"Light has mass" by Alan H. Guth

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Offline PmbPhy

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"Light has mass" by Alan H. Guth
« on: 19/03/2015 11:50:08 »
I was at MIT yesterday visiting Alan Guth. He's assisting me on a portion of my website called Common Misconceptions in Physics. I have to edit the video to make the end cut better but until then you can see he talk about why cosmologists tend to view light has having mass (not proper mass but relativistic mass). The video is at:
http://www.newenglandphysics.org/Science_Literature/Journal_Articles/DSC_0003.MOV

« Last Edit: 20/03/2015 16:57:51 by PmbPhy »

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Offline David Cooper

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Re: "Light has mass" by Alan H. Guth
« Reply #1 on: 20/03/2015 18:14:09 »
Lack of replies does not mean lack of interest - I'm looking forward to seeing it. (I can't watch videos until the middle of a month if I have enough data allowance left to use up. If the file size isn't too big, I should be able to see it eventually.)

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #2 on: 20/03/2015 18:15:55 »
Lack of replies does not mean lack of interest - I'm looking forward to seeing it. (I can't watch videos until the middle of a month if I have enough data allowance left to use up. If the file size isn't too big, I should be able to see it eventually.)
If you'd like I can download  all of them onto a DVD and mail them to you or anybody else who wants to see them.

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #3 on: 20/03/2015 22:09:15 »
I'll watch them online Pete. It's cheaper for you that way.

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Offline Colin2B

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Re: "Light has mass" by Alan H. Guth
« Reply #4 on: 20/03/2015 22:29:34 »
If you'd like I can download  all of them onto a DVD and mail them to you or anybody else who wants to see them.

Sorry, I've been wasting my time on The Box - not the TV! I should be able to watch this w/e.
Grandchildren tomorrow, bees Sunday, video in between.
Have a good w/e.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #5 on: 21/03/2015 02:22:02 »
Quote from: Colin2B
Sorry, I've been wasting my time on The Box ....
Yeah. I hear ya. I too have wasted my time trying hopelessly to help people understand physics. I know that I shouldn't try when all I get is them telling me that I'm not as smart as they are with all their months of education behind them and how that they're true geniuses and I'm just someone who memorizes textbooks.

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Offline David Cooper

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Re: "Light has mass" by Alan H. Guth
« Reply #6 on: 21/03/2015 18:47:49 »
What are the file sizes? If they're we'll compressed it shouldn't be a problem, but I normally avoid watching videos until the 14th and 15th of the month when I can use up any unused data allowance without any risk of being cut off from the Net for too long (I get a new 5GB allocation every month on the 16th, but this allowance is shared with other people, and that's why things are so tight).

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #7 on: 21/03/2015 23:48:02 »
What are the file sizes? If they're we'll compressed it shouldn't be a problem, but I normally avoid watching videos until the 14th and 15th of the month when I can use up any unused data allowance without any risk of being cut off from the Net for too long (I get a new 5GB allocation every month on the 16th, but this allowance is shared with other people, and that's why things are so tight).
They range in size from approximately 300 Mb to 500 Mb. I'll try to compress them.

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Offline David Cooper

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Re: "Light has mass" by Alan H. Guth
« Reply #8 on: 22/03/2015 20:26:21 »
They range in size from approximately 300 Mb to 500 Mb. I'll try to compress them.

They are every bit as big as I'd feared then. I don't know if uploading them to youtube would make it easier - they usually let you choose the resolution you want to view things in, and I don't think people have to upload multiple versions for that to be available.

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Offline Colin2B

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Re: "Light has mass" by Alan H. Guth
« Reply #9 on: 23/03/2015 00:18:58 »
I don't know if uploading them to youtube would make it easier - they usually let you choose the resolution you want to view things in, and I don't think people have to upload multiple versions for that to be available.
David
Not sure what is possible, I could have a look over the next few days at options.
If it doesn't work, are you in UK? I might be able to download & copy for you.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

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Offline David Cooper

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Re: "Light has mass" by Alan H. Guth
« Reply #10 on: 23/03/2015 01:05:07 »
The key thing isn't really about making it easier for me to access them - I'll certainly be able to see them eventually, but they need to be available to anyone else with limited data options too, and posting disks around isn't going to fix that, but thanks for offering.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #11 on: 23/03/2015 04:45:10 »
Quote from: David Cooper
The key thing isn't really about making it easier for me to access them - I'll certainly be able to see them eventually, but they need to be available to anyone else with limited data options too, and posting disks around isn't going to fix that, but thanks for offering.
I'm sorry about that guys. I tried to compress them but the decrease if file size is insignificant to the original file so its not worth doing. E.g. if the original file size is 320 Mb then the compressed size was 310 Mb.

