with light, only we can't see this....

The universe IS "filled with" light, and we can see it. Just look at all those stars. The reason the space between the stars look dark is not that there is no light passing through it, but that light does not reach our eyes. Light travels in straight lines (barring gravitational lensing), and you cannot see a light ray "from the side".

Thank You for your answers. [/B]

Mmm, what are you really getting at here?

Hans

 

[Matabiri]

Can invisible light penetrate the reflective properties of mirrors?
(I am thinking that they could.)

The reflective property of most mirrors comes from a thin layer of aluminum. Possibly this layer is transparant to some wavelengths, in which case your statement is correct.

Not really - a conductive layer is essentially a perfect Faraday cage - except for wavelengths longer than the surface - but that's not really "transparency".

A Bragg mirror, on the other hand...

Mmm, what are you really getting at here?

Good question. It would be nice for people to actually read a basic physics textbook before asking questions like that.

Good answers as well; I was putting answering them aside as I wasn't sure where to start amongst that jumble...

 

[Zombified]

A material which is above 0 degrees K (even by a very tiny amount) emits electromagnetic radiation (e.g. photons). Get it hot enough - red hot, that is - and you get actual visible light. Make it even hotter and it glows white (think of an incandescent light bulb).

The hotter you make something, the higher the frequency of the emitted light (on average). This is called black-body radiation (the odd name just means the light is purely thermal in origin and has nothing to do with the composition of the material itself - it works the same for any material). Basically, collisions and/or vibrations in the hot material have energy associated with them, and sometimes that energy is emitted as a photon.

Now, as you cool down, you go from white-hot to red-hot and then down into near infrared, which is invisible. You can't see any light any more, but it still feels warm if you hold your hand near it. That's the radiation you're feeling. Cool further, and you get further into the infrared.

(Regarding "invisible light", think of your eyes as a tuner that can get signals in a certain frequency range. It can't tune signals outside its range, any more than you can get an FM station on an AM-only radio. But they're still all the same thing: electromagnetic radiation.)

If you get the material cold enough, you'll eventually get down into microwaves (radio) and further. For example, the microwave background radiation in space corresponds to about 3 degrees kelvin - extremely cold, but still enough warmth to generate radio waves.

At absolute zero, there is no transfer of energy at all between adjacent atoms or material. This means not only kinetic energy, but radiated energy, either. The frequency of light goes to zero as the temperature does. At absolute zero, there is no light at all.

The interaction of light with matter basically transfers the energy of a photon to or from another particle, like an electron. Photons can transfer potential energy (e.g. knocking an electron out of an atom or a metal conductor) or kinetic energy (e.g. scattering an atom in a material).

It is not the case that all matter is made of light - light consists of a particular type of particle (photons), whereas matter is made out of a bunch of different particles, especially electrons, protons, and neutrons. Those particles seem less ephemeral than photons because they have rest mass, so they're still there when they don't have any kinetic energy, whereas photons are all kinetic energy. Plus, those types of particles "stack up" and are in some sense solid (I am really trying to keep it simple here) whereas photons pass right through each other.

An "ideal" perfect mirror would reflect all electromagnetic waves of any frequency, but no such thing exists in reality. Low frequencies that have longer wavelengths than the thickness of the reflector will pass through, at least partially, and very high frequencies (X, gamma rays) are very penetrating and aren't reflected by matter.

 

[Zombified]

Regarding "slowing" light, light always slows down when it goes through matter. Light going through water or glass goes slower than it does through air, and that's (slightly) slower than going through a vacuum.

It's related to light "bending" when it goes through a piece of glass (like a prism or a lens).

When light gets slowed to ridiculously slow speeds, it just means somebody's found a state of matter where light propogates through the material very, very slowly. These funny states of matter generally require very low temperatures - otherwise, the heat jostles the atoms out of their precise alignment.

But the slowing itself is nothing unusual, just the amount of it.

 

[hammegk]

I'm hoping one of the resident PhDs will provide some insight into Navigator's question that leads to many odd places.

Does the concept of a single boson have meaning?

If we have 2 bosons, is temperature definable? Does "distance apart" have bearing on the answer?

Will the zero-point field ever disappear assuming a heat death scenario?

 

[richardm]

Regarding "slowing" light, light always slows down when it goes through matter. Light going through water or glass goes slower than it does through air, and that's (slightly) slower than going through a vacuum.

Does it? I was told that light travels at a constant speed, but in some materials it bangs around a lot and takes longer to make its way through.

