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In my current science-based project, I have this idea for a string of rotating black holes (bonus points if you can figure out how they can be lined up without gravitating towards each other) that are used for their ergospheres.

Because ergosphere spacetime flows FTL, I think a vessel within it would also be propelled FTL, without actually violating causality, since it's spacetime that's moving so fast, not the vessel. Therefore, this string of ergospheres could be used as a means of effective-FTL travel without negative energy. Side note: I'm aware that there are Alcubierre drive theories that only require positive energy, but it's still way too much energy.

Basically, it would end up something like this (probably not to scale):enter image description here

Does this make any sense scientifically?

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  • $\begingroup$ @JBH hi there i swear i remember hearing that spacetime itself moves FTL in an ergosphere, mostly from this video: youtu.be/vuyp1885Bx4?t=640 but im glad u like my idea! i like it too. $\endgroup$
    – Zanly
    Aug 15, 2022 at 20:35
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    $\begingroup$ I do not think a line of black holes is going to work in a stable configuration. Best you can do is probably having two black holes orbit each other or as Dragongeek suggested having all of them orbit something. However that would require a VERY big black hole to act as a centre with all of the other smaller black holes all perfectly orbiting it to be aligned however then I find it difficult that the black holes could be even remotely close to each other because they will certainly gravitate into each other. $\endgroup$
    – Venik Hue
    Aug 15, 2022 at 21:22
  • $\begingroup$ You'd be better off with a cosmic string. $\endgroup$ Jun 21, 2023 at 23:22

3 Answers 3

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Because of this dragging effect, an object within the ergosphere cannot appear stationary with respect to an outside observer at a great distance unless that object were to move at faster than the speed of light (an impossibility) with respect to the local spacetime. (Source)

The Good, the Bad, and the Ugly

When Sergio Leone directed that Clint Eastwood movie, little did he know that he'd created one of the most useful phrases in human history.

Ugly First

You're asking us if an idea is scientific but, per the quote above, your idea is based on a scientific impossibility. So, no. Your idea is not scientific. I can't even imagine how to get black holes to line up in a straight line in a universe where everything is in constant motion and, generally, a circular motion.

The Bad

Everything you're talking about is a best-guess based on mathematics and remote observations. One of our celestial machinists could give you a boatload of math I can barely remember from college to explain the what-you-can and what-you-can't of the situation. But the simple truth is that there's no empirical evidence the consequences of the ergosphere are correct.

But this being bad is actually good for you! The fact that there isn't any actual science (per se) behind the idea you're trying to exploit means you can exploit away.

The Good

The number of people who will read your story and know to judge that it's not scientific is darn close to zero. Of those (assuming you have a well-written story) the number of those who will judge you for not being scientifically perfect is zero.

And I'm a fan of reminding people that what we know of science is far from gospel truth. We're constantly learning new things and discovering hidden mysteries. It's a huge mistake to believe that what we know of "science" today in any represents a cut-in-stone understanding — especially when you're dealing with massively theoretical things like what happens inside the event horizon of a black hole.

Bear in mind you're in great company. When Larry Niven first wrote about the Ringworld he didn't realize he'd written an entire story about a scientific impossibility. MIT students at a scifi conference marched the halls chanting "The Ringworld is unstable!" as if they'd cured cancer. What did he do?

He fixed it in his next book.

My point? This is a cool idea. Run with it and ignore anyone who thinks otherwise.

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  • $\begingroup$ repeat comment because u seem to have deleted ur previous comment: hi there! i swear i remember hearing that spacetime itself moves FTL in an ergosphere, mostly from this video: youtu.be/vuyp1885Bx4?t=640, but im glad u like my idea! i like it too. $\endgroup$
    – Zanly
    Aug 15, 2022 at 21:22
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    $\begingroup$ @Zanly Yeah... I decided to turn it into an answer. I understand that's what people are saying, but take frequently sensationalist YouTube videos with a (large) grain of salt. There's no empirical evidence. In other words, our mathematical models only work if something inside the ergosphere is traveling FTL. That more than likely means our models are wrong since science, today, believes c is inviolable. Math is predictive, not omniscient. $\endgroup$
    – JBH
    Aug 15, 2022 at 21:41
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So you are talking about Lense-Thirring effect (frame dragging effect) a consequence of General Relativity. Ergosphere is a safe region around a black hole within which you can revolve around the black hole and still be able to maintain the velocity required to escape. This means that even at a sub-luminal speed (speed < $c$) you can easily escape the black hole's gravity.

Since, in your string of black holes and their ergospheres, you are using FTL frame of reference (the space-time itself), you could consider your observer to be a part of the FTL dragging frame. You also mentioned that a vessel inside FTL frame could be FTL for a region outside, you may make the string of ergospheres a vessel in the space-time that is dragging.

This all means that the observer is within the FTL dragging frame (not inside the ergospheres because that would require consistent motion) and the traveller is moving within the string of ergospheres which is itself within the FTL frame. So this highway you constructed will make a good sense even within the limits of relativity.

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I suspect that might not work.

The idea behind warp drives etc. is that spacetime is a riemannian manifold you can deform, and by deforming spacetime properly one may "generate" shorter geodesic paths between two points in spacetime, which a spaceship may then take.

The problem is that black holes (as far as I know, im actually not at all good at general relativity) as well as other massive objects, tend to increase the "distance" between points, so work in exactly the wrong way.

However if you used white holes (and im not mistaken) you may actually directly shrink the "distance" between source and destination, so then that should work.

So if you use white holes instead of black holes things may work out just as presented.

However I would certainly love to see the opinion of someone actually knowing general relativity on this. Would certainly expand my knowledge.

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