Light - solid, liquid or gas?

[PsychoticEpisode]

This may sound dumb to the educated but I'm just a hayseed from the country and I know absolutely nothing about light. Photons have been described as being both a wave and a particle. If it is a particle then is it a liquid, gas or solid? They fly thru space, affected by gravity, bounce off things and mix with water, so if not one of those three then is it in a special class by itself?

[orcot]

try putting light in a jar. If it hovers around it would be like a gas.
If it moves when you move the jar, it would be like a liquid, and if it keeps still it would be a solid.

[abyssoft]

In a perfectly mirrored jar, light would react most like a gas

[Light]

This may sound dumb to the educated but I'm just a hayseed from the country and I know absolutely nothing about light. Photons have been described as being both a wave and a particle. If it is a particle then is it a liquid, gas or solid? They fly thru space, affected by gravity, bounce off things and mix with water, so if not one of those three then is it in a special class by itself?

Don't be concerned about asking the question, it's probably one that others wonder about also.

It's non of the three. It's a small bundle of energy that exhibits some of the properties of a particle. It also exhibits the properties of a wave. One term that been in use for a long time is better at describing it - the term is "wavicle" (also sometimes spelled "waveicle") which indicates that it behaves as both particle and wave.

It's not really correct to say they "mix with water." They simply pass through it just as they do air or glass unless they are absorbed.

[PsychoticEpisode]

Don't be concerned about asking the question, it's probably one that others wonder about also.

It's non of the three. It's a small bundle of energy that exhibits some of the properties of a particle. It also exhibits the properties of a wave. One term that been in use for a long time is better at describing it - the term is "wavicle" (also sometimes spelled "waveicle") which indicates that it behaves as both particle and wave.

It's not really correct to say they "mix with water." They simply pass through it just as they do air or glass unless they are absorbed.

Does that mean the light energy bundle is half particle - half wave? or are certain circumstances required for either the wave or particle property to manifest itself?

[Light]

Does that mean the light energy bundle is half particle - half wave? or are certain circumstances required for either the wave or particle property to manifest itself?

Another good question! :)

No, it isn't a half-and-half situation, it's simply that it acts like one or the other under certain conditions - which also answers your second question.

For example, a photon will travel in a straight line if unimpeded and gravity will change it's path. There it is acting as a particle.

But a beam of light can be refracted and diffracted and if passed through a narrow slit will form a spreading wavefront. There it is acting like a wave.

[PsychoticEpisode]

The doppler effect, is that because of wavelengths or does each primary color have its own speed?

[Light]

The doppler effect, is that because of wavelengths or does each primary color have its own speed?

It streches the wavelengths because the source of light is moving away from the person seeing it. All colors are stretched out.

All colors travel at exactly the same speed through a vacuum - the speed of light - as does all other forms of electromagnetic energy. Radio waves, microwaves, X-rays, infrared, ultraviolet and the like.

[PsychoticEpisode]

It streches the wavelengths because the source of light is moving away from the person seeing it. All colors are stretched out.

All colors travel at exactly the same speed through a vacuum - the speed of light - as does all other forms of electromagnetic energy. Radio waves, microwaves, X-rays, infrared, ultraviolet and the like.

I picked up Hawking's "A Brief History of Time" and started reading it last nite. I'm wading thru the light section and just trying to understand it, thus all the questions. I have another that popped into my head while reading and not sure if this has ever been asked.... Can more than one observer see the same photon?

[Light]

I picked up Hawking's "A Brief History of Time" and started reading it last nite. I'm wading thru the light section and just trying to understand it, thus all the questions. I have another that popped into my head while reading and not sure if this has ever been asked.... Can more than one observer see the same photon?

Good - you're on a nice path. And your questions are more than welcome. :)

No, only a single observer. In order to see the photon, it has to be absorbed in the retina of the eye. Thus it ceases to exist and cannot be detected by anyone else.

[PsychoticEpisode]

Good - you're on a nice path. And your questions are more than welcome. :)

No, only a single observer. In order to see the photon, it has to be absorbed in the retina of the eye. Thus it ceases to exist and cannot be detected by anyone else.

