[Halc (OP)]

you need to find a solution in where every point of that ship is in a same frame of reference relative the push of what engine you imagine, I don't see how that can be done myself.

That was my solution.  The entire ship is always stationary in its own frame at all times.
If you feel a need to limit acceleration, fine, but it will add to your minimum trip time.

[Bored chemist]

Can someone help me out here please?
I'm meant to be leading this "ship"- it's actually a flotilla of little ships.
All the pilots know that they can accelerate their craft at well defined rates and they all move at the same speed WRT the launch pad (which they left long enough ago that its local gravity isn't a factor)
 I know that I can hold the fleet together simply by making sure all the little bits of my ship accelerate at the same rate.
But someone is now saying that , in spite of being deliberately held together, it will fall apart.

He refuses to give a mechanism, but my crew are still starting to get jumpy.

What should I tell them?
Do I tell them to rely on common sense, or do I tell them that magic gremlins are pulling the ship apart?

(These are experienced spaceship pilots. they consider relativity to be common sense)

[David Cooper]

My method of moving the ship is quite simple.  You have a length L that is the distance between the starting point of the tail of the ship and the destination point of the nose of the ship.  Accelerate the tail as quickly as possible (instantly?) to whatever speed is required to contract L down to the length of the ship.  That brings the nose to its destination (or actually brings the destination to the nose).  Now we instantly stop the nose, which springs L back to its original length, bringing the tail to its final destination.  We're done.  The time it takes to do that is the same as the amount the clocks get out of sync between the nose and the tail.

What stops you accelerating it faster than that? If you can individually accelerate each atom to a tiny fraction below c and get the timing right, all of them can then move at that speed with a delay until the front of the ship is moving too, and then when you stop, the rear ones stop first, being decelerated to a speed that makes them fully happy to sit next to the atoms stopped around them - the length of the trip shouldn't limit you to slower speeds than that on shorter trips. With the right kind of launch and catch system, this could be done in such a way that nothing breaks despite the astronomical acceleration force because the arrangement of atoms isn't broken in any way - they are just momentarily the wrong distance apart, but that's put right again at a rate that propagates along the ship at a fraction below the speed of light.

[Toffo]

Can someone help me out here please?
I'm meant to be leading this "ship"- it's actually a flotilla of little ships.
All the pilots know that they can accelerate their craft at well defined rates and they all move at the same speed WRT the launch pad (which they left long enough ago that its local gravity isn't a factor)
 I know that I can hold the fleet together simply by making sure all the little bits of my ship accelerate at the same rate.
But someone is now saying that , in spite of being deliberately held together, it will fall apart.

He refuses to give a mechanism, but my crew are still starting to get jumpy.

What should I tell them?
Do I tell them to rely on common sense, or do I tell them that magic gremlins are pulling the ship apart?

(These are experienced spaceship pilots. they consider relativity to be common sense)

This is launchpad personnel's view:

If the readings of the accelerometers of the ships are the same, then the increasing time delay of light signals traveling towards the front causes a pilot to see a ship at the rear becoming increasingly retarded. Said pilot should see an elastic band connecting the ships stretching, otherwise there is a inconsistency: Pilot sees other ship falling behind, but does not see the connecting elastic band getting longer.


If on the other hand the observed accelerations observed by the pilots are the same, then the readings of the accelerometers are not the same.




Oh yes, it was the pilots' view that was requested. Well it's that everything happens slower on ships closer to the rear. Whatever happens inside a rocket motor happens slower, whatever happens inside an accelerometer happens slower. Slowed down accelerometer measures slowed down motor to accelerate the ship 'normally'. 

[Halc (OP)]

What stops you accelerating it faster than that?

I need to stop when I get there.  I accelerate the tail just enough to contract the distance I need to travel to be exactly even with the nose of the ship.  If it goes any faster, the nose overshoots the place where it needs to stop.
Keep in mind that during the acceleration phase, the tail of the ship never moves.  If I accelerate over finite time, then yes, the tail moves.

If you can individually accelerate each atom to a tiny fraction below c and get the timing right, all of them can then move at that speed with a delay until the front of the ship is moving too,

That makes parts of the ship not stationary, and other parts stationary.  It can't take that (right ?!).  Perhaps it is a mathematical stipulation that the ship always be stationary in its own frame.

