Light "SPEED" Questions, Thoughts.

Light \"SPEED\" Questions, Thoughts.

Me and a buddy of mine often talk about the properties and actions of light. Hmmm, go figure, look where I am. /ubbthreads/images/graemlins/rolleyes.gif

For those of you that havent heard, scientist has actually stopped light. Or at least had it VERY close to stopped. I forget the exact details, some sorta solution was used in a chamber.

This got us to thinking. The speed of light actually slows down when it passes threw water. Which like most of the things light does, doesnt make any sence, seeing how scientists say it doesnt have mass. Kinda like how gravity can effect light.
Well, if light slows down while passing threw water, would it speed back up after it exited the water, back into the air? I know this doesnt make sence, and it shouldnt, but, I wouldnt be surprised if it did.

Heres another strange light fact.
If you are riding in the bed of a truck going 100 mph, and throw a ball at 60 mph in the direction the truck is traveling, that ball is now traveling at 160 mph.
Well, if you are in that truck, and shine a light, that light is NOT traveling 100 mph faster. The trucks speed is not added to it as it was with the ball. Whats up with that?

Wave / Pulse:
I know that light is one of the few , if the only things that can be a pulse and a wave. Its kinda both at once sometimes. Theres the trick of taking a piece of paper, and making 2 small slits in it, very close together. You would expect that if you shined a light threw the slits, that 2 lines would be cast on the wall. It Doesnt! It makes a wave pattern on the wall. Its hard to put in words, but, it is a large patch, that fades from bright to dim to bright to dim. (a wave)
Why is this really strange? Light is in pulse form. After passing threw the slits, its a wave. Like I said, this is all hard to put in words, I saw a show in it years ago, and beleave me, visual aid helps. /ubbthreads/images/graemlins/smile.gif Try to think of what water ripples would do, if they passed threw to slits, what would happen when the ripples passing threw those 2 slits would do as it met on the other side.

Im sure I know a bunch of other odd light oddities, but I cant remember now.

Feel free to post any oddities you know of.
I have always thought light is one of the stragest acting and behaving things in this world.

Re: Light

OH, heres another one.
If you are in a spaceship going the speed of light, and you are sitting in a chair facing the back of the ship. If you take a light, point it at your face, and turn it on, will you see the light?

I would guess no.

Re: Light

you cannot guess. Because you cannot travel in the speed of light. Speed of light only apply on light itself

as i believe, anything with mass cannot go as fast as speed of light.

Re: Light

jtice,

There was a recent thread here in the CAFE about relative speed and light. Check it out as it may clarify the mystery of light and relative speed (but not resolve the mystery).

Yes, light travels at different speeds in different media, and so when it leaves the water and goes to the air, *BING* it's faster again. It has no mass, so can be instantaneously accelerated.

As for the wave-particle duality, ALL matter, ALL particles, exhibit this behavior. An electron, a photon (which is a bit of light, the smallest bit of light), an atom, etc. they all can act like a wave in some situations, and a particle in others. The short way to think of it for light, is that it travels like a wave, but interacts like a particle. On the large, everyday scale, the particle nature of light is not seen. It is very easy to see the wave nature of light, as in the double slit experiment, or the fresnel bright spot in the shadow of a sphere (very uncanny, that one), but it takes some very fine appartus and careful experiements to show that light is "quantized". For most applications light is handled as a wave and is described by Maxwells equations for electrodynamics. Or if the wavelength is very small compared to the aperature (most optical intstruments) it is described by classical geometrical optics.

At any rate, you have mentioned the essential oddities of light in a very easy to understand way. Well put.

Re: Light

"or the fresnel bright spot in the shadow of a sphere (very uncanny, that one), "

Can you expound, please?

