# Does Light have Mass?
**URL:** https://boards.straightdope.com/t/does-light-have-mass/296288
**Category:** Factual Questions
**Created:** [March 26, 2005, 12:57am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288 "2005-03-26T00:57:40Z")
**Posts on this page:** 20
**Page:** 1
### Author:  [@appleciders](https://boards.straightdope.com/u/appleciders)
#### Post date: [March 26, 2005, 12:57am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/1 "2005-03-26T00:57:40Z")
I think the title covers it. I’m so confused!
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### Author:  [@rfgdxm](https://boards.straightdope.com/u/rfgdxm)
#### Post date: [March 26, 2005, 1:00am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/2 "2005-03-26T01:00:37Z")
> [@appleciders](#):
>
> I think the title covers it. I’m so confused!
Yes.
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### Author:  [@Shagnasty](https://boards.straightdope.com/u/Shagnasty)
#### Post date: [March 26, 2005, 1:03am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/3 "2005-03-26T01:03:08Z")
IANA Physicist but I can give you the very basic answer that yes, light does have mass. The powerful gravity of a black hole sucks light into it because it has mass.
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [March 26, 2005, 1:17am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/4 "2005-03-26T01:17:15Z")
No. Absolutely not. Photons do not have mass. That’s the reason that they move at the speed of light, which no particle with mass can do.
They do have energy, however, which is why they behave like a particle with mass in certain situations.
See [Does light have mass?](http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html) for a physicist’s better explanation.
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [March 26, 2005, 1:29am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/5 "2005-03-26T01:29:18Z")
INANA Physicist either, but photons do NOT have mass.
Here’s a concurring physicist:
[Does light have mass?](http://math.ucr.edu/home/baez/physics/Relativity/SR/light_mass.html)
> [@](#):
>
> The short answer is “no”, but it is a qualified “no” because there are odd ways of interpreting the question which could justify the answer “yes”.
>
> Light is composed of photons so we could ask if the photon has mass. The answer is then definitely “no”: The photon is a massless particle. According to theory it has energy and momentum but no mass and this is confirmed by experiment to within strict limits. Even before it was known that light is composed of photons it was known that light carries momentum and will exert a pressure on a surface. This is not evidence that it has mass since momentum can exist without mass. [For details see the Physics FAQ article [What is the mass of the photo](http://math.ucr.edu/home/baez/physics/ParticleAndNuclear/photon_mass.html) ?].
Also see wikipedia on
[photons](http://en.wikipedia.org/wiki/Photon).
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### Author:  [@Squink](https://boards.straightdope.com/u/Squink)
#### Post date: [March 26, 2005, 1:31am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/6 "2005-03-26T01:31:54Z")
Curse you **Exapno** , and your little **mapcase** too! 😉
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [March 26, 2005, 1:41am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/7 "2005-03-26T01:41:04Z")
What, it took you more than 12 minutes to find that site?
Googleloser. 🙂
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### Author:  [@Mathochist](https://boards.straightdope.com/u/Mathochist)
#### Post date: [March 26, 2005, 4:19am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/8 "2005-03-26T04:19:19Z")
Photons have no _rest_ mass. Also, it’s a little disingenuous to distinguish mass and energy.
Oh, and Baez is a mathematician. He knows how to use logic properly 😃
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [March 26, 2005, 4:53am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/9 "2005-03-26T04:53:19Z")
A single photon has no mass, but a system of two photons moving anti parallel to each other does have mass.
Mass and energy are not _things_ they’re properties of a system. And a good definition of the mass of a system is the energy of that system which cannot be transformed away by a change of reference frame.
In the case of a single photon the energy can be transformed away to a level that is as close to zero as you would like. However a system of two photons moving anti parallel to each other has a zero center of momentum frame and therefore its energy cannot be transformed away. And it therefore has mass.
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [March 26, 2005, 4:57am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/10 "2005-03-26T04:57:27Z")
> [@Mathochist](#):
>
> Also, it’s a little disingenuous to distinguish mass and energy.
Not true. Mass and energy are most definitely not the same thing and one cannot be transformed into the other.
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### Author:  [@CynicalGabe](https://boards.straightdope.com/u/CynicalGabe)
#### Post date: [March 26, 2005, 5:01am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/11 "2005-03-26T05:01:55Z")
> [@rfgdxm](#):
>
> Yes.
Thats not a complete sentence!!! :mad:
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### Author:  [@Hermitian](https://boards.straightdope.com/u/Hermitian)
#### Post date: [March 26, 2005, 5:50am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/12 "2005-03-26T05:50:29Z")
> [@Ring](#):
>
> Not true. Mass and energy are most definitely not the same thing and one cannot be transformed into the other.
Um, I think Einstein cleared this up 100 years ago. Energy and mass can be transformed from one to the other. This happens every time a nuclear bomb goes off.
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### Author:  [@appleciders](https://boards.straightdope.com/u/appleciders)
#### Post date: [March 26, 2005, 6:19am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/13 "2005-03-26T06:19:19Z")
I’m sure if I read that enough times, it would all make sense, but I have at least a vague notion. Thanks!
