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Lies in education [was Re: The "loop and a half"]

On 2017-10-05 06:24, Steve D'Aprano wrote:
> On Thu, 5 Oct 2017 02:14 pm, Stefan Ram wrote:
>> Chris Angelico <rosuav at gmail.com> writes:
>>> You can get through a lot of life believing that mass is conserved,
>>> but technically it is not, as can be proven.
>>   Well, in fact, it is conserved.
> It certainly is not. The whole point of Einstein's equation E = mc? is that
> energy and mass are freely convertible. Neither energy nor mass alone are
> conserved.

You're both right, you're just defining "mass" in different ways.

>>   When an electron and a positron annihilate to give a gas of
>>   two photons, this gas as a whole still has the same mass as
>>   the electron before. And its center of gravity moves with
>>   less than c.
> That doesn't sound like any description of matter/anti-matter annihilation
> I've every seen before.
> I think you may be conflating the concepts of clouds of virtual
> electron/positron particles with actual annihilation events between real
> electron/positron particles.
> In actual annihilation events, there is (as far as I know) generally a single
> real photon produced, with momentum equal to the sum of the momentum vectors
> of the original electron and positron. That moves away from the point of
> production at the speed of light.