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I've a weird question which is more a sci-fi question than a physics question, but i need people who know physics to have any hope of interesting answers :p

I was rewatching the scene of Interstellar (spoilers!)

They are on a planet covered in water. There, when one hour passes, 7 years passes on earth, because the ocean planet is near a black hole, and due to the dilation of time (general relativity), time is desynchronized with the earth.

But i was wondering, imagine if we were able to create a portal from earth to this planet, but aside from this special detail, the rest of physics rules as we know them remains. How would that portal behave? If we throw an object from earth to this planet, through the portal, would its trajectory or speed be normal? Or would it have weird effect like being (or appearing) slowed down (as it would if it was approaching the black hole from space, in the normal way, from the perspective of a distant observer, i guess?)? What other weird behaviour would occur, if any?

What would an observer see through the portal?

I know that to answer this, we would have to postulate first a few characteristics of the portal but i'm curious.

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  • $\begingroup$ If I built a shed, what color would it be? $\endgroup$
    – WillO
    Oct 14, 2022 at 21:00
  • $\begingroup$ i assume you say that because from a fictional object (portal) you can imagine everything and so it's not possible to answer. But i'm not asking for "the real answer" here, i know there isn't. I'm asking, given we define what would be a portal, if we can deduce some consequences of it :) Nothing strange here, just funny questions which can have interesting answers $\endgroup$ Oct 14, 2022 at 21:06
  • $\begingroup$ given we define what would be a portal, it's properties will depend entirely on how we've defined it. $\endgroup$
    – WillO
    Oct 14, 2022 at 22:07

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The beauty of general relativity is the local behavior of any object (trajectories, time passage, distances) is always the same! I.e. an insect riding on your rock thrown through the portal (perhaps an Einstein Rosen bridge) would not notice any difference between Earth and the other planet.

The differences come when someone observes events from far away, in a different gravitational potential. This is why in Interstellar the astronaut who stayed in the ship aged much differently than the ones on the planet's surface. The astronauts looking down with a telescope to the planet would see the rock exit the portal in slow motion (as well as seeing everything else on the planet happening in equally slow motion).

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  • $\begingroup$ yes but what would an observer of earth see through the portal? $\endgroup$ Oct 14, 2022 at 20:56
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    $\begingroup$ @Daemonsoadfan Keep in mind that the portal's ends cannot remain synchronized. Synchronization over long distances isn't even a meaningful concept! So the observer would not see anything special, but after enough time, the portals could be used for time travel. $\endgroup$
    – Sten
    Oct 15, 2022 at 7:37

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