I need help understanding light.

[syberscott]

I'll get right to it. Do photons have mass or not?
If so then how do they travel as fast as they do? (I'm assuming that anything with mass cannot travel at a velocity of c, but I'm not too familiar with particles so I may be off track here.)
And if photons don't have mass, then how do they carry momentum and energy?

 

[f95toli]

Photons do not have mass.
There is no way to "understand" how photons carry momentum an energy, they just do, it is a consequence of the fact that the speed of light is constant.
The best thing is to look this up in some book about special relativity, there are a few good books that are easy to understand.

 

[AdvancedRobotics]

While it does seem logical that light would have mass, as it is affected by gravity, it in fact does not. It is due to the fact that the speed of light is constant, that light does not contain mass but has momentum. It gets quite blurry trying to explain these things, and it's best to just accept that it is true.

Also, it is understood that the speed of light is only achieveable by massless objects, such as photons. We as human beings can only travel a bit below the speed of light.

 

[Ameesh]

Originally posted by: AdvancedRobotics
While it does seem logical that light would have mass, as it is affected by gravity, it in fact does not. It is due to the fact that the speed of light is constant, that light does not contain mass but has momentum. It gets quite blurry trying to explain these things, and it's best to just accept that it is true.

that is the lamest answer i have ever heard.

its like saying, god exsists, i cant explain, just accept that its true.

As for my answer:

i can say that all things that have mass are affected by gravity, but the reverse isnt neccesarly true (all things affected by gravity have mass). IN the case of light being affected by gravity (lets take the black hole example) . Photons are not being affected dirrectly but rather the immense gravity of the black hole is affected (bending/distoring) space-time which cases the light's path to change.

as far as how photons have momentum and not mass, that is a much longer discussion and i suggest you read up on Einstiens theory of General Relativity.

on a more interesting note, scientists have not been able to accuratly "weigh" a photon but they have been able to put an upper limit on its mass if it had one, its something like ~ 4E-50 which is a lot less then an electron wieghing in at 9E-28

einstiens theories have stood up to a lot of testing, but they still hold, but people still try.

pardon my atrouicious spelling.

 

[Cogman]

I was taught that Light at rest has no mass, but a moving beam of light does have mass. In a science book that we had they compared a beam of light to an atom and they both looked exactly the same. I belive that light is the best example of converting mass into energy. when light stops moving it will create heat (this is true with all light, even if the heat is insignifigant it is still created). BTW, as to show that light had mass, I had a object with extremly light winglets and when placed in light it would start turnning from the current of the light (It was a very interesting device but it was later broken )

 

[Bowfinger]

I know that scientists somewhere demonstrated that a laser could levitate a tiny glass sphere. I thought this was due to the mass of photons. Are you saying instead that it was due to their momentum?

 

[Amorphus]

hee. now, if I wanted to confuse him even more, I could bring up this question:

if light has mass, how can it be a wave? aren't waves massless, since they describe a regular change in something?

yeah, I know, the definition of "wave" escapes me at the moment.

now, about the "light is not affected by gravity" - whats this junk I see in textbooks saying "the gravitic pull of a black hole is so strong that even light cannot escape!" ?

 

[Cogman]

like I said, As long as it is moving it has mass. But it also forms a wave, I cant remember the definition, but I do remember that a photon was defined as object (for lack of better words) that shows both wave and partical atributes. (moves like a wave, and yet has mass when moving)

 

[Shalmanese]

Theres no such thing as a stationary photon. Thus, rest mass has a academic meaning but you never encounter it in real life.

That photons have momentum can be easily deonstrated using a device thing which i have temporarily forgotten the name of. Its basically a very thin piece of foil suspended on a bearing. If you let sunlight shine on it, it will spin due to the momentum imparted from the photons.

