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This video is about Bell's Spaceship Paradox of Special Relativity, wherein a pair of rockets (or spacecraft) connected by a weak thread accelerate with uniform acceleration, maintaining the same separation, and the question is: does the thread break? And if so, why?
REFERENCES
Interactive Spacetime Globe by Alexander Wu:
https://alexonscience.com/projects/spacetimeglobe/
Read an overview on Wikipedia:
https://en.wikipedia.org/wiki/Bell%27s_spaceship_paradox
John Baez on Bell's Spaceship Paradox, Rindler Acceleration, etc
https://math.ucr.edu/home/baez/physics/Relativity/SR/BellSpaceships/spaceship_puzzle.html
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How do you measure the lenght of the rope/distance of the two spaceships? I hope in a way that you make sure, that the places where the two ends of the rope are are measured at the same time from the standpoint of the observer (nomatter where he or she is). Otherwise it is not an accurate measurement.
@Duracellmumus Says:
This method makes the rockets to turn when any force is aper at the end of the traverse.
You need to put the string ends exactly the places where the acceleration force is aper on the rocket body, but it is impossible to do.
@PurpleYoshiEgg Says:
How would the first rocket accelerate first from any perspective if we already established the assumption that both rockets accelerate the same amount at the same time? There is some rest of the owl stuff in this video that make it seem like this is not well thought out science at all.
@ralanham76 Says:
How does it look from the front rocket ?
Why not have the second rocket start just before ?
@Kholdilocks Says:
I'm sorry I got distracted when your stick figure cat got ripped in two by accelerating rockets.
@soloshottie Says:
that "from a moving perspective" doesnt make sense since being on the ship doesnt really change our perception of events, especially not to the degree of perceiving completely different motion from reality (barring unusual factors of course)
@KubeSquared Says:
Wait... Does this mean that any relativistic spacecraft is limited to one engine?
@ballyandreen405 Says:
I'm afraid the explanation has some major logic flaws.
1. Simultaneity issues are not involved. The rockets fire at the same time in the same original reference frame (obviously allowed by relativity), and then they run at constant speed with no further need to synchronize.
2. It violates the notion of no absolute zero velocity and violates the equivalency principle to suppose that the string stretches and breaks due to increasing "spacetime warpage" due to increasing velocity. In other words, at v=0.00001c, the string doesn't break. At v=0.05c or so, the string breaks. At v=0.1c, the gap between broken strings grows, etc. You've invented a speedometer that measures absolute velocity, and you've found a difference between identical-acceleration-reference frames (based on some unexplained changes in electromagnetic forces). Those things are not allowed by the theory of relativity.
3. There is nothing magic about the second (behind) ship being accelerated by pulling with a string as opposed to engine thrust. It is just two different mechanisms for accomplishing the acceleration.
The real solution is actually simpler. In an accelerating reference frame, clocks (and all processes) run at a slower rate at the floor than at the ceiling (just as they do in a gravity field; see Pound-Rebka or GPS clocks). The behind ship crew would age a little slower than the ahead ship crew. You need to adjust the acceleration of the behind ship to account for that. The adjustment is made "naturally" when the string is pulling the ship behind. An observer back in the original launch reference frame would calculate the trailing ship has a higher acceleration by, not coincidentally, just the right amount to result in the Lorentz contraction. But within the 2-ship system, there would be no way to measure velocity. If you impose the identical acceleration as seen by the still observer, the two crews would see that the lead ship had the greater acceleration and it would pull away and break the string. But their view would be simply that - the lead ship had greater acceleration. It wouldn't give them any sort of "electrostatic force" related speedometer. At best they could calculate the speed of the observer that would allow the two accelerations to be identical in the observer's reference frame, but that is not an absolute velocity thing.
@m.j.v.4463 Says:
This doesn't make sense. If both rockets and the string accelerate at the same time and rate, they must behave like an inertial system subjected to uniform gravity. Either the string breaks at the start, or it doesn't at all, even at relativistic speeds. Nothing should change from the perspective of the rockets and the string.
@backwashjoe7864 Says:
What magnitudes of acceleration and velocity are needed for this to happen in a real experiment? What other practical considerations are there in actually doing this? Length of the string to separate the rockets some minimum amount? How to protect the string from the rocket exhaust? How to make the thrusts of both rockets colinear with the string? How small must other forces be on the system, such as gravity from a planet they might be orbiting? What else??
