# relativity and accelerating BBs

#### dseppala

Woody,
Are you saying that it is impossible for observers in F0 to build a device that uniformly accelerates each BB if the BBs start from different positions in the device? In the scenario, the accelerations of the two BBs are identical as measured in F0. Observers in F0 start the motion of both BBs simultaneously. So if you say that F0 observers don't always measure the same separation between the BBs, the BBs must be accelerating at different rates as measured in F0 since F0 observers started the acceleration of each BB simultaneously.
David Seppala
Bastrop TX

#### Woody

Strange, but that is what Relativity says.

Yes, that would have to be the case.

Objects that are accelerating identically relative to reference frame F1
(along the direction of relative difference of velocity of F1 and F0)
must be measured by an observer in F0 as accelerating at different rates.

Indeed if you measure any object that is accelerating (relative to you)
you will find that its length is changing (a tiny amount) due to the change in relative velocities.
So, as observed from your inertial frame, the front and the back of the object must be accelerating at different rates.

#### dseppala

Woody,
In this scenario, I stated that the two BBs are accelerating identically as measured in frame F0, not as measured in frame F1. So F0 starts the BBs to start their acceleration simultaneously, and they accelerate in the identical way as measured in F0, so they always keep a separation of 5 meters as measured in frame F0.
David Seppala
Bastrop TX

#### Woody

That is a different scenario

OK in that case, as you say, the observer travelling with the BBs
will see them accelerating at different rates such that they move together then apart again.

One way to interpret the relativistic contraction effect is as a "rotation" in 4Dimensional space-time.
An object moving relative to you is rotated slightly into the time dimension
(clocks will be observed to run slower)
This means that you are viewing it from an angle, so it appears shorter.

#### dseppala

Woody,
What I am trying to understand is how does the co-accelerating observer explain the BBs moving toward each other and then at a later time moving away from each other without observing any change of force on the BBs
David Seppala
Bastrop TX

#### Woody

You can't have it both ways.

If they are to maintain 5m separation as viewed from F0 then they will have to accelerate differently in frame FA.
This does imply that the forces on them, as seen by the co-accelerating observer would not be identical.

You can't have it both ways,
either they have identical acceleration and stay at a constant separation as observed by the co-accelerating observer,
in which case the separation viewed by the inertial observer (F0) will change over time.

Or if they remain at a constant separation as viewed from F0, then the separation viewed by the co-accelerating observer must change,
which means that they must have different forces exerted on them to accelerate at different rates.

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1 person

#### dseppala

Yes, so why doesn't a co-accelerating observer feel any change in force if the separation is constant as measured in F0?
David Seppala
Bastrop TX

#### Woody

Who says he doesnt

1 person

#### dseppala

Woody,
Every inertial reference frame says the force is always constant and in a constant direction. They say the acceleration never changes. Why do you think a co-accelerating observer in this force field feels a change in direction of the force?
David Seppala
Bastrop TX

#### Woody

This is the source of the argument.

Since we are viewing distance distortions in frames moving at different velocities relative to our own,
We must, as a consequence, be seeing distortions of the forces, accelerations, and any other parameter which corresponds to this distance.

Note however that the distance distortion is not the only one,
there is also a related time distortion.

So the accelerations, forces etc. in the (relatively) moving frame will be affected by both time and distance distortions.

2 people