I think I see what you're envisioning, sort of compressing the ship at light speed like a slinky.  Any two parts of the ship going a different velocity are always separated in a space-like manner, never a time-like manner, so the discrepancy cannot cause breakage.  It violates my mathematical stipulation, but can a brittle ship take that?  I cannot identify what would break.  What the tail is doing at event X is of no concern to parts of the ship outside X's light cone.

[Halc (OP)]

If on the other hand the observed accelerations observed by the pilots are the same, then the readings of the accelerometers are not the same.

What are the pilots reading if not the accelerometers?  How do they otherwise decide that they're the same?

Second note is, same as what? Identical to the value measured on other ships, or just identical from moment to moment?  Nobody seems to be proposing that a particular ship vary its acceleration during the process (except David just now), but it isn't off the table either.

An accelerometer on a ship will measure proper acceleration.  Not sure what meter the lauchpad guy is reading, but that one will read acceleration in his frame, not the ship's proper acceleration.  The former falls off as speed grows, while proper acceleration should be constant for the duration.

[Halc (OP)]

With the right kind of launch and catch system, this could be done in such a way that nothing breaks despite the astronomical acceleration force because the arrangement of atoms isn't broken in any way - they are just momentarily the wrong distance apart, but that's put right again at a rate that propagates along the ship at a fraction below the speed of light.

This is actually a cool idea.  The example of my 100 LY ship moving only one light hour would have the tail start and stop long before the nose ever moves.  The wave of movement would propagate up to the front (at what rate?).
The thing would move exactly like a caterpillar.
Would that be faster?  My ship took 55 days (see post 37), but a 100 LY ship would seem to need more time than that for the 'wave' to reach the front.

It seems not to violate the brittle-ship thing.  There is a point in the ship where all the matter to the rear is moving nearly at c, but the stuff in the other direction is stopped.  If that persisted for even a moment, it would shatter, but it doesn't.  The wave passes before any stress/strain can build up.  Still, the wave would seem to propagate at 2c at best.  It would take 50 years to move my big ship that way, not just 55 days.
For slower speeds (well under c), the wave propagates faster than 2c.  We need to find a sweet spot that balances wave speed with physical speed.  It would depend on the ratio of ship length to trip length it seems.  My method I suspect is independent of trip length, at least for short trips.  I think it takes 55 days to go any short distance.  The figure is not specific to  travel of one light hour.

[Halc (OP)]

Still, the wave would seem to propagate at 2c at best.

That was wrong.  If we accelerate the tail to light speed, the wave propagates at light speed, but the wave seems to move faster with slower ship speeds, so for instance if we accelerate the tail to .866c, the wave moves at about 1.732c (sqrt(3)), if I did that correctly.  The caterpillar gets there faster if it moves slower.  Whodathunkit?

[Toffo]

What are the pilots reading if not the accelerometers?  How do they otherwise decide that they're the same?

Second note is, same as what? Identical to the value measured on other ships, or just identical from moment to moment?  Nobody seems to be proposing that a particular ship vary its acceleration during the process (except David just now), but it isn't off the table either.

An accelerometer on a ship will measure proper acceleration.  Not sure what meter the lauchpad guy is reading, but that one will read acceleration in his frame, not the ship's proper acceleration.  The former falls off as speed grows, while proper acceleration should be constant for the duration.

Bell's spaceships are seen to accelerate at the same rate by the launchpad ... and clocks in the ships are seen to tick at the same rate. And accelerometers screwed on the ships and observed by a telescope from the launchpad are seen to read the same value.


A pilot of a normal spaceship sees all parts of his ship to accelerate at the same rate.

[Halc (OP)]

Bell's spaceships are seen to accelerate at the same rate by the launchpad ... and clocks in the ships are seen to tick at the same rate. And accelerometers screwed on the ships and observed by a telescope from the launchpad are seen to read the same value.

A pilot of a normal spaceship sees all parts of his ship to accelerate at the same rate.

Agree to all but the last one.  Our pilot would need an accelerometer bolted to either end of his ship, and if he looked at them, they'd read a different value.  If the ship is short as most are, they'd not read very different, but it gets quite apparent with longer ships.  They're getting shorter in launchpad frame, so the front isn't getting up to the same velocity in that frame.  In ship frame, the front clock is running faster, so it takes more time to do the same acceleration.  In both frames that spells different reading on the accelerometers at either end of the ship.