I'll look into streaming the video. I don't know how to do that right now but will try to learn.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #12 on: 23/03/2015 16:34:07 »
Pete, the file video must have a virus or something alike: it's authomatically deleted from my two computers (a desktop and a notebook).

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #13 on: 23/03/2015 19:33:09 »
Pete, the file video must have a virus or something alike: it's authomatically deleted from my two computers (a desktop and a notebook).

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lightarrow
My antivirus software didn't detect anything. I myself took the videos. Those files went from my camera, to my computer and then to the website.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #14 on: 24/03/2015 10:44:59 »
Pete, the file video must have a virus or something alike: it's authomatically deleted from my two computers (a desktop and a notebook).

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lightarrow
My antivirus software didn't detect anything. I myself took the videos. Those files went from my camera, to my computer and then to the website.
Ok, I must have some other problem, then, don't know which. Sorry for my previous post.

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Re: "Light has mass" by Alan H. Guth
« Reply #15 on: 27/03/2015 04:58:57 »
Peter (I'm guessing that's your name, although forgive me if it's not), if you don't have a problem with this video being public... I can upload it on my website and anyone who's interested in watching it can straight away, without download, watch it there. (Yes, I will credit you for the video and no my website is not famous.)
« Last Edit: 27/03/2015 05:13:31 by Jasper Hayden »

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #16 on: 27/03/2015 16:07:01 »
Peter (I'm guessing that's your name, although forgive me if it's not), if you don't have a problem with this video being public... I can upload it on my website and anyone who's interested in watching it can straight away, without download, watch it there. (Yes, I will credit you for the video and no my website is not famous.)
Thanks for the offer, Jasper. I have to wait for Alan to give the go ahead and that can take an extremely long time. He's very busy and it's a rare thing for him to return a call or e-mail.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #17 on: 28/03/2015 09:57:31 »
About the video: probably I haven't understood completely the language, but the phrase "light has mass" is dangerous,  for people who took it decontextualized. All considered, if I had to choose for a single simple sentence, I'd prefer to say: "light has zero mass".

(What has mass is not "light" but "light present in a stationary region of space", which is very different).

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« Last Edit: 28/03/2015 10:01:45 by lightarrow »

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Offline CZARCAR

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Re: "Light has mass" by Alan H. Guth
« Reply #18 on: 28/03/2015 10:16:02 »
light is present in the stationary region of space like a ghost? or do ghosts have mass?

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #19 on: 28/03/2015 11:00:49 »
light is present in the stationary region of space like a ghost? or do ghosts have mass?
Sorry, I can't understand your question.

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Offline JohnDuffield

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Re: "Light has mass" by Alan H. Guth
« Reply #20 on: 28/03/2015 17:31:03 »
I haven't looked at the video, but can I volunteer this: light has a non-zero "inertial mass", and a non-zero "active gravitational mass". The issue here is that the word mass is ambiguous, and the meaning has changed somewhat over the years. Nowadays when people say "mass" on its own, they tend to mean "rest mass". Light has no rest mass because it isn't at rest. However when you trap light in a mirror-box, it's effectively at rest. It's still moving at c, but because it isn't going anywhere, it's like it's at rest, and it increases the mass of the system. Then when you open the box, it's a radiating body that loses mass. See Einstein's E=mc˛ paper and think of rest mass as energy that's effectively at rest. Note the last line:

"If the theory corresponds to the facts, radiation conveys inertia between the emitting and absorbing bodies."

Light conveys inertia. That's why it has a non-zero inertial mass.  IMHO the thing to appreciate with all this is that photon momentum is a measure of resistance to change-in-motion for a wave moving linearly at c. And that when the photon is in the box, the extra rest mass is a measure of a measure of resistance to change-in-motion for a wave moving back and forth at c. Note that Einstein refers to a body and the electron on the same line. IMHO the electron can be likened to a photon in a box of its own making.
« Last Edit: 28/03/2015 17:37:10 by JohnDuffield »

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #21 on: 29/03/2015 00:12:31 »
About the video: probably I haven't understood completely the language, but the phrase "light has mass" is dangerous,  for people who took it decontextualized. All considered, if I had to choose for a single simple sentence, I'd prefer to say: "light has zero mass".

