# Is acceleration absolute

1. Jan 10, 2005

### frb

is it, cause velocity isn't.

2. Jan 10, 2005

### dextercioby

Since coordinates,velocity and acceleration are ALL OF THEM CINEMATIC QUANTITIES,they need to be defined wrt a reference system.Therefore their character is RELATIVE AND NOT ABSOLUTE.

Daniel.

3. Jan 10, 2005

### kirovman

No, because if two objects accelerate towards each other you get the sum of their acceleration.

Same with forces, since F=ma.

For example ficticious forces centrifugal force, and centrifugal acceleration, are not considered in all reference frames.

4. Jan 10, 2005

### Andrew Mason

It depends on what you mean by absolute. It is not invariant under relativity, but it is a quantity which all observers in all reference frames would observe and be able to measure.

If Einstein's principle of equivalence is correct, since acceleration and gravitation are equivalent, acceleration must be observed in all reference frames and observers in all reference frames must agree.

Ignoring relativistic effects, observers in all inertial frames would agree on the measure of acceleration, even though they are travelling at different speeds. If they are travelling at relativistic speeds, they would have to apply the Lorentz transformations.

AM

5. Jan 10, 2005

### Andrew Mason

It depends on what movie you are watching?

AM

6. Jan 10, 2005

### Gokul43201

Staff Emeritus
I agree with AM.

You CAN determine acceleration absolutely, at least at low velocities. It is always possible to know if a frame is accelerating , using a simple pendulum or glass of water, or some such thing.

7. Jan 11, 2005

### ramollari

Let's see it from a Newtonian perspective (low speeds). There are two inertial frames of reference that observe a moving body. According to frame 1 it is v1 and according to frame 2 it will be v2. So velocities are relative. If frame 2 is moving at a speed v' relative to frame 1, then according to the classic addition of relative velocities, v2 = v1 + v'

then,

a2 = dv2/dt = d(v1 + v')/dt = dv1/dt + 0 = a1,

since v' is constant (inertial frames of reference).

If velocities are measured in 3 dimensions, just split to their components, and apply the same method. You will get the same x, y, and z component for acceleration.

8. Jan 11, 2005

### marlon

Acceleration is absolute in every theory before General Relativity. So, Newtonian physics (here, also speed is absolute), Special relativity (no absolute speeds, only absolute acceleration)

It is in General Relativity that acceleration is no longer absolute because every reference frame is equivalent.

regards
marlon

9. Jan 11, 2005

### Chronos

Hmmm, where to start. The difference between general and special relativity is general theory treats all reference frames, including rotating and accelerating frames as equal. Special relativity treats accelerating frames differently. In SR, acceleration is an absolute and velocity is relative. In GR, all motion is relative. GR requires curved spacetime to make this work. This idea is not the least bit exotic. Solving classical mechanics problem in curved coordinate systems [e.g., polar coordinates] is a common and fully accepted practice. It makes sense and is much easier to use to analyze and make concrete predictions.

I should point out that if you adopt the proper coordinate system in SR, it produces the same result as GR. GR restricts your choice of coordinate systems, SR does not. That leads to many perceived paradoxes. It is also explains why Einstein took 10 years to publish his work on GR. The concept was too vitally important for Einstein to present anything other than an air-tight case. He has done a pretty amazing job so far... 90 years later and no one has yet punched an irrefutable hole in that theoretical edifice.

10. Jan 11, 2005

### marcus

can an observer decide whether his or her frame of reference is rotating or not? can different observers agree?

11. Jan 11, 2005

### Rybo

Rotation is constant/absolute accelearation is not.

Marcus, yes to both questions, provided they have the skill and observational equipment to calulate the realtionship of two or more frames outside of their immediate/local frame of refereence.

All things are in motion(energetic) ergo all things are either accelerating(out) or decelerating(in) to one or more other things in Universe.

Rybo

12. Jan 11, 2005

### jdavel

marlon,

You said,

"Acceleration is absolute in every theory before General Relativity. So, Newtonian physics (here, also speed is absolute), Special relativity (no absolute speeds, only absolute acceleration)"

But unlike Newtonian physics, according to SR the magnitude of acceleration is different in different inertial frames.