 

[ShawnD1]

you can figure out the theoretical mass of the photon by combining the energy formulas relating to mass and to frequency - planck and einstein.

http://edmonton-elite.cjb.net/junk/mass.jpg

 

[CTho9305]

Originally posted by: Cogman
I was taught that Light at rest has no mass, but a moving beam of light does have mass. In a science book that we had they compared a beam of light to an atom and they both looked exactly the same. I belive that light is the best example of converting mass into energy. when light stops moving it will create heat (this is true with all light, even if the heat is insignifigant it is still created). BTW, as to show that light had mass, I had a object with extremly light winglets and when placed in light it would start turnning from the current of the light (It was a very interesting device but it was later broken )

That doesn't work because of photons pushing it. It is much simpler.... the white side reflects light. The black side absorbs light. The black side gets hot. The air behind the black side expands. It moves. . If it was photons pushing it, it would spin the other way.

 

[AdvancedRobotics]

Originally posted by: Ameesh

Originally posted by: AdvancedRobotics
While it does seem logical that light would have mass, as it is affected by gravity, it in fact does not. It is due to the fact that the speed of light is constant, that light does not contain mass but has momentum. It gets quite blurry trying to explain these things, and it's best to just accept that it is true.

that is the lamest answer i have ever heard.

its like saying, god exsists, i cant explain, just accept that its true.

As for my answer:

i can say that all things that have mass are affected by gravity, but the reverse isnt neccesarly true (all things affected by gravity have mass). IN the case of light being affected by gravity (lets take the black hole example) . Photons are not being affected dirrectly but rather the immense gravity of the black hole is affected (bending/distoring) space-time which cases the light's path to change.

as far as how photons have momentum and not mass, that is a much longer discussion and i suggest you read up on Einstiens theory of General Relativity.

on a more interesting note, scientists have not been able to accuratly "weigh" a photon but they have been able to put an upper limit on its mass if it had one, its something like ~ 4E-50 which is a lot less then an electron wieghing in at 9E-28

einstiens theories have stood up to a lot of testing, but they still hold, but people still try.

pardon my atrouicious spelling.

Before you bash other people, try to learn something. Lemme correct something in this statement:

>> "As for my answer:"

Don't you mean someone elses. I searched on google and found this exact thing that you said but much longer. Your attempt at pretending to be smart has failed!

>> "pardon my atrouicious spelling"

You must really suck at copying and pasting then.

Originally posted by: Cogman
I was taught that Light at rest has no mass, but a moving beam of light does have mass. In a science book that we had they compared a beam of light to an atom and they both looked exactly the same. I belive that light is the best example of converting mass into energy. when light stops moving it will create heat (this is true with all light, even if the heat is insignifigant it is still created). BTW, as to show that light had mass, I had a object with extremly light winglets and when placed in light it would start turnning from the current of the light (It was a very interesting device but it was later broken )

You were taught correctly. At rest, light has no mass. However, this is trivial as a photon can never be at rest, it is always traveling at a constant speed.

Originally posted by: Amorphus
hee. now, if I wanted to confuse him even more, I could bring up this question:

if light has mass, how can it be a wave? aren't waves massless, since they describe a regular change in something?

yeah, I know, the definition of "wave" escapes me at the moment.

now, about the "light is not affected by gravity" - whats this junk I see in textbooks saying "the gravitic pull of a black hole is so strong that even light cannot escape!" ?

A wave is something that moves energy without moving matter. A photon is both a particle and a wave.

 

[Bowfinger]

Originally posted by: AdvancedRobotics

Originally posted by: Cogman
I was taught that Light at rest has no mass, but a moving beam of light does have mass. In a science book that we had they compared a beam of light to an atom and they both looked exactly the same. I belive that light is the best example of converting mass into energy. when light stops moving it will create heat (this is true with all light, even if the heat is insignifigant it is still created). BTW, as to show that light had mass, I had a object with extremly light winglets and when placed in light it would start turnning from the current of the light (It was a very interesting device but it was later broken )

You were taught correctly. At rest, light has no mass. However, this is trivial as a photon can never be at rest, it is always traveling at a constant speed.

A couple of nits. First, light travels at a constant speed in a vacuum. It travels at other speeds in other mediums. Second, it is possible for photons to come to a rest. Within the last year or so a group announced that they had successfully stopped photons within an experiment, then released them. In this context, "stopped" meant caused them to stop moving, not absorbed or stored.