Re: Light

AilSnail,

During the great wave vs. particle debate about light--NOT the most recent one w/ Einstein & quantum stuff, but before that--Huygens advocated the notion that light was a wave. Some very intelligent people pointed out that if that was the case, that when two waves met they could either cancel each other out or reinforce each other, AND (and here's the kicker) they showed that if light really was a wave, that there would have to be a bright spot in the center of the shadow of a sphere. It takes certain conditions to set this effect up, so you can't just hold a golf ball in front of a lamp or anything like that, but these people set up the experiment, pulled down the window blinds, turned on the light, and LO AND BEHOLD, there was a bright spot in the center of the shadow thrown by a sphere. Nothing like the most outlandish, unforeseen consequence of a theory showing up in an experiment to convince people that it is true. Light was "proven" to be a wave, after many years of the dominance of Newton's corpuscular theory. Ironic that even later on, light was shown to come in packets. This stuff started to rear its head during the attempts to describe a "black body" radiator using classical physics. (Well, at the time, it was all they knew.) Max Plank quantized the radiation into discrete levels and got theory and reality to agree, but he didn't understand the real significance of what he had done. He just speculated that it was some function of the walls of the black body that caused the light to be quantized, but never dreamed that light itself was quantized. The other experiment had to do with the "photo-electric" effect. I will stop myself here. I'm rambling. Sorry.

Re: Light

[ QUOTE ]
jtice said:
If you are in a spaceship going the speed of light, and you are sitting in a chair facing the back of the ship. If you take a light, point it at your face, and turn it on, will you see the light?

[/ QUOTE ]
In theory: yes, you would see the light. The speed of light is relative to the observer, regardless if you are going that fast. The flashlight, and you (in theory) would be stationary. Suppose the Earth were travelling through space at the speed of light, and you are on the ground and turn on a flashlight.... would you not still see the beam?

Re: Light

Just a quick note in case no one else has mentioned it already, light does indeed have mass, but it is so minute as to be almost immeasurable. This is why gravity can and does have an effect on it. Some scientists may say that it doesn't have mass, but this is probably because it has so very little mass, that for most calculations, it is nearly irrelevant to take it into consideration. Kind of like how the mass of electrons is sometimes ignored because it has such a small mass in comparison to the mass of protons and neutrons. Then, compared to the mass of an electron, the mass of a photon of light is nearly insignificant.

We're talking about a very insignificant amount of mass here, well, insignificant for most considerations anyway, but not all, obviously. Remember, E could not, after all, equal MC squared if photons did not indeed have mass. The law of conservation of mass-energy and all of that, you know. I hope that this little tid bit is of some use in your speed of light quandaries. /ubbthreads/images/graemlins/smile.gif

Re: Light

X-CalBR8,
I work in nuclear physics (really) but I do not claim to be an expert. In everything I was taught, it has always been stated that light/photons were energy. Only matter has mass.
E=mc^2 does not say anything about photons having mass. It says that mass and energy are different forms of the same thing. It is a WAY WAY over simplified formula saying that if matter is accelerated to SOL it becomes energy.

Hmmm...after looking at what I just wrote here I can see why there might be some confusion.
Now where did I put that Physics text book?

Re: Light

@AlphaTea: Feel free to double check what I just said, for I am certainly human, and as such, very much capable of error, but I feel very confident that what I said is 100% accurate because, if I recall correctly, I double checked it with a physics professor at a major university just a little over a year ago. I think you will indeed find that a photon has mass, for the reasons that I gave previously. Don't feel bad, I went through I don't know how many science classes over the years before I was finally exposed to this information just a few years ago. /ubbthreads/images/graemlins/smile.gif

BTW, I once had the statistic of how much light it would take to weigh just as much as a single snow flake and it was some unbelievably large amount. I may try to look that up again later tonight if I have the time. This discussion has piqued my interest somewhat. /ubbthreads/images/graemlins/smile.gif

Re: Light

I'm puzzled by speed. Is there such a thing as absolute speed? How would it be measured? All the speeds I know of are speeds relative to something else. For instance when you go 60 miles per hour in your car, all that means is your car is going 60 MPH relative to the road.

But the road is moving too. The earth makes a complete spin around it's axis each day. And the earth makes a complete orbit around the sun each year. Taking all that into account would tell you how fast you are moving relative to the sun.

