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Comment by ouid | Hacker News Reader

Interstellar had really bad orbital mechanics. You remember when they used the staged rocket to get off earth? Do you remember later, when they landed on that other planet with 1.5 times the mass of earth? Do you remember the machine that they used to leave that planet?

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Remember when he let go of the spacecraft and fell into the black hole?

I recall a distinct lack of spaghettification.

I don't recall the exact scene, but wouldn't the different relative centers of mass, and the corresponding difference in distance from the black hole cause some drift. I assume it would be slow. Possibly VERY slow, so it likely wouldn't look like falling, but I think they would separate, right?

I read that as you fall into a black hole, there comes a point where the gravitational difference between your head and toes would be large enough to rip you apart. (Not that you wouldn't have other problems anyway :-) )

If they're as close to a decently-sized black hole as dramatic movie pictures require, I suspect they would be close enough for the separation not to be gentle at all, assuming their individual limbs wouldn't separate.

they were spinning, right?

I haven't see the movie, but this only makes sense if the spacecraft is actively using thrusters, and the human obviously is not.

Mass has little do escape velocity, it's density. Consider that any planet/sun could be shrunk until it's a black hole, with effectively infinite escape velocity.

Earth density: 5.51 g/cm³, escape velocity: 11.2 km/s

Jupiter density: 1.33 g/cm³, escape velocity: 59.5 km/sec

I think it's more than just density.

first of all, mass contributes more to escape velocity than density. V_e=k*(m^(1/3)d^(1/6)). (scaling the mass is quadratic compared to scaling the the density). Second, the density of rocky planets is bounded from below pretty close to the density of earth.

A black hole has the same escape velocity as a star of the same mass, as long as you are well outside the swarzchild radius. Inside or near it, the effect of general relativity makes things very weird.

In case of mostly anything else, Newtonian physics apply correctly, and the relevant thing is mass (and distance from the center). The escape velocity from a body is sqrt(2GM/d) where d is the distance from the center of mass, M is the mass of the object, and G is the universal gravitational constant.

Note that if the sun was replaced with a black hole of the same mass, the orbit of the earth wouldn't change.

escape velocity is calculated from the surface.

Not necessarily, for example we'd measure escape velocity from our system as starting from Earth, so effectively we'd be 1 AU above Sun's surface.

It's unspoken, but I assume the staged rocket was used to conserve the antimatter the ship was carrying for its rocket-less launches.

I wouldn't be surprised if that's in the script somewhere but ended up on the cutting room floor.

Except if the antimatter engines were so reliable, why risk blowing up your last hope to conserve the obvious surplus of antimatter you had for the mission anyway?

There were several plotpoints about how there was NOT surplus of antimatter (or whatever propellant they used).

And in case of "blowing up" they could use those am engines as LAS (they apparently had enough thrust), so what risk are you speaking about?

Seriously, bitching about science stuff in Interstellar, very large but actually very well crafted scifi is kind of dumb. It's big enough that you find SOME mistakes, but most of the time you'll be wrong (or blaming them for not explaining every detail), and it has pretty correct science overall. Just hating for sake of hating.

I think you're giving them too much credit. Happy that it didn't "unsuspend disbelief" for you, but it's too easy to make creative excuses for movie fridge logic.

Christopher Nolan seems like a pretty smart guy. It must have crossed his mind at some point.

Plus, the ship's engines don't appear to use propellant - they use the standard sci fi "eeire blue glow" trope. Seems like antimatter is the best explanation to me.

> Christopher Nolan seems like a pretty smart guy. It must have crossed his mind at some point.

Also, he had Kip Thorne as an advisor. I remember criticism of the "slow-time planet" (including by Bad Astronomy) being shot down by Kip pointing out he actually calculated it as an edge-case solution for a specific rotating black hole.

even the way time passes faster on The planet with tidal waves is complete bullshit. It does not match any of the assumptions aT all and is just used as a stupid trope.

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