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Light has mass?


lucifer
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from the day one i started doing science there is one unanswered question.... whether the light has mass?

so anybody has a answer!!!

is dat true light has mass in partical nature phenomena like compton effect,photo electric effect?

Edited by lucifer
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yes dude it must have but i think we dont have had enough exposure n tech development to measure it........ with out any mass no pure energy exists.........

from the day one i started doing science there is one unanswered question.... whether the light has mass?

so anybody has a answer!!!

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Because neither requires it to have mass. The equation of motion in general relativity, the most up-to-date mainstream theory of gravity, does not (explicitly) contain a mass-dependence. How do you produce the question marks with a broken shift key, btw?

Edited by timo
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then how come light bends due to gravity?

Short answer: In general relativity it is energy rather than mass that gravitates in general relativity. Better said: It is energy, momentum, stress and pressure that gravitate in GR. It's called the stress-energy tensor. Mass gravitates because mass is one kind of energy. Light gravitates because it is another kind of energy.

 

Note that mass is a somewhat ambiguous term in special relativity, and even more ambiguous in general relativity.

 

There are several other threads on this topic right here at this site, and even more discussions elsewhere. Here are few things almost everyone agrees on (there are always nuts who won't agree to anything):

  1. The intrinsic mass of a single photon is identically zero.
  2. Light is affected by gravity.
  3. Light gravitates.

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There was a science fiction book where the ancient weapon left by the obligatory disappeared progenitor race was a concentration of light which was so 'dense' (remember, fiction) that the gravitational field generated by the energy formed a 'massless black hole' which of course was used against the ultimate Evil from intergalactic space.

Edited by ACG52
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from the day one i started doing science there is one unanswered question.... whether the light has mass?

so anybody has a answer!!!

is dat true light has mass in partical nature phenomena like compton effect,photo electric effect?

That depends on what you mean by the term mass.

Does light have inertial mass? Yes.

Does light have proper mass? No.

 

However there are circumstances in which it can be said that the rest mass of light is zero. Consider a gas of massless photons. The gas will have a frame in which the total momentum of the photons is zero. The mass of the system of photons will then equal the total energy of the photons divided by c2. So in this case the light has non-zero rest mass.

 

I have a set of lecture notes from a course that Alan Guth teaches. In it he says that light has mass since it has energy. Guth is a smart man. :) The course is the about the early universe. In a course like that its useful to think of light having mass.

 

See also http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html

 

Because gravity bends space/time and light will always follow the geodesic.

Gravity doesn't always bend spacetime. That only happens when there are tidal gradients present. In a uniform gravitational fiel the spacetime is zero and yet light is deflected by the gravitational field.

 

The equivalence principle states that inertial mass is proportional to gravitational mass. For this reason all particles fall at the same rate. It is this reason why geodesics can be created in which light is deflected, and again, that's because it has mass.

 

Consider Feynman's view on this. From the Feynman Lectures, Vol-I, page 7-11

In the Einstein relativity theory, anything that has energy has mass - mass in the sense that it is attrracted gravitationallly. Even light, which has energy, has a "mass." When a light beam, which has energy in it, comes past the sun there is an attraction on it by the sun. Thus the light does not go straight, but is deflected.

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There was a science fiction book where the ancient weapon left by the obligatory disappeared progenitor race was a concentration of light which was so 'dense' (remember, fiction) that the gravitational field generated by the energy formed a 'massless black hole' which of course was used against the ultimate Evil from intergalactic space.

Fredrik Pohl, Hechee Rendezvous?

 

In any case, what you are talking about is called a kugelblitz, which is the German term for ball lightning.

 

A bucket full of photons (a closed bucket with perfect mirrors for sides, top, and bottom) weighs more than does the empty bucket. Put enough photons in the bucket and it will form a black hole. One way of looking at it: It's energy rather than mass that gravitates in general relativity.

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A bucket full of photons (a closed bucket with perfect mirrors for sides, top, and bottom) weighs more than does the empty bucket. Put enough photons in the bucket and it will form a black hole. One way of looking at it: It's energy rather than mass that gravitates in general relativity.

