# Can energy be destroyed? **URL:** \ **Category:** Factual Questions\ **Created:** [March 29, 2007, 12:50am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916 "2007-03-29T00:50:40Z")\ **Posts on this page:** 11\ **Page:** 2
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [April 1, 2007, 3:17am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/21 "2007-04-01T03:17:30Z")
[QUOTE=Mathochist] Gravity is coupled to momentum, not mass. Photons have no rest mass, but they damn sure have momentum. The rough idea is that Heisenberg lets “virtual” particle-antiparticle pairs pop into and out of existence, as long as they’re gone before too much time elapses (uncertainty relation sez how much time that is). If it happens right near a black hole, one might get sucked in, and to conserve momentum the other one gets kicked away. Not much, but _just enough_ to keep it outside the event horizon. [/QUOTE] Which brings me to something that has puzzled me. As I understand things, if a photon exists it travels at the speed of light. Given an excited atom when an electron falls to a lower energy level a photon is emitted. Is it going at the speed of light at the instant of creation? And what accelerated it to that speed? Is this one of those things that just is? ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [April 1, 2007, 3:19am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/22 "2007-04-01T03:19:34Z")
If anyone cares here’s my simplified explanation of Hawking radiation. Virtual particles are not real particles. They exist on borrowed energy and must quickly annihilate and repay this energy. However, the extreme tidal gravitation of a BH can pull them apart with enough force that they gain enough energy to become real long lived particles and at the same time pay back the energy debt to the nearby negative energy regions of space. So when the hole captures one of these now real particles, it in effect radiates the other one. It has therefore supplied enough energy to create two particles, but only gets the energy of one back. Thus it loses mass. (Warning: Hawking Radiation is actually much more complicated than this, however I only understand it at this level.) ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 1, 2007, 3:29am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/23 "2007-04-01T03:29:13Z")
[QUOTE=David Simmons] Which brings me to something that has puzzled me. As I understand things, if a photon exists it travels at the speed of light. Given an excited atom when an electron falls to a lower energy level a photon is emitted. Is it going at the speed of light at the instant of creation? And what accelerated it to that speed? Is this one of those things that just is? [/QUOTE] There’s no real “acceleration” like you’re thinking of from classical mechanics. It basically just happens, as we understand it. Actually, think of it as three particles (un)colliding. There’s an electron and a photon going out from the collision into the future, and you’d say there’s an electron coming in from the past. I’d say it’s a positron (anti-electron) going out to the past. Anyhow, as long as various numbers line up – the spins, the 4-momenta, the charges… – the particles are allowed to interact. So if we go back to the electron-emits-a-photon picture, the photon is just made from the loss of 4-momentum in the electron, and _all_ of that 4-momentum goes into “kinetic energy”. It isn’t accelerated to the speed of light, it just travels at that speed from the get-go. That just what photons do. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [April 1, 2007, 3:53am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/24 "2007-04-01T03:53:29Z")
> [@](#): > > That’s interesting. I would think that the only thing that could radiatewould be a massless “particle” like a photon. As soon as anything with mass popped up outside, it would be pulled back in. First of all virtual particles aren’t on the mass shell which means that m[sup]2[/sup] = E[sup]2[/sup] - p[sup]2[/sup] doesn’t apply, and secondly they’re created and made real outside the event horizon so if one of them has enough energy it can escape. This is allowed as long as the other has equal and opposite momentum. ---
