# How can a massless particle like a photon have transferable momentum? **URL:** \ **Category:** Factual Questions\ **Created:** [October 6, 2005, 5:21am UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011 "2005-10-06T05:21:38Z")\ **Posts on this page:** 16\ **Page:** 2
**Author:** ![toadspittle](https://avatars.discourse-cdn.com/v4/letter/t/c0e974/32.png) [@toadspittle](https://boards.straightdope.com/u/toadspittle)\ **Post date:** [October 6, 2005, 7:01pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/21 "2005-10-06T19:01:25Z")
> [@CalMeacham](#): > > They get hotter. Photons get absorbed by something (even if you have highly reflecting internal facets, as with the “light pump” in Theodore Sturgeon’s “Microscopic God” , you have a _huge_ number of bounces. Eventually, even with teeny absorption per bounce, it all gets absorbed). Some of the energy might get re-emitted as photons (of a different wavelength), but at least some and usually all of it ends up going into heating the object, so it gets hotter. The photons have momentum but no mass, so it doesn’t get heavier. Some of the heat re-emerges as infrared photons of vastly different wavelength. most of the heat ends up being conducted away by objects your black object is in contact with, or by the surrounding air. So in a way, yes, black objects do get heavier (since mass and energy are interchangeable). You are adding energy to the objects, which we’re calling “heat”. Remember–particle mass is measured in “electron volts.” ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 6, 2005, 7:04pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/22 "2005-10-06T19:04:44Z")
I knew someone would bring up this nitpick. In a practical sense, it’s not heavier. Put it on a lab scale and weigh it – you won’t see a difference. ---
**Author:** ![alterego](https://avatars.discourse-cdn.com/v4/letter/a/6bbea6/32.png) [@alterego](https://boards.straightdope.com/u/alterego)\ **Post date:** [October 7, 2005, 2:50am UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/23 "2005-10-07T02:50:13Z")
Then in that same sense there’s no relativity. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [October 7, 2005, 8:24am UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/24 "2005-10-07T08:24:40Z")
> [@Mycroft Holmes](#): > > Bingo! In the next step, he divides both sides of the equation by a, which has to be zero to satisfy a=2a. Dividing by zero is a big no-no! So if a proof comes down to a=2a, you’ve proven that a=0? Cool… Let a = b = 1 a[sup]2[/sup] = ab a[sup]2[/sup] - b[sup]2[/sup] = ab - b[sup]2[/sup] (a+b)(a-b) = b(a-b) a+b = b 2a = a Therefore a = 0, therefore 1 = 0. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 7, 2005, 12:01pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/25 "2005-10-07T12:01:03Z")
> [@](#): > > Then in that same sense there’s no relativity. In that sense, yeah. Do you add velocities relativistically out on the highway? ---
**Author:** ![Go\_You\_Big\_Red\_Fire\_Engine](https://avatars.discourse-cdn.com/v4/letter/g/eada6e/32.png) [@Go\_You\_Big\_Red\_Fire\_Engine](https://boards.straightdope.com/u/Go_You_Big_Red_Fire_Engine)\ **Post date:** [October 7, 2005, 12:03pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/26 "2005-10-07T12:03:50Z")
Slightly related Q: in FRET, do they actually know how the energy of the photon is transferred from donor to acceptor? Or is it just, you know, “magic”? ---
**Author:** ![Mangetout](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/mangetout/32/19_2.png) [@Mangetout](https://boards.straightdope.com/u/Mangetout)\ **Post date:** [October 7, 2005, 12:45pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/27 "2005-10-07T12:45:46Z")
Am I right in assuming that a photon acquires momentum from whatever emits it? - i.e. that not only is the spot projected by my laser pointer _pressing ever so slightly against the wall_, but the laser pointer itself is pressing ever so slightly against my hand in the opposite direction? It must be, from the POV of conservation of momentum, mustn’t it? ---
**Author:** ![Quercus](https://avatars.discourse-cdn.com/v4/letter/q/7ab992/32.png) [@Quercus](https://boards.straightdope.com/u/Quercus)\ **Post date:** [October 7, 2005, 1:00pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/28 "2005-10-07T13:00:08Z")
> [@Mangetout](#): > > Am I right in assuming that a photon acquires momentum from whatever emits it? - i.e. that not only is the spot projected by my laser pointer _pressing ever so slightly against the wall_, but the laser pointer itself is pressing ever so slightly against my hand in the opposite direction? It must be, from the POV of conservation of momentum, mustn’t it? Yup. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 7, 2005, 1:00pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/29 "2005-10-07T13:00:57Z")
