[Still off-top] Physics [was Requests author discusses MentalHealthError exception]

[Steven D'Aprano]

On Fri, 4 Mar 2016 07:20 am, alister wrote:

> On Thu, 03 Mar 2016 11:03:55 -0700, Ian Kelly wrote:

>> Antimatter has positive mass.
> 
> Are you sure?
>  mix 1 atom of hydrogen + 1 of anti hydrogen & you end up with 0 mass (+
> LOTTS of energy)
> 
> To be honest it is all over my head

It's good to be honest :-)


Yes, anti-matter has positive mass. There was still some tiny lingering
doubt up until (by memory) 20 years ago, at which time physicists actually
managed to make sufficient anti-matter that they could assemble it into
slow-moving atoms and observe the effect of gravity on it, and sure enough,
anti-matter falls due to gravity the same as regular matter.

(There wasn't really any serious doubt about this, since you can pull and
push anti-matter with electric and magnetic fields and it behaves exactly
the same way as regular matter. But if gravity had turned out to be
different, it would have been a truly paradigm-changing discovery.)

As far as the reaction of matter and anti-matter, we've known for about a
century that mass and energy are related and freely convertible from one to
the other. That's the famous equation by Einstein: E = m*c**2. Even tiny
amounts of energy (say, the light and heat released from a burning match)
involve a correspondingly tiny reduction in mass.


-- 
Steven