@keenanpepper Says:
> normal objects aren't like that - instead one part is pushed or pulled and then the intermolecular forces in the object transmit that acceleration
yeah, which is overall significantly WORSE for the "tear yourself apart" issue
@declanabdf4 Says:
But what it doesn't
@krumpliii Says:
No length doesn't contract in real life. It's theoretical. It's not real science.
@lorigulfnoldor2162 Says:
but wait isn't it so that gravity accelerates you at each part independently? why does not gravity pull things apart, not towards?
@carpedm9846 Says:
2:10 but the rocket is not a single object, its made up of plates and pipes and fuel tanks. Shouldnt there be no difference between a screw holding the control panel in place and the string?
@tinglin6121 Says:
This video is misleading. What's snapping the rocket is the acceleration, and you can easily adjust the acceleration time of the two rockets to prevent it from snapping.
Here is the counterargument: if the rockets are at rest and the observer is accelerating passing by the rockets, will the rope snap according to the observer?
@adamrussell658 Says:
Objects dont shrink. The space they occupy shrinks. Since it all shrinks together, the string will not break.
@Galimeer5 Says:
Instructions unclear: tore myself in half flying a rocket
@tarjeibaerland Says:
Aaaand now I'm scared to drive a 4WD.
@seededsoul Says:
The string would break from the inertia of the string being accelerated by the lead rocket.
@ConradSpoke Says:
This is incorrect.
@joelhall3646 Says:
So, I will still argue they will not break. What I argue is, a complete reundersanding of QM. While yes, by the current mainstream QM view, this would be accurate "ish", the link between QM and GR isn't established. What you propose is a false link, an assumption. It states that both QM and GR are both correct but a link between them just hasn't been found yet. That is a fallacy of scientific epic proportions. That is to assume true two opposing theories based on faith. This is the modern failing of science turned to dogma.
@alienck3901 Says:
Arent length contraction is just happens to the observer?
@Mangadextrious Says:
Thought i finally got one, figured it would be something about inertia snapping the string 😔
@shotcrete3473 Says:
And if the spacecraft time it so that they are accelerating simultaneously from their point of view.
@navidahmadrahat6843 Says:
Great video ❤ YouTube recommend me more
@fluffysheap Says:
Well this is just completely wrong in every way. Moving objects don't "feel" their own length contraction, nor that of objects moving with them in the same reference frame. They look completely normal to themselves, but see the rest of the universe distorted.
If the rope breaks, it's for ordinary stretchy reasons. If the rockets are far apart, the rocket in the back would see the rope go slack (while the rocket in front sees it stretch) because the rope has to pull its own weight, and the rocket in back can't help because you can't push a rope. But this has nothing to do with relativity.
@rphb5870 Says:
Imagine a man whose arms are tied to the back of a car and his legs tied to the front of another, and for the sake of arguments the rope he is bound by is stronger then his bones. Can ye really see any good outcome for the man when these two cars start moving?
@lolxnn Says:
You cant accelerate two rockets at the same time for the same reason you cant synchronize two clocks exactly
@alexeecs Says:
If you pay close attention to 1:36, you can see that the globe dude actually relived the same duration of rocket dude twice! After accelerating, part of that history is repeated. That's real life travel to the past! But because the past is so far away they wouldn't be able to influence it without breaking the speed of light
@awareqwx Says:
I wonder how hard it would be to calculate at which time the rear rocket would need to fire before the forward one to exactly cancel out the length contraction once everything got up to speed
@89technical Says:
Q: Suppose the second rocket starts moving Infinitesimally before the first rocket, so it allows the smallest amount of give in the rope: Would the acceleration of the first rocket, sill cause the rope to snap as it contracts moving towards the speed of light?
@skepular Says:
Just leave some slack in the string
@NobodyYouKnow01 Says:
At what point, approaching the speed of light, does a closed system become an open one that could rip itself apart?
