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Author Topic: How can light have no mass?  (Read 14549 times)

Offline lightarrow

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How can light have no mass?
« Reply #25 on: 10/07/2010 15:19:01 »
Then, why the Particle Data Group says that photon's mass is zero?
Because you're supposed to know  that by mass they mean proper mass, not inertial mass.
Yes, of course. The point is not that I intended to say that you misunderstood the term proper mass, but that in particle physics the concept of relativistic mass is not used, at least because you wouldn't know what value to write...
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If you also look at the site you might see that the lifetime of a free neutron you'll see that it says 15 minutes. You're also supposed to know that they mean proper lifetime. We all know that the life time of a particle increases with speed. But that isn't evident in the lifetime that they give of 15 minutes.

Those things are read and used by professional physicists and whoever reads them knows what those terms mean. I know that it refers to proper mass (aka rest mass). Nobody has to tell me that. But that's particle physics. If you expect to go to other branches of physics and expect the term to mean what you think it does then you'll be sorely mistake. E.g. Misner, Thorne and Wheeler's text "Gravitation" uses mass in one key derivation where it means "inertial mass" (what you'd call relativistic mass) but merely says "mass." Other texts are the same.

Show me where you think I made a mistake.
And what if I would say it's the Sun to orbit around the Earth and not the other way...around?  :)  Can you show me I'm wrong? You probably can't (at least I think so) but we find it more convenient to say it's the Earth to orbit around the Sun. In a similar way, most physicists find it more convenient to consider mass as invariant and not speed dependent; I mean, nothing forbids that tomorrow most physicists will change theyr mind, of course.
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And please don't tell me that one definition is "right" and the other is "wrong" because that's not a valid arguement when the relativity literature uses the same term both ways. Mostly as a matter of taste. Some people get tired or writing "rest" and others get tired of writing "inertial" in front of the term "mass". That's the only real difference.

Please recall that I was answering Ben's questioi
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Hi, how light can travel at c, have energy but no mass even though E=mc^2 ?

how does this work? 
And it is this question that I'm responding to. As such I was basically explaining what the "m" in that expression stands for and my answer is that it refers to the inertial mass of a body, not its proper mass. The proper mass of a photon is zero while the inertial mass is not. There are other kinds of mass too,i,e, passive gravitational mass, active gravitational mass.
Since you talked about inertial mass, I have a question for you, I know that you know more than me about it (I speak seriously). Spacetime curvature near an object with proper mass = M is independent of the frame of reference from which you measure ithe curvature, isnt'it? And what about near a zero mass object as a photon?
« Last Edit: 10/07/2010 15:21:55 by lightarrow »
 

Offline CZARCAR

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How can light have no mass?
« Reply #26 on: 10/07/2010 19:05:29 »
how do the lightmills, spinning mills inside a glass enclosure, spin faster when placed in direct sunlight & effect a spin & not just heat up if photons have no mass?
 

Offline lightarrow

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How can light have no mass?
« Reply #27 on: 10/07/2010 19:45:44 »
how do the lightmills, spinning mills inside a glass enclosure, spin faster when placed in direct sunlight & effect a spin & not just heat up if photons have no mass?
1. In those devices the effect is actually given by the (spare) air molecules heated from the black part of the blades (infact it rotates in the opposite sense respect to the one it should).
2. Even if it would work correctly and the force were given by the light reflecting back from the polished part of the blade, the effect would be simply described by classic electrodynamics: light has momentum even if it has no mass.
Hope you don't ask why does light have momentum without having mass, we have discussed this subject hundreds of times... :)
« Last Edit: 10/07/2010 19:49:02 by lightarrow »
 

Offline Geezer

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How can light have no mass?
« Reply #28 on: 10/07/2010 19:46:04 »
There does seem to be something slightly paradoxical going on here.

As I understand it, it takes an infinite amount of energy to accelerate any mass to the speed of light.

But photons are accelerated to the speed of light, apparently without consuming an infinite amount of energy, so how can they have mass?
You're thinking of Euler's definition of mass, not Newton's. Newton defined inertial mass as that which determines momentum, not that which determines acceleration. And thats how its used in modern physics.

Ah! So that means I could travel at the speed of light then.
 

Offline CZARCAR

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How can light have no mass?
« Reply #29 on: 10/07/2010 20:02:11 »
how do the lightmills, spinning mills inside a glass enclosure, spin faster when placed in direct sunlight & effect a spin & not just heat up if photons have no mass?
1. In those devices the effect is actually given by the (spare) air molecules heated from the black part of the blades (infact it rotates in the opposite sense respect to the one it should).
2. Even if it would work correctly and the force were given by the light reflecting back from the polished part of the blade, the effect would be simply described by classic electrodynamics: light has momentum even if it has no mass.
Hope you don't ask why does light have momentum without having mass, we have discussed this subject hundreds of times... :)
long time since ive seen the lightmill. whats its proper name? i wanna research it.....thanx
 

