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Boris
2004-Oct-08, 06:07 PM
My son is in third grade (public school in central VA), where in science class they are trying to teach him that LIGHT IS MATTER.

With the possible exception of the crazy quantum mechanics, I've never seen any source claim that light (or radiation) is matter. My understanding is that light is not matter practically by definition: matter is all things that are not radiation.

Certainly this would be true in the relatively simple science that 3rd graders are getting, wouldn't it?

(Granted, light is treated as a _particle_ to explain certain interactions with matter, like the photoelectric effect. Also granted that light/radiation/energy can be transmuted into matter, such as for example gamma rays spontaneously transmuting into electron/positron pairs and vice-versa).

Related question for the physicists on the board:

What is the definition of matter?

What THINGS in our universe, other than light, are not matter?

Can the _impulse_ of a photon be taken as evidence that it is matter?

Boris



PS

What really angers me is that they set up an experiment in the class to determine what things are matter and what are not. Using a glass of water, full to the brim, things were introduced into the water (rock, air, and light) to see if the glass would overflow (thus showing the thing had volume, a defining quality of matter I suppose, next to mass).

My son did the experiment, determined that shining light into the glass did not make it overflow, and concluded that light was not matter. He was told he must have made a mistake! What the &^$#!? (Other kids found that shining light into the glass DID make it overflow. Our future in science! Go figure...)

I agree kids should be taught critical thinking, but this goes a bit too far;-)


PPS

I'm not sure the water glass / light experiment would result in an overflow even if light WAS matter... surely light would not displace very much water, even if it did stay in the glass for longer than two femtoseconds. What's worse, enough light would heat the water - i.e. enough light would be absorbed - causing it to expand slightly and the glass to overflow, showing that light was matter. Because photons absorbed by water would add to the water's volume. (This is a rhetorical statement, folks. I'm playing the devil's advocate as best I can. That's a scientific method, is it not?)

ToSeek
2004-Oct-08, 06:20 PM
I would say that light is not matter, and if a glass of water overflows when you shine a light on it, then odds are you bumped the table. Also note that in that experiment, if light were matter, then the water should continue to overflow until the glass is empty of water and full of light. Somehow I doubt that happens.

eburacum45
2004-Oct-08, 06:51 PM
energy and matter are equivalent in some respects, of course, and if you shine a torch onto a black hole it gets more massive.

is that what they are trying to say?

kanon14
2004-Oct-08, 07:32 PM
probably the other kids just threw the whole flashlight into the glass #-o

Cl1mh4224rd
2004-Oct-08, 08:14 PM
probably the other kids just threw the whole flashlight into the glass #-o
That's exactly what I was thinking... #-o

crazy4space
2004-Oct-08, 08:54 PM
I am just a dumb old hick from nowhere Missouri however, didnt Mr. Einstein say E=MC2. Isnt mass, matter and energy interchangable? If you do the equation backwards you still get the same result?

ToSeek
2004-Oct-08, 08:59 PM
I am just a dumb old hick from nowhere Missouri however, didnt Mr. Einstein say E=MC2. Isnt mass, matter and energy interchangable? If you do the equation backwards you still get the same result?

You can trade money for food; it doesn't mean you can eat money. To be less flippant, yes, matter and energy can be converted back and forth, and even energy has mass. But I'd still consider them to be two different things.

crazy4space
2004-Oct-08, 09:07 PM
can trade money for food; it doesn't mean you can eat money. To be less flippant, yes, matter and energy can be converted back and forth, and even energy has mass. But I'd still consider them to be two different things.[/quote]

Isnt the whole point this - that they do have mass otherwise they would be undetectable?

Normandy6644
2004-Oct-08, 09:46 PM
I think matter and energy, while certainly related, are two different things. If you considered light to be bundles of photons, then (although massless) I think you can consider it matter. Though something tells me this is a bit sophisticated for third graders. :D

dgavin
2004-Oct-08, 11:48 PM
There are a lot of theroies attempting to explain what 'light' or photons are.

The problem is they usually either favor light being a particle alone, or light being a energy wave alone. When it's already been proven to exhibit the properties of both.

I cannot remember the source of this, but one of the theroies is that a photon is a combination of a +1/3 mass particle and a -1/3 mass particle. (where as an electron would be a +1 mass particle). It starts diving into what I call as a layman, mass/anti mass? particle interaction. The net effect of the two particles is a rest mass of zero, but as each particle is an opposite, they repel then atract, collide and convert to energy, which imediately cools down back into similar particles. They travel in the direction that the intial release of energy was going when it cooled to form the photon.

I'm not cetrain but I think this one is really saying that light is the only know case of two stable virtual particles.

It's still a theroy, but the only one I remember that explained how it might be both a particle, and an energy wave, and maintain itself without a net loss of speed or material. Basically a case of cyclic entropy. It's energy, cools to matter, collides, producing same amount of energy, and keeps doing this until it is absordbed by real matter (usually resulting in a shift of the electrons orbits)

Anyone know more about this theroy? I read about it over two years ago, and only understood the basics. But it did make a lot of sense.

CUStudent
2004-Oct-08, 11:51 PM
I think matter and energy, while certainly related, are two different things. If you considered light to be bundles of photons, then (although massless) I think you can consider it matter. Though something tells me this is a bit sophisticated for third graders.

