Re: Dumb Relativity Question

From: Jesse Mazer (vze2ztqw_at_mail.verizon.net)
Date: 02/11/05 

Date: Fri, 11 Feb 2005 01:52:44 GMT

Ben Rudiak-Gould wrote:

> [alt.punk and alt.gothic removed from newsgroups line, since I fail to
> see the relevance]
>
> Jesse Mazer wrote:
>
>> The Drums of Fascinet wrote:
>>
>>> So, when you have two objects inelastically collide at relativistic
>>> speeds such that they squish together and have no velocity in what was
>>> the CM frame, their new mass is greater than the sum of the original
>>> objects' masses in their respective frames.
>>
>>
>> No, it's impossible for them to collide in such away that they squish
>> together and come to rest without radiating any energy away as
>> electromagnetic waves, flying chunks, etc. Proof:
>>
>> [...]
>>
>> E3 = (m1 + m2)*c^2
>>
>> But clearly E3 is smaller that E1+E2, indicating that energy would
>> not be conserved if this were possible, therefore this type of
>> collision could never happen.
>
>
> Why do you assume that the rest mass of the final object is the sum of
> the rest masses of the original objects? Rest mass isn't conserved.
> There are lots of examples of inelastic collisions like the OP
> described -- the time reversal of radioactive decay, for example.

Because that was what was specified in the original problem. If some of
the energy was carried away by particles with nonzero momentum, then
that would fall into the category of "radiating any energy away as
electromagnetic waves, flying chunks, etc."

Jesse

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