Offline Geezer

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How can light have no mass?
« Reply #30 on: 10/07/2010 20:16:50 »
how do the lightmills, spinning mills inside a glass enclosure, spin faster when placed in direct sunlight & effect a spin & not just heat up if photons have no mass?
1. In those devices the effect is actually given by the (spare) air molecules heated from the black part of the blades (infact it rotates in the opposite sense respect to the one it should).
2. Even if it would work correctly and the force were given by the light reflecting back from the polished part of the blade, the effect would be simply described by classic electrodynamics: light has momentum even if it has no mass.
Hope you don't ask why does light have momentum without having mass, we have discussed this subject hundreds of times... :)
long time since ive seen the lightmill. whats its proper name? i wanna research it.....thanx

http://en.wikipedia.org/wiki/Crookes_radiometer
 

Offline Pmb

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How can light have no mass?
« Reply #31 on: 10/07/2010 22:33:48 »
Yes, of course. The point is not that I intended to say that you misunderstood the term proper mass, but that in particle physics the concept of relativistic mass is not used, at least because you wouldn't know what value to write...
That's because particle physicists concern themselves with intrinsic properties. I didn't know that you were focused on particle physics.

And what if I would say it's the Sun to orbit around the Earth and not the other way...around?  :)
Actually that's a very valid view in general relativity where preferred reference frames are not singled out.
  Can you show me I'm wrong?
I'd say that you'd be justified in doing so if you were to say that you're speaking from a GR point of view and then made it clear that you're referring to the Earth frame of reference.
You probably can't (at least I think so) but we find it more convenient to say it's the Earth to orbit around the Sun.
I agree that its convenient. But physics deals with the laws of nature, regardless if something is convenient for humans to work with. :)
In a similar way, most physicists find it more convenient to consider mass as invariant and not speed dependent; I mean, nothing forbids that tomorrow most physicists will change their mind, of course.
I find that to be a bit misleading. It is certainly true that particle physicists tend to do that, if for no other reason than brevity of writing. A friend of mine used to teach particle physics at Tufts University and when we speak about this he keeps reminding me to "choose what you mean and then stick with it." and leaves it at that. He doesn't make a big point out of it. However for someone whose field is relativity then that's not the most useful notion. Especially since its not one that can be given a general meaning.
Since you talked about inertial mass, I have a question for you, I know that you know more than me about it (I speak seriously). Spacetime curvature near an object with proper mass = M is independent of the frame of reference from which you measure the curvature, isn't it?
It depends on what you mean by "curvature". Spacetime curvature is a tensor quantity and as such the components are frame dependant. However the tensor itself is a geometric object whose absolute meaning is in dependant of the frame. But when you measure things you must choose a frame of reference, or to be more precise, an observer. There are some interesting exercises in MTW regarding this.
And what about near a zero mass object as a photon?
It still has zero proper mass. :)
 

Offline Pmb

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How can light have no mass?
« Reply #32 on: 10/07/2010 22:35:35 »
how do the lightmills, spinning mills inside a glass enclosure, spin faster when placed in direct sunlight & effect a spin & not just heat up if photons have no mass?
As mentioned above, those devices don''t work by photons impacting on the vains. However photons do exert a force on the vains merely because they carry momentum.
 

Offline Pmb

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How can light have no mass?
« Reply #33 on: 10/07/2010 22:36:52 »
Ah! So that means I could travel at the speed of light then.
Nope. Anything with non-zero proper mass can not travel at the speed of light. Your proper mass is non-zero.
 

Offline CZARCAR

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How can light have no mass?
« Reply #34 on: 10/07/2010 22:46:51 »
Einstein predicted a shifting of the stars, planetary alignment & increased gravity affect, to prove that light has matter?
 

Offline Geezer

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How can light have no mass?
« Reply #35 on: 10/07/2010 23:51:20 »
Ah! So that means I could travel at the speed of light then.
Nope. Anything with non-zero proper mass can not travel at the speed of light. Your proper mass is non-zero.

Oh. So photons only have improper mass then? Yes! Yes! That must be it, I'm sure.

(I'm beginning to think there's a "word-trick" going on here  ;D)
« Last Edit: 10/07/2010 23:53:19 by Geezer »
 

Offline Pmb

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How can light have no mass?
« Reply #36 on: 11/07/2010 00:01:20 »
Oh. So photons only have improper mass then? Yes! Yes! That must be it, I'm sure.
:)  The term "proper" here does not mean "correct". It means "characteristic".
 

Offline Pmb

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How can light have no mass?
« Reply #37 on: 11/07/2010 00:03:23 »
Einstein predicted a shifting of the stars, planetary alignment & increased gravity affect, to prove that light has matter?
Actually in Einstein's 1916 GR paper he defined the word "matter" so as to include EM fields and thus EM radiation.
 

Offline Geezer

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How can light have no mass?
« Reply #38 on: 11/07/2010 00:21:31 »
Oh. So photons only have improper mass then? Yes! Yes! That must be it, I'm sure.
:)  The term "proper" here does not mean "correct". It means "characteristic".

I knew it! A word trick!
 