(What has mass is not "light" but "light present in a stationary region of space", which is very different).

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lightarrow
Alan explained that since light has energy it has mass. We might retake it to drive this point home though.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #22 on: 29/03/2015 20:16:44 »
About the video: probably I haven't understood completely the language, but the phrase "light has mass" is dangerous,  for people who took it decontextualized. All considered, if I had to choose for a single simple sentence, I'd prefer to say: "light has zero mass".

(What has mass is not "light" but "light present in a stationary region of space", which is very different).

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lightarrow
Alan explained that since light has energy it has mass. We might retake it to drive this point home though.
You make it even worser...
"since light has energy it has mass" is wrong: a light pulse has energy but zero mass ...

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« Last Edit: 29/03/2015 20:18:46 by lightarrow »

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Offline Ethos_

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Re: "Light has mass" by Alan H. Guth
« Reply #23 on: 29/03/2015 20:32:56 »
You make it even worser...
"since light has energy it has mass" is wrong: a light pulse has energy but zero mass ...

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lightarrow
Wrong.............light has zero proper mass but it does have energy which takes the form of inertial mass. There needs to be an understanding here about the difference between proper mass and inertial mass.
« Last Edit: 29/03/2015 20:35:28 by Ethos_ »
"The more things change, the more they remain the same."

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #24 on: 30/03/2015 01:29:35 »
Light has kinetic energy. This is not calculated via a rest mass. That is (1/2)mv2 because the m used here cannot be used in an equation for the kinetic energy of light. Pete went over this in another thread. He has a page on this very subject. I have to say that I agree with Alan and Pete.

EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
« Last Edit: 30/03/2015 01:36:24 by jeffreyH »

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #25 on: 30/03/2015 06:26:31 »
You make it even worser...
"since light has energy it has mass" is wrong: a light pulse has energy but zero mass ...

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lightarrow
Wrong.............light has zero proper mass but it does have energy which takes the form of inertial mass. There needs to be an understanding here about the difference between proper mass and inertial mass.
And you seem not to understand that what you call "proper mass" is actually called "mass", in physics (that is, in most of the more recent books-by most of the physicist, included nuclear/elementary particle physicists).

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #26 on: 30/03/2015 06:36:51 »
Light has kinetic energy. This is not calculated via a rest mass. That is (1/2)mv2 because the m used here cannot be used in an equation for the kinetic energy of light. Pete went over this in another thread. He has a page on this very subject. I have to say that I agree with Alan and Pete.

EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
The term "rest mass" is meaningless for light speed moving bodies as light. If you want to talk about relativistic mass and you want to distinguish it from what is usually called "mass", you can call this last one "invariant mass".
About light speed in presence of gravitational fields: it doesn't vary. If you want to talk of variation of it you have to talk of: coordinate speed, non-local effects, spacetime drag.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #27 on: 30/03/2015 15:52:22 »
Quote from: lightarrow
The term "rest mass" is meaningless for light speed moving bodies as light.
I think everyone knows that. It's merely the term that's been used to refer to the magnitude of the photons 4-momentum. I myself call it proper mass.

Quote from: lightarrow
About light speed in presence of gravitational fields: it doesn't vary.
You're quite incorrect. It's well known that it does in fact.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #28 on: 31/03/2015 12:23:26 »
Quote from: lightarrow
About light speed in presence of gravitational fields: it doesn't vary.
You're quite incorrect. It's well known that it does in fact.
So the correct interpretation of Pound-Rebka experiment would be the varying speed of light during its travel from the tower to the ground?

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #29 on: 31/03/2015 15:05:26 »
Quote from: lightarrow
About light speed in presence of gravitational fields: it doesn't vary.
You're quite incorrect. It's well known that it does in fact.
So the correct interpretation of Pound-Rebka experiment would be the varying speed of light during its travel from the tower to the ground?

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lightarrow
That experiment measures change in frequency, not change in speed. The Shapiro experiments on time delay are what measures changes in the speed of light in a gravitational field. Shapiro himself in the paper where he reports the results of his experiments explains that the varying speed of light in a gravitational field is the fourth prediction of general relativity (or something like that). In any case Shapiro states in no uncertain terms that the speed of light changes in a gravitational field.

