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Imagine a planet - henceforth Planet X - that is orbiting very close to a black hole. The space-time distortion will be very high there and according to the theory of relativity, the time there will pass slower than for the rest of the universe.

For those who have watched the movie Interstellar, it's similar to the first planet the mission visits (the one close to the Gargantua black hole).

In order to make the things more visual, let us assume a planet where the distortion is so strong that we have the next equivalence:

1 hour in Planet X = 10 years in the rest of the universe.

Now imagine the Earth sends several missions to Planet X, 1 every 10 years for 100 years.

The planet will receive those missions within the same day. The first mission to arrive will have to wait only 1 hour to meet the second mission that departed from Earth 10 years later.

Therefore, we see here that Planet X is very vulnerable in the way that a potential enemy could gather many "ships" and "troops" there in only a few hours, while actually taking decades to build that army. This way, the planet would be easily invaded.

However, the enemy would need a lot of time in order to invade Planet X, so much that it would not make sense to actually do it. Any resources on Planet X would need many years to be exported to the enemy planet.

What do you think about this perspective?

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migrated from space.stackexchange.com Feb 13 '18 at 16:20

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    $\begingroup$ "1 hour on planet X = 10 years in the rest of the universe" - this is not how relativity works... $\endgroup$ – Aify Feb 13 '18 at 16:30
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    $\begingroup$ So... What problem are you trying to solve? $\endgroup$ – Frostfyre Feb 13 '18 at 16:40
  • $\begingroup$ Welcome to WorldBuilding.SE! I've edited your question slightly to try and make it easier to understand; if it conflicts with your intent, please feel free to revert it. I must admit, however, that I'm not sure what your actual question is. Could you edit it further to make it clearer what you're asking? $\endgroup$ – F1Krazy Feb 13 '18 at 16:40
  • $\begingroup$ “Safe” from what perspective? Vs an enemy army you have kind if answered your own question. The only defense the slow planeteers have is that no one will care enough to start a campaign against a planet that will end in 1000s of years. Also why not just point 10,000 nukes at this planet and have them enter from all sides and see the inhabitants scramble to combat the synchronized attack? Why bother sending people? $\endgroup$ – Tyler S. Loeper Feb 13 '18 at 17:46
  • $\begingroup$ A friend of mine just told me about a book called Incandescence by Greg Egan which is basically about this. $\endgroup$ – Muuski Feb 13 '18 at 19:37
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Thankfully for Planet X, 10 years of ships would not arrive over the course of 1 hour because the ships would be experiencing the same time dilation that Planet X has as they approached. Assuming you had some way of observing Planet X without light also suffering from the time distortion:

From the view of someone observing Planet X: If you were watching these ships approach Planet X, you would see them traveling at a normal speed at first. However, as the ships approach Planet X, the ships will appear to slow down(and stretch), and will move slower and slower until they are going at the same speed as Planet X. They would look like they are piling up just outside of Planet X, very slowly getting closer and closer.

From the view of someone on the ship: Time on the ship looks perfectly normal for you the whole trip. However, if you looked at Planet X everything would look very slow at first, and speed up as you approached. Similarly, if you looked back where you came from, everything would start moving faster as you moved further away.

From the point of view of someone on Planet X: You would see, far in the distance, 10 years worth of ships approaching every hour. The buildup of each group would look almost instantaneous. As they get closer, they begin to slow down, more and more. Eventually, when they are finally near enough to the planet, each group reaches you every ten years.

As I said, you would only be able to observe the effects like this if you could get light to ignore the time dilation. If you couldn't, then from outside Planet X, the ships would appear to pile up and take 10 years to finally reach the planet. From inside Planet X, light from the ships' starting point would take 10 years to reach you, so you would see 10 years of build up approach over the course of 10 years, since the light would slow as it approached you.

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    $\begingroup$ Exactly this, the perfect answer to perception across a relativistic distortion. $\endgroup$ – Ash Feb 13 '18 at 17:17

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