Where’s All The Antimatter?

One of the biggest mysteries in the Universe is the fact there there’s matter, and not antimatter. Where did it all go?

When we look around, everything we can see is made of matter. For every type of matter from electrons, protons and quarks there is a similar type of matter known as antimatter. So why aren’t there piles of antimatter rocks, cars and chocolate bars just lying around? Why does Scotty always have a little extra kicking around in his liquor cabinet? And where do I get mine?

The primary difference between matter and antimatter is that they have opposite electric charge. Which seems pretty mundane. The negatively charged electron has an antiparticle known as the positron, which has a positive electric charge.

Anti-protons have a negative charge, and are just flat out grumpy. We’ve been able to create these particles in the lab, and have even been able to create small amounts of anti-hydrogen consisting of a positron bound to an antiproton, when examined closely there’s were shown to have a goatee and a little colorful sash or dagger.

When we create particles in accelerators such as the Large Hadron Collider, we seem to get equal amounts of matter and antimatter. This suggests that when particles were formed soon after the big bang, there should have been equal amounts of matter and antimatter.

Particle Collider
Large Hadron Collider (CERN/LHC/GridPP)

But the universe we observe is only made of matter, and… here’s the best part… we have no idea why. Why didn’t the matter and antimatter completely annihilate each other? How come we ended up with a little more matter? This delightful mystery is known as baryon asymmetry.

We do have a few ideas, and by we, I mean some giant brained crackerjacks have come up with a few plausible options. The most popular is that somehow antimatter behaves a little differently than matter. This could cause an imbalance between matter and antimatter. After particles collided in the early universe, there would still be matter left over, hence the matter we observe.

Another idea is that the observable universe just happens to be in a region dominated by matter. Other parts of the multiverse could have observable universes dominated by antimatter. Baryon asymmetry is one of the big mysteries of modern cosmology.

Zero Gravity Flight
Stephen Hawking, weightless (courtesy Zero Gravity Corporation)

There is an even crazier idea. Antimatter might have anti-gravity. In other words, an atom of anti-hydrogen would fall up instead of down. If that is the case, then matter and antimatter would repel each other, and you might have matter universes and antimatter universes that are forever separate.There have been some initial experiments to test this idea, but so far there have been no conclusive results.

What do you think? Where’s all our antimatter and how do we track it down? I’d sure love to bring some home and show my friends…

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Charge of Electron

Charge of Electron

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The charge of the electron is equivalent to the magnitude of the elementary charge (e) but bearing a negative sign. Since the value of the elementary charge is roughly 1.602 x 10-19 coulombs (C), then the charge of the electron is -1.602 x 10-19 C.

When expressed in atomic units, the elementary charge takes the value of unity; i.e., e = 1. Thus, the electron’s charge can be denoted by -e. Although the proton is much more massive than the electron, it only has a charge of e. Hence, neutral atoms always bear the same number of protons and electrons.

JJ Thomson is the undisputed discoverer of the electron. However, despite all those experiments he performed on it, he could only manage to obtain the electron’s charge to mass ratio. The distinction of being the first to measure the electron’s charge goes to Robert Millikan through his oil-drop experiment in 1909.

The Millikan Oil-Drop Experiment

Here’s the basic idea. If you know the density and dimensions (thus subsequently the volume) of a substance, it’s going to be easy to calculate its mass and the force that gravity exerts on it, a.k.a. weight. If you recall, weight is just m x g.

Now let’s assume these substances to be charged oil drops. If you subject these drops to gravity alone, they’ll fall freely. However, if they are allowed to fall in a uniform electric field, their trajectory will be altered depending on the direction and magnitude of the field.

If the forces due to the field are directed opposite to gravity, the downward velocity of the particles may decrease. At some point, when the upward force is equal to the downward force, the velocities may even go down to zero and the particles will stay in mid-air.

At this specific instance, if we know the magnitude of the electric field (in N/C, units defining the force per unit charge) and the weight of each particle, we can calculate the force of the electric field on a single particle and finally derive the charge.

Thus, a basic Millikan Oil-Drop Experiment setup will include an enclosure containing falling charged oil drops, a device to measure their radii, an adjustable uniform electric field, and a meter to determine the field’s magnitude.

By repeating the experiment on a large number of oil drops, Millikan and his colleague, Harvey Fletcher, obtained electron charge values within 1% of the currently accepted one.

We have some articles in Universe Today that are related to the charge of the electron. Here are two of them:

Physics World also has some more:

Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:

Sources:
Wikipedia
GSU Hyperphysics
University of Alaska-Fairbanks