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [March 26, 2005, 6:53am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/14 "2005-03-26T06:53:51Z")
Well, at least now you know it’s not a simple qustion, which is a start…
Photons do not have _rest mass_, i.e. the kind of mass you can measure by bringing the particle to a stop and putting it on a scale. But it does have _relativistic_ mass, i.e. energy is mass, and a photon definitely has energy. I don’t think it even needs to be a _system_ of photons - if you trap a photon in a 100% reflective box and put the box on a scale, the box will weigh more than it would without the photon.
I have no idea what **Rign** meant by his post, was that supposed to be a joke?
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### Author:  [@PETERFI](https://boards.straightdope.com/u/PETERFI)
#### Post date: [March 26, 2005, 7:50am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/15 "2005-03-26T07:50:39Z")
> [@appleciders](#):
>
> I think the title covers it. I’m so confused!
Perhaps the right question is whether Light bothers to go to Mass on Easter Sunday which is of course a Holy Day of Obligation, presuming that Light even observes. Of course, it would be rather ungreatful of Light not to go to Mass because after all, it is written,
“And God said Let there be Light! And there was Light!”
I don’t want to string you along or be to particular about it but, the theories these days have me pretty quarky with all their muonces.
This silly string theory might mean that the whole mass, no mas problem might be interdimensional and we might not know here foam here from there anymore.
So to let the cat out of the bag, if Schrodingdong won’t mind, it’s mostly no mass, except that it could be a little, just as light acts as particles, except when its acting like waves.
Going both waves before it was fashionable, I guess.
I know I’ve probably just confused you, so that puts you just about in the same indeterminate state of kantiousness that I am. I mention Kant because of his Copernican Revolution. I leave it to you to google that.
The point being that we are at a state where it is possible that within 10 years or so everything we think we know that matter-s about the universe could be turned upside down again. Or at least a little inside out.
So, you’ve got good answers from those above.
For now.
Fondly,
Peter Pun
[The Peter Files](http://ThePeterFiles.blogspot.com)
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### Author:  [@Tangent](https://boards.straightdope.com/u/Tangent)
#### Post date: [March 26, 2005, 7:58am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/16 "2005-03-26T07:58:35Z")
Uh oh, someone **GRAPED** the thread!
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### Author:  [@Desmostylus](https://boards.straightdope.com/u/Desmostylus)
#### Post date: [March 26, 2005, 10:10am UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/17 "2005-03-26T10:10:48Z")
> [@scr4](#):
>
> I have no idea what **Rign** meant by his post, was that supposed to be a joke?
No joke. **Ring** is a Feynman QED hardliner, although I think **Ring** is still an undergrad. There’s a reasonable case to be made for **Ring** ’s interpretation. E.g. in a bomb, the energy released came from the nuclear binding energy. It was energy before the explosion, and it was still energy afterward. Mass defect isn’t really mass defect, it’s improperly accounted for energy.
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### Author:  [@Ring](https://boards.straightdope.com/u/Ring)
#### Post date: [March 26, 2005, 3:14pm UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/18 "2005-03-26T15:14:00Z")
This has all been discussed before. In the below thread Chronos agrees with my position – and that’s all the verification I need.
[http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712)
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### Author:  [@Exapno\_Mapcase](https://boards.straightdope.com/u/Exapno_Mapcase)
#### Post date: [March 26, 2005, 3:27pm UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/19 "2005-03-26T15:27:49Z")
> [@scr4](#):
>
> Photons do not have _rest mass_, i.e. the kind of mass you can measure by bringing the particle to a stop and putting it on a scale. But it does have _relativistic_ mass, i.e. energy is mass, and a photon definitely has energy.
Just a comment on “relativistic mass” from the Does Light have mass? link:
> [@](#):
>
> Sometimes people like to say that the photon does have mass because a photon has energy E = hf where h is Planck’s constant and f is the frequency of the photon. Energy, they say, is equivalent to mass according to Einstein’s famous formula E = mc2. They also say that a photon has momentum and momentum is related to mass p = mv. What they are talking about is “relativistic mass”, an outdated concept which is best avoided [See Relativity FAQ article Does mass change with velocity?] Relativistic mass is a measure of the energy E of a particle which changes with velocity. By convention relativistic mass is not usually called the mass of a particle in contemporary physics so it is wrong to say the photon has mass in this way. But you can say that the photon has relativistic mass if you really want to. In modern terminology the mass of an object is its invariant mass which is zero for a photon.
**ring** , can you define your use of “anti parallel”?
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### Author:  [@scr4](https://boards.straightdope.com/u/scr4)
#### Post date: [March 26, 2005, 3:49pm UTC](https://boards.straightdope.com/t/does-light-have-mass/296288/20 "2005-03-26T15:49:34Z")
> [@Ring](#):
>
> [http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712](http://boards.straightdope.com/sdmb/showthread.php?s=&threadid=111712)
OK, in that context I understand and mostly agree with what you’re saying. I think the only disgreement is whether “relativistic mass” is a valid and useful concept. To me, a system or object with relativistic mass still acts like it has mass, and therefore it’s a useful idea. Kind of like centrifugal force.
I still think you went too far with this following statement, to the point of being overly confusing in the context of this thread:
> [@](#):
>
> Mass and energy are most definitely not the same thing and one cannot be transformed into the other.
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