[Bored chemist]

I think I might be able to see what you are on about.
Imagine I assemble my crew of pilots + ships in space then set them off at 1 second intervals.
As they leave the launch point they are quite close together.
At their destination, they also arrive at 1 second intervals. And now they are doing 100,000 miles a second
But they are now travelling at high speed so 1 second is long enough to travel 100,000 miles
So the flotilla has "stretched" in transit.

But the problem there is that you can't say at what time I launched the flotilla. Was it when I set off the first ship, the last one, tee one in the middle or what?
If there wasn't a flotilla at the start, how can you say it stretched?

(Obviously, this isn't a relativistic effect)

[Halc (OP)]

I think I might be able to see what you are on about.
...
But the problem there is that you can't say at what time I launched the flotilla
...
(Obviously, this isn't a relativistic effect)

That is not at all what anybody is on about.  The long ship, or the flotilla that paces it, are all launched at the same time in the initial rest frame of everything.

It is very much a relativistic effect that I'm talking about.  The thing you mention there is indeed not a relativistic effect.

[alancalverd]

If an array of particles all launch at the same time, with the same acceleration vector, their mutual relative velocities will remain zero so they will  remain at the same separation relative to one another. No relative motion = no relativistic effects.

If the initial array was a straight line, then an observer at the launch point will see the line contract in the radial direction, but not in the tangential direction, as they approach c relative to the launch point. Relative motion = relativistic effects.

[Toffo]

Agree to all but the last one.  Our pilot would need an accelerometer bolted to either end of his ship, and if he looked at them, they'd read a different value.  If the ship is short as most are, they'd not read very different, but it gets quite apparent with longer ships.  They're getting shorter in launchpad frame, so the front isn't getting up to the same velocity in that frame.  In ship frame, the front clock is running faster, so it takes more time to do the same acceleration.  In both frames that spells different reading on the accelerometers at either end of the ship.

Yes, the accelerometers show different readings, and from that we know that I did not say or mean that the accelerometers show the same readings.

I meant the eyes of the pilot tell him that the accelerations are the same, the same way as the eyes of drag racers tell the guys in the cars what the difference of accelerations is.

[Halc (OP)]

If an array of particles all launch at the same time, with the same acceleration vector, their mutual relative velocities will remain zero so they will  remain at the same separation relative to one another. No relative motion = no relativistic effects.

This is all from the inertial frame perspective.  Their mutual relative velocities will remain zero in the original stopped inertial frame, yes.  There will be relativistic effect: time will pass for the particles more slowly but identically because they're moving.

If the initial array was a straight line, then an observer at the launch point will see the line contract in the radial direction, but not in the tangential direction, as they approach c relative to the launch point. Relative motion = relativistic effects.

No.  They're all accelerating identically, so no contraction.  See my very carefully worded example at the bottom of post 45 which describes exactly this kind of uniformly accelerating array.  For contraction to happen, the tail needs to accelerate more than does the front, bringing it closer to the front in the original inertial frame.

I encourage you to reword that example with the same acceleration to give the coordinates of the various parts of the ship after the acceleration completes.  Each piece accelerated identically for 1.3 years (ship time) or about 1.7 years in the inertial frame.  They all move exactly one light year (which is why those numbers were chosen), and that makes them all still the same distance apart.  No contraction of the space between them.  If you contract that space, some of the ships need to move far faster than light to get the ship length down in time.

Read and comment on the example (bottom of post 45) and stop waving it off because you know a different answer.  All I get is you holding your ears going "Lalala" when the contradiction is pointed out, or when I find several articles on the web (post 48) that support what I've been trying to say.

Describing in the frame of the array cannot be done because there is no one such frame.  Each of the elements is moving at a different speed because the ones at the front have been accelerating for a longer time than the ones in the rear.  Their clocks were in sync in the inertial frame, so they cannot be synced in any other (except you appear to also deny relativity of simultaneity).  That means they've accelerated for different amounts of time, and that's why they pull apart from each other in the frame of any of the pieces.

My thread isn't even about this.  I assumed one knows that different parts of the ship accelerate differently, and the question was if this limits a long ship from going places anytime quickly.

[Halc (OP)]

I meant the eyes of the pilot tell him that the accelerations are the same, the same way as the eyes of drag racers tell the guys in the cars what the difference of accelerations is.

The drag racers are side by side, not end to end, and not moving at relativistic speeds, and are doing a non-local observation.

