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<blockquote data-quote="Umbran" data-source="post: 6687528" data-attributes="member: 177">There is always a possibility.&nbsp; But if you want a percentage chance, I can't give you one.&nbsp; We have observed and experimented with anti-electrons, anti-protons, and anti-neutrons, and measured their masses.&nbsp; They have the expected mass.You probably read such, but it doesn't mean what you probably think it means.&nbsp; I'll try to avoid going too much into terminology, as when we start talking about quantum mechanics and &quot;spin&quot;, there are a lot of terms.There are a bunch of numbers we use to describe a particle, often called &quot;quantum numbers&quot;.&nbsp; Speaking broadly - an anti-particle will have all its quantum numbers reversed from a matter particle produced in the same way.&nbsp; So, say we have a *really* energetic photon.&nbsp; Rather than just fly along forever, it may at some point spontaneously create a particle-antiparticle pair out of its energy.&nbsp; Say it creates a positron and electron.&nbsp; The electron has some angular momentum intrinsic to it.&nbsp; In order to conserve angular momentum at the moment of creation, the positron must be created with the opposite angular momentum.&nbsp; In that sense, we may say it has the opposite spin.In astrophysics, &quot;visible&quot; universe and &quot;known&quot; universe are synonymous.&nbsp; Yes, there is a limit to the volume of space we know anything about.&nbsp; It is possible that nearly anything could be going on outside that visible volume of space.&nbsp; However, I note that the laws and constants we see are apparently *extremely* consistent across what space we can see.&nbsp; You'd need a particular reason to justify why those laws then become different somewhere else.&nbsp; Moreover, you'd have to deal with how, *just by coincidence* that weird thing happens to be out where we cannot see it.</blockquote>

[QUOTE="Umbran, post: 6687528, member: 177"] There is always a possibility. But if you want a percentage chance, I can't give you one. We have observed and experimented with anti-electrons, anti-protons, and anti-neutrons, and measured their masses. They have the expected mass. You probably read such, but it doesn't mean what you probably think it means. I'll try to avoid going too much into terminology, as when we start talking about quantum mechanics and "spin", there are a lot of terms. There are a bunch of numbers we use to describe a particle, often called "quantum numbers". Speaking broadly - an anti-particle will have all its quantum numbers reversed from a matter particle produced in the same way. So, say we have a *really* energetic photon. Rather than just fly along forever, it may at some point spontaneously create a particle-antiparticle pair out of its energy. Say it creates a positron and electron. The electron has some angular momentum intrinsic to it. In order to conserve angular momentum at the moment of creation, the positron must be created with the opposite angular momentum. In that sense, we may say it has the opposite spin. In astrophysics, "visible" universe and "known" universe are synonymous. Yes, there is a limit to the volume of space we know anything about. It is possible that nearly anything could be going on outside that visible volume of space. However, I note that the laws and constants we see are apparently *extremely* consistent across what space we can see. You'd need a particular reason to justify why those laws then become different somewhere else. Moreover, you'd have to deal with how, *just by coincidence* that weird thing happens to be out where we cannot see it. [/QUOTE]

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