What is theoretically wrong with this concept of faster-than-light communication?

Before I get to the description of this stupid hypothetical invention, I have to say that I know that it is a fatally flawed idea. There is absolutely no practical way to ever create a device like I’m going to describe, so I’m not too interested in a long list of what’s wrong with it practically. What I want to know is what’s wrong with it theoretically.

Okay, so imagine a tug of war game. The two sides pull against each other. When one team pulls, it exerts a force on the other team that is felt immediately. When you tug on one end of a string, the other end moves at the same time and rate as the end you’re pulling. If you had a hypothetical “string” that was a lightyear long, pulled taut, when you pulled one end of the string, wouldn’t the other end move immediately? What if the string was attached at the other end to a device that pulled it back to its original position, every time you pulled your end? You could communicate instantly with Morse code using a method like this, even though the receiver is a lightyear away, right?

So what theoretically prevents this from happening? Forget about issues like keeping the string taut, or planetary orbits disrupting the string, the string not being strong enough, or any other “practical” problem. Obviously information can’t travel faster than light, even theoretically, because of a causality paradox (right?), so something has to be flawed in the theory of this idea. What is it?

Quite simply, no, the other end doesn’t move immediately. The tug propagates along the rope at a finite speed (yes, even with a taut rope).

That is, when you tug on one end of a taut rope, the bit of the rope at that end moves towards you, then a little while later the bit next to it moves towards you, then a little while later the bit next to that moves towards you, so that always a part of the rope is stretched a bit and then later it relaxes as the part next to it is stretched a bit, till eventually this reaches the other end and is felt as a tug there. But this all happens at a finite propagation speed, just one that seems instant in everyday experience.

(Standard disclaimer on science matters: Correct me if I’m wrong)

I take it that if you replaced the rope with a solid rod it would make no difference, and that the stretching/compression would still go on? I am glad someone asked this question - it’s crossed my mind before, but equally I thought it must be ill-fated…

Yes, that would be correct.

A previous thread on the same topic: Relativity and very long sticks.

It doesn’t really seem instant even in everyday experience either. Thunder and sonic booms come to mind.

Everyday experience pulling taut ropes and pushing solid rods, I mean. It takes some non-everyday thought to see other longitudinal waves as equivalent.

For those not inclined to follow the link:

The speed of sound in a rope or a rod is a lot slower than the speed of light. And that’s the speed any physical motion propagates at.

Which is why everyday experience is such a bad intuitive guide to things that occur on much larger or smaller scales.

This also indicates a fundamental problem with “statis fields” and the like. Take a monofilament thread stretched from Earth’s orbit to Wunderland, surround it wil a Known Space-type statis field, and move it back and forth. Voila, instant communications without all that mucking about in hyperspace. The only problem is you severely violate both Special Relativity and causality by connecting two non-local points instantaneously, which disturbs everyone but David Bohm, and even he won’t like the idea that the connection isn’t irretrievably hidden behind the stochastic veil of quantum mechanics.


I think SF writers could probably work around that by saying that the stasis field would behave as an extraordinarily rigid, but still normal object (so your induced movements would propagate along it fast, but not instantaneously), whereas the space inside it remains geometrically static (and because no time is passing in there, it’s not a problem).

It’s not as if the concept is otherwise sound anyway, and just flawed by the issue of rigidity.

But if it’s “geometrically static” inside, and time doesn’t pass, how can it be anything but completely rigid? Sure, like a “hyperdrive” it’s just a conceit of sci-fi authors who need a plot device that works outside of physics as we know it, but it also creates a serious problem that one can’t wave away as a simple loophole in the unknowns; it’s a real and problematic violation of Special Relativity.


I don’t kn… I am not sure how to tell you this. I am… umm, this is awkward. I’m terribly sorry to have to tell you like this, but David Bohm passed away fifteen years ago. Although, it might be possible to use a an infinitely rigid device that connects to non-local points instantaneously to bring him back :wink:

He’s just sleeping.


Just how much force would it take on our end of the rod to have the molecules bump into each other for a length of over 4 light years? Assuming a standard steel rod. Then Unobtanium. Seems like the “tap” on our end would destroy both the rod and maybe the Earth along with it!

While relativity does provide absolute limits on the rigidity of materials, it does not provide any such limits on how dissipative a material can be (though thermodynamics might). There is no reason in principle why a lightyears-long rod could not maintain a sound wave at approximately constant amplitude along its whole length. I doubt that any known real-world material would be adequate for the task, but it’s a more accessible level of unobtainium.



If time doesn’t pass inside, then the question of rigidity is moot, because movement is impossible anyway, in fact you could just as easily argue that the inside doesn’t effectively exist for as long as the stasis field is switched on, so put a glass rod in a stasis field and bend it double, then straighten it and turn off the field and nothing is broken, because breakage can only happen when time is passing.

So it could be bendy from the outside, effectively rigid (in fact, static, hence the name) on the inside.

just think about the amount of force alone it would take to pull all that mass. A 6,000,000,000,000 mile long piece of rope? I hope you’ve been working out! :wink:

How do you bend the outside without rendering some kind of volume change on the inside? You can’t, or at least not without creating some kind of seriously problematic discontinuity (as if halting the passage of time and attendant problems the the third law of thermodynamics wasn’t enough). So it’s not possible, even in theory.

Per Chronos’ comment, I’ve been trying to ponder the thermodynamic restrictions on such a substance. Eschewing the material properties of any extant material, I think you’d have to delve down into quantum mechanics to develop a theoretical limit of propogation and dissipation for the theoretical unobtanium. There is, I think, a definite limit, but I don’t know that it’s readily calculable. I’ll have to think about it some more. You could, on the other hand, create some kind of 1-dimensional defect in space like a hypothetical superstring. There’s no limit to how fast space itself can stretch, or what its “material” properties are, although I’d suspect there would be some property analogous to inertia that would prevent you from transmitting information faster than light, or perhaps even at all. Plus, you’d have some serious mathematical difficulty with the endpoints, I think.


Forgetting a rigid rod for the moment…

Does one side a particle move instantaneously with the other side?

I’m sorta kidding and I’m guessing the problem is nonsensical until we figger out what particles are, but in principle the problem is no different for a crystal rod than for a particle, is it? It’s only a question of scale.

And don’t bring up that collider thing in Europe to frighten me like you did last time, Stranger.