At the same time, aren't we doing them a disservice by giving them the wrong impression? Sure, you can tell the kids that light is energy, but teaching them that light is definitly matter at this early age is going to cause some confusion later on. Something tells me that this is just a confused third grade teacher. No offense to teachers, but I doubt an elementary ed major in college took enough physics to understand Einstein.

Normandy6644
2004-Oct-09, 12:10 AM
I think matter and energy, while certainly related, are two different things. If you considered light to be bundles of photons, then (although massless) I think you can consider it matter. Though something tells me this is a bit sophisticated for third graders.

At the same time, aren't we doing them a disservice by giving them the wrong impression? Sure, you can tell the kids that light is energy, but teaching them that light is definitly matter at this early age is going to cause some confusion later on. Something tells me that this is just a confused third grade teacher. No offense to teachers, but I doubt an elementary ed major in college took enough physics to understand Einstein.

I agree with you, though at the same time I'm not sure the students will absorb it enough to really have it make a difference for any future physicists. :D

Kebsis
2004-Oct-09, 12:35 AM
I am just a dumb old hick from nowhere Missouri however, didnt Mr. Einstein say E=MC2. Isnt mass, matter and energy interchangable? If you do the equation backwards you still get the same result?

Well, mass and matter are not interchangable, especially in Einstein's equations. And light, to my knowledge, has zero rest mass.

Normandy6644
2004-Oct-09, 12:37 AM
I am just a dumb old hick from nowhere Missouri however, didnt Mr. Einstein say E=MC2. Isnt mass, matter and energy interchangable? If you do the equation backwards you still get the same result?

Well, mass and matter are not interchangable, especially in Einstein's equations. And light, to my knowledge, has zero rest mass.

Yeah, the energy of light comes from E^2=(pc)^2+(moc^2)^2. Since m is 0, E=pc, where p is the momentum.

Boris
2004-Oct-09, 12:44 AM
I agree with you, though at the same time I'm not sure the students will absorb it enough to really have it make a difference for any future physicists. :D

Although I think the question of whether light is matter is a good one, and I'd still like to have an authoritative answer (please!), my point with the teacher (and possibly the school, and maybe the school system or even the state!) is that they've set up this experiment to show that light is matter, and then they ignore the experimental results. Now granted the experiment may be flawed (see my PPS in the initial post), the fact is my son conducted the experiment correctly, found a negative result, and now the teacher is telling him he's done it wrong.

THAT is the problem. No, he hasn't done it wrong. Just because the result is not what the teacher expected, does not make it wrong. But before I get much farther into this with the "authorities," I thought I'd better check here to make absolutely sure I haven't missed anything.

I've already corresponded with the teacher, to ask her what source(s) indicate to her that light is matter. Depending on her reply (not yet received) I may have to take this further up the educational command chain.

Again, I agree that teaching the FACT that light is matter is not such a big problem. They will learn the "truth" sooner or later, whatever it may be. But if it is a FACT that light is NOT matter, and if a student's work has shown that light is not matter, then I do think it is a big problem if the teacher asks the kid to sweep his results under the table just so the teacher's scientific cart is not upended. This the exact opposite of what kids should learn about science.

(Thanks to all this they may soon learn how much controversy science research can generate!)

If I may assume that light is NOT matter, then the situation right now in this class is that shoddy experimenting (or worse: fraud) is being rewarded by this teacher, and careful experimenting is being punished.

So yes, I claim this is a crucial issue for our future physicists.

Boris
2004-Oct-09, 12:52 AM
Yeah, the energy of light comes from E^2=(pc)^2+(moc^2)^2. Since m is 0, E=pc, where p is the momentum.

This is interesting to me. Does this explain why a photon can exert a force (on say a solar sail) even though it has no mass? How can something without mass have momentum? (We know that classical, newtonian momentum p = mv) It's got to have something to do with relativity I guess.

This relates to my question (#3 in the initial post), can the impulse of a photon be taken as evidence that the photon is matter.

Boris

Boris
2004-Oct-09, 01:05 AM
What is the definition of matter?

What THINGS in our universe, other than light, are not matter?

Can the _impulse_ of a photon be taken as evidence that it is matter?


I appreciate everyone's contributions so far - they've not yet shaken my understanding of what light and matter are, and that they are different!

But I notice no one has tried to address these three questions that I posted originally, which I consider crucial to this topic. I'll offer my own answers, by way of example, and maybe an expert can give me some support or shoot me down.

1. matter is all things that have mass and take up space (volume)

2. one thing other than matter in the universe might be a magnetic field, easily demonstrated to third graders. Thus "fields" or "forces" are something tangible, yet not matter.

3. the impulse of a photon does not make it matter see def. #1. Just because light may mediate a transfer of energy between two separate objects in space, does not make it matter. Magnetic fields do the same thing, and they are not matter either.

Any other ideas?

Boris

Boris
2004-Oct-09, 01:15 AM
Now that I've offered up a definition for matter, I can't help trying to define light... and I've had some strange thoughts!

Matter on the one side is clear to me, easily defined.

Forces or fields on the other side are more mysterious, but maybe can be defined generally as interactions between discrete matter particles. (these interactions can be described using particle exchanges, or distortions in space, etc.).

This sort of gives us a definition for all things not matter, except... light.

