Relative Dead Stop in Deep Space

[bland]

TL;DR Summary

A bit of confusion about cessation of acceleration being taken as a dead stop, without any momentum effects.

So there's two spaceships in deep space. at rest with respect to each other. Then one of them shoots off at some huge speed and everyone feels it. Then they shut the engines off suddenly. No one is going to suddenly lurch forward, they will at the instant the engines are shut off effectively be at rest with respect to the other spaceship that they were at rest with initially.

Do you see the problem here, do you say 'well we know they were moving so now they must be coasting' with respect to the other ship. But if suddenly someone was teleported to the ship as soon as the engines were shut off, and if someone told him they were just accelerating etc, he would just say, as far as I'm concerned you're stationary, I can't tell, why should I believe you.

The problem I'm having is working out how they were able to come to a dead stop. I mean if it was a dead stop the occupants momentum should carry them forward. Yes somehow they are still at rest when the engines are shut down.

 

[Ibix]

Summary: A bit of confusion about cessation of acceleration being taken as a dead stop, without any momentum effects.

Then they shut the engines off suddenly. No one is going to suddenly lurch forward, they will at the instant the engines are shut off effectively be at rest with respect to the other spaceship that they were at rest with initially.

No, they'll be traveling at some huge speed with respect to the other spaceship. The point is that they can regard themselves as initially at rest, and then accelerating to a huge speed, or initially moving at a huge speed and decelerating to rest. Neither view is better than the other.

 

[bland]

You've just broken my brain. I've spent 5 minutes writing and rewriting my reply. I think you may have misunderstood me.

Let me try and put it another way.
Ok there's two spaceships coasting away from each other at a constant speed.
Now either one can call themselves at rest. Nothing to see here.
This is the point that the observer now teleports onto the ship, right when they've shut their engines down and stopped accelerating, but the observer doesn't know this.
Is this right so far?

Now the observer knowing that both ships are coasting away from each other decides to call his ship stationary for the purposes of measuring the other ships speed. Now there's nothing wrong with this is there, I mean he really is stationary with respect to himself in the same way that I really am not moving with respect to the room I'm now in.

BUT one second before the observer beamed aboard, the ship was accelerating. And they shut the engines off just as he came aboard. So as far as the other occupants of the ship are concerned they definitely are coasting at high speed with respect to the other ship. Or are they. If they are then how are they going to prove this to the observer.

 

[Ibix]

Both are coasting at high speed with respect to each other. Either can consider themself at rest and the other as moving, or themself as moving and the other at rest, or both as moving. There's no experiment that can tell you if one view is correct or another - all are equally valid.

After acceleration you know only that you changed speed. But since there's no absolute truth to the claim that the ship was stationary (or not stationary) before accelerating, there's no absolute truth to the claim that the ship is not stationary (or is stationary) afterwards.

Speed is always measured relative to something (typically the surface of the Earth in our everyday lives). But there is no absolute sense of "stationary", so you can always pick something else as the thing that you say is stationary and define your speed with respect to that.

 

[A.T.]

So as far as the other occupants of the ship are concerned they definitely are coasting at high speed with respect to the other ship.

Everyone agrees that they move with respect to the other ship. But that is the movement in the rest frame of the other ship, not in their own rest frame.

 

[Mister T]

Ok there's two spaceships coasting away from each other at a constant speed.
Now either one can call themselves at rest. Nothing to see here.
This is the point that the observer now teleports onto the ship, right when they've shut their engines down and stopped accelerating, but the observer doesn't know this.
Is this right so far?

No. I don't understand how they can stop accelerating if they are moving at a constant velocity. Their acceleration was zero all along and the engines were off that entire time.

By the way, the phrase "dead stop" means nothing without a reference to something else. Like, a ship on the sea can come to a dead stop when it stops moving relative to the sea. But a ship in space can't do that because there is no such thing as being at rest relative to space.

 

[bland]

It's OK, I see where I went wrong. Yes 'dead stop' was mentioned with reference to the other ship. Somehow I had this idea in mind that when they stop accelerating they continue going forward. I'm not sure how I mixed that up, but I visualised the acceleration they'd feel as if they were standing on the Earth then the Earth was suddenly removed, of course they'd then just be floating in space. Sorry to bother anyone.

 

[anorlunda]

You don't need high speeds or teleportation or space to do this. Just think of two boats on a lake going different directions, different speeds. and a person stepping from one to the other as they pass close. We had a PF thread on that in the past.

https://www.physicsforums.com/threads/conservation-of-momentum-of-two-boats.791732/#post-4972782

 

[Mister T]

Somehow I had this idea in mind that when they stop accelerating they continue going forward.

If by accelerating you mean speeding up, then of course they would continue to move after the acceleration stops. That's how engineers navigate space ships through our solar system.