Stephen Hawking writes in his book “A Brief History of Time” of the possibility of there being antiworlds and and antipeople made out of antimatter. Yet he warns that if I should happen to meet my antiself, I should not shake hands due to the fact that I would be annihilated. What he does not explain is what my antiself would look like. How will I be able to recognize my antiself? Will he look like a photonegative? Will he be a mirror image?
A concerned fledgling physicist
I think Stephen Hawking was trying to be humorous when he talked about shaking hands with your antiself. You could not stand on the same ground as your antiself; You could not even breath the same air. I doubt he was being literal.
A particle and its antiparticle are reversed images through CPT reversal. C - Charge is easy. Negative charge becomes positive. Your charge is zero, or close enough to it. Your antiself would also have zero charge. P - Parity or handedness. Handedness is reversed. However I’m not sure what connection the particles’ handedness has to macroscopic handedness. But let’s say your antiself would be a mirror image. T - Time. This is a very tough one. Your antiself could be time reversed. I have a problem imagining time reversal on a macro scale though.
So, what would your antiself look like? Just like you. Light does not know matter from anit-matter (at least as far as color goes). So he or she would look just like you, possibly mirror reversed.
.ti tbuod I tub ,sdrawkcab epyt dluow ehs ro eh ebyaM
So that’s why you lepton her!
Sorry, I’m in a funny mood tonight
Now that’s funny!
This must be why my wife always experiences an anticlimax when I experience the opposite.
Not quite, DrMatrix. CPT gets you right back where you started. C all by itself gets you from matter to antimatter: Despite the name, flavor, lepton number, and all those other “internal” qualities are reversed along with charge. Also note that the time reversal operator here only refers to subatomic properties, and does not have any effect on the Second Law, which is responsible for most of the time asymmetries we’re familiar with. Parity would be reversed, but the parity of a complex structure like a whole person has essentially no bearing on the parity of the particles composing it, so it’s plausible to have an antidouble with its heart on the left side or on the right.
In any event, photons are the same as antiphotons, so your antidouble would look perfectly normal (at least, to the extent that you look normal).
Matrix, you kill me. 
To expand a bit on the good Doctor’s excellent response, the reason you’d annihilate if you shook hands with your opposite is that a particle annihilates when it collides with its antiparticle. The electrons in your hand, for example, would come into contact with the positrons in antiyou’s hand, and they would annihilate in a burst of very unfriendly gamma rays. The atoms of your hand would thus be disrupted, freeing their positively charged nuclei, which contain protons and neutrons, which would be attracted to the nuclei of antiyou, which are negatively charged, and composed of antiprotons and antineutrons. More gamma rays. Bad, bad scene.
But, as DrMatrix pointed out, if the antiyou suddenly appeared on Earth, he’s got more than you to worry about–there are electrons, protons and neutrons in the ground and the air, too, which will be happy to annihilate with antiyou’s positrons, antiprotons, and antineutrons. So you’ll never lay eyes on antiyou–upon appearing on Earth, he’d just instantly detonate, releasing more energy than any nuclear bomb. Same thing if you suddenly appeared on Antiearth–except you’re the one blowing up.
But, assuming you met in a reasonably good vacuum, and you had a spacesuit and he had an antispacesuit, better not shake hands. : )
Are you sure about that? If I’m not mistaken, neutrinos are all left-handed particles, but anti-neutrinos are right-handed. So, we see a parity reversal here when we go from matter to antimatter. I agree that parity we see in the macro world is not related to the parity of the quantum world. I am still struggling to try to understand time asymmetry.
Podkayne and Chronos:
The last time we were in the same thread. Podkayne mercilessly ripped apart my excellent (at least, in my own mind) posts and Chronos defended me. Here, I get “Not quite” from Chronos and praise from Podkayne (thanks, BTW). It is obvious that you two have gotten together and decided to try to mess with my head. Simul-posting was the dead giveaway. Well, it’s not going to work, because I’m on to you two!
Dr. Edward Teller once wrote of meeting his anti-self, which prompted this poem written by Harold P. Furth and originally published in New Yorker magazine:
Full poem here: http://www.cardhouse.com/berk/science99_2.htm
Ignoring the fact that the OP is hypthetical and non-literal and whatnot… what are the odds that there is an anti-you? I mean, it would seem likely that the anti-particles to all the particles that make up ME never met each other and are making up billions of different anti-planets or anti-dogs or whatever… I claim essentially no knowledge of anti-matter except the rudimentary basics (and hell maybe not even those)… i’m just curious
Let’s start with the odds that there is another you on some other planet (made of matter). Which, by my rough calculations, is approximately slim and none. We multiply this probability by the ratio of anti-matter to matter in the universe. The universe is almost entirely matter with very little antimatter. So, the odds of there being an antiyou are approximately zero times slim and none.
Wouldn’t it be balanced? I mean for every anti-particle destroyed a regular particle is destroyed… I suppose i’m not too clear on how a-p’s are created (and how that would lean more towards r-p’s [regular particles]). I just would have thought if one is the ‘mirror’ of the other, the numbers would be the same
also, my question wasn’t literally “what are the odds”, but instead my question was whether or not it follows that because I formed, all the anti’s to my particles also formed the same being except with negative particles… or instead would the actual particles that were formed at the same time as MY particles doing totally different things that in no way relate to anti-me
Chronos wrote:
“A robot must obey all commands given by a human, except when in violation of the First Law”? 
(Well, they do have positronic brains…)
Just keepin’ you on your toes, Matrix. Anyway, I defer to Chronos in all things, so you better listen to him, not me. : )
And that’s why I’m going to let **Chronos[\b] answer Kaje’s question about matter-antimatter asymmetry, 'cause I’m a wimp, and my particle physics is rusty. Short story, Kaje, to tide you over, is that it turns out that ther are slight differences in the way that matter particles and antimatter particles decay into other particles, or maybe that some particles have a slight tendency to decay more often into antimatter than matter. . . hell, I’ve never gotten that straight. :o If you want to be as confused as I am, do a web search on “antimatter asymmetry” or “CP violation.”
Give me F=ma anyday.
Hey, was that \ a parity violation, or what?
Silly me!
I read more of Hawking’s book and later on he states that my antiself would be a mirror image of myself according to P symmetry (I think). So thanks for your answer and I’ll keep up with the physics studies.
Off to “The Elegant Universe”!
Wait, here’s another question, could there be a “super” me, using the superparticle of supersymmetry? Would the SuperMe have “super” powers? (That last one was a joke) Though not of the caliber y’all were making before.
Your antiself would consider you his/her antiself. 
Also do not shake hands with my antiself :mad:. He’s a mean so and so.