Thanks for the help. I'm having a bit of trouble trying to understand how a photon goes from zero to c instantaneously. When I try to think of why, I get an image in my head where spacetime is actually moving at c (like a river) and the photon gets swept up upon its creation. Aside from what my thoughts are, what provides the instantaneous propulsion of a photon?

[Light]

Thanks for the help. I'm having a bit of trouble trying to understand how a photon goes from zero to c instantaneously. When I try to think of why, I get an image in my head where spacetime is actually moving at c (like a river) and the photon gets swept up upon its creation. Aside from what my thoughts are, what provides the instantaneous propulsion of a photon?

Yet another valid question. ;)

A photon is generated whenever an electron drops from a higher level of excitation (higher energy level) to a lower one. This happens instantanously and the energy released is conserved ( saved/converted) into the exact amount of energy carried by the photon. It's the fact that this happens with different kinds of atoms where the electrons exist at differing energy levels (compared to other atoms) that result in the photons created having different energy levels. And those different energy levels are what determine the wavelength of the emission. The higher the energy, the shorter the wavelength.

[PsychoticEpisode]

Yet another valid question. ;)

A photon is generated whenever an electron drops from a higher level of excitation (higher energy level) to a lower one. This happens instantanously and the energy released is conserved ( saved/converted) into the exact amount of energy carried by the photon. It's the fact that this happens with different kinds of atoms where the electrons exist at differing energy levels (compared to other atoms) that result in the photons created having different energy levels. And those different energy levels are what determine the wavelength of the emission. The higher the energy, the shorter the wavelength.

Is this brightness? You say that energy is released. Is this the same as what happens when nuclear fusion takes place. Hawking says that according to Einstein's E=mc² that an equal amount of mass is also lost when enrgy is released. This leads to my next question, how goes an electron that generates a photon lose no mass but still loses energy. Is this energy released first absorbed from an external source then released? I think I'm not reading something right here.

I would recommend this book (A Brief History of Time) to anyone who has trouble getting thru normal scientific verbiage. I still have to read things more than once in an attempt to understand but at least its possible to do so.

Space and time appear linked and I was wondering if light and time are thought to be similarly linked?

[Light]

Is this brightness? You say that energy is released. Is this the same as what happens when nuclear fusion takes place. Hawking says that according to Einstein's E=mc² that an equal amount of mass is also lost when enrgy is released. This leads to my next question, how goes an electron that generates a photon lose no mass but still loses energy. Is this energy released first absorbed from an external source then released? I think I'm not reading something right here.

I would recommend this book (A Brief History of Time) to anyone who has trouble getting thru normal scientific verbiage. I still have to read things more than once in an attempt to understand but at least its possible to do so.

Space and time appear linked and I was wondering if light and time are thought to be similarly linked?

No, it's not brightness (intensity), that depends on the number of photons being released at the same time. And it isn't related to fusion or any other nuclear activity. It happens at what we would consider fairly ordinary temperatures and with no real pressure applied. Such as in a light bulb or burning match or candle.

There's no loss in mass involved. It can best be thought of as like the release of energy when someone throws a ball very hard and you catch it barehanded. The burning, stinging sensation is due to the release of energy when you stop the ball from moving. That energy of impact is converted directly into heat and pressure and it's clear that no loss of mass has occurred. The ball weighs the same before, during and after the event.

Yes, you've figured it out. :) The energy comes first from elsewhere. In a light bulb, it's the electricity. In a burning match or candle, it's from the chemical conversion (burning/oxidation) that's taking place. As you know, there's no such thing as a free lunch and the same applies to energy - it must come from somewhere, excites the atom which raises the energy level of the electron, and is then passed along in the form of a photon when the electron drops back to it's rest level.

[PsychoticEpisode]

The double slit experiment shows the wave/particle duality of light. My problem with this experiment is that light is massless and travels at c but the electron has mass, cannot travel at c and should slow down after being fired at slits. Does this not mean the experiment is possibly flawed? Do we really know enough about light to say this is what's happening? Is there something else going on we don't see?

