[Kryptid]

Ok, when the elevator accelerates forward, basic question is what makes the light ray to accelerate downwards.

The fact that you are accelerating upwards is what makes the light ray look like it is accelerating downwards. It really is just that simple.

[pasala (OP)]

Ok, when the elevator accelerates forward, basic question is what makes the light ray to accelerate downwards.

The fact that you are accelerating upwards is what makes the light ray look like it is accelerating downwards. It really is just that simple.

Based on these thought experiments, Einstein deducted that Gravity bends light.  Whether it  is simple or big, as per Einstein there is bending of light. 

What I am saying, if it is gravity, Einstein did not take up this one, to bend light it must move down, isn't it?.  When the elevator accelerates forward, frame of the elevator, literally, lifts the gravity field and gravity concentration at the bottom increases. 

So, Finally we can draw a conclusion that Gravity is a field present on Earth.

[Colin2B]

When the elevator accelerates forward, frame of the elevator, literally, lifts the gravity field and gravity concentration at the bottom increases. 

So, Finally we can draw a conclusion that Gravity is a field present on Earth.

No, the gravity field is not lifted nor does the gravity concentration increase, you can demonstrate that in a rocket where the field strength decreases with height.
What you are describing here does not show that a gravity field exists on earth.

Let's modify slightly the scenario and place the source of light and a sesnor in a stationary frame (e.g. on the 1st floor), while removing the side walls in that elevator - so that the light will pass through the elevator, which accelerates upwards and reach the stationary sensor on the opposite side:

The whole point of Galileo’s ship and Einstein’s elevator experiments is that they are closed systems without information from outside. It doesn’t matter whether the source or sensor are outside, you are using a different reference frame to the elevator.
The elevator needs to be either in a gravitational field and not moving or in no gravitational field and accelerating; in both cases the light beam will bend. Unfortunately, @pasala is mixing the 2 cases and causing himself confusion.

[Kryptid]

When the elevator accelerates forward, frame of the elevator, literally, lifts the gravity field and gravity concentration at the bottom increases.

There is no gravity in the elevator (at least not beyond the extraordinarily weak field produced by the elevator's mass). You are misunderstanding the equivalence principle.

[Zer0]

@pasala



Thanks for your Response.
🙏

A few points i wish to clarify with you...

1) Gravity is a force field present on Earth.
Only on Earth?
Moon as well Right?
All Mass has a Gravity field?
(I'm saying Yes to all Above)
🌚

2) Acceleration can mimic or say give identical effects as a Gravity field.
Correct?
(Yes)
⚖️

3) We can Pack gravity inside a Rocket & take it elsewhere...
Hmm?
(Noo)
How could we fold & unfold Gravity?
🚀

4) An Elevator needs to be Accelerating at 9.8 m/s/s.
That is a Tremendous rate of change of Velocity, Right?
(Ofcourse)
🦽
(& I'm also Wondering at this point, just what was with Einstein & Elevators man! )
🤔

5) For the sake of a different flavour, let's Imagine an Apple falling of a tree.

5(a) Now...
Gravity of Earth would attract Apple towards itself.
(Yes)
🌏
Gravity of Apple would attract Earth towards itself.
(Yes)
🍏

5(b) How about Visualising this scenario in terms of Geodesics & SpaceTime.
🌈

Apple breaks/snaps away from the tree & stays Stationary.
(Yes)
⏸️

It follows Geodesic path of the fabric of Space which is Curved.
(Yes)
💃

Einstein said
 " SpaceTime is Curved "...So...
Apple isn't just moving thru Curved Space but also thru Time.
Since the moment the Apple snapped off of the Tree...Time is Ticking...hence it is Stationary in position, still the Position itself is what's moving & evolving & bringing it closer to the surface of the Earth.
(Yes!)
⏳

     The Concept/Supposition that a Gravitational Field could be Dense or Light seems rather weird.
🤔
Are there any observable regions in the Universe where We could visually see this happening?
(No)
😑
Perhaps not until You bring in Dark Matter into the conversation.
(Yes)
👻




P.S. - Dark Matter is Not Dark, rather Transparent.
& It Ain't Matter!
✌️

[pasala (OP)]

When the elevator accelerates forward, frame of the elevator, literally, lifts the gravity field and gravity concentration at the bottom increases.