But surely the sun is moving too. They tell me all the stars in the universe are moving away from each other. Surely there is not one star in the middle of the universe that's standing still. It would have to be standing still relative to something, wouldn't it? And how would you know which one it was?

So just how fast are we moving?

If the mass of an object increases as it's speed increases, just what speed are we talking about. Is it speed relative to Peoria Illinois, or relative to the sun, or relative to Alpha Centauri? Or is it absolute speed?

If it's absolute speed, how fast are we going absolutely?

Re: Light

Ah, here we go. While looking for my previously mentioned statistic of how much light it would take to weigh as much as a snowflake, I got lucky and happened across this web site. This web site list the mass of some very interesting things, various forms of light and a snowflake being among them.

It list "an hour's worth of visible light from a 100 watt lightbulb" as having a mass of .0000000000004 grams. LOL. I told you guys it was small. /ubbthreads/images/graemlins/grin.gif

Here is an even better one, "a red photon" has the mass of .0000000000000000000000000000000034 grams. Now *that* is small! /ubbthreads/images/graemlins/smile.gif

I'm not sure where they got their numbers from, but they sound about right from what I've read in my physics books before. This should make for some interesting discussion, eh? /ubbthreads/images/graemlins/smile.gif

Re: Light

So what color light is heavier?

Re: Light

X-Cal, the special theory of relativity doesn't permit us to view anything moving at the speed of light as having a "minute measure of mass". Any mass at all, regardless of how small, approaches infinite as it approaches the speed of light. Only a particle or phenomena that has zero mass is exempt.

Please provide more information to support your claim with something other than "I know a professor", or "here's a chart from a pen manufacturer".

Re: Light

@Empath: I make no claim, so I have nothing to prove. I only restated/regurgitated what I was taught to be the truth in college. Believe it or not, it makes me little difference, for it is all just theory anyway. I was only trying to share what I was taught because I thought that you guys would find it interesting. Little did I know that I would have it asked of me to show proof of something that even a doctorate level physics professor would struggle over trying to prove.

You may also belittle a chart from a "pen manufacturer", but they didn't just pull those numbers out of their butts. They obviously came from somewhere and they appear to be about the same range as I was expecting them to be in, so I expect that they are legitimate.

I've read physics web sites over the years that claim that a photon has no mass and others that claim to know exactly how much mass one has. Decide for yourselves.

Even if I were to take the considerable time to find and quote my college physics book, somebody here would probably ask me to support the claim with proof and I would bet that the author of the book himself would be hard pressed to prove it one way or the other, assuming that it's even *possible* to prove it one way or the other. As I said before, it's all just theory anyway.

Frankly, I don't really believe the half of what I was taught in college about subatomic particles anyway. I believe that they are far from figuring out the puzzle, despite how they like to brag about having it all figured out now days.

After all, I still have a set of encyclopedias from just before World War II that says something to the effect that scientist have *proved* that it is absolutely impossible to ever split the atom. Shows what scientists know. I believe that a half century from now, people will all be laughing at the level of ignorance of scientists from this time period as well.

I just had company walk through the door so I don't have time to debate this further tonight, maybe tomorrow if I don't have to work. Good night all.

Re: Light

An hour's worth of heat from a light bulb has mass? And the energy from an A-bomb has mass? Or are they converting the energy to the eqiv. mass?
BTW the recent Light topic on CPF (or are they all light topics? No, some get heavy /ubbthreads/images/graemlins/smile.gif )
relative speed?

Re: Light \"SPEED\" Questions, Thoughts.

A couple links
This one didn't prove anything to me. Maybe I missed the point? How Heavy is Light?

No world shakers here eitherThe Properties of Light

And another that doesn't really answer the question but is talking about a neat art exhibit. New York Hall of Science From that comes this [ QUOTE ]
Mass (weight) of light can be calculated from E = mc2. The amount of solar energy falling on the entire surface of the earth is 8 kilograms / second. By comparison a super nova emits approximately 1 million million million tons per second.