All components of the stress-energy-momentum tensor contribute to the source of gravity. Stress is a source of inertia so it follows that its also a source of gravity. In the special case it is well noted that pressure is a source of gravity.

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Fredrik Pohl, Hechee Rendezvous?

 

In any case, what you are talking about is called a kugelblitz, which is the German term for ball lightning.

 

A bucket full of photons (a closed bucket with perfect mirrors for sides, top, and bottom) weighs more than does the empty bucket. Put enough photons in the bucket and it will form a black hole. One way of looking at it: It's energy rather than mass that gravitates in general relativity.

 

Einstein also stated that energy produces gravity.

 

In Pohl's Hechee series, there was an ancient race who used Black holes as a refugee while they caused a cycle of the universe to build a universe more suited to their race.

 

I was thinking of Alan Dean Foster's Flinx series, in particular Flinx Transcendent.

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In Pohl's Hechee series, there was an ancient race who used Black holes as a refugee while they caused a cycle of the universe to build a universe more suited to their race.

The Hechee used black holes as a hideaway from the Assassins (or the Foe or the Kugel). It is the latter who were the bad guys, not the Hechee. The Assassins were the ones who were killing off intelligent species willy nilly and who were adding energy to the universe in the form of kugelblitzes to make the universe collapse in a Big Crunch and re-emerge without all of that stupid mass. Yeah, yeah, silly sci fi.

 

I was thinking of Alan Dean Foster's Flinx series, in particular Flinx Transcendent.

More silly sci fi! Cool! It's better than all that fantasy nonsense that pervades the science fiction section of book stores today. I've read a one or two of his books, but none of that series. I have a plane flight tomorrow. Sounds like a good thing to grab for the flight.

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The Hechee used black holes as a hideaway from the Assassins (or the Foe or the Kugel). It is the latter who were the bad guys, not the Hechee. The Assassins were the ones who were killing off intelligent species willy nilly and who were adding energy to the universe in the form of kugelblitzes to make the universe collapse in a Big Crunch and re-emerge without all of that stupid mass. Yeah, yeah, silly sci fi.

 

 

More silly sci fi! Cool! It's better than all that fantasy nonsense that pervades the science fiction section of book stores today. I've read a one or two of his books, but none of that series. I have a plane flight tomorrow. Sounds like a good thing to grab for the flight.

 

 

Yeah, that's right, the Hechee were also hiding. I forgot that. It must be 30 years since I read it.

 

If you're going to start the Flinx series, start with the Tar-Aiym Krang. It's the first. published in 1972, so don't expect to find it on the shelves. (although Barnes and Nobel might have it).

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from the day one i started doing science there is one unanswered question.... whether the light has mass?

so anybody has a answer!!!

is dat true light has mass in partical nature phenomena like compton effect,photo electric effect?

I believe that there are a few other threads on this subject. Have you seem them? They might contain something you're interested in.

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oh sure i will...thanks for suggestion

I did a bit of foot work for you. See

 

http://www.scienceforums.net/topic/66737-what-is-mass/

 

http://www.scienceforums.net/topic/66669-photons/

 

All components of the stress-energy-momentum tensor contribute to the source of gravity. Stress is a source of inertia so it follows that its also a source of gravity. In the special case it is well noted that pressure is a source of gravity.

To be more precise, the source of gravity is mass, of which there are three kinds. These are useful to know if you're studying cosmology. E.g. see the definitions in Peebles text on cosmology on pages 269 Eq. (10.70) and page 453 Eq. (18.02). Peebles uses a perfect fluid as an example. Peebles gives expressions which he identifies as the active gravitational mass density, passive gravitational mass density and inertial mass density. The later two have the same value.

 

The following defines those terms

 

1) Inertial mass - T he mass of a body as determined by its momentum

 

For those of you who have trouble with this as the definition of inertial mass then please note that its

not something I conjured up. Although I've been saying that this is the definition all along that its what

I learned from relativity texts and mechanics texts and ajournal articles. Please see

http://www.thefreedictionary.com/inertial+mass

 

2) Active gravitational mass - The source of gravity

 

3) Passive gravitational mass - That on which gravity acts on.

