In a vacuum, Light moves at, well, the speed of light. But in water it travels slower than it does in a vacuum. But the space between atoms is nothing more than vacuum, so this little photon even in water is essentially still whizzing through a vacuum. So why does it take longer?
Light is an electromagnetic wave, and it will interact with the electrons and other charged particles in the medium it is traveling through. The result of this is the speed of light decreasing.
I remember many years ago hearing NPR play a dramatization of a SF short story about “slow glass” that slowed the speed of light almost completely. By looking through a pane of slow glass you could see the scene that had taken place on the other side sixty-odd years ago. Does anyone know what story that was? Does anyone know whether such a thing could be possible? I suspect for such a substance to exist, it would have to be incredibly dense (maybe on the order of a neutron star?) while retaining optical properties. Obviously it would probably be very impractical to create, but would the laws of physics allow it?
If someone somehow made a piece of glass that was as dense as a neutron star, than that thing would have a huge-ass gravitational field. Right after the guy invented it, he would have been pulled into it and crushed! Therefore, it is impossible for it to exist. Even if you’re thinking that you were just exaggerating it being as dense as a neutron star, I still don’t think physics would allow anything close.
“The most merciful thing in the world, I think, is the inability of the human mind to correlate all its contents.”
-H.P. Lovecraft, “The Call of Cthulhu”
The density/mass is not directly related to the speed of light. The speed is controlled by the electric permeability. It also is related to the magnetic permeability, but non-magnetic materials have the same permeability as free space.
Water has one of the highest permittivities of common materials, at 81 times that of free space, so light travels 9 times (square root of 81) slower in water than in free space. Most materials you deal with have permittivities around 2 to 6, so water at 81 is way out there.
If there were a material with a permittivity in the billions of times that of free space, it seems to me that the refraction might be a problem. The index of refraction of a material is the same as the relative speed of light, so if you remember Snell’s law, the light striking the surface at an angle would be bent at a much greater angle. It gets bent (as Bart says) back to the original direction when it reaches the other side of the glass, but this means that the image that comes out the objective side of the glass went into it in a smaller area. With an extreme case, the image you view is what would have struck the glass at a very small area, and would therefore be very dim.
Eventually, you’d have the problem that the image you’re seeing struck the subjective side of the glass in an area only a few atoms across, at which point the smoothness of the glass, not to mention quantum effects, would be the limit.
The speed of light in a given medium is basicaly the speed of light in vacuum divided by the refractive index of said medium. If your “slow glass” were about an inch thick, it must have a refractive index a bit higher than 2.2E19 (i.e. 2 with 19 trailing zeroes) in order to slow light down by 60 years.
Most materials known to human kind have a refractive index between 1 and 3. In fact, I’ve never heard of any substance with a refractive index higher than 3 alone, but I can’t gurantee you that they don’t exist. I still doubt you’ll even be able to get into the double digits however; there’s probably a theoritical limit as to the maximum refarctive index any material can have.
Seems to me that scenario would be an example of light slowing down through zero to a negative value. If you think of light as composed of photons, they’re particles having a positive speed, like other respectable particles, but with no rest mass, i.e., mass when they’re slowed to zero. “Tachyons” have been postulated so as to have negative speed or mass or something, but the last I knew (I’m not a physicist), these had not been “discovered” or fit theoretically into the Standard Model or any other reasonable (whatever that constitutes) model.
But I think you’re hijacking this thread from the author of the OP, who had a different question not yet well answered.
I can’t use the right physicists’ expressions to answer his question properly, but I’d say that you have to consider that all interpretations of reality are simply setups of neural modeling (a circular way of thinking, of course), and ones that have been around quite a while mesh well with others, producing common “intuitiveness”, while other newer ones, spread over the microscopic, everyday and megascopic worlds, suffer as to associated common intuition. It is highly useful to consider light (or electromagnetic energy of any wavelength) as both composed of particles and as having a wavelike distribution. There is math to handle both. Thinking intuitively at an everyday level, loss of momentum of some of the photons in collisions with particles of the medium through which it passes results in a lower average velocity of what can be otherwise interpreted as a wave effect, even though some photons might be held to have passed all the way through the extent of that medium, within the “vacuum” between its constituent particles. Compare the average velocity of sand particles falling through numerous layers of window screen to that of free-falling sand. Not really the same thing, but sort of like it.
Ray (ex-EE who is not sure what a physicist would think of this “answer”)
NanoByte:
The sand falling through window screen example is okay I guess. As to the rest… huh?