There is no gravity in the elevator (at least not beyond the extraordinarily weak field produced by the elevator's mass). You are misunderstanding the equivalence principle.

You are continuously saying that there is no gravity or simple inside the elevator.  Please remember it is not mine idea, but Einstein thought experiment only.  I think you are contradicting Einstein thought experiment.

As said by Einstein, if gravity is bending light, how and where it is coming.  In normal course  when the elevator is at rest on the ground there is strong gravity.  But it is not bending light.  It  is only after acceleration, please remember it is also as per Einstein, that gravity is bending light. 

It is surprise to see, how Einstein deducted that Gravity is bending light.  There is Gravity, when the elevator is at rest on the ground and also when it accelerates, but how it bends light, only when the elevator accelerates. As per Einstein, Gravity is bending light, if it is so, what changes are taking place in gravity when the elevator accelerates. 

When the elevator accelerates forward, I am proposing that there are changes in the gravity field within elevator and it is bending light and  it is  just filling gap of Einstein thought experiment.

[Kryptid]

You are continuously saying that there is no gravity or simple inside the elevator.  Please remember it is not mine idea, but Einstein thought experiment only.  I think you are contradicting Einstein thought experiment.

No, Einstein did not say that there is gravity inside of an accelerating elevator. What he said is that you can't tell whether the force you feel inside of the elevator is due to it accelerating upwards or from gravity (assuming you have no way to make observations outside of the elevator). He did not say that these two things are identical. Again, you are misunderstanding the equivalence principle.

but how it bends light, only when the elevator accelerates.

I already answered that.

[pasala (OP)]

When the elevator accelerates forward, frame of the elevator, literally, lifts the gravity field and gravity concentration at the bottom increases. 

So, Finally we can draw a conclusion that Gravity is a field present on Earth.

No, the gravity field is not lifted nor does the gravity concentration increase, you can demonstrate that in a rocket where the field strength decreases with height.
What you are describing here does not show that a gravity field exists on earth.

Let's modify slightly the scenario and place the source of light and a sesnor in a stationary frame (e.g. on the 1st floor), while removing the side walls in that elevator - so that the light will pass through the elevator, which accelerates upwards and reach the stationary sensor on the opposite side:

The whole point of Galileo’s ship and Einstein’s elevator experiments is that they are closed systems without information from outside. It doesn’t matter whether the source or sensor are outside, you are using a different reference frame to the elevator.
The elevator needs to be either in a gravitational field and not moving or in no gravitational field and accelerating; in both cases the light beam will bend. Unfortunately, @pasala is mixing the 2 cases and causing himself confusion.

Unfortunately, it is yourself confusing members. 
You are saying, Light beam will bend:
"The elevator needs to be either in a gravitational field and not moving or in no gravitational field and accelerating; in both cases the light beam will bend".

You are proposing that in a gravitational field, if the elevator is at rest then only light beam will bend.  If that is true, what is there need for Einstein thought experiments.  Einstein thought experiments propose that if the elevator attains acceleration, equal to acceleration light beam will bend, in case, if it attains velocity, equal to  velocity light beam will bend in opposite direction.

Further you have also proposed that in no gravitational field, acceleration bends light.  I don't think there is any such proposal by Einstein.  I think you are equating acceleration to gravity.  Ok, if that is true, if a space ship accelerates from Moon or from a space station, do acceleration equates gravity.

Well, Galileo's equivalence is different and it is in between, closed room of a ship to Earth.  Please go through it.  No where, much focus is laid on this equivalence.  What Galileo says, in a closed room of a ship, if the sailing is smooth, he could not differentiate whether he is in a ship sailing or on Earth.  It clearly tells us that Earth is closed. 

This is the core idea behind relativity, and is the same reason why we don’t feel our planet’s movement around the sun, or our solar system’s movement through the galaxy.

Yet we don't accept that Earth is a closed one.  If we say that doors and windows are not appearing, it is our incapacity and inefficiency to identify it.

Einstein equivalence is simply a comparison, saying that there is also equivalence. 