Light seems to be something in between the two. Light seems unique. (I should say electromagnetic radiation generally. Light is just a tiny subset of all electromagnetic radiation). On one hand light can be interpreted as a disturbance in the "non-matter" electromagnetic field, on the other hand it is seen to behave like a particle, nominally like matter, a photon.

So is light a third category of entity? Not matter, nor a field?

Boris

Tobin Dax
2004-Oct-09, 02:41 AM
Now that I've offered up a definition for matter, I can't help trying to define light... and I've had some strange thoughts!

Matter on the one side is clear to me, easily defined.

Forces or fields on the other side are more mysterious, but maybe can be defined generally as interactions between discrete matter particles. (these interactions can be described using particle exchanges, or distortions in space, etc.).

This sort of gives us a definition for all things not matter, except... light.

Light seems to be something in between the two. Light seems unique. (I should say electromagnetic radiation generally. Light is just a tiny subset of all electromagnetic radiation). On one hand light can be interpreted as a disturbance in the "non-matter" electromagnetic field, on the other hand it is seen to behave like a particle, nominally like matter, a photon.

So is light a third category of entity? Not matter, nor a field?

Boris

You're so close. Light *is* all electromagnetic radiation. Electric and magnetic fields are produced/governed (I can't think of better words right now) by exchanges of photons. Every force (e-m, gravity, strong and weak nuclear) is governed by a force-transmitting particle (photon, graviton, Z & W bosons, IIRC). Light, a photon, falls right into where you want to put it. It is a particle that gives us e-m forces.

DALeffler
2004-Oct-09, 04:37 AM
What THINGS in our universe, other than light, are not matter?

Time? Distance?

[Edit-1] What about the 4 states of matter (solid, liquid, gas, plasma)? If something's not one of those four things, can we say it's not matter?[/Edit-1]

worzel
2004-Oct-09, 11:36 AM
I think the teacher needs to be clear about whether they are talking about rest mass or relativistic mass, but apparently "relativistic mass"is old fashioned now anyway. Have a look at what the Usenet Physics FAQ (http://math.ucr.edu/home/baez/physics/) has to say about mass (http://math.ucr.edu/home/baez/physics/Relativity/SR/mass.html) and light (http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html).

Zamzara
2004-Oct-09, 11:43 AM
I'm guessing that some students got the water to overflow because it came out of the tap cold, and expanded as it slowly warmed up to room temperature. If the light was bright that could have warmed it a bit too.

Russ
2004-Oct-09, 06:31 PM
Please excuse me if I'm repeating someone elses answer. I just skimmed through the answers and didn't see this.

The obvious answer to the question: Is light matter? The answer can only be no. If light was matter it would have mass. If it had mass it could not travel at the speed of light because it would take more energy than exists in the universe to get it up to C.

It is a phase state of matter by virtue of E=mC^2 but that does not mean it is matter any more than ice is steam. :D

Boris
2004-Oct-10, 02:33 AM
Light seems unique. (I should say electromagnetic radiation generally. Light is just a tiny subset of all electromagnetic radiation).

You're so close. Light *is* all electromagnetic radiation.

I was trying to be clear that this topic is about electromagnetic radiation / photons generally. Because some readers here might consider light to mean just _visible light_, I wanted to clarify that I am using the word to mean the entire spectrum of e-m radiation, i.e. e-m radiation generally, or photons.






Electric and magnetic fields are produced/governed (I can't think of better words right now) by exchanges of photons.

Now this I don't understand. You are treating light as a field?


Every force (e-m, gravity, strong and weak nuclear) is governed by a force-transmitting particle (photon, graviton, Z & W bosons, IIRC). Light, a photon, falls right into where you want to put it. It is a particle that gives us e-m forces.

So you mean when some electron transitions happen in the tungsten of a light bulb filament, the e-m field is disturbed (mediated by photon), and this disturbance ultimately makes it's way to some electrons in my retina, where they are affected by the disturbance, exciting a cell, signalling my brain, etc....? I've never thought of light that way, but I guess it could work! (It sounds a lot like the aether).

Tobin Dax
2004-Oct-10, 02:57 AM
Light seems unique. (I should say electromagnetic radiation generally. Light is just a tiny subset of all electromagnetic radiation).

You're so close. Light *is* all electromagnetic radiation.

I was trying to be clear that this topic is about electromagnetic radiation / photons generally. Because some readers here might consider light to mean just _visible light_, I wanted to clarify that I am using the word to mean the entire spectrum of e-m radiation, i.e. e-m radiation generally, or photons.

Gotcha. It is hard to tell sometimes. Plus, I'm still getting blank stares from students when I tell them that radio waves travel at the speed of light, over 6 weeks into the semester.






Electric and magnetic fields are produced/governed (I can't think of better words right now) by exchanges of photons.

Now this I don't understand. You are treating light as a field?

No, I'm treating light as a particle (which it is). Although it looks like I said "fields" when I was thinking "forces." The fields do influcence the forces, but photons are the particles that transmit the force. (There's the word I should have used.)






Every force (e-m, gravity, strong and weak nuclear) is governed by a force-transmitting particle (photon, graviton, Z & W bosons, IIRC). Light, a photon, falls right into where you want to put it. It is a particle that gives us e-m forces.

