# Thread: Is there a preferred reference frame?

1. Special relativity tells us that all inertial frames are equally valid. And when it comes to acceleration the acceleration is measured with respect to any inertial frame. My question is, that given at any point in space there is only one truely inertial frame (the one that is in free-fall under local gravity conditions), does this form a preferred frame of reference?

2. And when it comes to acceleration the acceleration is measured with respect to any inertial frame
No. Acceleration is not relative, it's absolute.

3. Mmmm, absolute with respect to what? You have to be accelerating with respect to something.

4. Acceleration takes place with respect to everything. That's what absolute means. SR does not deal with acceleration, that's the realm of GR.

5. Acceleration is with respect to EVERY inertial frame. However for any point in space there is ONLY ONE true inertial frame. That is the frame that has no gravity, the frame that is in free-fall.

6. Originally Posted by Jilan
(the one that is in free-fall under local gravity conditions)
This does not uniquely specify an inertial frame of reference. Consider two objects dropped from different heights. At a given height, both objects will be in free-fall, yet have different velocities.

7. Originally Posted by Jilan
Mmmm, absolute with respect to what? You have to be accelerating with respect to something.
You measure acceleration with an accelerometer. Its readings do not depend on the identification of a specific frame. If you are in a rocket and you throttle up, you will feel an acceleration, yes? Would your sensation change if the reference frame changed from Mars to Ceti Alpha V?

8. Originally Posted by AlexG
SR does not deal with acceleration, that's the realm of GR.
Actually, that isn't correct. SR is perfectly able to describe accelerated frames as well, it is just that those frames are not related via Lorentz transformations :

Acceleration in Special Relativity

9. Originally Posted by tk421
You measure acceleration with an accelerometer. Its readings do not depend on the identification of a specific frame. If you are in a rocket and you throttle up, you will feel an acceleration, yes? Would your sensation change if the reference frame changed from Mars to Ceti Alpha V?
At maximum thrust I guess it would feel just the same, but depending on the mass of Ceti Alpha you may not actually be going anywhere!

10. Originally Posted by Markus Hanke
Actually, that isn't correct. SR is perfectly able to describe accelerated frames as well, it is just that those frames are not related via Lorentz transformations :

Acceleration in Special Relativity
I like this. "In General Relativity all motion is relative"

11. Originally Posted by Jilan
At maximum thrust I guess it would feel just the same, but depending on the mass of Ceti Alpha you may not actually be going anywhere!
You miss the point entirely, it would seem (perhaps you're joking, but I can't separate that from a lack of understanding). Your accelerometer reading does not in any way depend on declaring that your reference frame is here or Ceti Alpha.

12. Originally Posted by Jilan
Special relativity tells us that all inertial frames are equally valid. And when it comes to acceleration the acceleration is measured with respect to any inertial frame. My question is, that given at any point in space there is only one truely inertial frame (the one that is in free-fall under local gravity conditions), does this form a preferred frame of reference?

For every point in space, there are a number of trajectories for freely falling particles that will pass through this point. All the freely falling frames that can be attached to these particles seem to be Lorentz transformations of each other at the given point at the same time.