I don't think eyes can detect acceleration.  If I put a camera in a car pointing down at the back seat (not out the window), and somewhere in a minute of footage the car accelerates, I could not tell from watching the camera footage when that acceleration starts.  Acceleration is felt, and possibly visually detected by seeing something bend to the side or some other accelerometer, and it will be felt very different at one end of the ship than the other.

In my example of a long ship moving a short way (post 37), the people at either end of the ship die the strawberry jam death, and the ones in the middle die of boredom.  That would be something the pilot could see except the trip is shorter than the time needed for the image of the dead people at either end to reach the pilot in the middle.

[Toffo]

The drag racers are side by side, not end to end, and not moving at relativistic speeds, and are doing a non-local observation.

I don't think eyes can detect acceleration.  If I put a camera in a car pointing down at the back seat (not out the window), and somewhere in a minute of footage the car accelerates, I could not tell from watching the camera footage when that acceleration starts.  Acceleration is felt, and possibly visually detected by seeing something bend to the side or some other accelerometer, and it will be felt very different at one end of the ship than the other.

In my example of a long ship moving a short way (post 37), the people at either end of the ship die the strawberry jam death, and the ones in the middle die of boredom.  That would be something the pilot could see except the trip is shorter than the time needed for the image of the dead people at either end to reach the pilot in the middle.

Do I really need to explain that one drag racer gets ahead of the other one when the accelerations are not the same
 

[Bored chemist]

Do I really need to explain that one drag racer gets ahead of the other one when the accelerations are not the same

Apparently, to some folk here, yes. You need to explain it.
You also need to explain that drag racers actually look out of the window.

[Halc (OP)]

Thus the entire ship must accelerate as a single entity. There being no change in length, there can be no relative velocity or acceleration between the front and the back of the ship and thus no change in perceived clock rates between observers on the ship.

We need to pinpoint those rules.
There can be no change in proper length.  But there is very much going to be changes in relativistic length.
For it to have a proper length, it needs to be stationary in its own frame.  I fretted a lot about that one since it seems to be difficult to avoid, but decided it was a mandatory requirement, and that the solution to the problem lies exactly in that requirement.

Clearly my assessment of the rules was wrong.  It seems a brittle ship doesn't need to be stationary in any frame, and thus its proper length is only defined as the collective proper length of the parts.
In particular, a ship that is accelerating has parts that are necessarily moving at different velocities in other frames (like the original 'stopped' frame), and my description kept the ship stationary only in its own frame. If it can move at different speeds in that frame, it can do it in its own, and that means it might not even have a frame that is its own.

So David's post clued me into that.  You can have the bottom half of the ship going at .9c and the top half stopped.  That doesn't break the brittle ship unless any time is allowed to pass without moving the line between the fast stuff and the slow stuff.

So picture a causal diagram for the exact point/event E in the ship where this velocity difference exists.  Draw a big X with the event in the middle.
The stuff at the bottom is the past light cone, and nothing is moving in that cone.  As far as E has measured, the ship is stationary and has no reason to feel stress or strain.
The region above the X is the future light cone, and everything moves at .9c there.  As far as E will measure, the ship will always be uniformly fast and has no reason to feel stress or strain.  The stuff to the left and right are very different, but out of the causal cone.  The ship at E will need to accelerate to .9 instantly as the wave passes by.  It does not see the wave coming since it travels at faster than light, so the computer needs to know the plan, but we always said that.

Interestingly, the wave travels at just over light speed if the velocity change is near c, but far faster than light if the velocity change is low.  Because of this, the fastest way to get from A to B is to go slow.  I think my original plan is still the optimal one.  I could not think of a way to get the ship from A to B faster by allowing parts to move at different velocities like that.
I moved my ship one light hour in 55 days by keeping the speed down, but if we go at .999c and let the wave move up the ship, it takes 50 years (plus an hour).  I need to run the numbers for a wave resulting from an abrupt acceleration to 452 km/sec, which was the winning speed before.

[David Cooper]

Can you combine the two methods? Your way produces a faster time for the front of the ship (by setting it moving sooner), while mine produces a faster trip time for the back of the ship. Apply your method first to set a wave of acceleration through it much faster than the speed of light, then apply my method afterwards to increase the speed of the back end to nearly c with this wave running through the ship at a slower speed. The next thing to look at would be whether the trip time for the middle of the ship can be improved too.