<h2>Tl;DR</h2>

<ul>
<li>Detection via <strong>polarized light</strong> - Antimatter interaction with polarized light could be detected by vector rotation;</li>
<li>We're <strong><em>mostly</em> sure</strong>, because absence of gamma rays and characteristic Faraday polarization indicates absence of observable antimatter in meaningful amounts.</li>
</ul>

<h2>Long answer</h2>

<p>I do believe @userLTK to be correct on his comment.</p>

<p>To my limited knowledge, the absence of gamma-ray bursts that characterizes matter-antimatter interactions hints of baryonic matter prevalence throughout the visible universe; an anti-galaxy near a baryonic galaxy would show a healthy amount of gamma rays originating from boundary particle collisions.</p>

<p><img src="https://i.sstatic.net/LyAjq.png" alt="enter image description here"></p>

<p>This effect would be observable between structures of any scale - e.g. star/anti-star, galaxy/anti-galaxy or supercluster/anti-supercluster.</p>

<p><a href="https://books.google.com/books?id=H3fpCAAAQBAJ&amp;printsec=frontcover#v=onepage&amp;q&amp;f=false" rel="noreferrer">"Light from the Depths of Time"</a>, by Rudolph Kippenhahn, mentions that</p>

<blockquote>
  <p>[...] Even when, like a star, it is radiation, its spectrum remains
  exactly the same, quite independent of whether atoms or anti-atoms are
  responsible for the light.</p>
</blockquote>

<p>So the light originating from anti-galaxies would look the same as ordinary galaxies. But what about gravity?</p>

<p>A quote from <a href="https://books.google.com/books?id=T43ZoeIan64C&amp;pg=PA317&amp;lpg=PA317&amp;dq=light%20originating%20from%20anti-matter&amp;source=bl&amp;ots=u6_o3ZWqEv&amp;sig=TzYMlXCz2SzPKCmsXspvOWQ6MW4&amp;hl=en&amp;sa=X&amp;ei=lsldVfnJI4GqNvCkgagN&amp;ved=0CCAQ6AEwAA#v=onepage&amp;q=light%20originating%20from%20anti-matter&amp;f=false" rel="noreferrer">"Isodual Theory of Antimatter: with applications to Antigravity, Grand Unification and Cosmology"</a> may help us further:</p>

<blockquote>
  <p>[...] However, the photons is invariant under charge conjugation and
  travel at the maximal causal speed in vacuum, <em>c</em>. Therefore, the
  photon could well result to be a superposition of positive and
  negative energies, perhaps as a condition to travel at the speed <em>c</em>,
  in which case the photon would be an isoselfdual state, thus
  experiencing attraction in both fields of matter and antimatter.</p>
</blockquote>

<p>Meaning that antimatter would also cause gravitational effects, such as lensing.</p>

<p>That would make remote detection of anti-matter quite hard - it would emit and bend light in exactly the same fashion as common matter.</p>

<p>Faraday polarization rotation could give us some hope:</p>

<blockquote>
  <p>Polarized light, e.g., from non-thermal synchrotron sources, that
  passes through gas with a non-zero magnetic field will have its
  polarization vector rotated by the process of Faraday rotation. [...]
  Note that regions dominated by antimatter (positrons) cause a rotation
  <strong>opposite</strong> to that caused by regions dominated by matter (electrons).</p>
</blockquote>

<p>(<a href="http://www.slac.stanford.edu/econf/C040802/papers/L017.PDF" rel="noreferrer">SLAC Summer Institute on Particle Physics (SSI04), Aug. 2-13, 2004</a>)</p>

<p>But the same source mentions that </p>

<blockquote>
  <p>The effect has been measured many times and the amount of rotation,
  usually expressed in terms of the so-called “rotation measure” is
  given by the line-of-sight integral [...] The fact that we observe an
  effect at all means that on average, we can’t have equal amounts of
  antimatter and matter along our various lines of sight.</p>
</blockquote>

<p>So it seems that the visible universe is missing some antimatter after all.</p>

CC BY-SA 3.0
2015-05-21T12:14:29.030Z
2015-05-21T20:46:48.667Z
2015-05-21T20:46:48.667Z
OnoSendai
<p>I just was wondering and searching on the internet with little luck in the topic. On Antimatter Wiki they tell the observable universe is built up by matter. I read antimatter can be detected in cosmic rays in very small quantity. But let's go in theories and our practical knowledge, my question is:</p>

<p>How could we detect if a set of objects (like an entire galaxy) is completely built up by antimatter? Since matter and antimatter look like "mirror image" on current, could they form a similar object which is indistinguishable from "our" matter objects? I mean, would spectrum analysis for example show that it is not matter, but built up by antimatter?</p>

<p>Linked question: Are we sure about that what we see in universe is matter?</p>

CC BY-SA 3.0
2015-05-21T08:40:04.420Z
I think the answer is we can't.   An entire galaxy made up of anti-matter would look exactly like a galaxy made up of matter.  (as far as I know).   But maybe some experts should answer this with more certainty.
userLTK
<observational-astronomy><antimatter>
How can we tell the difference between matter and antimatter by observation in space?
CsBalazsHungary
# How can we tell the difference between matter and antimatter by observation in space?