@riverseeber514 Says:
to make the last part clearer, think about if only the leading (front) rocket had turned on its thrusters. The string might break, or it might not, depending on how strong it is. The rockets don't 'break' because they're strong enough to handle the forces placed on them by the thruster. If the rocket was made of glass, and the thruster had a very short burst of energy all at once, it very well might 'break'. Relativity makes the example more complecated by adding questions about *when* it started accelerating, or perhaps by making spacetime shrink, but if it can handle the force of the thruster, then presumably, it should also be able to handle the shrinking of space time (or the fact that the thruster is slightly off on time compared to a Newtonian expectation)
@douginorlando6260 Says:
The confusion goes away when they include this fact … the two spaceships are not identical, one operates at ever increasing thrust
@Mixa_Lv Says:
But at the same time, because of the relativity, from the rockets's points of view they never start moving at all, but the universe around them accelerates on the opposite direction, do it's the universe that is shrinking, not the rockets.
@kyzer42 Says:
The reason the string snaps is because, *if* the two ships appears to accelerate at the same time for us ("stationary" observers), then *the two ships don't perceive themselves as accelerating at the same time* . Each ship perceives itself as starting to accelerate before the other, and thus each ship perceives the rope getting stretched and snapped.
@mayththemyth Says:
Now my question is, if we assume that the string is exactly uniformly strong, then where woukd it snap?
@christophercampanella9897 Says:
Are you saying it's the object and not SpaceTime that's contracting? I'm just starting to read this stuff and
@zigrastical6646 Says:
what happends if the back one moves first so from the moving perspective they move at the same time(at least from that perspective)
@FLORIDIANMILLIONAIRE Says:
It's good to learn some engineering after becoming a physicist or a physician I'm speaking from experience in both the fields.
@devindaniels1634 Says:
This is either completely wrong or setup dishonestly. Lorentz transformations are done using relative velocity, as there's no such thing as absolute velocity.
Ergo, the whole system accelerates as one, the whole system moves as one, there is one reference frame for the whole system and no relative length contraction.
The only way for what you're saying to occur is if the system is not behaving identically, and then other forces will break the string long before relativity comes into play.
In addition, relativity never even comes into play in this system. Why? Relativistic velocities are, big surprise, relative. In order for relatively to be relevant here, there would need to be relativistic velocity *differences* inside the system.
@nerdyengineer7943 Says:
No.
@AvinashSPhysicsMathTutor Says:
This has more detailed explanation. https://www.youtube.com/watch?v=BBlZNcUt4Jk
@mihaleben6051 Says:
Yes the string breaks. Its not that hard
@Fox_8ball Says:
So for every action an equal and opposite reaction would say that if when the rocket expands it breaks but when it contacts it doesn't crush. And that makes sense how?
@Woofingchristhopin Says:
Ok I kinda get how even if an object is Prepelled buy two propellants at exactly same speed it can rip part, but I can't understand how if u moving at there end speed u would Perceive the first object moving first. I honestly feel like u leave alot of info out that would be way hard to explain in a short video just to make use think about it but it still doesn't seem possible
@jaymoore332 Says:
If the spacecraft start moving at the same time, the string snaps. But if the spacecraft behind starts first, the string must go slack. So if you wave your hands and mumble, “Mean Value Theorem,” you see that there must be some schedule of accelerations for which the string remains exactly the same length. What does that look like?
@lycerisrico8943 Says:
This just feels wrong to me tbh. It's clear to me that the rope doesn't break.
In stationary speed, both rocket and the rope are a whole system. From rocket perspective, they're stationary, so no contraction and the rope doesn't break. From frame perspective, they got contracted and the space between rocket is also contracted at similar rate as the rope, so the rope also doesn't break
And then both rockets are accelerating at the same time and the same rate. I'm assuming that both rocket are entangled or there's a tool that gives signal with c speed in the middle. At frame reference, they appear accelerating at the same time, so the rope doesn't break. From the front rocket reference, it appears that front rocket are accelerating first because of the light delay from back to front, so it gives the rope a pull. BUT, from back rocket reference, due to light delay from front to back rocket, it's also appears that back rocket are accelerating first, so it gives some slack to the rope. A pull force at front side of the rope, a slack at the back of the rope, they're equal, so the rope doesn't break, unless the acceleration are too big so it breaks due their own weight.
Idk, how you can reach the conclusion that one of the rocket are accelerating first compared to other from frame reference? It's just doesn't make sense to me
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