[pasala (OP)]

You are continuously saying that there is no gravity or simple inside the elevator.  Please remember it is not mine idea, but Einstein thought experiment only.  I think you are contradicting Einstein thought experiment.

No, Einstein did not say that there is gravity inside of an accelerating elevator. What he said is that you can't tell whether the force you feel inside of the elevator is due to it accelerating upwards or from gravity (assuming you have no way to make observations outside of the elevator). He did not say that these two things are identical. Again, you are misunderstanding the equivalence principle.

but how it bends light, only when the elevator accelerates.

I already answered that.

Ok, then how Einstein deducted that Gravity bends light.

[Kryptid]

Ok, then how Einstein deducted that Gravity bends light.

It follows from the equivalence principle that light's behavior in a gravitational field is indistinguishable from light's behavior from the point of view of an observer inside of an accelerating elevator. That does not mean that gravity and acceleration are literally the same thing, only that they have the same effects in certain scenarios.

[CrazyScientist]

The whole point of Galileo’s ship and Einstein’s elevator experiments is that they are closed systems without information from outside. It doesn’t matter whether the source or sensor are outside, you are using a different reference frame to the elevator.
The elevator needs to be either in a gravitational field and not moving or in no gravitational field and accelerating; in both cases the light beam will bend. Unfortunately, @pasala is mixing the 2 cases and causing himself confusion.

Thanks! That's what I wanted to hear. I admit, that I might also cause some confusion with my sccenario, as it has more to do with SRT than GRT. What I had in mind is the idea that motion path of light will curve in the stationary frame of light source just due to placing the sensor inside a moving frame (elevator), while curving in the frame of moving elevator only due to placing the sensor in the same frame as the light source. It has more to do with my model of constant c in relative motion than with the subject discussed in this thread, so sorry for causing confusion...

[Colin2B]

Thanks! That's what I wanted to hear. I admit, that I might also cause some confusion with my sccenario, as it has more to do with SRT than GRT. What I had in mind is the idea that motion path of light will curve in the stationary frame of light source just due to placing the sensor inside a moving frame (elevator), while curving in the frame of moving elevator only due to placing the sensor in the same frame as the light source. It has more to do with my model of constant c in relative motion than with the subject discussed in this thread, so sorry for causing confusion...

I’d prefer not to confuse op further by going through this, but when I’ve finished you will see what an outside observer will see in both situations.

[Colin2B]

You are proposing that in a gravitational field, if the elevator is at rest then only light beam will bend.  If that is true, what is there need for Einstein thought experiments.

 
The need for the thought experiment is to show that acceleration and gravity are equivalent. This can be shown by comparing the case above with an elevator in no gravitational field, but accelerating. The light beam bends in both cases, they are equivalent.

Einstein thought experiments propose that if the elevator attains acceleration, equal to acceleration light beam will bend,

i think you missed out the words due to gravity so it should read

“if the elevator attains acceleration, equal to acceleration due to gravity light beam will bend”.
In that case, yes this is correct.

in case, if it attains velocity, equal to  velocity light beam will bend in opposite direction.

not so, don’t know where you got that from. Show your source. It has nothing to do with velocity, just acceleration.

Further you have also proposed that in no gravitational field, acceleration bends light.  I don't think there is any such proposal by Einstein. 

Yes, that is the proposal from Einstein.

I think you are equating acceleration to gravity. 

YES, YES, YES. but not equating as in they are the same thing, but that the effects of both are equivalent.

Ok, if that is true, if a space ship accelerates from Moon or from a space station, do acceleration equates gravity.

If you are in a spaceship travelling at constant velocity you will feel weightless. If your pilot then accelerates at 9.8m/s² then you will feel a force pushing you against the back of your seat equivalent to the force of gravity. That force will be equivalent to your weight on earth. Yes, the acceleration is equivalent to gravity.

Forget light for a moment and consider a tennis ball.
Spaceman in no gravitational field (or freefall) feels weightless. If they push a tennis ball away from them it will go in a straight line to the opposite wall (as would light).
Now the spaceship starts to accelerate, the ‘bottom’ wall of the ship moves up and pushes against the feet of spaceman so he has to use leg muscles to stand; if he now pushes the tennis ball away it will fall towards his feet, exactly the same as if he were standing on earth in a gravitational field. Acceleration and gravity are equivalent.