[Billy T]

To PsychoticEpisode:

Light has been basically correct but made a two errors.

(1) An atom in an excited state does weight more by m = E/c^2 and this is reduced when the photon is emitted. For example, some excited states are meta-stable and may persist for micro seconds, storing the mass/energy until the photon is emitted. Perhaps they were excited by a collision with an electron or are the lower state (but not the ground state) of an earlier transition from an even higher upper state. (Often an atom may be collisionally excited into a relative high excited state and then cascade down thru several levels with the emission of several photons, one after the other.) It may help you to think of florescent crystal where the light that excited it has been turned off and even 30 minutes later, some of the energy that has been stored in it (probably as “quasi free electrons” fall into a lattice defect trap) is released as a photon. Yes, this excited crystal is losing mass as it glows (as time passes).

(2)The emission process does take time. If the transition probability is low then the photon emitted is long, perhaps several meters long. You can measure it length as follows: First you must accept the strange fact that in a two path interferometer, EACH photon goes by both paths. If you make one path 10 cm longer than the other the interference pattern is still sharp and clear. As you increase the path difference, the pattern fades and does not exist if the path length difference exceeds the photon length. For the photons associated with the “Sodium D” pair of yellow lines from a typical high pressure sodium lamp, you will find that they are about 30 cm long. I have done this experiment and the pattern was completely gone with path length difference of 50 cm. It is impossible to have a classical concept of the emission of a photon, but you will not be far wrong if you think of it as if you were holding a static stretched rope with the far end tied to a distant tree and then began to oscillate your hand to “pump out” a set of waves in the rope. If you stop hand motion when the set of waves is half way to the tree, you can see your finite length “photon” travel to the tree and know it took a finite time to make it. Low probability transitions, make long photon, all else being equal. (If you know about Fourier transforms, you will understand why these long ones correspond to very well defined frequencies and the shorter one to broader spectral lines.)

[Azeperak]

If I remember a course I took in modern physics - The double slit experiment shows that light exhibits a classic interference pattern - however, light also behaves as a particle in other circumstances.

The compromise model of light is that it is made up of pieces of waves. A discrete package made of a wave section is the description of a photon that I was given - but that was 20 years ago.

[Light]

To PsychoticEpisode:

Light has been basically correct but made a two errors.

(1) An atom in an excited state does weight more by m = E/c^2 and this is reduced when the photon is emitted. For example, some excited states are meta-stable and may persist for micro seconds, storing the mass/energy until the photon is emitted. Perhaps they were excited by a collision with an electron or are the lower state (but not the ground state) of an earlier transition from an even higher upper state. (Often an atom may be collisionally excited into a relative high excited state and then cascade down thru several levels with the emission of several photons, one after the other.) It may help you to think of florescent crystal where the light that excited it has been turned off and even 30 minutes later, some of the energy that has been stored in it (probably as “quasi free electrons” fall into a lattice defect trap) is released as a photon. Yes, this excited crystal is losing mass as it glows (as time passes).

(2)The emission process does take time. If the transition probability is low then the photon emitted is long, perhaps several meters long. You can measure it length as follows: First you must accept the strange fact that in a two path interferometer, EACH photon goes by both paths. If you make one path 10 cm longer than the other the interference pattern is still sharp and clear. As you increase the path difference, the pattern fades and does not exist if the path length difference exceeds the photon length. For the photons associated with the “Sodium D” pair of yellow lines from a typical high pressure sodium lamp, you will find that they are about 30 cm long. I have done this experiment and the pattern was completely gone with path length difference of 50 cm. It is impossible to have a classical concept of the emission of a photon, but you will not be far wrong if you think of it as if you were holding a static stretched rope with the far end tied to a distant tree and then began to oscillate your hand to “pump out” a set of waves in the rope. If you stop hand motion when the set of waves is half way to the tree, you can see your finite length “photon” travel to the tree and know it took a finite time to make it. Low probability transitions, make long photon, all else being equal. (If you know about Fourier transforms, you will understand why these long ones correspond to very well defined frequencies and the shorter one to broader spectral lines.)

