Interstellar: A Visual Masterpiece with Disappointing Writing and Physics

[OmCheeto]

mix two threads together: this one and the dark mater one and make "lovions" able to travel by dark mater transmission so two people in love can send their love across the universe in a form of FTL communications. the governments start a program of collecting people with a deep love bond and sending each in opposite directions across space to create an interstellar communications network...etc...

Bah! You've not listed an antagonist. Throw in Brad Pitt, and we've got the sequel to Thelma and Louise.

script to follow, after my nap... zzzz...

 

[Greg Bernhardt]

 

[Matterwave]

I was most confused how there could be a 7year to 1 hour time dilation factor between being on the surface of a planet, and orbiting the planet...anyone have any indications on how that happened? I mean the surface of the planet is what a few hundred km away from the orbit? A few thousand? How can they travel through such a huge gravitational potential difference in such a short distance?

Unless the planet itself was a black-hole, I don't see how this could possibly work...

 

[DaveC426913]

The whole planet was in a high grav potential.

In a nutshell, getting to the planet required them to get nearer the BH.

They had a quick diagram showing this.

 

[nikkkom]

The whole planet was in a high grav potential.

In a nutshell, getting to the planet required them to get nearer the BH.

They had a quick diagram showing this.

View attachment 75767

You do realize that this requires delta-V in the neighborhood of 0.5c?

 

[DaveC426913]

Yeah. The more I think about this particular maneuver, the more I see the egregious inaccuracy in it.

It's a dilation factor in excess of 60,000.

That's equivalent to 0.9999999999c or approaching the speed of light to within one part in 10 billion.

 

[hankaaron]

You do realize that this requires delta-V in the neighborhood of 0.5c?

And I thought the shuttle craft escaping 1.3G was far-feteched. Geez.

 

[hankaaron]

The whole planet was in a high grav potential.

In a nutshell, getting to the planet required them to get nearer the BH.

They had a quick diagram showing this.

View attachment 75767

A couple of questions.

1. If the Endurance is in orbit around the planet wouldn't it eventually end up in the high gravity well too.
2. So 23 years passes for Romily (who remained in Endurance) and seven hours passed for Cooper and Brand on the planet. Let's say in that seven hours Cooper and Brand measure the planet rotate one-quarter spin on it's axis. Does Romily measure the planet making a one-quarter spin in 23 years?

 

[Fredrik]

That's equivalent to 0.9999999999c or approaching the speed of light to within one part in 10 billion.

But they're near a supermassive black hole, so it's probably not about speed at all. I'm more concerned that the engines would burn up all the fuel rather than produce anywhere near enough thrust, and even if the necessary thrust was obtained, it would crush the astronauts against the floor. Meanwhile, the mothership would probably have used up all its fuel trying to keep up with the planet. (If it's in orbit around the black hole at a higher altitude, then its speed should be slower than that of the planet). Of course, I haven't actually tried to figure out what's going on near a rotating black hole. I'm just naively applying what I remember about Schwarzschild black holes. We would really need to look at the Kerr solution to understand this.

 

[DaveC426913]

A couple of questions.

1. If the Endurance is in orbit around the planet

It isn't.

This is worth emphasizing, so people don't get the wrong idea:
The Endurance did not orbit the planet while Coop and whats-her-face made planetfall. The Endurance orbited the black hole, in a higher orbit than the planet, so less time dilation. This was shown in the film.

See diagram in post 120.

But they're near a supermassive black hole, so it's probably not about speed at all. I'm more concerned that the engines would burn up all the fuel rather than produce anywhere near enough thrust, and even if the necessary thrust was obtained, it would crush the astronauts against the floor.

Equivalence Principle. Acceleration and gravity are equivalent. It's just easier to grasp just how ridiculous the thrust would need to be when you see it as acceleration as opposed to fighting gravity.

Meanwhile, the mothership would probably have used up all its fuel trying to keep up with the planet. (If it's in orbit around the black hole at a higher altitude, then its speed should be slower than that of the planet). Of course, I haven't actually tried to figure out what's going on near a rotating black hole. I'm just naively applying what I remember about Schwarzschild black holes. We would really need to look at the Kerr solution to understand this.

No need to keep up. They'll pass each other often enough.

 

[Rive]

The Endurance did not orbit the planet while Coop and X made planetfall.