[CrazyScientist]

Acceleration and gravity are equivalent.

There are some differences: acceleration of a frame doesn't curve the space time in that frame - however it curves the worldline of an accelerating object. But it's kinda complicated subject...
https://physics.stackexchange.com/questions/197546/does-acceleration-warp-space

[Colin2B]

Acceleration and gravity are equivalent.

There are some differences: acceleration of a frame doesn't curve the space time in that frame - however it curves the worldline of an accelerating object. But it's kinda complicated subject...

It’s not really complicated, but it is why the effects are equivalent rather than acceleration and gravity being equal.
Worth noting that Einstein was assuming a uniform gravitational field so tidal forces are not an issue. Point source fields are only locally uniform at a fair distance from eg centre of planet, so an elevator on surface of earth is ok.

[Janus]

I know, that it doesn't have too much with the gravity, but since we're talking about a scenario with the elevator, I wonder what would happen if we would modify it and place the sensor inside the moving elevator, while keeping the source of light in stationary frame. Let's say, that light is being emitted by the stationary source in the moment, when it is at the same level, as the sensor placed inside the elevator moving upwards - will the light reach that sensor or not? If it won't reach it, then the motion of elevator will become absolute/definitive, what will violate the relative nature of relative motion. If it will reach the sensor, then path of light will become curved upwards in the frame of stationary source. Which option is the valid one?

Let's, for the moment, exclude the acceleration of the Elevator, and just imagine the path of the light if the Elevator and external light source have a relative motion with respect to each other. Light entering the hole from the source will not hit the sensor opposite the hole.  However, you cannot conclude from this that there is absolute motion on the part of the Elevator.  This is due the the aberration of light, which is caused by the Relative motion between source and Elevator.  It happens exactly the same whether you consider the Elevator moving and the source stationary or the Elevator stationary and the Source moving.  Thus it can only tell you that they are moving relative to each other, and not which one is "really" moving.
If you add the acceleration of the Elevator back in, it's effect will be compounded on top of the aberration. The light beam's path will still curve( as measured from the Elevator), but the angle at which it initially enters the hole will depend of the relative velocity difference between the Source ( at the moment of emission) and the Elevator( at the moment of entering the hole.)

[CrazyScientist]

I know, that it doesn't have too much with the gravity, but since we're talking about a scenario with the elevator, I wonder what would happen if we would modify it and place the sensor inside the moving elevator, while keeping the source of light in stationary frame. Let's say, that light is being emitted by the stationary source in the moment, when it is at the same level, as the sensor placed inside the elevator moving upwards - will the light reach that sensor or not? If it won't reach it, then the motion of elevator will become absolute/definitive, what will violate the relative nature of relative motion. If it will reach the sensor, then path of light will become curved upwards in the frame of stationary source. Which option is the valid one?

Let's, for the moment, exclude the acceleration of the Elevator, and just imagine the path of the light if the Elevator and external light source have a relative motion with respect to each other. Light entering the hole from the source will not hit the sensor opposite the hole.  However, you cannot conclude from this that there is absolute motion on the part of the Elevator.  This is due the the aberration of light, which is caused by the Relative motion between source and Elevator.  It happens exactly the same whether you consider the Elevator moving and the source stationary or the Elevator stationary and the Source moving.  Thus it can only tell you that they are moving relative to each other, and not which one is "really" moving.
If you add the acceleration of the Elevator back in, it's effect will be compounded on top of the aberration. The light beam's path will still curve( as measured from the Elevator), but the angle at which it initially enters the hole will depend of the relative velocity difference between the Source ( at the moment of emission) and the Elevator( at the moment of entering the hole.)

