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

From	Gene Heskett <gheskett@wdtv.com>
Newsgroups	comp.lang.python
Subject	Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception]
Date	2016-03-05 10:15 -0500
Message-ID	<mailman.226.1457190954.20602.python-list@python.org> (permalink)
References	<56d16e6a$0$1593$c3e8da3$5496439d@news.astraweb.com> <djuvt8FlbhtU1@mid.individual.net> <CAHVvXxSaOVx=pMcg05xc+B3OOk2Uguzx-2c0f51_EG8CLb3wbQ@mail.gmail.com>

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On Saturday 05 March 2016 08:11:46 Oscar Benjamin wrote:

> On 5 March 2016 at 02:51, Gregory Ewing <greg.ewing@canterbury.ac.nz> 
wrote:
> >  The masslessness of photons comes from an extrapolation
> >
> >> that leads to a divide by infinity: strictly speaking it's just
> >> undefined.
> >
> > No, it's not. The total energy of a particle is given by
> >
> >    E**2 == c**2 * p**2 + m**2 * c**4
> >
> > where p is the particle's momentum and m is its mass.
> > For a photon, m == 0. No division by zero involved.
> >
> > For a massive particle at rest, p == 0 and the above
> > reduces to the well-known
> >
> >    E == m * c**2
>
> The distinction I'm drawing is between physical fact and mathematical
> convenience. For other particles we can say that the 1st formula above
> holds with m taken to be the mass of the particle at rest. We can
> extend that formula to the case of photons which are never at rest by
> saying that in the case of photons m=0. That's nice and it's
> mathematically convenient in the calculations. It's analogous to
> extending the natural definition of the factorial function by saying
> that 0!=1. We can't prove that 0!=1 but it's useful to define it that
> way. It wouldn't be a disaster to simply leave 0! undefined: it would
> just make some equations a little more complicated.
>
> Since the generally accepted physical fact is that photons are never
> at rest we are free to define their "rest mass" (use any term you
> like) to be anything that is mathematically convenient so we define it
> as zero because that fits with your equation above. Turning full
> circle we can then use the equation above to say that they are
> massless since they would hypothetically be massless in some other
> situation even though genuinely massless photons are not thought to
> exist in physical reality (unless I'm really out of date on this!).
>
> >> Something I don't know is if there's some theoretical reason why
> >> the binding energy could never exceed the sum of the energies of
> >> the constituent particles (resulting in an overall negative mass).
> >
> > Conservation of energy would be one reason. If you
> > put two particles together and got more energy out than
> > went in, where did the extra energy come from?
>
> That's the point: the energy balance would be satisfied by the
> negative energy of the bound particles. The binding energy can be
> defined as the energy required to unbind the particles (other
> definitions such as André's are also possible). From this definition
> we see that the binding energy depends on the binding interaction
> (electromagnetic or whatever) that binds the particles together.
>
> The only examples I know of where the binding energy is computed
> approximately for e.g. a hydrogen atom predict that the binding energy
> is proportional to the (rest) mass of the bound particle(s). If it's
> guaranteed that the binding energy always somehow comes out
> proportional to the mass of the particles with a coefficient
> necessarily smaller than 1/c**2 then you could say that the bound
> product could never have negative energy. I just can't see off the top
> of my head an argument to suggest that this is impossible.
>
> --
> Oscar

I've never heard of a massless photon, and they do exert a push on the 
surface they are reflected from, its even been proposed to use it as a 
space drive.  The push is miniscule indeed at normal illumination levels 
but some have calculated how much laser power it would take to move 
something like a solar sail. Practically, the cost of the energy and the 
size of the laser needed are impractical.