Hello, Billy T,

Actually, they weren't errors at all. :) This individual is very new to the whole concept and I was purposly making things overly simple in order to get the basic concepts across. I think it's a mistake to jump into the deep water before first giving them time to get their feet wet. ;)

For example, you're quite correct about the added energy increasing the mass but once the photon(s) have been released, the mass returns to it's original value. And there was never any nuclear reactions involved, that's one of the primary points I wanted to make.

[The Pie Swindler]

http://www.newscientist.com/article.ns?id=dn2497

article on how they propose to develop liquid light, how could be considered a (unique) gas, literally, and some of the possible uses for this liquid light (optical computing)

Interesting.

[Billy T]

...And there was never any nuclear reactions involved, that's one of the primary points I wanted to make.I will risk confusing some again it the interest of being as fully correct as possible: There are no "nuclear reactions" in the sense you intend (some "nuclear chemisty" taking place), but the nucleus is often influencing the exact frequency of the light emitted. First, by the isotope effect and if the nucleus has "spin" by sort of a Zeeman effect.

In a classical model of a nucleus with one electron in orbit, it is really both orbit about the center of mass and this makes the quantized levels of the "electron's orbit" depend upon the mass of the nucleus. For a specific example, the line spectra from atomic hydorgen, atomic deterium and tritium are all slightly different. (Once the second or the meter, I forget which, was defined using the radiation of mercury, but it had to be the specified isotope of mercury.)

To understand the "spin effect" (better called the "fine structure") note that if both the spin of the electron and the proton in the hydrogen atom are "aligned" the energy levels are slightly higher (wider spaced) than the case where one is "spin up" and the other is "spin down"

But as you said, the beginner should not worry about these small effects. You need good spectral resolution to see them.

However, I do think that when the truth can be told without being burried in sophisticated math, it should be. I rarely "baby down" physics even for beginners. I want them to know it is complex, strange, and exciting and that most of us will never fully understand more than a small part of it. That is I want them to know that if they go into physics, a great adventure is ahead and that it will last a lifetime and probably provide them a reasonable living too!

[Cyperium]

I have a related question, how is the red, green and blue derived from light?

Sure if we scatter it, they will appear, but in the light where is the red, green and blue?

Why red, green and blue?

Also, can radiowaves etc. also be broken down into red, green and blue, if not, why?

[Light]

I will risk confusing some again it the interest of being as fully correct as possible: There are no "nuclear reactions" in the sense you intend (some "nuclear chemisty" taking place), but the nucleus is often influencing the exact frequency of the light emitted. First, by the isotope effect and if the nucleus has "spin" by sort of a Zeeman effect.

In a classical model of a nucleus with one electron in orbit, it is really both orbit about the center of mass and this makes the quantized levels of the "electron's orbit" depend upon the mass of the nucleus. For a specific example, the line spectra from atomic hydorgen, atomic deterium and tritium are all slightly different. (Once the second or the meter, I forget which, was defined using the radiation of mercury, but it had to be the specified isotope of mercury.)

To understand the "spin effect" (better called the "fine structure") note that if both the spin of the electron and the proton in the hydrogen atom are "aligned" the energy levels are slightly higher (wider spaced) than the case where one is "spin up" and the other is "spin down"

But as you said, the beginner should not worry about these small effects. You need good spectral resolution to see them.

However, I do think that when the truth can be told without being burried in sophisticated math, it should be. I rarely "baby down" physics even for beginners. I want them to know it is complex, strange, and exciting and that most of us will never fully understand more than a small part of it. That is I want them to know that if they go into physics, a great adventure is ahead and that it will last a lifetime and probably provide them a reasonable living too!

Hello, Billy T,

I've no disagreement with the science you've presented but from the experience as having been a teacher for many years, I do have to mildly disagree with your approach. :)

If too much depth is presented too early, the result - all too often - is a confused, bewildered student who feels overwhelmed. Too many of them will wrongly assume that "this stuff is just deep for me."

On many occasions, I've had a bright student raise a topic in class - a valid question that arose from something we were currently studying - and while attempting to answer it saw dismay spread across several other faces. They simply were not ready for that level of the topic.