That part is the other side of this business. To fall so deep to that gravity well would mean the same speed to loose before they can enter that atmosphere.
That speed is not something to burn up with aerobrake...

 

[dragoneyes001]

lol they had a ready supply of anchors to toss out into the upper atmosphere to slow them down without the use of fuel

 

[DaveC426913]

That part is the other side of this business. To fall so deep to that gravity well would mean the same speed to loose before they can enter that atmosphere.
That speed is not something to burn up with aerobrake...

Agreed. I noticed that when I started thinking about just how far apart the two orbits must be (in distance, but more importantly in orbital delta v!) to get a factor 60,000 time dilation between them.

So, we;re not just talking about a climb out of a gravity well, we're also talking about an orbital speed-matching.I am beginning to see that this one plot point is a far more egregious scientific error than all the rest put together.

KT really should have made the dilation factor much, MUCH smaller - say 3,000 times smaller - and found some reason to strand them on the planet for months or a year. Then that 20 years could have passed with a slightly less outrageous science blunder.

 

[DaveC426913]

Though I suppose, technically, the Endurance could have sat in the L2 Lagrange point above the planet... :D

 

[QuantumPion]

Perhaps they used the BH to perform a gravity assist or somehow used the properties of the BH to extract energy from it (Penrose Process?)

 

[Rive]

KT really should have made the dilation factor much, MUCH smaller - say 3,000 times smaller - and found some reason to strand them on the planet for months or a year. Then that 20 years could have passed with a slightly less outrageous science blunder.

Practically any dilation factor with noticeable effect would be the same. At that point of the story we already had some hints about the magnitude of Δv available - two years from Earth to the wormhole! - and it's nowhere to the necessary to achieve any real dilation factor // climb in or out of any gravity well with real dilation factor.

It cannot be helped. They had to cheat.

 

[Fredrik]

I doubt that velocities are even relevant to this problem. We have to compare the proper time of the world line of an object in orbit around a rotating black hole, to the proper time of the world line of an object that starts and ends in that same orbit, but takes a detour down to a lower orbit and stays there for a while before it climbs back up. I expect the contribution from the velocity difference to be negligible compared to the contribution from the "altitude" difference.

 

[Rive]

Altitude change is velocity (usage of the Δv available for the device).

 

[Fredrik]

Altitude change is velocity (usage of the Δv available for the device).

Obviously you can't change your position without changing your velocity, but this isn't special relativity. In principle, you can get a huge time difference by descending straight down at walking speed and then coming back up just as slowly. A clock that hovers at a fixed lower altitude accumulates less time than a clock that hovers at a fixed higher altitude, because even though their coordinate velocities are zero, the clock at the lower altitude has a greater proper acceleration. (Less proper acceleration = closer to inertial motion = closer to maximizing the proper time).

 

[Rive]

You are right, we are just following a slightly different track.
It's as DaveC426913 said: it's easier to grasp it if we calculate with speed (change) instead of gravity potential. But both view is 'right'. Yours might be a bit more accurate (and also harder to discuss as in this case it's about a really special case of general relativity).

 

[Fredrik]

You are right, we are just following a slightly different track.
It's as DaveC426913 said: it's easier to grasp it if we calculate with speed (change) instead of gravity potential. But both view is 'right'. Yours might be a bit more accurate (and also harder to discuss as in this case it's about a really special case of general relativity).

Mine is an oversimplification, since I'm ignoring that the black hole is rotating and that the mothership and planet are both in orbit around the black hole. It might be close enough to the actual answer, but this is far from certain. The velocity argument is completely missing the detail that's the reason for the age difference in the oversimplified picture, so it's very likely to be completely wrong.

The only way to settle this is to calculate the proper times of the relevant world lines in a Kerr spacetime.

 

[OmCheeto]

Agreed. I noticed that when I started thinking about just how far apart the two orbits must be (in distance, but more importantly in orbital delta v!) to get a factor 60,000 time dilation between them.

So, we;re not just talking about a climb out of a gravity well, we're also talking about an orbital speed-matching.I am beginning to see that this one plot point is a far more egregious scientific error than all the rest put together.

KT really should have made the dilation factor much, MUCH smaller - say 3,000 times smaller - and found some reason to strand them on the planet for months or a year. Then that 20 years could have passed with a slightly less outrageous science blunder.