> [@](#): > > Am I right in assuming that a photon acquires momentum from whatever emits it? - i.e. that not only is the spot projected by my laser pointer pressing ever so slightly against the wall, but the laser pointer itself is pressing ever so slightly against my hand in the opposite direction? It must be, from the POV of conservation of momentum, mustn’t it? > > * * * In general, no – reflected light is the same color as the incident light. There are situations in which you do get a change in momentum. These cases are called _inelastic scattering_, and there are books and conferences devoted to it, because it is comparatively a rare phenomenon. In general, light being reflected or scattered has the same wavelength (and therefore the same momentum) as the incident light. The same sort of reflection and scattering without momentum change can happen with electrons, too. Look up Mossbauer scattering and Mossbauer spectroscopy. ---
**Author:** ![Caligulas](https://avatars.discourse-cdn.com/v4/letter/c/a5b964/32.png) [@Caligulas](https://boards.straightdope.com/u/Caligulas)\ **Post date:** [October 7, 2005, 1:01pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/30 "2005-10-07T13:01:42Z")
> [@Mangetout](#): > > Am I right in assuming that a photon acquires momentum from whatever emits it? - i.e. that not only is the spot projected by my laser pointer _pressing ever so slightly against the wall_, but the laser pointer itself is pressing ever so slightly against my hand in the opposite direction? It must be, from the POV of conservation of momentum, mustn’t it? Correct. ---
**Author:** ![CalMeacham](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/calmeacham/32/35_2.png) [@CalMeacham](https://boards.straightdope.com/u/CalMeacham)\ **Post date:** [October 7, 2005, 1:19pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/31 "2005-10-07T13:19:41Z")
Ahhh – misread your question. ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [October 7, 2005, 5:24pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/32 "2005-10-07T17:24:17Z")
> [@Malacandra](#): > > So if a proof comes down to a=2a, you’ve proven that a=0? Cool… > > Let a = b = 1 > > a[sup]2[/sup] = ab > > a[sup]2[/sup] - b[sup]2[/sup] = ab - b[sup]2[/sup] > > \*\*(a+b)(a-b) = b(a-b) > > a+b = b\*\* > > 2a = a > > Therefore a = 0, therefore 1 = 0. You divided by 0 again (bolding mine) To get from the first bold line to the second you divide by (a-b), but since a=b, (a-b)=0. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [October 7, 2005, 8:44pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/33 "2005-10-07T20:44:23Z")
> [@CalMeacham](#): > > I knew someone would bring up this nitpick. > In a practical sense, it’s not heavier. Put it on a lab scale and weigh it – you won’t see a difference. It most certainly is not a nitpick. The fact that a system of photons with a center of momentum frame has mass goes right to heart of reality as described by Special Relativity. The complete equation for relativistic energy is: E[sup]2[/sup] = m[sup]2[/sup]c[sup]4[/sup] + p[sup]2[/sup]c[sup]2[/sup] Setting c = 1 for simplification: E[sup]2[/sup] = m[sup]2[/sup] + p[sup]2[/sup] So if the system of photons has a center of momentum frame p equals zero and E = m or m = E. So the container has more mass even if the photons don’t get absorbed but just keep bouncing around. Mass equals energy that cannot be transformed away. In fact with enough photon energy in a small enough volume you could form a black hole. ---
**Author:** ![Malacandra](https://avatars.discourse-cdn.com/v4/letter/m/45deac/32.png) [@Malacandra](https://boards.straightdope.com/u/Malacandra)\ **Post date:** [October 7, 2005, 9:15pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/34 "2005-10-07T21:15:40Z")
> [@zoid](#): > > You divided by 0 again (bolding mine) > To get from the first bold line to the second you divide by (a-b), but since a=b, (a-b)=0. I know. I was just pointing out the flaw in the earlier “refutation” that claimed that getting down to a=2a proved that a=0. ---
**Author:** ![zoid](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/zoid/32/336_2.png) [@zoid](https://boards.straightdope.com/u/zoid)\ **Post date:** [October 7, 2005, 10:48pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/35 "2005-10-07T22:48:29Z")
Oh - never mind 😃 ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [October 8, 2005, 4:16pm UTC](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011/36 "2005-10-08T16:16:00Z")
> [@Malacandra](#): > > I know. I was just pointing out the flaw in the earlier “refutation” that claimed that getting down to a=2a proved that a=0. Three men are travelling through the desert, and one of them pisses off the other two. One of _them_ poisons the first guy’s canteen while he sleeps, but later in the night the other one (independently) drills a hole in the canteen, letting all the (poisoned) water out. Who’s guilty of murder? [Previous page](https://boards.straightdope.com/t/how-can-a-massless-particle-like-a-photon-have-transferable-momentum/325011.md?page=1)