**Author:** ![David\_Simmons](https://avatars.discourse-cdn.com/v4/letter/d/9de053/32.png) [@David\_Simmons](https://boards.straightdope.com/u/David_Simmons)\ **Post date:** [April 1, 2007, 5:22am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/25 "2007-04-01T05:22:10Z")
[QUOTE=Mathochist] So if we go back to the electron-emits-a-photon picture, the photon is just made from the loss of 4-momentum in the electron, and _all_ of that 4-momentum goes into “kinetic energy”. It isn’t accelerated to the speed of light, it just travels at that speed from the get-go. That just what photons do. [/QUOTE] That’s what I suspected. It just is, at least so far. > [@Ring](#): > > First of all virtual particles aren’t on the mass shell which means that m2 = E2 - p2 doesn’t apply, and secondly they’re created and made real outside the event horizon so if one of them has enough energy it can escape. This is allowed as long as the other has equal and opposite momentum. Oh I semi understood your post. Well, since you said you semi understood maybe I only hemi-semi understood. 🙂 ---
**Author:** ![Critical1](https://avatars.discourse-cdn.com/v4/letter/c/3e96dc/32.png) [@Critical1](https://boards.straightdope.com/u/Critical1)\ **Post date:** [April 1, 2007, 6:43pm UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/26 "2007-04-01T18:43:06Z")
[QUOTE=Mathochist] So you don’t believe Hawking (among others)? [/QUOTE] the last read I did from Hawking on the subject has him stating that the information is lost. while it can be emitted again it will have no memory or relationship to the energy that went into the hole in the first place. ---
**Author:** ![Mathochist](https://avatars.discourse-cdn.com/v4/letter/m/c89c15/32.png) [@Mathochist](https://boards.straightdope.com/u/Mathochist)\ **Post date:** [April 1, 2007, 8:02pm UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/27 "2007-04-01T20:02:46Z")
[QUOTE=Critical1] the last read I did from Hawking on the subject has him stating that the information is lost. while it can be emitted again it will have no memory or relationship to the energy that went into the hole in the first place. [/QUOTE] But [Hawking conceded that point in 2004](http://en.wikipedia.org/wiki/Thorne-Hawking-Preskill_bet). ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [April 1, 2007, 11:10pm UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/28 "2007-04-01T23:10:16Z")
An odd post of an odd fact. One of the stranger aspects of Hawking radiation is that it can both exist and not exist depending on whether the observer is hovering above the horizon or free falling into it. In the first case the horizon appears as a hot radiating thermodynamic membrane and in the second it’s nothing at all. ---
**Author:** ![bonzer](https://avatars.discourse-cdn.com/v4/letter/b/45deac/32.png) [@bonzer](https://boards.straightdope.com/u/bonzer)\ **Post date:** [April 2, 2007, 12:51am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/29 "2007-04-02T00:51:28Z")
[QUOTE=Ring] One of the stranger aspects of Hawking radiation is that it can both exist and not exist depending on whether the observer is hovering above the horizon or free falling into it. In the first case the horizon appears as a hot radiating thermodynamic membrane and in the second it’s nothing at all. [/QUOTE] But that’s hardly stranger than the [Unruh effect](http://en.wikipedia.org/wiki/Unruh_effect). Surprising when it’s first pointed out to you. Ultimately pleasingly consistent. ---
**Author:** ![Ring](https://avatars.discourse-cdn.com/v4/letter/r/6a8cbe/32.png) [@Ring](https://boards.straightdope.com/u/Ring)\ **Post date:** [April 2, 2007, 3:29am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/30 "2007-04-02T03:29:40Z")
[QUOTE=bonzer] But that’s hardly stranger than the [Unruh effect](http://en.wikipedia.org/wiki/Unruh_effect). Surprising when it’s first pointed out to you. Ultimately pleasingly consistent. [/QUOTE] But there are obviously some differences (as I know you’re fully aware). For instance, an inertial observer can see past the Rindler horizon, and Rindler spacetime is globally flat whereas any true gravitational phenomenon (say, a BH) necessarily involves spacetime curvature. Bonzer please don’t think I’m trying to tell you something you don’t already know; I’m well aware you are a googleplex more knowledgeable than I will ever be. ---
**Author:** ![Jinx](https://avatars.discourse-cdn.com/v4/letter/j/c6cbf5/32.png) [@Jinx](https://boards.straightdope.com/u/Jinx)\ **Post date:** [April 2, 2007, 10:46am UTC](https://boards.straightdope.com/t/can-energy-be-destroyed/397916/31 "2007-04-02T10:46:03Z")
Sorry! Erroneous Posting! Sorry! :eek: [Previous page](https://boards.straightdope.com/t/can-energy-be-destroyed/397916.md?page=1)