Can anyone help me understand the Equivalence Principle

[LSMOG]

Equivalence principles explains why lite bend towards massive objects, Einstein uses a moving lift to illustrate this, the light will seem to be bending if the lift is moving, but for a stationary lift, it will not because the position it strikes is stationary. So I think it is not correct to use the lift as Einstein did to explain the bending of light.

 

[jbriggs444]

if the lift is moving

The [inertial] movement of the lift is irrelevant. Its acceleration is what matters.

 

[Dale]

So the equivalence principle says roughly that any experiment performed in a lift which is accelerating at 1 g in the absence of gravity will have the exact same result as an experiment performed at rest in the presence of a 1 g gravity field.

In this case, we can easily calculate the deflection of a beam of light in a lift accelerating at 1 g in the absence of gravity. Therefore, we also know the deflection of a beam of light in a lift at rest in a 1 g gravitational field.

 

[LSMOG]

So the equivalence principle says roughly that any experiment performed in a lift which is accelerating at 1 g in the absence of gravity will have the exact same result as an experiment performed at rest in the presence of a 1 g gravity field.

In this case, we can easily calculate the deflection of a beam of light in a lift accelerating at 1 g in the absence of gravity. Therefore, we also know the deflection of a beam of light in a lift at rest in a 1 g gravitational field.

In these cases, is the light source assumed to be at rest

 

[Dale]

In these cases, is the light source assumed to be at rest

With respect to the lift, yes.

You could certainly revise it to use a light source moving relative to the lift

 

[LSMOG]

With respect to the lift, yes.

You could certainly revise it to use a light source moving relative to the lift

Okay. Thanks. I thought the reason for light to look bending is because the life is moving because as the lift moves, the light touches different positions on a lift. If it is stationary, I don't understand why the light should bend

 

[Ibix]

Okay. Thanks. I thought the reason for light to look bending is because the life is moving because as the lift moves, the light touches different positions on a lift. If it is stationary, I don't understand why the light should bend

The reason that the light's path appears curved in an accelerating lift is the acceleration.

If the lift isn't accelerating then the light moves straight across the lift. If you regard the lift as in motion at constant speed then you will find that the light is moving diagonally so that its upwards speed is the same as the lift.

However, if the lift is accelerating the light has no reason to change direction. So, as measured by observers attached to the walls of the lift, it appears to curve downwards.

 

[LSMOG]

Now the light from the star has no reason to bend towards the Earth because it is no in the acceleration lift

 

[jbriggs444]

Now the light from the star has no reason to bend towards the Earth because it is no in the acceleration lift

If one accepts the equivalence principle then the [local] behavior of light near the Earth is is approximated by considering the surface of the Earth [and anything a fixed distance from the surface of the Earth] as if it were indeed in a lift accelerating upward.

If an apple drops from a tree, the relevant picture is that the apple is stationary while the ground and tree accelerate upward.

 

[Ibix]

Just to add to what @jbriggs444 said, the key insight from the equivalence principle is that the "natural motion" (Edit: perhaps "un-forced motion" is a slightly less Aristotelian way of phrasing that ) is free-fall, whether that is a straight line in deep space, or a tight circle around a black hole, or a ballistic arc. Sitting on the ground is not free-fall - there is an upwards force on you from the ground, just as there is in the accelerating lift. So, from a local perspective, light appears to follow a curved path because you aren't free-falling but it is.

This is the key conceptual difference from Newtonian physics. In Newtonian physics things move in straight lines unless acted on by a force. In relativity things follow paths called "geodesics" unless acted on by a force. A geodesic is the generalisation of a straight line to curved spacetime. Their spatial part may be curved near a source of gravity.

 

[Dale]

If it is stationary, I don't understand why the light should bend

Because of gravity.