So you mean when some electron transitions happen in the tungsten of a light bulb filament, the e-m field is disturbed (mediated by photon), and this disturbance ultimately makes it's way to some electrons in my retina, where they are affected by the disturbance, exciting a cell, signalling my brain, etc....? I've never thought of light that way, but I guess it could work! (It sounds a lot like the aether).[/quote]

That sounds like a decent explanation, though I'm honestly not sure how good it is. Though, in light of my misspeaking (miswriting?) earlier, it might be better to say that the tungsten atom exerts a slight electromagnetic force on your retina, transmitted via the emitted photon. (Wow, that sounds better now than when I began to write it. :))

Evan
2004-Oct-10, 04:57 AM
Hmm, speed of light. It is only constant in a vacuum. Where exactly is a vacuum to be found? Interstellar space is thought to contain around 100,000 to 1,000,000 hydrogen atoms per cubic meter. A very thin and transparent gas to be sure, but not a vacuum. Also, the speed of light through transparent matter is frequency dependent, hence the prism and the rainbow. The speed of radio waves through a coaxial cable is around 1/2 C. The denser the medium the slower it is. Then we have a couple of exceptions. The Casimir effect (http://www.daviddarling.info/encyclopedia/C/Casimir.html).

Then there is quantum tunneling.

"According to Nimtz, Mozart's 40th Symphony hopped across 12 centimeters of space at 4.7 times the speed of light. What's more, Nimtz actually had a recording to prove it. To his now bemused audience, he played a tape in which among the background hiss strains of Mozart could be heard. This was the 'signal' that had traveled faster than light."

http://www.wsws.org/public_html/prioriss/iwb9-9/light.htm

The equivalence of matter and energy is not really debated. It is more a matter of semantics. There is interesting reading on this here:

http://plato.stanford.edu/entries/equivME/

Tobin Dax
2004-Oct-10, 07:13 AM
Am I missing some conclusion here, or are you just throwing out more bits to speculate on?


Also, the speed of light through transparent matter is frequency dependent, hence the prism and the rainbow.

I'd say it's wavelength dependent. The frequency stays the same (in most cases) while the wavelength changes.

worzel
2004-Oct-10, 11:47 AM
Hmm, speed of light. It is only constant in a vacuum.
A photon always travels at c. If they are travelling through a material they may appear to travel slower because they will get absorbed and re-emitted by atoms of the material. They travel at c between the emission/absorbtions.

Where exactly is a vacuum to be found?
Between the atoms of every material :)

Evan
2004-Oct-10, 02:39 PM
Frequency determines wavelength. Either is valid to express peak to peak time or distance of an electromagnetic oscillation.

Frequency does not stay the same as wavelength changes, it is the reciprocal of wavelength.

Worzel said "Between the atoms of every material"

The Casimir effect indicates otherwise. It is not conjecture either, it is an experimentally verfiable effect.

worzel
2004-Oct-10, 03:15 PM
Worzel said "Between the atoms of every material"

The Casimir effect indicates otherwise. It is not conjecture either, it is an experimentally verfiable effect.
Just google'd "Casimir Effect". Interesting, but from what I read it appeared to be about a force between to surfaces in a vacuum.

Tobin Dax
2004-Oct-10, 06:33 PM
Frequency determines wavelength. Either is valid to express peak to peak time or distance of an electromagnetic oscillation.

Frequency does not stay the same as wavelength changes, it is the reciprocal of wavelength.

Not inside of materials. The speed of light changes inside of materials, so lamdba no longer equals c/nu--it's now c/n*1/nu. Just as the permeablility can stay the same as the permittivity changes, the frequency can stay the same as the wavelength changes.

Evan
2004-Oct-11, 03:18 AM
Worzel,

If you check the link I posted above you will see this:

"There is another interesting possibility for breaking the light-barrier by an extension of the Casimir effect. Light in normal empty space is " slowed" by interactions with the unseen waves or particles with which the quantum vacuum seethes. But within the energy-depleted region of a Casimir cavity, light should travel slightly faster because there are fewer obstacles. A few years ago, K. Scharnhorst of the Alexander von Humboldt University in Berlin published calculations4 showing that, under the right conditions, light can be induced to break the usual light-speed barrier. Under normal laboratory conditions this increase in speed is incredibly small, but future technology may afford ways of producing a much greater Casimir effect in which light can travel much faster. If so, it might be possible to surround a space vehicle with a " bubble" of highly energy-depleted vacuum, in which the spacecraft could travel at FTL velocities, carrying the bubble along with it. "

This experiment has been performed. Under these conditions light travels faster than "C" in a "normal" vacuum. So then, what is the velocity of light?

Evan
2004-Oct-11, 03:37 AM
Tobin,

The wavelength and frequency of light in vacuum (whatever that really is) is directly related as the reciprocal. That will determine the amount of refraction when that light enters a transparent medium. Yes, the wavelength will be compressed depending on the refractive index of the medium. The frequency will not change since that would mean the light would change color.

eburacum45
2004-Oct-11, 05:48 AM
Surroundimg a spaceship with a bubble of negatively charged energy in order to travel faster than light is a real possibility, explored in the so-called Alcubierre-Broeck drive concept;

unfortunately the details of this concept are a little difficult to achieve, as the spacecraft has to be shrunk to the effective size of an atom in order to fit inside the bubble.
Maybe one day, but not in the immediate future.

Quantum tunneling is a real FTL effect, like quantum entanglement; but these phenomena are not believed to carry usable information at faster than light speeds.