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #30 on: 31/03/2015 17:12:01 »
Quote from: lightarrow
About light speed in presence of gravitational fields: it doesn't vary.
You're quite incorrect. It's well known that it does in fact.
So the correct interpretation of Pound-Rebka experiment would be the varying speed of light during its travel from the tower to the ground?

--
lightarrow
That experiment measures change in frequency, not change in speed. The Shapiro experiments on time delay are what measures changes in the speed of light in a gravitational field. Shapiro himself in the paper where he reports the results of his experiments explains that the varying speed of light in a gravitational field is the fourth prediction of general relativity (or something like that). In any case Shapiro states in no uncertain terms that the speed of light changes in a gravitational field.

I din't realize that Shapiro used the Schwarzschild solution. You learn something new every day.  [;D]

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Offline JohnDuffield

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Re: "Light has mass" by Alan H. Guth
« Reply #31 on: 31/03/2015 17:34:12 »
EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
The answer is that the speed of wave depends on the properties of the medium. In mechanics  a shear wave travels at speed v = √(G/ρ) where G is the shear modulus of elasticity, and ρ is the density. In electrodynamics the equation is c = √(1/ε0μ0), where ε0 is electric permittivity and μ0 is magnetic permeability. People tend to say space isn't a medium, but that's not what Einstein said. Google it.

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Offline PhysBang

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Re: "Light has mass" by Alan H. Guth
« Reply #32 on: 31/03/2015 19:12:55 »
EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
The answer is that the speed of wave depends on the properties of the medium. In mechanics  a shear wave travels at speed v = √(G/ρ) where G is the shear modulus of elasticity, and ρ is the density. In electrodynamics the equation is c = √(1/ε0μ0), where ε0 is electric permittivity and μ0 is magnetic permeability. People tend to say space isn't a medium, but that's not what Einstein said. Google it.
Yeah, this is pretty much a bunch of nonsense when applied to the question at hand. The question is about the speed of light in a vacuum, not in a medium, and a google search for the supposed holy words of a single scientists is not relevant when evaluating scientific claims.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #33 on: 01/04/2015 03:31:46 »
Quote from: JohnDuffield
The answer is that the speed of wave depends on the properties of the medium.
Don't you ever get tired of being wrong? In this case that's off-topic. When light moves through a gravitational field in a vacuum it locally moves at c = 3x108 m/s. However when using coordinates which are used by external remote observers the time it takes to move through a distance is slowed down because of gravitational time dilation. When that is taken into account we get a slower speed of light. When determining how long it takes for light to move round trip to a planet and back we have to take that into account in order to explain the time delay.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #34 on: 01/04/2015 03:37:32 »
Quote from: jeffreyH
Light has kinetic energy. This is not calculated via a rest mass. That is (1/2)mv2 because the m used here cannot be used in an equation for the kinetic energy of light. Pete went over this in another thread. He has a page on this very subject. I have to say that I agree with Alan and Pete.
I'm sure Alan would be pleased! :)   Please understand that  (1/2)mv2 is not the expression of the kinetic energy of an object. The kinetic energy of a point mass m is defined as the work required to accelerate an object from 0 to v. It can also be defined as the K in

E = K + E0

where E0 is the rest energy. For light E0 is zero. However K is not zero for light and has the value E = hf

Quote from: jeffreyH
EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
What was the purpose of asking this question in this thread? JD used that as an in to start ranting again.

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #35 on: 01/04/2015 09:15:52 »
Quote from: jeffreyH
Light has kinetic energy. This is not calculated via a rest mass. That is (1/2)mv2 because the m used here cannot be used in an equation for the kinetic energy of light. Pete went over this in another thread. He has a page on this very subject. I have to say that I agree with Alan and Pete.
I'm sure Alan would be pleased! :)   Please understand that  (1/2)mv2 is not the expression of the kinetic energy of an object. The kinetic energy of a point mass m is defined as the work required to accelerate an object from 0 to v. It can also be defined as the K in

E = K + E0

where E0 is the rest energy. For light E0 is zero. However K is not zero for light and has the value E = hf

Quote from: jeffreyH
EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration. In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
What was the purpose of asking this question in this thread? JD used that as an in to start ranting again.

I know and I should have known better. My apologies.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #36 on: 01/04/2015 10:11:43 »
Quote from: jeffreyH
I know and I should have known better. My apologies.
No problemo my good man. We all make mistakes. :)

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #37 on: 02/04/2015 11:51:51 »
Quote from: lightarrow
About light speed in presence of gravitational fields: it doesn't vary.
You're quite incorrect. It's well known that it does in fact.
So the correct interpretation of Pound-Rebka experiment would be the varying speed of light during its travel from the tower to the ground?