CurtC:
I think you should check your numbers again. Your average water only slows visible light by about 25%…
I read that SF story too! IIRC, a murder was committed in front of a pane of “30 year” glass. Thirty years later, the glass was being watched in a courtroom to see whodunnit.
Anyway, such glass would have to be able to absorb a huge amount of energy. For an extreme example, say you had some 30 year glass (i.e. light that goes in one side comes out the other side 30 years later). Keep it in the dark for 30 years, to reset it. Then expose it to a 100 watt light bulb for 30 years. Assume all 100 watts goes into the glass (say you’ve got mirrors set up to reflect all the light into it …), thats 100 watts x 24 hours/day x 356.25 days/year x 30 years = 25.65 MJ of energy. That’s going to be a hot piece of glass!
Arjuna34
It’s more like 95GJ (giga-joules) for your 30 year glass, Arjuna34. As large a number as that sounds however, it only corresponds to about 1 micrograms of mass. If you could really find a material with the afformentioned refractive index, I wouldn’t be surprised if it had some other weird properties as well. Heck, I probably wouldn’t be surprised by anything else either…
I’ve read that story too - and I KNOW I’ve got the paperback somewhere. Guess I’ve got to go read it again.
IIRC, it wasn’t a murder mystery - it was more of a - heck, my English vocabulary fails me here, please excuse me - “mood piece”, using the slow glass gimmick to drive a plot about love lost and love not-yet-quite-lost. (No spoilers here, read it yourself, it’s beautiful, almost poetic).
Back on track: I seem to recall a female scientist (a Dane! :-)) reaching a breakthrough in this area recently, working in a U.S. laboratory. Does anybody recall anything of this ?
Norman
I recall hearing a piece on NPR, scientific not SF, that someone had come up with a substance that slowed light down to something like 2 feet and hour. And no one was sucked into its gravitational pull and crushed.
Howardsims, your premise misses a key point. Following the particle theory of light’s behavior, the photons do strike some water molecules, you know. Hence, reflection and some absorption manifested as a slight temperature rise. Then, there’s the wave behavior of light where one can explain reflection and refraction. And then, there’s the “wavicle” theory to light’s behavior. In any sense, whatever theory you prefer, water is a medium whereas a vacuum is lacking medium.
I don’t think many of us would agree that a vacuum and a tank of water are equivalent. Along such lines, your reasoning is similar to the arguement to why can’t we throw a brick through a window without breaking it, etc. There’s been a thread on that topic which you might wish to browse, too.
A clarification: The refractive index of water at visible frequencies is about 1.34. The refractive index of 9 is valid below about 10 GHz.
It is too clear, and so it is hard to see.
At least one of the stories was
“The Light of Other Days”. By, I think, Bob Shaw. He wrote at least a couple of stories about slow glass. All of them dark, depressing pieces.
Ok, I’ve been lurking for about 6 months now and this thread finally prompted me to join the ranks of posters.
The mention of “Slow Glass” reminded me of an article I read a week or so ago. Apparently physicists have managed to slow the speed of light to a paltry 38 miles per hour. If you want to know the details here’s the link:
Physicists Slow Speed of Light
The scientist used a strange form of matter called a Bose-Einstein condesate and took advantage of a process called “quantum interference” to slow the speed of light down.
In the future they hope to slow the speed further to 1/2 inch per second. At this speed you would need a about 15,000 miles of the stuff for light to take 60 years to cross it. Not too bad.
gEEk
ZenBeam wrote:
Thanks, ZB. I’ve studied systems that use water capacitors because of the high permittivity, but these were below 10 GHz. I didn’t realize the refraction would change so drastically at different frequencies.
Oops, I forgot to figure in the number of seconds in an hour :(.
CurtC- Did you hear of any problems with using water as a dielectric? Unless it’s very pure, I would think you’d get a lot of leakage. I’d like to find out more about the practical problems with water capacitors. Is there info on the web?
Arjuna34
FYI, some indexes of refaction for yellow sodium light, 589 nm.
Ice 1.309
Zircon 1.923
Diamond 2.417
Glass 1.52-1.8
Benzene 1.501
air 1.0003
Diamond is the highest of any normal material.
BTW, the equivalent index of refraction for the Bose-Einstein condensate I mentioned above is about 17.6 million.
Not that a Bose-Einstein condesate could be considered a “normal material”
gEEk