Evan
2004-Oct-11, 05:49 AM
One more thing. According to Einstein:

""the mass of a body is a measure of its energy-content; if the energy changes by L, the mass changes in the same sense by L/9 × 10^20, the energy being measured in ergs, and the mass in grammes"

This is easily experimentally verified.

Evan
2004-Oct-11, 06:03 AM
Eburacum45,

Quantum tunneling can carry usable information at FTL speed. It is subject to restrictions, but not those envisioned by Einstein.

Read more here:

http://www.physics.sfsu.edu/~ebierman/research/FastLightCausality.pdf

worzel
2004-Oct-11, 08:22 AM
Worzel,

If you check the link I posted above you will see this:

[snip]

This experiment has been performed. Under these conditions light travels faster than "C" in a "normal" vacuum. So then, what is the velocity of light?

Oh, you're talking about c, when you quoted me you seemed to be questioning my statement about vacuums.

Again, what you post is interesting. But the effect is very small and difficult to reproduce compared to the apparent slowing down of light through a medium. The Cassimir effect not withstanding do you agree with what I originally posted?

eburacum45
2004-Oct-11, 11:21 AM
There seems to be some possibility of faster than light transmisson over very small distances by quantum tunneling, or though vacuum containing negative energy; but the information seems to be subject to uncertainty, so needs a classical channel message as well to set up a comparison; this classical channel message is slower than light.

However, if any of these tiny effects do allow FTL information transfer at some time in the future, the door then opens to causality paradoxes; you can't have messages passing around in relativistic space time without eventually running into causality paradoxes.

This may or may not rule out any FTL information transfer altogether.

Spaceman Spiff
2004-Oct-11, 02:04 PM
What is the definition of matter?

What THINGS in our universe, other than light, are not matter?

Can the _impulse_ of a photon be taken as evidence that it is matter?


I appreciate everyone's contributions so far - they've not yet shaken my understanding of what light and matter are, and that they are different!

But I notice no one has tried to address these three questions that I posted originally, which I consider crucial to this topic. I'll offer my own answers, by way of example, and maybe an expert can give me some support or shoot me down.

1. matter is all things that have mass and take up space (volume)

2. one thing other than matter in the universe might be a magnetic field, easily demonstrated to third graders. Thus "fields" or "forces" are something tangible, yet not matter.

3. the impulse of a photon does not make it matter see def. #1. Just because light may mediate a transfer of energy between two separate objects in space, does not make it matter. Magnetic fields do the same thing, and they are not matter either.

Any other ideas?

Boris

1. That's good, but let's make that "rest mass". And taking up space isn't necessary. No experiment has ever found the electron to take up any space, except that "space" dictated by the Heisenberg Uncertainty principle.

2. magnetic fields, electric fields, electromagnetic fields, .... A field is an entity with the potential to cause a change in momentum with time (or equivalently, cause a spatial gradient in potential energy). Certainly, magnetic fields are easy to demonstrate.

3. That's right. Real photons can exert momentum, and virtual photons are the exchange particle of the Electromagnetic force (but I don't think that discussions of quantum field theory and virtual exchange particles are appropriate for 3rd graders).

Either your son's teacher or the science dept or, ... has their head in a fog about these things. This is clearly BAD Science, no question, based upon interconnected layers of misconceptions.

TravisM
2004-Oct-11, 03:27 PM
Interstellar space is thought to contain around 100,000 to 1,000,000 hydrogen atoms per cubic meter.

I heard more like 3 atoms per cubic meter... a near vaccum...

Evan
2004-Oct-11, 03:46 PM
Worzel,

If you mean do I agree with the absortion and re-emission statement, then, yes. The question is "what is a true vacuum?" The classical definition of a vacuum breaks down when quantum properties of matter and space are considered. As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".

TravisM,

Perhaps you are thinking of the density per cubic centimeter. Check here:

http://hypertextbook.com/facts/2000/DaWeiCai.shtml

Spaceman Spiff
2004-Oct-11, 04:17 PM
Hmm, speed of light. It is only constant in a vacuum. Where exactly is a vacuum to be found? Interstellar space is thought to contain around 100,000 to 1,000,000 hydrogen atoms per cubic meter. A very thin and transparent gas to be sure, but not a vacuum.



Here is a site (http://hypertextbook.com/facts/2000/DaWeiCai.shtml)listing representative gas densities of the medium filling the Galaxy.

This one (http://hypertextbook.com/facts/2000/ChristinaCheng.shtml)tabulates representative densities averaged over very large volumes of space. There is a lot of mostly empty space between galaxies and especially between galaxy clusters. Best estimate for this average is about 0.25 protons per cubic meter.

worzel
2004-Oct-11, 07:55 PM
Worzel,

If you mean do I agree with the absortion and re-emission statement, then, yes. The question is "what is a true vacuum?" The classical definition of a vacuum breaks down when quantum properties of matter and space are considered. As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".

Fair enough. I was just giving the normal answer to the misconception about light only going at c in a vacuum. I really don't understand all this vacuum pressure, energy, etc. at all.

Kaptain K
2004-Oct-11, 08:43 PM
As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".
Sorry, it wasn't Eddington, but J.B.S. Haldane!

