# Simple E=mc^2 question

**URL:** <https://boards.straightdope.com/t/simple-e-mc-2-question/243708>\
**Category:** Factual Questions\
**Created:** [May 5, 2004, 8:16pm UTC](https://boards.straightdope.com/t/simple-e-mc-2-question/243708 "2004-05-05T20:16:57Z")\
**Posts on this page:** 2\
**Page:** 2

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**Author:** ![Shade](https://avatars.discourse-cdn.com/v4/letter/s/2bfe46/32.png) [@Shade](https://boards.straightdope.com/u/Shade)\
**Post date:** [May 6, 2004, 4:39pm UTC](https://boards.straightdope.com/t/simple-e-mc-2-question/243708/21 "2004-05-06T16:39:27Z")

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> [@Libertarian](#):
>
> Probably a dumb question, but how do they know that some of the stars/galaxies aren’t made entirely of anti-matter?

From MikeS’s cite:

> [@](#):
>
> The Universe at large: This is tougher. If there were antimatter galaxies then we should see gamma emissions from annihilation. Its absence is strong evidence that at least the nearby clusters of galaxies (e.g., Virgo) are matter-dominated. At larger scales there is little proof.
> 
> However, there is a problem, called the “annihilation catastrophe” which probably eliminates the possibility of a matter-antimatter symmetric universe. Essentially, causality prevents the separation of large chucks of antimatter from matter fast enough to prevent their mutual annihilation in the early universe. So the Universe is most likely matter dominated.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [May 6, 2004, 8:51pm UTC](https://boards.straightdope.com/t/simple-e-mc-2-question/243708/22 "2004-05-06T20:51:11Z")

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> [@](#):
>
> Except, I just started thinking about electrons. Is that antimatter charge the same type of negative as the charge on an electron, or are they different?
> 
> If not, why does antimatter and matter go boom whereas protons and electrons don’t?

I don’t think that this has been completely answered. A positron has the same charge as a proton, but they’re different in other ways. A positron is opposite to an electron in every way except mass, not just in charge. A proton and an electron can’t annihilate, because there are other properties like baryon number and lepton number that don’t balance out between them. In fact, the lowest-mass combination of particles you can get with those values of total baryon number, lepton number, and charge is… An electron and a proton. So you can’t combine an electron and a proton to produce any other particles, unless you add more energy to the system.

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