- #1
Naty1
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If photons don't have mass, why do their paths "bend" in a gravitational field?
This is question #8 in the FAQ and the answer provided is this:
I'm trying to understand the last sentence.
I must be having a mental block: why would thinking of "relativistic mass" cause identical geodesics? Is this statement saying light of all frequencies follows the same geodesics?
(edit: that part seems ok)
If a given region of spacetime has some initial curvature, then as photons of different energy (or relativistic mass) pass that region should they not cause a slight change in the curvature ...and I therefore the geodesics for photons of different energy would be slightly different...??
(edit: No they are not..had to think about this; so it's just the "relativistic mass" that's now confusing me.
This is question #8 in the FAQ and the answer provided is this:
I'm trying to understand the last sentence.
In general relativity, gravitation is a manifestation of the curvature of spacetime. The motion of all objects is affected by this curvature, regardless of whether they have mass or not. Light follows geodesic paths in spacetime, which are straight lines in flat spacetime, and curved paths in curved spacetime.
Note that by "mass" above I mean "invariant mass" as discussed in post #6 above, because it is the invariant mass that is zero for a photon. If you prefer to think in terms of "relativistic mass" (which is related to energy via E = mrelc2), note that all photons follow the same geodesics, regardless of their energy.
I must be having a mental block: why would thinking of "relativistic mass" cause identical geodesics? Is this statement saying light of all frequencies follows the same geodesics?
(edit: that part seems ok)
If a given region of spacetime has some initial curvature, then as photons of different energy (or relativistic mass) pass that region should they not cause a slight change in the curvature ...and I therefore the geodesics for photons of different energy would be slightly different...??
(edit: No they are not..had to think about this; so it's just the "relativistic mass" that's now confusing me.
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