- #1
oldman
- 633
- 5
In the cosmology forum I have admitted to understanding GR only dimly despite, in a previous thread in this forum ("Metrics and Forces"), having been educated by Pervect to a slightly improved understanding. Here I would like to continue this education.
Consider a big observer inside an opaque box who is falling freely, in two extreme situations. First, let him be falling towards a nearby single massive object (say a neutron star). He will be unaware of gravity qua gravity since he is falling freely.
But he will be very aware of tidal forces, since he is big. He'll find himself settling into one orientation (head or heels toward the star, squashed sideways, stretched longways, feeling somewhat as if he were rotating head over heels at the orbital frequency appropriate to his distance from the star).
A General Relativist may explain to him that he is feeling dizzy because spacetime's spatial sections are curved by a nearby mass.
Consider a second extreme situation. Let the observer be surviving somehow in a flat inflating universe, where the Hubble constant is increasing exponentially fast. Here again he will experience strong tidal forces (as Pervect taught me to appreciate); in this case a symmetric dilating disruption.
Here the General Relativist can't attribute the disruption to curved space sections, because the inflating universe is flat.
Will he in this case invoke a curved time section?
Does this tell us that time sections would on a universal scale be almost flat? in a universe where H is almost constant: one that is hardly decelerating because of gravity, or accelerating due to dark energy, because the amount of mass/energy is well below the critical density, and where the Hubble flow is almost pure kinematic or coasting motion?
If so, one could invert this reasoning, and say that gravity and/or the antigravity of dark energy (both of which are attributed to the influence on the geometry of spacetime of mass/energy --- by means quite unknown) are simply manifestations of a curved time dimension. Too simple, I guess.
Consider a big observer inside an opaque box who is falling freely, in two extreme situations. First, let him be falling towards a nearby single massive object (say a neutron star). He will be unaware of gravity qua gravity since he is falling freely.
But he will be very aware of tidal forces, since he is big. He'll find himself settling into one orientation (head or heels toward the star, squashed sideways, stretched longways, feeling somewhat as if he were rotating head over heels at the orbital frequency appropriate to his distance from the star).
A General Relativist may explain to him that he is feeling dizzy because spacetime's spatial sections are curved by a nearby mass.
Consider a second extreme situation. Let the observer be surviving somehow in a flat inflating universe, where the Hubble constant is increasing exponentially fast. Here again he will experience strong tidal forces (as Pervect taught me to appreciate); in this case a symmetric dilating disruption.
Here the General Relativist can't attribute the disruption to curved space sections, because the inflating universe is flat.
Will he in this case invoke a curved time section?
Does this tell us that time sections would on a universal scale be almost flat? in a universe where H is almost constant: one that is hardly decelerating because of gravity, or accelerating due to dark energy, because the amount of mass/energy is well below the critical density, and where the Hubble flow is almost pure kinematic or coasting motion?
If so, one could invert this reasoning, and say that gravity and/or the antigravity of dark energy (both of which are attributed to the influence on the geometry of spacetime of mass/energy --- by means quite unknown) are simply manifestations of a curved time dimension. Too simple, I guess.
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