Thanks! I like that you found the connection to aberration of light - that's exactly what I had in mind However in the scenario which I proposed, there are no holes in the side walls of the elevator. In the original form of my scenario there are no side walls at all - light is passing through the accelerating elevator and reaches a stationary sensor on the opposite side, what results in a curved path of light in the frame of that elevator:

Now let's put back one side wall in the elevator (the one which is placed further from the source) and place the sensor in the middle of that wall. If we keep in mind the fact that the path of light emitted by a moving source is not influenced by the motion of it's source in the frame of a stationary observer, then light emitted in the moment when the source and the sensor are placed on the same level, will reach that sensor despite the relative motion of both frames - just like on the image below (showing the frame of elevator):

However this would mean that path of light becomes curved upwards in the frame of stationary source.

I have also an issue with the animation from wikipedia: https://en.wikipedia.org/wiki/Aberration_(astronomy)

According to it light behaves just like any other physical object (e.g a bullet) and the perpendicular motion of the light source is being added to the vector of light propagation - so motion of the light beam is a sum of 2 perpendicular vectors. However as I said earlier motion of the light source shouldn't affect the motion path of light in stationary frame.

It's an interesting subject, but I'm not sure if this is the right thread to discuss it. I think that my own thread would be a better place - sadly it was closed by Halc some time ago...

[charles1948]

All the above posts about "What is Gravity", make me wonder whether a similar question could be asked, such as:

What is the "Strong Nuclear Force".

This "force", as far as I understand it, makes protons in the nucleus of an atom gather together.  Despite the protons' mutual positive charges.  Which ought electrically to repel them from each other, and make them fly apart.

This "Strong " force seems to be accepted as a "Fundamental Force of Nature".  Without needing an explanation.

If this is so, can't the "Gravitational"  force , which makes atoms gather together, also be accepted as a "Fundamental Force of Nature".   Without needing an explanation?

Why can't we  treat "Gravity" like the "Strong Force" -  ie, as just the way things are in the Universe?

[CrazyScientist]

All the above posts about "What is Gravity", make me wonder whether a similar question could be asked, such as:

What is the "Strong Nuclear Force".

This "force", as far as I understand it, makes protons in the nucleus of an atom gather together.  Despite the protons' mutual positive charges.  Which ought electrically to repel them from each other, and make them fly apart.

This "Strong " force seems to be accepted as a "Fundamental Force of Nature".  Without needing an explanation.

If this is so, can't the "Gravitational"  force , which makes atoms gather together, also be accepted as a "Fundamental Force of Nature".   Without needing an explanation?

Why can't we  treat "Gravity" like the "Strong Force" -  ie, as just the way things are in the Universe?

I think that the main purpose of science is to understand the mechanics of reality in which we exist. It's not that strong force doesn't need to be explained - it's just the science, which is unable to explain it at this time, so there's no other choice than to accept it's existance without understanding it's nature.

BTW atoms in molecules stick together due to electrostatic force rather than graviational one...

[Janus]

I have also an issue with the animation from wikipedia: https://en.wikipedia.org/wiki/Aberration_(astronomy)

According to it light behaves just like any other physical object (e.g a bullet) and the perpendicular motion of the light source is being added to the vector of light propagation - so motion of the light beam is a sum of 2 perpendicular vectors. However as I said earlier motion of the light source shouldn't affect the motion path of light in stationary frame.

I

I'm going to add a couple of animations of my own to try and clarify what is going on here:
First from the source frame:

abrrtn1.gif (61.86 kB . 576x576 - viewed 8153 times)
The pulse is emitted downward and the observer moves to the right to intercept it. The red line shows the pulse's relationship to the source during its trip.
Now from the observer frame:

abrrtn2.gif (109.42 kB . 576x576 - viewed 8161 times)
Note that in order for things to be physically consistent with the source frame, the pulse must at all times stay on the red line which extends downward from the source just like it did in the Source frame. In other words, the pulse must always be directly below the source.
This, means, the relative to the observer, it follows the Green line.  It isn't about the source "adding" any velocity to the pulse, it is about keeping the two frames physically consistent.*

*One caveat to this: in these animations I did not try to account for the fact that the light pulse must travel at c as measured from either frame.  In reality, the pulse takes longer to make the trip as measured in the observer frame, which means the source travels a greater distance relative to the observer during the trip, and would have to be a bit further away from the observer when the Pulse leaves, than what the Source frame would measure.  This, in turn has an effect on the exact angle of the Green line measured by the Source frame.

.