# The Naked Scientists Forum

### Author Topic: "Light has mass" by Alan H. Guth  (Read 18146 times)

#### 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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#### 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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#### 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.

#### 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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#### 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.

#### 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?

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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.

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

#### 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.

#### 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.

#### 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.

#### 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.

#### 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.

#### 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. :)

#### 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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lightarrow

#### 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.

#### 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.>>

<<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 »

#### 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.

#### 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.

#### 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.

#### 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.

#### The Naked Scientists Forum

##### Re: "Light has mass" by Alan H. Guth
« Reply #43 on: 15/04/2015 01:08:48 »