Spaceman Spiff
2004-Oct-11, 09:12 PM
Worzel,

If you check the link I posted above you will see this:

"There is another interesting possibility for breaking the light-barrier by an extension of the Casimir effect. Light in normal empty space is " slowed" by interactions with the unseen waves or particles with which the quantum vacuum seethes. But within the energy-depleted region of a Casimir cavity, light should travel slightly faster because there are fewer obstacles. A few years ago, K. Scharnhorst of the Alexander von Humboldt University in Berlin published calculations4 showing that, under the right conditions, light can be induced to break the usual light-speed barrier. Under normal laboratory conditions this increase in speed is incredibly small, but future technology may afford ways of producing a much greater Casimir effect in which light can travel much faster. If so, it might be possible to surround a space vehicle with a " bubble" of highly energy-depleted vacuum, in which the spacecraft could travel at FTL velocities, carrying the bubble along with it. "

This experiment has been performed. Under these conditions light travels faster than "C" in a "normal" vacuum. So then, what is the velocity of light?

There is the speed of light in a vacuum = 2.99792458e8 m/s, and then there is anomalous dispersion and quantum entanglement. For an explanation of the former, go here (http://www.astro.ucla.edu/~wright/anomalous-dispersion.html).

Robert Andersson
2004-Oct-11, 10:15 PM
As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".
Sorry, it wasn't Eddington, but J.B.S. Haldane!
Maybe you should edit your original post. Even though we are all aware that we shouldn't believe everything we read, many people pick up bits like that :)

Evan
2004-Oct-12, 04:22 AM
It appears that quote is attributed to a number of people including Eddington, Haldane, Clarke et al.

http://www.quotationspage.com/quote/27537.html

http://www.quoteshead.com/show_authors_quotes/Sir-Arthur-Eddington/2/

Not proof he said it but it was something he would say.

Spaceman,

Please define "vacuum".

Ut
2004-Oct-12, 05:05 AM
I'm sort of concerned that no one bothered to point out that this experiment is entirely fruitless. There's ALWAYS light shining into the glass, at least if there's a third grader near it. I assume this experiment is done during the day time, or in a room with, say, a light turned on. Or even if it's not, you can leave a glass of water on the counter in the middle of the day, and watch it not overflow for hours on end.

Did someone forget to inform this teacher that light is the thing that lets you see, and not just some magic beam that comes from a flashlight?

Spaceman Spiff
2004-Oct-12, 02:26 PM
Spaceman,

Please define "vacuum".

Ok, I will try, though quantum field theory is not my bag; I am an astronomer.

Spacetime is filled with quantized "force fields": electric, magnetic, electromagnetic, etc. That is the vacuum as we understand it today. Light is a disturbance (or traveling wave) in the quantized electromagnetic field. When this disturbance interacts with say an "electron field" (not to be confused with an electric field), energy is exchanged in the form of quanta (photons, electrons). An electron is said to 'exist at a location' where its "electron field" (square of wave function) is a maximum. As long as the electromagnetic disturbance is passing through space that is essentially 'empty' of matter fields with maximum amplitudes, then light is said to be traveling through a vacuum and does so at speed c. I probably haven't explained this precisely right, but it's probably not too far off. I invite someone else closer to the "field" to spiff this up. :D

The expression "speed of light in a vacuum" has its origins in Maxwell's equations that predates quantum mechanics, and so what is meant by such a "vacuum" has changed since Maxwell.

Spaceman Spiff
2004-Oct-12, 02:29 PM
As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".
Sorry, it wasn't Eddington, but J.B.S. Haldane!
Maybe you should edit your original post. Even though we are all aware that we shouldn't believe everything we read, many people pick up bits like that :)

I believe JBS Haldane used the qualifier "queerer" rather than "stranger". It is possible that Eddington was paraphrasing Haldane.

Evan
2004-Oct-12, 03:03 PM
Some interesting comments here:

http://www.colorado.edu/philosophy/vstenger/Briefs/c.pdf

"...there is no such thing as a perfect vacuum in the universe and so any
observed light beam must travel through a medium and will have an effective speed c/n, where n is the index of refraction of the medium. While this usually yields speeds less than c, some media, such as highly ionized plasma, can have an index of refraction less than 1 over a limited light frequency range. In that case, the effective light speed can exceed c."

Kaptain K
2004-Oct-12, 03:40 PM
As Sir Arthur Eddington said, "Not only is the universe stranger than we imagine, it is stranger than we can imagine".
Sorry, it wasn't Eddington, but J.B.S. Haldane!
Maybe you should edit your original post. Even though we are all aware that we shouldn't believe everything we read, many people pick up bits like that :)

I believe JBS Haldane used the qualifier "queerer" rather than "stranger". It is possible that Eddington was paraphrasing Haldane.
Yes, JBSH used the word "queerer" (back before "queer" had its current connotation). I should have mentioned that in my original post.

Bob
2004-Oct-12, 05:05 PM
The project will not only cost more than you imagine, it will cost more than you can imagine.

Spaceman Spiff
2004-Oct-12, 07:00 PM
Some interesting comments here:

http://www.colorado.edu/philosophy/vstenger/Briefs/c.pdf

"...there is no such thing as a perfect vacuum in the universe and so any
observed light beam must travel through a medium and will have an effective speed c/n, where n is the index of refraction of the medium..."

Well, yeah, if you want to get pedantic about it, he's right. Although for most photons (or disturbances in the electromagnetic field) traveling through the 'medium of space', this index of refraction is not consequential.
He's also gotta be careful by what he means by c/n > 1. There are also some recently discovered media with negative indeces of refraction (n < 0). Does that mean that v_photon < 0? I don't imagine so.