[/ QUOTE ]
See what happens when you use Google (or Boogle today /ubbthreads/images/graemlins/smile.gif )

Now my own question. On earth is there an easy way to determine somethings Mass (not weight) Scales don't work they determine weight.

Re: Light

Wits' End said: "On earth is there an easy way to determine somethings Mass (not weight) Scales don't work they determine weight."

For that you would use a balance, with what you want to know the mass of on one side and weights of a known mass on the other side. Let me know when you find a way to make light sit still on a balance though. I think there could be a Nobel prize in it for you. /ubbthreads/images/graemlins/wink.gif

Maybe it could be done with that experiment that was posted on CPF a while back that freezes and stops light in it's tracks. I just wonder what would happen if you stopped enough light in one place? Could you weigh it? Would it cause a light explosion when it warmed up? Who knows? We are still learning about such things.

Then there are things like black holes that have enough gravity to trap light using extreme gravity. That brings up the question of whether strong gravity waves could be used to trap, store and concentrate light, enough to measure the mass. Of course to try that, we would have to learn how to generate our own artificial gravity waves and if we could do that, we might all be flying around in flying saucers someday. /ubbthreads/images/graemlins/smile.gif

Like I said earlier though, when you are talking about something like a subatomic particle, you are completely within the realm of theory anyway. How accurate those theories are today is anyone's guess. They could be spot on or they could be miles away.

I just don't like how most of the authors of the books and the people in the various scientific communities try to act all arrogant and act like all popular modern day theories are 100% correct, just because they happen to be the popular theory of the day. A theory is called a theory and not a fact for good reason. Just like my previous example of how scientist not so many decades ago, in all their arrogance, "proved" that the atom could never be split, and we all know how that turned out.

Re: Light

Ok guys. Here is a web page taken directly from an actually physics web site so that there can be no grumbling about where the data came from and it is something that you can go to and see for yourselves so that you don't have to take my word for it that it came from a physics book. Maybe that will make everybody happy.

It says basically that according to special relativity that a photon's "rest mass" is considered to be zero, but since a photon is supposedly never at rest, it does have mass because it has energy and that mass can be calculated with the following formula: mc2=hf => m=hf/c2

In this formula, h is a constant (Planck's constant) and f is the frequency of the photon. With this formula you can calculate the mass of any photon of any wavelength and yes, the higher frequency wavelengths are the ones with the most mass because they contain the most energy. So a blue photon is more massive than green which is in turn more massive than a red one.

I even went to the trouble of finding Planck's constant just in case anyone here cares to do the math. Planck's constant is approximately 6.626176 × 10-34 joule-seconds. That is 10 to the negative 34 power. This board doesn't seem to want to allow me to show the -34 as a power, so that's why I thought that I should clarify even though most of you reading this far are probably already familiar with Planck's constant somewhat anyway.

The energy contained within a single photon is represented by E=hf where e=energy in joules h=Planck's constant and f=frequencey in hertz or E=(6.626176x10-34)f

Here is a direct quote taken from the site: "though photons don't have rest mass, they do have energy and thus they have mass. The photons are wave particles. This means that they act as waves and as particles as well. This is the duality of the nature of light (and of every particles). And so as particles they have mass, and as waves they have frequency."

I hope that this helps to clear things up somewhat and doesn't just make everyone here even more confused. /ubbthreads/images/graemlins/smile.gif

Re: Light

Now this leaves one to wonder whether or not the theory of special relativity will have to be revised due to the recent experiments where they are supposedly stopping light these days. If they can stop light so that it has zero velocity, then according to special relativity, the photon at rest would have zero energy and therefor zero mass. Now if they can stop a photon and then prove that the photon still has mass, that will be a major physics breakthrough that will require the revisement of a lot of widely accepted physics theory. Just something else to ponder.

Now I think I'm going to go take an aspirin because my brain hurts from all of this unaccustomed overexertion this morning. /ubbthreads/images/graemlins/smile.gif