When two massive bodies collide we treat them simply by their mass interactions with momentum and energy conservations. Down at the molecular level it must be more like a field particle collision for the particles coming in close contact, why dont we consider all these effects ? Are they included in our equations already ?


2 Answers 2


No, they are not included because they don't affect our results. In Classical Mechanics we don't worry about what's happening at fundamental level, that's the real of QM/QFT. Note that at fundamental level energy and momentum is still conserved.

If we tried to study a classical collision we would immediately fail, because we would have to do $10^{100^{100}}$ calculations. The reasoning is similar to this answer.

To conclude, if Classical Mechanics works, why change it?

  • $\begingroup$ This is exactly what me and my friend concluded, but still I hypothesised that maybe our classical mechanics had somehow averaged and already included those details. As most probably after that rigorous calculation we would come to approximately the same answer. Any comments ? Is it possible ? $\endgroup$ Commented Oct 28, 2013 at 17:49
  • $\begingroup$ @rijulgupta Newton probably knew nothing about molecules, so he couldn't even imagine to include them in his theory. $\endgroup$
    – jinawee
    Commented Oct 28, 2013 at 17:59
  • $\begingroup$ This so extremely fascinating to think. :) was quantum or microscopic physics developed to not to interfere with macroscopic properties ? Like macroscopic objects dobt have wave nature.. $\endgroup$ Commented Oct 28, 2013 at 18:10
  • $\begingroup$ @rijulgupta You're touching a fundamental point. See: en.wikipedia.org/wiki/Correspondence_principle $\endgroup$
    – jinawee
    Commented Oct 28, 2013 at 18:13
  • $\begingroup$ So it is true, what more could a beginner ask for, its 12 here. So what a great start of the day. $\endgroup$ Commented Oct 28, 2013 at 18:21

At the subatomic level, particles interact with each other by the exchange of virtual particles. These particles happen to be always bosons: Gluons for the strong force, W/Z bosons for the weak force, photons for the electromagnetic force and a hypothesized graviton for gravitation. These processes are described by Quantum Field Theory, to illustrate them we can use Feynman diagrams.

So when two macroscopic bodies collide, there will be a massive exchange of these virtual particles, in this case mostly photons, as the electromagnetic happen to be the most important one in this case. Energy and momentum conservation also holds at the subatomic level.

  • $\begingroup$ I have edited the question to better target my problem regarding the matter, maybe you would want to change your answer in accordance. Sorry for the inconvenience $\endgroup$ Commented Oct 25, 2013 at 13:48
  • $\begingroup$ I thought virtual particles were just an abstraction used within perturbation theory. If you use a different method, they don't "exist". Maybe I'm confused though... $\endgroup$
    – Nick
    Commented Oct 28, 2013 at 18:31
  • $\begingroup$ Well, we don't observe them but they pop up in our mathematical framework. I guess it's up to you to decide if they exist or not. $\endgroup$
    – pfnuesel
    Commented Oct 29, 2013 at 10:55

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