Make no mistake here, I'm absolutely not in favor of "dumbing down", but one has to recognize that there is certain danger when moving too fast. Sadly, I've seen it happen more times than I care to count. Students require time to develop a feeling of confidence and comfortable familiarity at each plateau along the way before being forced to take the next plunge.

[Billy T]

I have a related question, how is the red, green and blue derived from light?
Sure if we scatter it, they will appear, but in the light where is the red, green and blue?
Why red, green and blue?
Also, can radio waves etc. also be broken down into red, green and blue, if not, why?light and radio waves are the same sort of thing (described by the same Maxwell's equations, but following Light's advice, I suggest you not bother with that now.) They do not really have any color. They have frequency and intensity. Color is a perceptual thing that happens in the brain, and it does not take much of one to derive "color" experience. Gold fish can do it better than humans can, and so can bees. I.e. the range of frequencies they can see and interpret as colors is larger than one octave, almost two while humans respond to less than one octave. (All the flowers you see as white, have very distinct colors to the bee, presumably beautiful colors, as that is how they know which is currently giving nectar. Bees see well out into the UV.)

If you are "color blind," you have only two different types of "cones" in your retina but normally there are three for humans and four for gold fish. These cones respond to the frequencies differently. Low frequency visible light will make the "red" cones respond more etc.

So coming to the brain are three different sets of responses of different type of nerves in the eye. It is sort of like a triangle - you could describe the location of any point in the triangle by telling the three distances for each of the corners. Or you could tell other combinations of information.

The "red, green and blue" dots you see on your TV up close are approximately best matches for the three different color cones in your eye, but even in grade school you learned of the three primary colors. They could be any set of three that are quite different from each other and any particular mix of the three corresponding intensities would still specify (to the brain) what the color is.

We call a set of basics descriptors "the basis set" (reasonable name is it not?). The red/ green/ blue basis set is not the only one used in the brain. In fact in V5 area of the brain (if memory is not failing me) we unconsciously convert to another "basis set." For example, in the red/green axis set of nerves, a high rate of neural firing means "Red" and a low rate means "blue" (or the other way round - I forget which way it is.)

The activity of one subset of V5 nerves encodes the "red/green axis" another subset encodes the "blue/yellow axis" and still a third set the "light dark axis." If you have been following you might say: "Hey you said there were three primary colors, how can you encode color in only two subset of nerves?"

Well, think of that triangle again. (You may have even seen a "color triangle" in a paint store, if not a psychology book.) If I tell only how far the color spot inside the traingle is from two of the corners, you know what color I am referring to. Beyond V5 in the brain, that is all your color system works on.

You can confirm that in some sense "blue is the opposite of yellow" at the perceptive level: Fixate a bright yellow spot (stare at it for two minutes, then look at a white wall - you will see a blue spot of the same size.

99% of the people who have any explanation for this will give you, at best, a half correct answer. It is true that you have metabolitically exhausted the cells that respond to yellow light, but not just in the retina. In V5 also and there, AFTER the transformation from the original basis set to the one used later in the brain, is where "blue is the opposite to yellow."

But as Light said/ advised me/ this is too much all at once, so just file it away and recall later.

[Light]

light and radio waves are the same sort of thing (described by the same Maxwell's equations, but following Light's advice, I suggest you not bother with that now.) They do not really have any color. They have frequency and intensity. Color is a perceptual thing that happens in the brain, and it does not take much of one to derive "color" experience. Gold fish can do it better than humans can, and so can bees. I.e. the range of frequencies they can see and interpret as colors is larger than one octave, almost two while humans respond to less than one octave. (All the flowers you see as white, have very distinct colors to the bee, presumably beautiful colors, as that is how they know which is currently giving nectar. Bees see well out into the UV.)

If you are "color blind," you have only two different types of "cones" in your retina but normally there are three for humans and four for gold fish. These cones respond to the frequencies differently. Low frequency visible light will make the "red" cones respond more etc.

So coming to the brain are three different sets of responses of different type of nerves in the eye. It is sort of like a triangle - you could describe the location of any point in the triangle by telling the three distances for each of the corners. Or you could tell other combinations of information.