--
lightarrow
That experiment measures change in frequency, not change in speed.
It doesn't matter. What matters is that the change in frequency is correctly described by red/blue shift assuming constant light speed.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #38 on: 02/04/2015 12:11:25 »
Quote from: lightarrow
It doesn't matter. What matters is that the change in frequency is correctly described by red/blue shift assuming constant light speed.
The emitter sends out photons of a known frequency and the detector receives the photons and measures their frequency. The local speed of light at the emitter and detector is c = 3x108 m/s. The coordinate speed of light does not equal this value. No assumption about the coordinate speed of light enters into the experiment and the local speed of light matters since neither emitter nor detector depends on the local speed of light so no assumption is used.

Note: When discussing the speed of light in a gravitational field like this please note which speed you're talking about: coordinate speed or local speed. Otherwise its unclear what you mean and one has to guess.

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Offline lightarrow

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Re: "Light has mass" by Alan H. Guth
« Reply #39 on: 03/04/2015 13:28:06 »
Quote from: lightarrow
It doesn't matter. What matters is that the change in frequency is correctly described by red/blue shift assuming constant light speed.
The emitter sends out photons of a known frequency and the detector receives the photons and measures their frequency. The local speed of light at the emitter and detector is c = 3x108 m/s. The coordinate speed of light does not equal this value. No assumption about the coordinate speed of light enters into the experiment and the local speed of light matters since neither emitter nor detector depends on the local speed of light so no assumption is used.

Note: When discussing the speed of light in a gravitational field like this please note which speed you're talking about: coordinate speed or local speed. Otherwise its unclear what you mean and one has to guess.
Which speed to talk about should have been specified by jeffreyH, infact he wrote:

<<EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field? It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated.>>

Instead, in my reply n. 26 I wrote:

<<If you want to talk of variation of it you have to talk of: coordinate speed, ...>>

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lightarrow

« Last Edit: 03/04/2015 13:30:22 by lightarrow »

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #40 on: 04/04/2015 20:34:59 »
It cannot be observed directly because of the coordinate basis of the spacetime. That was implied. I should have stated it explicitly.

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Offline David Cooper

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Re: "Light has mass" by Alan H. Guth
« Reply #41 on: 14/04/2015 22:42:29 »
I had enough data allowance left to watch it, so I've just given it a go. For half an hour I watched a white screen with a Q in the middle of it (Q for Quicktime) while 300MB slowly ticked up on the 4G-WIFI device. Eventually a "?" appeared on top of the Q, and nothing else happened at all after that. 300MB lost! Not a complete disaster though as I then remembered that if you right-click on a link you have an option to save the thing as a file and can bypass whatever software the browser picks to try to play it directly, and fortunately I had also managed to store up enough unused data allowance to get a couple of shots at this. I've now played it with VLC media player - it gave me all the sound, but the picture is stuck on a single frame. Still, it's the sound that matters most, and at least I've got the file now - I can put it on a flash drive and try it in another machine.

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Offline PmbPhy

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Re: "Light has mass" by Alan H. Guth
« Reply #42 on: 14/04/2015 23:21:40 »
Quote from: jeffreyH
EDIT: The question is not whether or not light has mass but why does the light speed up as it leaves a gravitational field?
Why would you assert such a thing? Clearly the purpose of that talk is whether light has mass or not. That was precisely what I asked Alan for and that's precisely what he wanted to say in response. So the question is not about whether light speeds up as it leaves the gravitational field.

Quote from: jeffreyH
It is slowed by gravitation but as the strength of the gravitational field decreases the effective velocity of light must increase. This cannot be observed directly but can be calculated. Normally we would call this an acceleration.
Because that is precisely what it is. :)

Quote from: jeffreyH
In the case of light we can't do this as locally no change in speed will be noted. The value for c is constant to a local observer.
For some reason people think that only the local speed of light can be measured. That's not true at all.

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Offline jeffreyH

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Re: "Light has mass" by Alan H. Guth
« Reply #43 on: 15/04/2015 01:08:48 »
I wasn't responding to the video as I haven't had the time to watch it. I ma currently stuck reading lots of complex maths so my brain is a bit fried.