 

If I can find out how to post Latex then I'll quote the equations. Can someone tell me where there is a Latex tutorial? I think there's one on this site but I don't know how to find it.

Edited by pmb
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from the day one i started doing science there is one unanswered question.... whether the light has mass?

so anybody has a answer!!!

is dat true light has mass in partical nature phenomena like compton effect,photo electric effect?

 

Light does not have any mass because is made of photons which are massless particles

 

http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/960731.html

 

The massless property of light has been well-tested

 

http://en.wikipedia.org/wiki/Photon#Experimental_checks_on_photon_mass

 

then how come light bends due to gravity?

and how light got sucked into black hole?

 

As ACG52, timo, and DH reported before. Massless particles are also affected by gravity. If you want to go into the details, search the term "massless" in the next introduction to General Relativity and you will find the equation of motion for light signals.

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well light does not get affected by gravity.But light gets affected by bending of space and time. There is a principle called least-time principle in which light always follow the least time to travel a distance.In our world straight line will take least time to reach a particular distance. in a medium light bends.In this also it follows least-time principle. Because when medium becomes dense least time to reach a distance will be bent line. So since sapce-time is curved,so to reach a perticular distance light would have to turn inorder to reach that distance in least-time!!! If you read relativity book written by albert einstein,you will understand it perfectly!!

 

ok thanks for answering everyone....compared to u guys i am a dumb a** :(...

 

 

 

No one is dumb.You are intelligent.You jst don't know some information.That is all. All of them might have had these doubts and got cleared just like you !!!

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If you read relativity book written by albert einstein,you will understand it perfectly!!!

 

There's actually a lot more to relativity than what Einstein wrote and not all of what he wrote in his papers was correct (which is to be expected of publishing a work-in-progress and getting feedback and more ideas). While he discovered it, it has been refined and advanced with contributions from many people.

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There's actually a lot more to relativity than what Einstein wrote and not all of what he wrote in his papers was correct (which is to be expected of publishing a work-in-progress and getting feedback and more ideas). While he discovered it, it has been refined and advanced with contributions from many people.

 

In this case Einstein's answer to the OP question is "no, light has no mass", because Einstein explicitly embraced the modern concept of mass in 1948.

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In this case Einstein's answer to the OP question is "no, light has no mass", because Einstein explicitly embraced the modern concept of mass in 1948.

Well this post is a good excuse as any to come back from my sabatical. What you claimed about Einstein (but again never proved) is wrong. In one of his early articles published between 1906 and 1908 Einstein said that radiation density (aka light) has mass density. This is the same thing that Alan Guth still uses in his cosmology course.

 

ok thanks for answering everyone....compared to u guys i am a dumb a** :(...

The term you're looking for is ignorant which means having a lack of knowledge. When people go to college they gather information/knowledge. They don't get smarter for the most part. There is a small increase in IQ in that the educated man has a better ability to present a more logical an argument than the uneducated person.

 

well light does not get affected by gravity.

I disagree. It sure does get affected by gravity.

 

But light gets affected by bending of space and time.

In a uniform gravitational field a beam of light is still deflected and in such a field the spactime curvature is zero.

 

If you read relativity book written by albert einstein,you will understand it perfectly!!

That is very very true. And anyone who reads Einstein's relativity book will learn that Einstein never said that gravity is a curvature in spacetime. Einstein disagreed with that interpretation.

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Well this post is a good excuse as any to come back from my sabatical. What you claimed about Einstein (but again never proved) is wrong. In one of his early articles published between 1906 and 1908 Einstein said that radiation density (aka light) has mass density.

 

What Einstein wrote in 1906/8 is not a disproof of whether Einstein accepted the modern interpretation in 1948.

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Well, well, well! It is a classical physics discussion BUT I THINK I WILL HAVE to bring special relativity in the case. Actually apparent mass=m/square root of 1-(v^2/c^2). Then if light will have mass, it will defy the special relativity. That means light when travelling at the velocity c will have infinite apparent mass which u can find by plugging the numbers in the above mentioned equation. So that is the fact behind it, u can also consult some postulates of Einstein's special theory of relativity to find a more convincing answer.

Edited by stancamp1
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