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Author Topic: "Light has mass" by Alan H. Guth  (Read 18876 times)

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 »


 

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.)
 

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.
 

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.
 

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.
 

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.
 

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).
 

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.
 

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.
 

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.
 

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.
 

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.
 

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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lightarrow
 

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.
 

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 »
 

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.
 

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 »
 

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?
 

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 »
 

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).

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

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).

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

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_ »
 

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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Re: "Light has mass" by Alan H. Guth
« Reply #24 on: 30/03/2015 01:29:35 »

 

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