Cheers, Gene Heskett
-- 
"There are four boxes to be used in defense of liberty:
 soap, ballot, jury, and ammo. Please use in that order."
-Ed Howdershelt (Author)
Genes Web page <http://geneslinuxbox.net:6309/gene>

Thread

[Off-topic] Requests author discusses MentalHealthError exception Steven D'Aprano <steve@pearwood.info> - 2016-02-27 20:37 +1100
  Re: [Off-topic] Requests author discusses MentalHealthError exception Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-02-27 09:50 +0000
  Re: [Off-topic] Requests author discusses MentalHealthError exception Ethan Furman <ethan@stoneleaf.us> - 2016-02-27 14:06 -0800
  Re: [Off-topic] Requests author discusses MentalHealthError exception Jason Friedman <jsf80238@gmail.com> - 2016-02-27 17:00 -0700
  Re: [Off-topic] Requests author discusses MentalHealthError exception Larry Martell <larry.martell@gmail.com> - 2016-02-29 11:21 -0500
  Re: [Off-topic] Requests author discusses MentalHealthError exception Ian Kelly <ian.g.kelly@gmail.com> - 2016-02-29 09:47 -0700
  Re: [Off-topic] Requests author discusses MentalHealthError exception Ben Finney <ben+python@benfinney.id.au> - 2016-03-01 03:56 +1100
    Re: [Off-topic] Requests author discusses MentalHealthError exception Rustom Mody <rustompmody@gmail.com> - 2016-02-29 09:08 -0800
      Re: [Off-topic] Requests author discusses MentalHealthError exception Ben Finney <ben+python@benfinney.id.au> - 2016-03-01 04:33 +1100
        Re: [Off-topic] Requests author discusses MentalHealthError exception Rustom Mody <rustompmody@gmail.com> - 2016-02-29 09:54 -0800
      Re: [Off-topic] Requests author discusses MentalHealthError exception Larry Martell <larry.martell@gmail.com> - 2016-02-29 17:40 -0500
      Re: [Off-topic] Requests author discusses MentalHealthError exception Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-02-29 23:29 +0000
      Re: [Off-topic] Requests author discusses MentalHealthError exception Joel Goldstick <joel.goldstick@gmail.com> - 2016-02-29 18:33 -0500
      Re: [Off-topic] Requests author discusses MentalHealthError exception Steven D'Aprano <steve@pearwood.info> - 2016-03-02 04:05 +1100
        Re: [Off-topic] Requests author discusses MentalHealthError exception Rustom Mody <rustompmody@gmail.com> - 2016-03-02 09:02 -0800
          Re: [Off-topic] Requests author discusses MentalHealthError exception Steven D'Aprano <steve@pearwood.info> - 2016-03-03 13:27 +1100
            Re: [Off-topic] Requests author discusses MentalHealthError exception Chris Angelico <rosuav@gmail.com> - 2016-03-03 13:35 +1100
              Re: [Off-topic] Requests author discusses MentalHealthError exception alister <alister.ware@ntlworld.com> - 2016-03-03 17:21 +0000
                Re: [Off-topic] Requests author discusses MentalHealthError exception Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-03 11:03 -0700
                Re: [Off-topic] Requests author discusses MentalHealthError exception alister <alister.ware@ntlworld.com> - 2016-03-03 20:20 +0000
                Re: [Off-topic] Requests author discusses MentalHealthError exception Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-03 13:33 -0700
                Re: [Off-topic] Requests author discusses MentalHealthError exception William Ray Wing <wrw@mac.com> - 2016-03-03 18:02 -0500
                Re: [Off-topic] Requests author discusses MentalHealthError exception alister <alister.ware@ntlworld.com> - 2016-03-04 14:05 +0000
                [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Steven D'Aprano <steve@pearwood.info> - 2016-03-04 11:04 +1100
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-03-04 10:19 +0000
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Marko Rauhamaa <marko@pacujo.net> - 2016-03-04 12:38 +0200
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Dan Sommers <dan@tombstonezero.net> - 2016-03-04 13:10 +0000
