# How can we explain the position of Mann's planet when travelling on Miller's planet in Interstellar movie?

In the middle of the movie Interstellar, a crew of astronauts land on Miller's planet. For them only one day passed. For the one astronaut left on the station, 23 years passed.

Imagine both look at Mann's planet (a very small point in the sky), would they agree about its position? When the crew leaves the station, Mann's planet is at position A. When they come back Mann's planet is at position B (1 day) for the crew and B' (23 years) for the station's astronaut. How can they both see together 2 different positions?

What parallels can we do with earth's views of the sky?

Instead of discussing Mann's planet, I'll draw an analogue with something simpler, but that works in the same way: a clock at the station, equipped with a calendar as well. Notice that both the astronaut at the station and the crew will need to agree on what reads on the calendar, even though 23 years go by for the astronaut while only a day goes by for the crew. Notice the issue is the same as with the position of Mann's planet, I'm just simplifying the situation so that we only need to deal with stuff at two places.

The key concept in here is that time is relative in a quite literal way. Time passes in the spaceship at a rate much different from the rate it passes at Miller's planet. To get things clearer, suppose the crew took a telescope with them down to Miller's planet and then looked at the spaceship so they could see the clock. What would they see? They'd see the days going by incredibly fast, because the spaceship time passes much faster when seen by them. Similarly, they see Mann's planet orbiting the black hole at a much faster rate than the spaceship sees. As a side note, note also that the telescope would only show the calendar a bit to the past, since it takes time for the light to reach the telescope. Also, there is no "good" definition of "present" in Relativity.

In summary, yes, once the crew returned to the spaceship, they would all agree on where they see the planet (I phrase it this way because, as mentioned, there is no suitable notion of present in General Relativity, so it doesn't make a lot of sense to say where the planet is, but it makes sense to say where they see the planet), but disagree on how long it took for it to complete each lap, similarly to how they would all agree that 23 years have passed on the calendar, but disagree on how long it took for the days marked on it to go by.

As for the parallels with Earth's view, similar effects will happen with astronauts on the International Space Station, for example. When they come back to Earth, we'll all agree on where we see Mars, but we might disagree on how long each lap it took around the sun took. Similarly, we'll all agree the rover Perseverance is on Mars, but we may disagree on how long its trip took.

You could ask the same question for the twin paradox, and need no general relativity. If the stay-at-home twin reckons that a month goes by, and the travelling twin reckons that a day goes by. How do they attribute the location of Earth in its orbit? They have to agree that it has moved 1/12 of the way around its orbit upon the return of the traveller, they just don't agree on how fast it moved there. The travelling twin is used to things in other places appearing to move rapidly as he/she is changing direction, and GR is like being in a constant state of changing direction. It all comes from the fact that there is not a unique way to match up the "nows." You need to say where the object is "now", and that's the rub. But the invariants must be agreed on in the end, like how many times a planet has orbited its star cannot be disagreed on once the various observers are reunited.

• It is not the same. You could imagine travel from one altitude to the other is instantaneous. Then the paradox is absolutely not the same as twin paradox. The question is how 23 years of MANN's planet trajectory can match with only 1 day of its trajectory. Commented Dec 28, 2016 at 19:23
• It still sounds the same to me, but either way, the answer is, the person who thinks it happened in 1 day just thinks it happened faster. When matching up "nows" nonlocally, whether in a noninertial frame or in the presence of gravity, you can get things to appear to move faster than an actual velocity that you would expect for an orbit, and so forth. Commented Dec 28, 2016 at 19:57
• You wrote: "in the presence of gravity, you can get things to appear to move faster than an actual velocity that you would expect for an orbit" A faster move in a dilated frame??? Commented Dec 28, 2016 at 20:04
• I mean that you can do a calculation, even using Newton's gravity, to calculate the speed the Moon needs to have to orbit Earth. Then you can go on an interstellar mission that takes you only one day, but is a month back on Earth. You cannot use the speed you calculated for the Moon and multiply by one day to get how far it moves, because you can easily calculate that a month will have gone by for the Moon. This doesn't mean the Moon is suddenly not obeying Newton's laws, it means you are in a noninertial frame and are matching up your "nows" with the Moon's in a new way. Commented Dec 28, 2016 at 20:10

Mann's planet appears to move much faster from the surface of the Miller's planet due to the effect of Time Dilation on the landing crew. Time is dilated because Miller's planet is nearer the black hole (called Gargantua), and thus suffering greater acceleration to counter gravitational attraction - in accordance with general relativity. This film is supposed to be about as good as it gets in terms of scientific accuracy.

• A faster movement in a dilated time frame??? Shouldn't movement be slower or at least equal following proper time or relativity and equivalence principles? Commented Dec 28, 2016 at 19:40
• Time is slower in the dilated time frame, for the landing party. So for them objects in other frames of reference away from the black hole seem faster. As you approach the black hole you are changing you frame of reference. Commented Dec 28, 2016 at 19:42
• I should say "as you move from one orbit to another that is nearer the black hole", rather than "as you approach". Commented Dec 28, 2016 at 19:55
• You wrote "So for them other frames of reference seem faster" If that were true there would be no gravitational Redshift, wrong? Commented Dec 28, 2016 at 19:59
• You are correct, it is wrong. Not sure I understand why you think there would be no redshift well enough to pinpoint what is wrong Photons experience time dilation as they leave a gravitational well, when they get into the observer's frame of reference they will have been red shifted. (NB: There are different causes of redshift. Photons also experience red-shift due to the relative velocity of the source and observer. This is a different effect.) Commented Dec 28, 2016 at 20:07