Offline Pmb

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How can light have no mass?
« Reply #39 on: 11/07/2010 00:56:16 »
Oh. So photons only have improper mass then? Yes! Yes! That must be it, I'm sure.
:)  The term "proper" here does not mean "correct". It means "characteristic".

I knew it! A word trick!
Not really. It's more about the various meanings of a given term. Sometimes I find it quite usefull to look up even simple words in a dictionary. For example; suppose you really wanted a new set of golf clubs and you told your wife "I need a new set of golf clubs." Your wife might respond "Since you're health and well being doesn't depend on you getting a new set of clubs then you don't really need them." Sounds reasonable, right? Now let's check the dictionary

http://www.merriam-webster.com/dictionary/need

need - 1a -  a lack of something requisite, desirable, or useful

You show your wife the dictionary and you get your way. LOL!

Same with the term "proper." I was a bit confused as why that word was chosen. It was merely a matter of checking the dictionary. So its not really a word trick. Its just something most people don't know and need to look up.
 

Offline Geezer

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How can light have no mass?
« Reply #40 on: 11/07/2010 01:17:51 »
Oh. So photons only have improper mass then? Yes! Yes! That must be it, I'm sure.
:)  The term "proper" here does not mean "correct". It means "characteristic".

I knew it! A word trick!
Not really. It's more about the various meanings of a given term. Sometimes I find it quite usefull to look up even simple words in a dictionary. For example; suppose you really wanted a new set of golf clubs and you told your wife "I need a new set of golf clubs." Your wife might respond "Since you're health and well being doesn't depend on you getting a new set of clubs then you don't really need them." Sounds reasonable, right? Now let's check the dictionary

http://www.merriam-webster.com/dictionary/need

need - 1a -  a lack of something requisite, desirable, or useful

You show your wife the dictionary and you get your way. LOL!

Same with the term "proper." I was a bit confused as why that word was chosen. It was merely a matter of checking the dictionary. So its not really a word trick. Its just something most people don't know and need to look up.

You'll be trying to tell me "normal" means perpendicular next.
 

Offline Pmb

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How can light have no mass?
« Reply #41 on: 11/07/2010 01:38:41 »
You'll be trying to tell me "normal" means perpendicular next.
Well, as you knnow, context is everything, right? :)
 

Offline lightarrow

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How can light have no mass?
« Reply #42 on: 12/07/2010 18:29:32 »

You probably can't (at least I think so) but we find it more convenient to say it's the Earth to orbit around the Sun.
I agree that its convenient. But physics deals with the laws of nature, regardless if something is convenient for humans to work with. :)
And which law of nature justifies to call "mass" the quantity E/c2 ?
 

Offline Pmb

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How can light have no mass?
« Reply #43 on: 13/07/2010 22:06:12 »
And which law of nature justifies to call "mass" the quantity E/c2 ?
The question, as stated, is not considered to be a meaningful question. It's like asking me "What law of physics justifies calling you Pete?" No law of physics can justify any definition of any of the basic quantities of nature. Thus nothing we see in nature can justify what we call "time" for the same reason. Same with "length/distance".

Since this seems to be taking a turn to opinions on how mass "should be" defined I'd rather merely post a link to an article I wrote on this subject and then you can read it and comment on that instead of me repeating what I already wrote in that paper. I wrote the paper just for conversations like this. There are a great deal of reasons that physicists chose to define the term "mass" to mean m = |p/v. Let me know what you think of the articles and the arguements it outlines. Thanks!

Note: Let me note that it was Einstein himself who assigned mass to EM radiation. He did this in one of his earliest E = mc^2 papers and is one of the reasons he referred to EM fields has having "matter." Would you like to read that paper? I have it in PDF format and can e-mail it to anyone who wishes to read it.
« Last Edit: 14/07/2010 23:01:35 by Pmb »
 

Offline yor_on

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How can light have no mass?
« Reply #44 on: 06/09/2010 17:27:41 »
There does seem to be something slightly paradoxical going on here.

As I understand it, it takes an infinite amount of energy to accelerate any mass to the speed of light.

But photons are accelerated to the speed of light, apparently without consuming an infinite amount of energy, so how can they have mass?

Ahh Geezer, welcome to the land of no return :)

You have just defined a photon as needing acceleration, even if only a 'instant' one, (just as I've done too at times, but don't tell anyone about that. It will ruin me reputation, what little i have left, and now after all, we're only considering yours. Yes geezer, the Nobel-Committee are assembled here, all beards and wrinkled brows.. :)

So0ooooo me lad, accelerating photons indeed, huh :)
=

But in a way you truly hit the head of the nail with that one. I'm flabbergasted too, every time I think of how a photon moves from a 'standing position' to 'c' without any acceleration involved?

On the other hand we can't observe that moment, can we? We only observe it dying.

Magic I say ..
And slippery too.
===

Or can we?
Bose Einstein condensates?
Freezing the light, but that are 'already made' photons, isn't it?
But perhaps we could observe the 'birth' of a photon in such a one?
« Last Edit: 06/09/2010 17:44:01 by yor_on »
 

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How can light have no mass?
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