Or is that just a meaningless quibble?

 

[Matabiri]

Does it? I was told that light travels at a constant speed, but in some materials it bangs around a lot and takes longer to make its way through.

Or is that just a meaningless quibble?

At some point you have to make a distinction between a macrostructural model (the material is a constant medium, and light passes through it slowly) and an atomic-scale model (light bangs around a lot but between-times travels at c, so the average speed is as measured above).

It's not a meaningless quibble, it's about scale, simplifications, and exactness of language.

 

[SGT]

* To give an example though - dark materials left in the sun on a sunny day will feel hot. If a car is left in the sun, visible light streams in through the glass, is "knocked down" into infra-red by interactions with surfaces inside the car, and cannot escape (glass transmits infra-red less efficiently than visible wavelengths). This is the greenhouse effect, basically.

Not really! Part of the energy from the surfaces id emmited as infra-red, but most of it is used to warm the adjacent molecules of air (The molecules of the material vibrate because of their thermal energy and transmit their kynectic energy to the air molecules).
The greenhouse effect happens because the warmed air molecules are trapped inside the greenhouse, or car, and cannot disperse in the atmosphere.

 

[Matabiri]

Not really! Part of the energy from the surfaces id emmited as infra-red, but most of it is used to warm the adjacent molecules of air (The molecules of the material vibrate because of their thermal energy and transmit their kynectic energy to the air molecules).
The greenhouse effect happens because the warmed air molecules are trapped inside the greenhouse, or car, and cannot disperse in the atmosphere.

That's also a major effect, true, but not the chief one. The much vaunted atmospheric Greenhouse Effect is also due to the inability of certain gases to transmit infrared.

Edit to add: were the effect mainly conduction, the hot surfaces would be able to radiate the heat straight back out of the car again, and there would not be such a dramatic effect.

Another edit to add: You will also find that the surfaces in a greenhouse car are much hotter than the air.

 

[Zombified]

If you have materials at different temperatures they will eventually exchange heat and reach equal temperatures, even if they aren't in contact, purely by radiating and absorbing infrared. So if two masses of air are seperated by glass, they would reach the same temperature if the glass was transparent to IR. Since glass isn't that transparent, the IR is reflected back into the car.

 

[Matabiri]

Originally posted by Zombified

Ta. Can I buy you some brains?

 

[Zombified]

I'm hoping one of the resident PhDs will provide some insight into Navigator's question that leads to many odd places.

Not a PhD, but while the cats are away...

Does the concept of a single boson have meaning?

It depends. There are certainly plenty of situations and experiments where that is a good approximation. If you're talking about a situation where thermodynamics is important, then you have photons constantly produced and absorbed. And in field theory you can have photons produced and absorbed in interactions where the "actual number" of photons isn't really definite.

This isn't limited to bosons. Any particle can be produced in a field interaction this way, if the interaction involves enough energy to produce one or more particles (often a pair). You need at least as much energy as the rest mass of the particles, but uncertainty in energy can permit these extra particles to appear for short periods of time.

If we have 2 bosons, is temperature definable? Does "distance apart" have bearing on the answer?

This one I'll really have to leave to the experts. My guess is you can if you think of temperature purely as a function of energy, but its not something you'd think of measuring with a thermometer.

Will the zero-point field ever disappear assuming a heat death scenario?

Are you referring to vacuum polarization? I'm guessing you'll always have that.

Heat death doesn't mean there's no energy. It just means the whole universe is in thermal equilibrium and you can't extra work from temperature differences. Vacuum polarization doesn't depend on energy or temperature differences, it's just a manifestation of uncertainty.

 

[Zombified]

Originally posted by Zombified

Ta. Can I buy you some brains?

Brains are smart food!

 

[wollery]

If we have 2 bosons, is temperature definable? Does "distance apart" have bearing on the answer?

This one I'll really have to leave to the experts. My guess is you can if you think of temperature purely as a function of energy, but its not something you'd think of measuring with a thermometer.

I guess I should field this one. Thermodynamic temperature is purely a function of the mean kinetic energy of the particles, whereas heat is the sum of the kinetic energies of the particles.

In astronomy you will often hear people talk about interstellar or intracluster gas being "hot". What it means is that the atoms and molecules are whizzing around really fast and their interactions give out X-rays (ie very high energy). If you were to take a thermometer and measure the physical temperature (heat) it would be very cold, because the gas has such a low density that collisions are pretty rare.