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-03-04 17:35 +0000
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] André Roberge <andre.roberge@gmail.com> - 2016-03-04 11:12 -0800
                Re: [Still off-top] Physics [was Requests author discusses   MentalHealthError exception] Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-05 15:51 +1300
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Chris Angelico <rosuav@gmail.com> - 2016-03-05 14:13 +1100
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Marko Rauhamaa <marko@pacujo.net> - 2016-03-05 10:31 +0200
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2016-03-05 13:11 +0000
                Photon mass (was: [Still off-top] Physics) Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-05 16:38 +0100
                Re: [Still off-top] Physics [was Requests author discusses MentalHealthError exception] Gene Heskett <gheskett@wdtv.com> - 2016-03-05 10:15 -0500
                Photon mass (was: [Still off-top] Physics) Thomas 'PointedEars' Lahn <PointedEars@web.de> - 2016-03-05 16:46 +0100
                Re: Photon mass (was: [Still off-top] Physics) Gene Heskett <gheskett@wdtv.com> - 2016-03-05 12:36 -0500
                Re: Photon mass (was: [Still off-top] Physics) Dennis Lee Bieber <wlfraed@ix.netcom.com> - 2016-03-06 12:16 -0500
                Re: Photon mass (was: [Still off-top] Physics) Larry Martell <larry.martell@gmail.com> - 2016-03-06 09:21 -0500
                Re: [Still off-topic] Physics Steven D'Aprano <steve@pearwood.info> - 2016-03-06 16:36 +1100
                Re: [Off-topic] Requests author discusses MentalHealthError   exception Gregory Ewing <greg.ewing@canterbury.ac.nz> - 2016-03-04 19:34 +1300
                Re: [Off-topic] Requests author discusses MentalHealthError   exception Marko Rauhamaa <marko@pacujo.net> - 2016-03-04 08:52 +0200
            Re: [Off-topic] Requests author discusses MentalHealthError exception Rustom Mody <rustompmody@gmail.com> - 2016-03-02 19:57 -0800
              Re: [Off-topic] Requests author discusses MentalHealthError exception Ian Kelly <ian.g.kelly@gmail.com> - 2016-03-02 23:44 -0700
              Re: [Off-topic] Requests author discusses MentalHealthError exception Steven D'Aprano <steve@pearwood.info> - 2016-03-04 02:50 +1100
              Re: [Off-topic] Requests author discusses MentalHealthError exception Mark Lawrence <breamoreboy@yahoo.co.uk> - 2016-03-03 16:21 +0000
      Re: [Off-topic] Requests author discusses MentalHealthError exception Anders Wegge Keller <wegge@wegge.dk> - 2016-03-03 08:23 +0100
    Re: [Off-topic] Requests author discusses MentalHealthError exception Paul Rubin <no.email@nospam.invalid> - 2016-02-29 16:35 -0800
      Re: [Off-topic] Requests author discusses MentalHealthError exception Ethan Furman <ethan@stoneleaf.us> - 2016-02-29 16:54 -0800
  Re: [Off-topic] Requests author discusses MentalHealthError exception Larry Martell <larry.martell@gmail.com> - 2016-02-29 17:36 -0500
  Re: [Off-topic] Requests author discusses MentalHealthError exception Larry Martell <larry.martell@gmail.com> - 2016-02-29 17:38 -0500
    Re: [Off-topic] Requests author discusses MentalHealthError exception Steven D'Aprano <steve@pearwood.info> - 2016-03-01 23:13 +1100
      Re: [Off-topic] Requests author discusses MentalHealthError exception "Sven R. Kunze" <srkunze@mail.de> - 2016-03-01 17:49 +0100
  Re: [Off-topic] Requests author discusses MentalHealthError exception "Mario R. Osorio" <nimbiotics@gmail.com> - 2016-02-29 20:11 -0800
  Re: Photon mass (was: [Still off-top] Physics) Gene Heskett <gheskett@wdtv.com> - 2016-03-06 16:46 -0500
  Re: Photon mass (was: [Still off-top] Physics) Larry Martell <larry.martell@gmail.com> - 2016-03-06 17:51 -0500

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