Edit: I take back this last comment. Go to this web page (http://physics.ucsd.edu/lhmedia/) (and click on "What is Left-handed material?") where they note that these 'metamaterials' with negative indeces of refraction have the property that the energy in the light wave moves in the opposite direction of the wave propagation.

Evan
2004-Oct-12, 07:05 PM
As the guy at the link points out a negative index of refraction implies a photon velocity >C in the medium, not <0.

Spaceman Spiff
2004-Oct-13, 01:45 PM
As the guy at the link points out a negative index of refraction implies a photon velocity >C in the medium, not <0.

Victor Stenger mentions a case of n < 1, not a negative index of refraction (n < 0).

"...some media, such as highly ionized plasma, can have an index of refraction less than 1 over a limited light frequency range. In that case, the effective light speed can exceed c."

I've posted a new link on such materials in my previous posting (now edited). But I think we've moved pretty far afield from the original post.

Ut
2004-Oct-13, 02:49 PM
So...
I'm the only one bothered by the fact that this teacher doesn't understand that the glass is bombarded by photons constantly, and not only when the flashlight is turned on?
Curious. Curious indeed...

Evan
2004-Oct-13, 02:50 PM
You're right, this is pretty far. But on this site (http://scienceworld.wolfram.com/physics/IndexofRefraction.html) it says "Amazingly, it is possible to construct exotic materials having frequency regions where the sign of (graphic formula) must be negative...".

I think that can be taken as a negative index.

Spaceman Spiff
2004-Oct-13, 04:46 PM
So...
I'm the only one bothered by the fact that this teacher doesn't understand that the glass is bombarded by photons constantly, and not only when the flashlight is turned on?
Curious. Curious indeed...

No, you're not the only one. I've already commented above that the teacher and/or the science dept have thoroughly confused themselves with misconceptions.

It's just that this topic has gone off on a tangent or two with some of Evan's inquiries. I and others contributed to the tangent, and everyone else probably lost interest. Apologies on my behalf. Hopefully, Boris got all he wanted to know.

Ut
2004-Oct-13, 06:05 PM
Yeah, I saw comments. It's just really eating at me that someone could forget that the light they see by also counts as light. And hey, I don't really know what everyone else is talking about ;)

John Dlugosz
2004-Oct-15, 09:49 PM
My son did the experiment, determined that shining light into the glass did not make it overflow, and concluded that light was not matter. He was told he must have made a mistake! What the &^$#!? (Other kids found that shining light into the glass DID make it overflow. Our future in science! Go figure...)




Try cotton balls. I guess they're not matter?

How about salt?

John Dlugosz
2004-Oct-15, 09:59 PM
Electric and magnetic fields are produced/governed (I can't think of better words right now) by exchanges of photons.

e and m fields are made of virtual photons, OK.

But I'm also told that light is made out of mutually-inducing e and m fields! So says Maxwell's equasions.

Isn't that going in circles?

John Dlugosz
2004-Oct-15, 10:04 PM
The Casimir effect indicates otherwise. It is not conjecture either, it is an experimentally verfiable effect.

I hadn't heard that the speed of light had been measured transverse to the plates of the casimir chamber. I read that it would be very difficult to do so, but that was some time ago. Do you have a link?

Evan
2004-Oct-15, 11:39 PM
The Casimir effect was quantitatively verified in 1996 by Lamoreaux to high precision. It follows according to QED (Feynman) and calulations by Scharnhorst that the speed of light must be affected by the reduction in zero point energy. Another paper here. (http://ernie.ecs.soton.ac.uk/opcit/cgi-bin/pdf?id=oai%3AarXiv.org%3Ahep-th%2F9408016)

I am not sure if anyone has experimentally verified the increase in C as yet but I seem to recall that it has been done. The effect was tiny, something like 10^-12 or some such. But it's not how well the bear dances, it's that he dances at all.

eburacum45
2004-Oct-18, 03:54 PM
That tiny volume of space between the Casimir plates is depleted in energy; in other words it has a lower energy than the ordinary vacuum...
I don't find it surprising that the speed of light is faster in such a volume of space- the problem is trying to use that effect.

I still think wormholes (which also use negative values of energy) offer a better prospect than faster than light drives.

But I am not going to hold my breath...

Boris
2004-Nov-03, 06:35 PM
Hopefully, Boris got all he wanted to know.

Yes, thank you all for very interesting posts and including the diversions into Casimir force, etc.

The teacher has recanted. Her source was not some state educational materials thank goodness (it was the internet! no surprise. madscinet to be precise). She said she told the class that light was not matter.

Then this past week my son took some science test, state mandated, in which the question came up (identify the item that is not matter. Among four choices was light). The correct answer in the test was that light is not matter.

So I am relieved.

Learned a lot about light the past few weeks!

Boris

Laser Jock
2004-Nov-03, 06:55 PM
Hopefully, Boris got all he wanted to know.

Yes, thank you all for very interesting posts and including the diversions into Casimir force, etc.

The teacher has recanted. Her source was not some state educational materials thank goodness (it was the internet! no surprise. madscinet to be precise). She said she told the class that light was not matter.

Then this past week my son took some science test, state mandated, in which the question came up (identify the item that is not matter. Among four choices was light). The correct answer in the test was that light is not matter.

So I am relieved.

