- #1

peterraymond

- 13

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- TL;DR Summary
- A mental experiment that shows that gravity bends light

I'm hoping this is basic and obvious, but assume it's not to the general public.

Ignore quantum mechanics and diffraction and assume a gun that can fire photons that each hit the center of a remote target. Place one of these and a conventional gun that shoots bullets at 1000 m/sec inside of a black box. Set the target range for your photon gun at 3000 km and for your conventional gun at 10 m, so travel time for each is 10 msec.

In a box where an observer feels weightless align the guns to their targets. No particular positioning in space is needed and you must still get direct hits if the box is moving in any way where the observer feels weightless throughout the box, because each condition is identical to the observer. Shots will still stay on target if for instance the box is accelerating at 100 m/sec^2 as it falls straight towards a far off super massive black hole.

If at the moment the shots are fired rocket motors start accelerating the targets "upward" at 10 m/s^2, over 10 msec each target will have moved 0.5mm. Or, the whole box could suddenly start accelerating at 10 m/sec^2, or that acceleration could be continuous. In all these cases the observer inside the box would see a miss at both targets because acceleration has moved the targets out of position during the time of flight. Direct measurements from the accelerating walls of the box though would make both paths appear curved.

Identical for the observer is having the box sit "stationary" on the surface of a large body where acceleration due to gravity is 10 m/sec^2. The observer can't tell the difference and the photon and the bullet must both still miss their targets by 0.5mm. Both appear to "drop" 0.5 mm over 10 msec. In this case though, Newtonian mechanics would say that the bullet path drops because the bullet is attracted to the mass of the large body. For the photon there is no mass, no gravitational force and no Newtonian way to calculate the apparent curve in its path. Both paths would be measured to curve, but only because this space was curved by gravity. If this were not true, the observer would need to somehow see through the walls of the box and some measures of motion would be absolute, not relative.

Yes?

Ignore quantum mechanics and diffraction and assume a gun that can fire photons that each hit the center of a remote target. Place one of these and a conventional gun that shoots bullets at 1000 m/sec inside of a black box. Set the target range for your photon gun at 3000 km and for your conventional gun at 10 m, so travel time for each is 10 msec.

In a box where an observer feels weightless align the guns to their targets. No particular positioning in space is needed and you must still get direct hits if the box is moving in any way where the observer feels weightless throughout the box, because each condition is identical to the observer. Shots will still stay on target if for instance the box is accelerating at 100 m/sec^2 as it falls straight towards a far off super massive black hole.

If at the moment the shots are fired rocket motors start accelerating the targets "upward" at 10 m/s^2, over 10 msec each target will have moved 0.5mm. Or, the whole box could suddenly start accelerating at 10 m/sec^2, or that acceleration could be continuous. In all these cases the observer inside the box would see a miss at both targets because acceleration has moved the targets out of position during the time of flight. Direct measurements from the accelerating walls of the box though would make both paths appear curved.

Identical for the observer is having the box sit "stationary" on the surface of a large body where acceleration due to gravity is 10 m/sec^2. The observer can't tell the difference and the photon and the bullet must both still miss their targets by 0.5mm. Both appear to "drop" 0.5 mm over 10 msec. In this case though, Newtonian mechanics would say that the bullet path drops because the bullet is attracted to the mass of the large body. For the photon there is no mass, no gravitational force and no Newtonian way to calculate the apparent curve in its path. Both paths would be measured to curve, but only because this space was curved by gravity. If this were not true, the observer would need to somehow see through the walls of the box and some measures of motion would be absolute, not relative.

Yes?