First off, I'd just like to say the science is now way over my head.
Secondly, I think the 60,000 factor may have some relevance for the sequel.
Do you remember the guy they left for dead? When Cooper flies back through the wormhole, he's going to get a scratchy message; "Hey! I'm still alive. Can you come pick me up, PLEASE..."

I'm basing this idea purely Hollywood physics, of course.

 

[QuantumPion]

First off, I'd just like to say the science is now way over my head.
Secondly, I think the 60,000 factor may have some relevance for the sequel.
Do you remember the guy they left for dead? When Cooper flies back through the wormhole, he's going to get a scratchy message; "Hey! I'm still alive. Can you come pick me up, PLEASE..."

I'm basing this idea purely Hollywood physics, of course.

Time dilation wouldn't make a message delayed and then suddenly appear decades later. It would make the message redshifted and slowed down. Even with a factor of 60,000, you could still receive about 1.4 seconds of message per day.

 

[dragoneyes001]

"I'm alive and now the interstellar record holder for biggest wave surfer in history! Aloha!"

 

[Matterwave]

The whole planet was in a high grav potential.

In a nutshell, getting to the planet required them to get nearer the BH.

They had a quick diagram showing this.

View attachment 75767

But how in the world do you fine tune the black hole's potential to give you high potential something like 100km away from low potential? It doesn't seem like it would work unless the black hole itself was ~a few 10's of km in size... and at that point I would be very doubtful that the planet can get so close and still have an orbit...

 

[DaveC426913]

But how in the world do you fine tune the black hole's potential to give you high potential something like 100km away from low potential?

Where did 100km come from? We have no idea how far the Endurance's orbit was above the planet's orbit.

It doesn't seem like it would work unless the black hole itself was ~a few 10's of km in size... and at that point I would be very doubtful that the planet can get so close and still have an orbit...

I've been thinking that myself.

Unfortunately, this plot point is getting squeezed between a rock and a hard place, because to have such a high gradient also means there is a high gradient across the planet's diameter. i.e. colossal tidal forces will rip the planet to rubble. Gotta be well within the Roche Limit.

 

[Matterwave]

Where did 100km come from? We have no idea how far the Endurance's orbit was above the planet's orbit.

I guess it sort of looked that way to me when I was watching the movie. I didn't see Endurance being a huge distance away from the planet.

 

[hankaaron]

Unfortunately, this plot point is getting squeezed between a rock and a hard place, because to have such a high gradient also means there is a high gradient across the planet's diameter. i.e. colossal tidal forces will rip the planet to rubble.

But the gradient shouldn't be greater than the inverse square law, right?

 

[DaveC426913]

But the gradient shouldn't be greater than the inverse square law, right?

The point is that they had to traverse a grav potential where the time dilation was 60,000.
If the grav pot across the planet is bigger than, like, a fraction of 1, then it would be torn apart.
Which means that they would have had to traverse a distance that is 60,000 x some inverse of that fraction x the diameter of the planet.

So, say, a planet falls apart when gravitational potential across the planet's diameter is big enough to cause a time dilation of, say 0.001 from one side to the other. (That's huge, it's probably orders of magnitude smaller).

So, now they must have passed 60,000 x 1000 x the diameter of the planet (let's call it 10,000 miles), for a total distance from Endurance to planet of 600 billion miles. these are just hypothetical numbers, but it's a real problem.

 

[Matterwave]

The point is that they had to traverse a grav potential where the time dilation was 60,000.
If the grav pot across the planet is bigger than, like, a fraction of 1, then it would be torn apart.
Which means that they would have had to traverse a distance that is 60,000 x some inverse of that fraction x the diameter of the planet.

So, say, a planet falls apart when gravitational potential across the planet's diameter is big enough to cause a time dilation of, say 0.001 from one side to the other. (That's huge, it's probably orders of magnitude smaller).

So, now they must have passed 60,000 x 1000 x the diameter of the planet (let's call it 10,000 miles), for a total distance from Endurance to planet of 600 billion miles. these are just hypothetical numbers, but it's a real problem.

Lol, 600 billion miles is well outside of our solar system (I think Neptune is ~1-2 billion miles away from the Sun?)...in what way is that called an "orbit" on the planet? In addition it took Endurance 2 years to reach Saturn, if Endurance was 600 billion miles from the planet, either the shuttle is orders of magnitude faster (then, why not just use the shuttle?) or it would have taken them ~300 years to get to the planet from Endurance, and then 300 years to get back (not counting all of that gravitational potential and time dilation they have to go through).