The "red, green and blue" dots you see on your TV up close are approximately best matches for the three different color cones in your eye, but even in grade school you learned of the three primary colors. They could be any set of three that are quite different from each other and any particular mix of the three corresponding intensities would still specify (to the brain) what the color is.

We call a set of basics descriptors "the basis set" (reasonable name is it not?). The red/ green/ blue basis set is not the only one used in the brain. In fact in V5 area of the brain (if memory is not failing me) we unconsciously convert to another "basis set." For example, in the red/green axis set of nerves, a high rate of neural firing means "Red" and a low rate means "blue" (or the other way round - I forget which way it is.)

The activity of one subset of V5 nerves encodes the "red/green axis" another subset encodes the "blue/yellow axis" and still a third set the "light dark axis." If you have been following you might say: "Hey you said there were three primary colors, how can you encode color in only two subset of nerves?"

Well, think of that triangle again. (You may have even seen a "color triangle" in a paint store, if not a psychology book.) If I tell only how far the color spot inside the traingle is from two of the corners, you know what color I am referring to. Beyond V5 in the brain, that is all your color system works on.

You can confirm that in some sense "blue is the opposite of yellow" at the perceptive level: Fixate a bright yellow spot (stare at it for two minutes, then look at a white wall - you will see a blue spot of the same size.

99% of the people who have any explanation for this will give you, at best, a half correct answer. It is true that you have metabolitically exhausted the cells that respond to yellow light, but not just in the retina. In V5 also and there, AFTER the transformation from the original basis set to the one used later in the brain, is where "blue is the opposite to yellow."

But as Light said/ advised me/ this is too much all at once, so just file it away and recall later.

That was excellent, Billy T.! :) Going just a bit farther usually doesn't create any problem and I don't feel that you overloaded him. In fact, I believe you employed the best style found in a teacher's toolbox - providing just enough beyond the answer to pique his curiosity. ;) Well done.

And I'll add just two more small things. First, in direct answer to his question "where is the red, blue and green in white light?", they (and the other colors) are all present IN the white light. Our color receptors and the mental interpretation we give to beam of light containing all the colors in the proper proportional balance is to "see" it as white.

While at another end of the spectrum (small pun), consider the action of pigments. We've just established that white light contains all colors, so what happens when white light strikes a pigmented object?

A pigment exhibits it's color by absorbing parts of the light and reflecting other parts. For example, blue paint is reflecting blue light. If we add red paint to the blue, it will now reflect green light. If we carry this experiment on to it's logical conclusion by adding paint of all the other colors, we wind up with black. No light is being reflected because all the colors (full spectrum) is now being absorbed by the paint.

So, let's follow this with a simple exercise. Take a brightly colored red rose into a totally darkened room, cover a flashlight (or torch, if you're British) with a transparent blue plastic film, and then shine the light onto the rose. What color would you expect to see?

[PsychoticEpisode]

Make no mistake here, I'm absolutely not in favor of "dumbing down", but one has to recognize that there is certain danger when moving too fast. Sadly, I've seen it happen more times than I care to count. Students require time to develop a feeling of confidence and comfortable familiarity at each plateau along the way before being forced to take the next plunge.

You have me pegged right as a beginner but alas, I am too old to be a student. I've reached an age in my life where I've decided to learn more about the universe I live in. I wish I had started earlier. I never knew I would like cosmology and related subjects until I started reading about it. I appreciate your's and eveyone else's help here. Thanks for going easy. I'll have more questions soon and I hope I get the same co-operation then.

[Light]

You have me pegged right as a beginner but alas, I am too old to be a student. I've reached an age in my life where I've decided to learn more about the universe I live in. I wish I had started earlier. I never knew I would like cosmology and related subjects until I started reading about it. I appreciate your's and eveyone else's help here. Thanks for going easy. I'll have more questions soon and I hope I get the same co-operation then.

I'm sure you will. :) Despite a lot of what you see going on here, there are actually several nice people on board.