13. Jan 12, 2005

### marlon

Well, the mere fact that you are talking about different inertial frames proves the fact that you need an absolute acceleration because how do you distinguish between an ordinary frame of reference and an inertial frame of reference.

Given the fact that all physical laws must be the same in each inertial frame i don't think i get your point. how do you mean acceleration is different???All physical laws (you know ma =...) are the same so ???

But nevertheless in SR acceleration is ABSOLUTE because of the very reason i gave above

regards
marlon

14. Jan 12, 2005

### ramollari

Why is angular velocity absolute and linear velocity relative? Isn't that a paradox? What makes a frame of reference inertial in terms of rotation is really a mistery to me.

15. Jan 12, 2005

### Andrew Mason

Actually it is not acceleration that is absolute. It is the rate of change of momentum. In SR, dp/dt is invariant and absolute.

AM

16. Jan 13, 2005

### jdavel

marlon,

Maybe use of the word "absolute" is causing our disagreement. Can we change to "invariant"? That's very clearly defined in the context of any relativity theory.

For example invariants in Galilean relativity include acceleration, force, mass, time intervals between events, etc. But in general, not velocity, momentum, kintetic energy, distance between events etc.

Invariants in SR include the speed of light (of course!), the interval between two events etc. And there are special cases where dv/dt and dp/dt are invariant, but in general they aren't.

17. Jan 13, 2005

### reilly

There's no way acceleration is anything but relative. It's the second deriv. of a distance, or coord, which necessarily refers to two points, and that's the source of the relative nature of spatial coordinates, and functions thereof.

Regards,
Reilly Atkinson

18. Jan 14, 2005

### salazar18

need to brush up on physics

it seems that this dialogue is lacking in depth of knowledge of momentum, angular momentum, time-space curvature, general relativity, and cyclical energy fluctuations

19. Jan 14, 2005

### Andrew Mason

Then perhaps you would care to enlighten us. Why is/is not dp/dt at least absolute and invariant?

AM

20. Jan 14, 2005

### Andrew Mason

I don't think that can be right. Although position is dependent upon a co-ordinate system, change in position and rate of change of position is not (at least to inertial observers where relative v<<c). Mass is not dependent upon the frame of reference either (if v<<c). To any inertial observer, (even those moving at speeds close to c) dp/dt is invariant and absolute under Newtonian/Galilean relativity and under relativity.

AM

21. Jan 15, 2005

### jdavel

Andrew,

"To any inertial observer, (even those moving at speeds close to c) dp/dt is invariant and absolute under Newtonian/Galilean relativity and under relativity."

That's not true! In general, a Lorentz transformation will change both the direction and magnitude of force. In fact force and the acceleration it causes can end up in mutually different directions after applying an LT!

Nature paid a huge price for insisting on there being one speed that would be the same in all reference frames! The price was that almost nothing else could be.

22. Jan 16, 2005

### reilly

Fred's watching me go down in a free falling elevator -- Albert Einstein Model # 32g.
Unless Fred is deranged, he will say that I'm moving down with acceleration g, and he's not.
Of course, I say something different. Fred's moving up with g, and I'm not accelerating.
That's about as relative as it gets.
regards,
Reilly Atkinson

23. Jan 16, 2005

### Andrew Mason

I don't follow you there. Force is defined as $\vec F = d\vec p/dt$. So it has to be in the direction of the momentum change, which means it has to be in the direction of the acceleration.

If dp/dt is not invariant under relativity then momentum cannot be conserved. Neither can energy because energy is related to momentum by:

$$E^2 - (pc)^2 = (mc^2)^2$$

If you can explain why it is not with an example, it might help. But right now, I don't see it.

AM

24. Jan 16, 2005

### Andrew Mason

I thought we were talking about inertial reference frames. Acceleration is relative under Newtonian mechanics if you use non-inertial frames of reference.

AM

25. Jan 17, 2005

### kawikdx225

An accelerometer mounted to you and Fred will determine which one of you is accelerating. You will measure that you are accelerating and Fred isn't.

A spaceship in deep space can also measure absolute acceleration using an accelerometer, acceleration can be relative but doesn't have to be.

A rotating object is also accelerating absolutly, just spin real fast in a circle and see what your arms tend to do.