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View Diary: Who can own the future? (262 comments)

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  •  Huh? It most certainly does. (2+ / 0-)

    It's just that there are different notions of mass. And for the purposes of acceleration, the one that matters is the relativistic mass, not the rest mass.

    I know you know what's going on, but pedantry doesn't help explain physics to the masses :-)

    •  Could you explain relativistic mass? (0+ / 0-)

      For the masses?  

      I've never been able to wrap my head around the speed of light limit, but then my physics education stopped at physical chemistry in college.  

      "When I was an alien, cultures weren't opinions" ~ Kurt Cobain, Territorial Pissings

      by Subterranean on Sun Jul 14, 2013 at 11:21:24 AM PDT

      [ Parent ]

      •  "relativistic mass" is an attempt (0+ / 0-)

        to keep some of the mathematics of Newtonian Mechanics intact while using relativistic mathematics.

        E.g., the relativistic momentum equation is mass times velocity divided by the square root of a velocity dependent term.  This denominator is the square root of the quantity (1 - speed squared/speed of light squared).  This denominator approaches zero as the speed approaches the speed of light.

        Note that mass times velocity is the expression for the momentum of objects with mass in Newtonian Mechanics.  It is easy to think of the denominator of the relativistic expression as dividing into just the mass.  Then define a relativistic mass as the so-called rest mass (mass at zero relative speed) divided by the new term.  This so-called relativistic mass then approaches infinity as the speed approaches the speed of light.

        But why should the velocity be ignored in the momentum and only the mass considered?  The speed, which is the amount of the velocity (doesn't include direction), is in the denominator.  It would be really odd to make the change based on speed but drop the velocity from the denominator, but that is what is often done.

        Really no one is going to get hurt if you think of mass as something that increases with speed.  You won't be able to get a job in a theory group at CERN, but otherwise, you'll be just fine.

        "Trust only those who doubt" Lu Xun

        by LookingUp on Sun Jul 14, 2013 at 01:13:07 PM PDT

        [ Parent ]

        •  I quote Einstein, when he teaches about (0+ / 0-)

          a body that moves with speed v and acquires additional energy E_0 from radiation:

          Thus, the body now has the same energy as a moving body of mass $m+E_0/c^2$.  We can say, therefore, that if a body acquires energy E_0, its mass increases by E_0/c^2; thus, the inertial mass of a body is not constant, but varies according to its energy.

          So, you may well be right that Einstein wouldn't be able to get a job at CERN, but not for the reasons you think.

    •  Not really (0+ / 0-)

      Newton's second law in the form F = ma doesn't work relativistically.  Oddly, it works fine in the form that Newton preferred, which is that the net force on an object equals the rate of change of the object's momentum with time.  It's just that the relativistic expression for momentum is different from the expression used by Newton (see my post below)

      There are different notions of mass.

      In Newtonian physics mass is either what is measured on a balance in a gravitational field (gravitational mass), or the ratio of the net force on an object to the resulting acceleration (inertial mass).

      In relativity the mass of a system is equivalent to the total energy of the system divided by the speed of light squared.  This turns out to have much deeper implications than I can explain in a short post.

      In the Standard Model of Particle Physics mass of fundamental particles is a result of how strongly they couple to the Higgs Field.

      All of these are different in sometimes small and other times big ways, but each exists within a particular theoretical framework.

      IOW, many concepts in science, including mass, are defined within particular theories,  Change the theory, and you change the definition.  But once you change the theory, you can't always use the definition from that theory within one of the other theories.

      "Trust only those who doubt" Lu Xun

      by LookingUp on Sun Jul 14, 2013 at 01:24:32 PM PDT

      [ Parent ]

      •  You're saying "not really" to the creator of the (0+ / 0-)

        theory of relativity; see the Einstein passage I quote in a previous comment.

        •  From wikipedia (0+ / 0-)

           

          It is not good to introduce the concept of the mass M = m/sqrt{1 - v^2/c^2} of a moving body for which no clear definition can be given. It is better to introduce no other mass concept than the ’rest mass’ m. Instead of introducing M it is better to mention the expression for the momentum and energy of a body in motion.

              — Albert Einstein in letter to Lincoln Barnett, 19 June 1948 (quote from L. B. Okun (1989), p. 42[1])

          Even Einstein could change his mind, and in this case he did.  You are also arguing from authority.  In the end no one person decides what works best in science.  In the physics community the idea of relativistic mass is considered as less objective than that of invariant mass.

          "Trust only those who doubt" Lu Xun

          by LookingUp on Mon Jul 15, 2013 at 06:40:27 AM PDT

          [ Parent ]

          •  thanks for the interesting comment. (0+ / 0-)

            I'll give the matter further thought.  

            You are also arguing from authority.
            As were you in saying "you wouldn't get a job at CERN".  The fact is that we all quote authorities every day -- we would get nowhere if we had to prove everything ab initio every time.
            In the end no one person decides what works best in science.
            This I can fully agree with.
            In the physics community the idea of relativistic mass is considered as less objective than that of invariant mass.
            That could well be; I have noticed that many physicists subscribe to the delusion that constructs such as mass, energy, quanta, orbitals etc. have an objective reality independent of the theory they are embedded in.  This was the case with time too, at one point.

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