robphy's latest activity
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You can't travel faster than ##c##. So your question is "how should I interpret some maths I've applied in a regime where the same maths... -
robphy replied to the thread High School Light & Time."Proper time" does not exist for the photon. However, there must be a sense of causality, The Emission-event occurs before the...
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robphy replied to the thread Undergrad Photons & Time: t=0 What?.I should probably write the relevant formula involving hyperbolic-trigonometry. ##\beta=(v/c)=\tanh\theta ## so...
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robphy replied to the thread Undergrad Photons & Time: t=0 What?.For a ship (with nonzero rest mass), it cannot achieve "the speed of light" physically. Although your Newtonian intuition suggests that...
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No. Light does not experience time. There is no valid frame of reference for a photon. This is a direct result of the postulates of...
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robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.Did you look at the Szekeres "Signal Space" reference? His S relation is reflexive, but not symmetric.
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robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.This might be of interest: E. H. Kronheimer & R. Penrose (1967) "On the structure of causal spaces" Mathematical Proceedings of the...
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robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.Along these lines (assuming (3+1)-spacetime): "The causal order C determines the conformal structure of space-time, or nine of the ten...
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robphy reacted to renormalize's post in the thread Graduate Can a causal or time-like structure emerge without assuming a metric? with
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Take a look at https://en.wikipedia.org/wiki/Causal_sets: "The causal sets program is an approach to quantum gravity. Its founding... -
robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.You might be interested in David Malament’s dissertation: “Does the Causal Structure of Space-Time Determine its Geometry”...
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robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.I think that’s a fair statement.
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robphy replied to the thread Graduate Can a causal or time-like structure emerge without assuming a metric?.AA Robb studied the “conical order” in terms of “before” and “after”.
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robphy replied to the thread Graduate Dirac's 1958 lecture on spinors.From https://www.nobelprize.org/uploads/2024/02/penrose-lecture.pdf You can read this 1936 Dirac reference here...
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robphy replied to the thread Graduate Dirac's 1958 lecture on spinors.You could try to browse Dirac's collected papers Cambridge U. - https://archivesearch.lib.cam.ac.uk/repositories/9/resources/1531...
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robphy replied to the thread Undergrad Gravitational analog of electromagnetic force.This reminds me of a very old post of mine (reconstructed here because the old-style LaTeX on PF appears broken): It might be...