 

[AdvancedRobotics]

Originally posted by: Bowfinger

Originally posted by: AdvancedRobotics

Originally posted by: Cogman
I was taught that Light at rest has no mass, but a moving beam of light does have mass. In a science book that we had they compared a beam of light to an atom and they both looked exactly the same. I belive that light is the best example of converting mass into energy. when light stops moving it will create heat (this is true with all light, even if the heat is insignifigant it is still created). BTW, as to show that light had mass, I had a object with extremly light winglets and when placed in light it would start turnning from the current of the light (It was a very interesting device but it was later broken )

You were taught correctly. At rest, light has no mass. However, this is trivial as a photon can never be at rest, it is always traveling at a constant speed.

A couple of nits. First, light travels at a constant speed in a vacuum. It travels at other speeds in other mediums. Second, it is possible for photons to come to a rest. Within the last year or so a group announced that they had successfully stopped photons within an experiment, then released them. In this context, "stopped" meant caused them to stop moving, not absorbed or stored.

Well I meant constant in that the speed of light is constant.

And do you have a link or something to that experiment? Sounds very interesting; would like to read.

Thanks!

 

[Cogman]

if you are talking about mine, No I dont (but it was a fun toy )

 

maybe not quite what you're looking for but I don't have enough time to look for a more scientific paper

http://www.msnbc.com/news/242698.asp?cp1=1

Also, I only read the intro so if its not the right article, don't bash me

 

[Bowfinger]

Originally posted by: JAGedlion
maybe not quite what you're looking for but I don't have enough time to look for a more scientific paper

http://www.msnbc.com/news/242698.asp?cp1=1

Also, I only read the intro so if its not the right article, don't bash me

I'm sure it's what I was thinking of.

My bad, they did not really stop photons. One experiment slowed them way down, "Last year, a research team at the Rowland Institute for Science and Harvard University, headed by Danish physicist Lene Hau, brought light waves down to a 1 mph crawl by putting them through a specially prepared haze of ultracold sodium atoms." Another, unrelated team, did an experiment where, "information about the light wave is gradually transferred to specially prepared atoms trapped within a glass chamber, and then turned back into a replica of the original light wave."

Interesting stuff, but not exactly what I reported. Sorry.

 

[PIMPBOT5000]

I think I read an article a while back that commented on an experiment that showed that there are different "speed grades" of light, that it doesn't always move at a constant speed.

 

[Shalmanese]

Originally posted by: Bowfinger
A couple of nits. First, light travels at a constant speed in a vacuum. It travels at other speeds in other mediums. Second, it is possible for photons to come to a rest. Within the last year or so a group announced that they had successfully stopped photons within an experiment, then released them. In this context, "stopped" meant caused them to stop moving, not absorbed or stored.

LIGHT can move at speeds other than c, PHOTONS cannot. Its a sublte distinction. When we say that light slows down, we are really saying that the aggregate waveform slows down. Each individual photon is still moving at c.

 

[trak0rr0kart]

I'm going to reply to everyone in this forum and no one in particuluar. I'm not bashing anyone, just telling you what I know so we can continue this discussion in a reasonable fashion I'm going off the assumption that light has mass for this article.

You cannot have momentum without mass. Momentum=mass*velocity.

Speed of light is fastest in a vacum ~300,000 KM/sec or 186,000 mi/sec + or -.

Take a diamond for a sec, light travels through it at about 140,000 KM/sec or so. Each different medium that light travels through will be at a different velocity. A glass jar for example will only let it travel through at about 200,000 km/sec. Water is about 206,000 km/sec.

An important distinction between Light and photons is that when the LIGHT slows down.. your really only increasing the frequency of the wave.. The actually photons are still travelling at the same velocity. The amplitude of the light will stay the same.. but when the frequency of light increases.. so does the path the photons must take to travel the same distance for the same time. I.E. Take 10 cm of air and compare the distance of the photons travel.. then take the distance the photons will travel in 10 cm of glass. You will see that the potons have to travel further in the glass because the frequency is higher then in the air.