Also, don't forget that no one is ever too old to be a student. I learn new things every single day. I never plan to stop - and I'm 60+. ;)

[PsychoticEpisode]

I'm sure you will. :) Despite a lot of what you see going on here, there are actually several nice people on board.

Also, don't forget that no one is ever too old to be a student. I learn new things every single day. I never plan to stop - and I'm 60+. ;)

I'm not quite that far but I'm gaining on ya. As you know I've been trying to learn as much as I can in a short time and am grateful for the answers to my queries. I have another one.

I hope I can word this properly. Light from the moon takes 1.5 seconds to get here. Pretend I have a spacecraft that can make the trip in whatever time it takes. Will the spacecraft because it has mass travel fewer miles than the light. Does light follow a geodesic thru spacetime whereas mass in motion is able to carve itself a shorter route thru spacetime? Mass will take longer but travel a shorter distance. Is this true or am I misunderstanding some principle?

[PsychoticEpisode]

OK....forget that one.

Do you think there is more than the spectrum of light that we know? Could there be light that actually has mass & can't be detected? Anything?

[devils_reject]

I lean towards gas, any particle that travels that fast has to be a gaseous like state. Besides anywhere there is high entropy or energy there is light.



Dedicated to light

[PsychoticEpisode]

I would ask if anyone can clear up this little aspect of light that is bothering me. Since an object with mass cannot travel faster than c can it travel at exactly c?

[guthrie]

To address your earlier point, Light travels in a straight line unless it is either reflected, or bent by a gravitational field. The spacecraft, if it coudl travel at the speed of light, woudl take the same amount of time.
Theoretically an object with mass could travel at C, I suppose (I'm no physicist) but the fact of the matter is that it would take an infinite amount of energy to get to that speed, ergo it would be impossible.

[DaleSpam]

Since an object with mass cannot travel faster than c can it travel at exactly c?The short answer is "no, that would require an infinite amount of energy". Thus, massive particles always travel at some v < c (higher energies -> higher v for a given particle) and massless particles always travel at c (higher energies -> higher frequency).

By the way, with respect to your initial question, of course it is none of the 3 since solid, liquid, gas, and plasma phases are all phases of matter. Having said that, I would say that light is most similar to an ideal gas. Photons have momentum so they can exert pressure like a gas. Photons do not interact with each other, one of the hallmark characteristics of an ideal gas. If you had a perfectly reflective container you could even "weigh" your photons just like you can weigh a gas and they would have inertia just like a gas would.

-Dale

[PsychoticEpisode]

I realize that darkness pervades the cosmos but is there anywhere one can go in space and not observe a photon? It doesn't seem likely that such a place exists. It would appear that photons dominate the space landscape, yet it is dark.

The stars of the night sky as I understand it are from our own galaxy. The view from a position between galaxies would be quite darker I suppose but even so we should be able to observe other galaxies from any position. Now if photons are awash in space, do they need room to move around in? Also light waves must be intersecting other light waves but with no noticeable affect on the observer, so do photons collide? Do photons occupy space, have volume? Could the production and abundance of photons require or force the expansion of the universe?

[DaleSpam]

If our eyes were sensitive to microwave range radiation (we would need huge eyes!) then we would see all of space glow with the cosmic microwave background radiation. Space would not appear dark.

Remember, you can think of light as both a wave and a particle. As particles, photons do not interact with each other, only with charged matter. E.g. you cannot collide two photons and have them bounce off in different directions. As waves, however, you can get interference patterns etc. An interference pattern is more like the sum of two waves rather than a collision. In other words, the waves don't "collide" to get an interference pattern, rather they just add themselves together.

-Dale

[guthrie]

Photons do collide, IIRC, but not very often and they woudl have to be able to interact by virtue of having the same wavelengths.
As for volume, they dont have volume, just length, I think. And the expansion of the universe probably doesnt have anything to dow ith photons, seeing as they are massless, but you need to go and read up some cosmology to make sure.

[PsychoticEpisode]

We are able to see the universe when it was approximately 300,000 years old when light finally escaped the cosmic cloud of the infant universe. Why can't we train our telescopes to look 4.5 billion years back and see the solar system forming? or can we?