(Asked by: username_0 on 2015-05-21)

I just was wondering and searching on the internet with little luck in the topic. On Antimatter Wiki they tell the observable universe is built up by matter. I read antimatter can be detected in cosmic rays in very small quantity. But let's go in theories and our practical knowledge, my question is:

How could we detect if a set of objects (like an entire galaxy) is completely built up by antimatter? Since matter and antimatter look like "mirror image" on current, could they form a similar object which is indistinguishable from "our" matter objects? I mean, would spectrum analysis for example show that it is not matter, but built up by antimatter?

Linked question: Are we sure about that what we see in universe is matter?

**Tags:** observational-astronomy, antimatter
> **Comments:**
> **username_1**: I think the answer is we can't.   An entire galaxy made up of anti-matter would look exactly like a galaxy made up of matter.  (as far as I know).   But maybe some experts should answer this with more certainty.  

---

## 1 answers

### Answer by username_2 on 2015-05-21. Score: 13 | Accepted answer

## Tl;DR

* Detection via **polarized light** - Antimatter interaction with polarized light could be detected by vector rotation;
* We're ***mostly* sure**, because absence of gamma rays and characteristic Faraday polarization indicates absence of observable antimatter in meaningful amounts.

## Long answer

I do believe @userLTK to be correct on his comment.

To my limited knowledge, the absence of gamma-ray bursts that characterizes matter-antimatter interactions hints of baryonic matter prevalence throughout the visible universe; an anti-galaxy near a baryonic galaxy would show a healthy amount of gamma rays originating from boundary particle collisions.

![enter image description here](https://i.sstatic.net/LyAjq.png)

This effect would be observable between structures of any scale - e.g. star/anti-star, galaxy/anti-galaxy or supercluster/anti-supercluster.

["Light from the Depths of Time"](https://books.google.com/books?id=H3fpCAAAQBAJ&printsec=frontcover#v=onepage&q&f=false), by Rudolph Kippenhahn, mentions that

> [...] Even when, like a star, it is radiation, its spectrum remains
> exactly the same, quite independent of whether atoms or anti-atoms are
> responsible for the light.

So the light originating from anti-galaxies would look the same as ordinary galaxies. But what about gravity?

A quote from ["Isodual Theory of Antimatter: with applications to Antigravity, Grand Unification and Cosmology"](https://books.google.com/books?id=T43ZoeIan64C&pg=PA317&lpg=PA317&dq=light%20originating%20from%20anti-matter&source=bl&ots=u6_o3ZWqEv&sig=TzYMlXCz2SzPKCmsXspvOWQ6MW4&hl=en&sa=X&ei=lsldVfnJI4GqNvCkgagN&ved=0CCAQ6AEwAA#v=onepage&q=light%20originating%20from%20anti-matter&f=false) may help us further:

> [...] However, the photons is invariant under charge conjugation and
> travel at the maximal causal speed in vacuum, *c*. Therefore, the
> photon could well result to be a superposition of positive and
> negative energies, perhaps as a condition to travel at the speed *c*,
> in which case the photon would be an isoselfdual state, thus
> experiencing attraction in both fields of matter and antimatter.

Meaning that antimatter would also cause gravitational effects, such as lensing.

That would make remote detection of anti-matter quite hard - it would emit and bend light in exactly the same fashion as common matter.

Faraday polarization rotation could give us some hope:

> Polarized light, e.g., from non-thermal synchrotron sources, that
> passes through gas with a non-zero magnetic field will have its
> polarization vector rotated by the process of Faraday rotation. [...]
> Note that regions dominated by antimatter (positrons) cause a rotation
> **opposite** to that caused by regions dominated by matter (electrons).

([SLAC Summer Institute on Particle Physics (SSI04), Aug. 2-13, 2004](http://www.slac.stanford.edu/econf/C040802/papers/L017.PDF))

But the same source mentions that

> The effect has been measured many times and the amount of rotation,
> usually expressed in terms of the so-called “rotation measure” is
> given by the line-of-sight integral [...] The fact that we observe an
> effect at all means that on average, we can’t have equal amounts of
> antimatter and matter along our various lines of sight.

So it seems that the visible universe is missing some antimatter after all.


---