*Einstein did not prove that light had mass conclusively.

Photons have mass. Space-time is basically a gravity map. You put a sun in the middle of the map.. you get a warped gravity field. Stick another in there.. you'll get a another warped gravity field. When Einstein was finally honored greatly is when he proved that photons had mass. He did this by predicting that when, in 1919, a solar eclipse would show that light was bending around the sun because the stars would be in different locations then normal, he did he prove that light could bend and therefore had mass. A by product of him proving this was the proving of the General theory of relativity.

AS far as light escaping a black hole is concerned I would say this. According to Stephen Hawking.. black holes have a thing called an event horizon. What this means is that at a certain radius from the center of the black hole (extremely massive body) there is a point where light can escape. Below this it will not, and above it.. it will escape at exactly C. So light that is bent at just above the critical angle for the space-time warp will escape then just above the surface. Because the black holes do not emit visable light does not mean that it dosen't emit any of the EMS (electro magnet spectrum) radiation. According to him and other scientists in the field (his coworkers), Higher level energy.. in the order of gamma rays etc.. are able to escape the black hole. The reason why you don't see any light at all.. is because none of the visible light is escaping from it.. but you can see it through gamma ray detection etc (all this is just theory as of yet and when it comes to extremely LARGe black holes then I think that even the gamma radiation might be extremely hard to detect as well).

Hope this helps a few of you out there with doubts.

 

[f95toli]

Originally posted by: trak0rr0kart
I'm going off the assumption that light has mass for this article. You cannot have momentum without mass. Momentum=mass*velocity. .

Where did you find that "definition"? It will only be valid in classical mechanics (speeds much lower than c) and has nothing to do with modern physics.
We stopped using that about 100 years ago, and it is not correct even for "normal" particles since it does not take relativistic corrections into account, and it is certainly not correct for massless particles like photons.

 

[syberscott]

Here is a brief quote by Stephen Hawking:
" As an object approaches the speed of light, its mass rises ever more quickly, so it takes more and more energy to speed it up further. It can in fact never reach the speed of light, because by then its mass would have become infinite, and by the equivalence of mass and energy, it would have taken an infinite amount of energy to get it there. For this reason, any normal object is forever confined by relativity to move at speeds slower than the speed of light. Only light, or other waves that have no intrinsic mass, can move at the speed of light."
To me, that says that photons have no mass.

 

[Shalmanese]

Uh, gravity states that all particles will move towards a mass, regardless of if the particle had mass or not. All the solar eclipse proved was that the sun warped gravity, not that photons have mass. momentum = mass x velocity is wrong

 

[trak0rr0kart]

Nobody has proved that light has mass conclusively or inconclusively. Here is an interesting quote though..

(( "We have, however, put an upper limit on the photon mass. In 1994, the Charge Composition Explorer spacecraft measured the Earth's magnetic field and physicists used this data to define an upper limit of 0.0000000000000006 electron volts for the mass of photons, with a high certainty in the results.

This number is close to zero; it is equivalent to 0.00000000000000000000039 times the mass of an electron (the lightest particle), says Turner. "

-Answered by April Holladay, science correspondent, November 22, 2000 ))

To me it says that it hasn't been answered correctly yet.. so that is why I made that assumtion for this article. I believe that it follows that path in space-time because it has mass.. not because of some magical reason. Then again.. I haven't proved that photons have mass yet either or seen it proved.. but the quote above is reassuring.

What if when the photons are released.. they had enough energy expelled from it that the little mass they have is released because of an exp dissipation of energy (theoritcal), E=mc^2, and then they become lighter as they approach the C so as to not violate the famous eq. and right when they hit c they where moving sooo fast that they broke the light barrier. and settled in at C. All of this happening within a 1e300 of a second or something. Just and idea.

Oh, Momentum=mass*velocity is correct.

 

[f95toli]

But it is easy to show that the momentum carrier by a photon can be much higher than what we get if we use the uppper limit in the relation m*v, and it does not explain why the momentum of a photon is frequency dependent.