Learned a lot about light the past few weeks!

Boris

That's great to hear! Thanks for the update.

Truth
2009-Nov-06, 07:17 PM
Light i not matter, because the definition of matter is:

´´Matter is everything that weighs and occupies a place``

You cannot touch or weigh light, there for it is not matter, light is an electromagnetic radiation of a wavelenght that is visible to the human eye!

I hope i helped the problem

mugaliens
2009-Nov-07, 08:43 AM
Just over five years!!!

Is this a new record for thread necromancy?

Maur
2009-Nov-07, 12:26 PM
And the debate rages on! Seriously, i though it's the opposite, that matter is a nice misconception useful when dealing with macroscopic world.

m74z00219
2009-Nov-09, 12:32 AM
e and m fields are made of virtual photons, OK.

But I'm also told that light is made out of mutually-inducing e and m fields! So says Maxwell's equasions.

Isn't that going in circles?

I think what you meant was that photons mediate the electromagnetic force.

----

I would say that energy is something apart from matter. There is no such thing as energy particles. Bosons (eg: photons) are matter just like fermions (the quarks that make up protons for instance or leptons like electrons).

Energy, on the other hand, is that which causes change (or prevents it in the face of competing forces). Like ionizing radiation causing an emission nebula to glow. The photons emitted by the local star have energy, but I wouldn't say that they are energy.

I could see how one would be tempted to call everything energy, because every thing possesses energy; however, it seems to me important to make room the subtle difference between having energy and being energy.




M74

NT2010
2010-Jan-06, 10:10 PM
Actually, it is both. You will never understand as long as you think quantum mechanics is 'crazy'.

Here is what 'crazy quantum mechanics' has done for us:

http://physicsworld.com/cws/article/news/41270

cosmocrazy
2010-Jan-10, 07:28 PM
Light i not matter, because the definition of matter is:

´´Matter is everything that weighs and occupies a place``

You cannot touch or weigh light, there for it is not matter, light is an electromagnetic radiation of a wavelenght that is visible to the human eye!

I hope i helped the problem

This would be how I would define matter, "when something occupies volume in space/time". Since light doesn't then no, in my definition of matter light is not matter.

RenaissanceMan
2010-Jan-16, 04:11 AM
Well, mass and matter are not interchangable, especially in Einstein's equations. And light, to my knowledge, has zero rest mass.

How does light "rest"? I thought it moved, and moved very fast at that.

To the extent that light has zero mass, what happens to the mass of hydrogen
as it fuses in the sun? It is lost. To what? To light.

So the mass of the sun is transmitted across space. When this ambient light
strikes earth, is it not converted back into mass?


This is an elegant explanation for the reason light is bent by stars.

I can't conceive of how space is "curved" when space, as we define it, is nothing. What is the difference between straight nothing and curved nothing? Let me hazard a guess: nothing.

Tensor
2010-Jan-17, 02:19 AM
How does light "rest"? I thought it moved, and moved very fast at that.

Who said it did rest? There is a difference between rest mass and resting.


To the extent that light has zero mass, what happens to the mass of hydrogen as it fuses in the sun? It is lost. To what? To light.

To be more precise, energy.


So the mass of the sun is transmitted across space. When this ambient lightstrikes earth, is it not converted back into mass?

Not necessarily.



I can't conceive of how space is "curved" when space, as we define it, is nothing. What is the difference between straight nothing and curved nothing? Let me hazard a guess: nothing.

Actually, your "guess" is quite wrong. Or, to be more precise, since you can't conceive of curved space, that means your understanding of it is "nothing".

JohnD
2010-Jan-17, 11:50 AM
Sorry, I'm not going to wade through this thread so I know no one else has said this, but......

Does it matter?

John

EDG
2010-Jan-17, 11:23 PM
Does it matter?

Now wait a minute, don't you start bringing matter into it again ;)

(I know I know, it's a very bad attempt at a pun...)

JohnD
2010-Jan-18, 03:47 PM
True, and I'm ashamed of myself for it, but how important at junior school level is it to quibble about light being matter or not? The argument seems to hinge on high level physics, and light is 'real'.
Now, discuss - Is light real?

John

mike alexander
2010-Jan-18, 03:58 PM
Adding a moderate amount of salt to water will cause the water volume to decrease. I guess that makes salt antimatter?

Jeff Root
2010-Jan-18, 04:33 PM
Since this thread was started in 2004 and most of the posts are from then,
before I joined BAUT, I didn't post in it before. My easy and unhesitating
reply to the question of the thread title is "Yes."

-- Jeff, in Minneapolis

EDG
2010-Jan-18, 04:44 PM
True, and I'm ashamed of myself for it, but how important at junior school level is it to quibble about light being matter or not?

John

I think it's important given that the OP's son was told that shining a light into water should make it overflow and that he made a mistake if it didn't. Something is going seriously wrong there if that's what was being taught to kids at school.

loglo
2010-Jan-18, 06:05 PM
Hopefully it was an aberration.

mike alexander
2010-Jan-18, 08:48 PM
On the other hand, it might explain rising ocean levels. The sun comes up, the ocean expands by absorbing light.

Tensor
2010-Jan-18, 10:39 PM
On the other hand, it might explain rising ocean levels. The sun comes up, the ocean expands by absorbing light.

Yeah, but then at night the levels would go down......wait a minute...... the tides, of course. :lol: