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Just Chat! / Re: What to do if your question is too long to fit in the title bar?
« on: Yesterday at 22:55:52 »
Worth remembering the visitor who asked a Dubliner the best route to Cork. "If I was going to Cork, sir, I wouldn't be starting from Dublin". That theme runs through my discussions with Hamdani on the "twin paradox" and is relevant here.
Simply, if you begin with classical/newtonian physics, you can't derive relativistic equations because the key element v/c (or more often v2/c2) is absent.
But if you start from a relativistic position, you can derive newtonian physics by simply putting v << c.
Consider a blind man running into an oncoming car. Until the instant of impact, he has no knowledge of their relative velocity, but at impact he has a very good idea of the kinetic energy of the collision.
Replace the car with a photon, and replace the man with the textbook "observer" - in this case a spectrometer. Since we have postulated that no information can travel faster than c, we are "blind" before the event and all we know is the energy of the photon/spectrometer event. We know by experiment that if we impose a relative velocity or gravitational field between the source and observer, we observe a proportionate doppler or gravitational shift of that energy.
Simply, if you begin with classical/newtonian physics, you can't derive relativistic equations because the key element v/c (or more often v2/c2) is absent.
But if you start from a relativistic position, you can derive newtonian physics by simply putting v << c.
Consider a blind man running into an oncoming car. Until the instant of impact, he has no knowledge of their relative velocity, but at impact he has a very good idea of the kinetic energy of the collision.
Replace the car with a photon, and replace the man with the textbook "observer" - in this case a spectrometer. Since we have postulated that no information can travel faster than c, we are "blind" before the event and all we know is the energy of the photon/spectrometer event. We know by experiment that if we impose a relative velocity or gravitational field between the source and observer, we observe a proportionate doppler or gravitational shift of that energy.
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