# How was Jim able to space walk when the ship was traveling at .5 c?

Until now any space movies that I've seen where they have shown space walk, they show that either the ship isn't moving or the character is on the spaceship because of the magnetic boots. However in Passengers, Jim was able to space walk at .5 c and not get smashed to the ship in the process.

With my understanding, the laws of physics should still apply at that situation even after being in the vacuum of space.

As the ship's engine are still firing. Why doesn't the ship accelerate away while he's not anchored to it?

• Because Jim is also moving at .5 c – Valorum Jan 11 '17 at 23:00
• Note we are hurtling through space at an insane speed at this moment (riding on the back of the earth); did you notice it? No, because you can't feel speed, you can only feel acceleration. "The wind in your face" makes you think you can feel speed, as do the bumps in the road but that's not the speed itself. That said, if the main engines are on presumably there is some acceleration coming from them (but perhaps not much) – user20310 Jan 11 '17 at 23:14
• How is 0.5c any different than the ~17K mph/27K km/h (relative to the earth) that spacewalking astronauts on the ISS experience? Then there's the Earth's motion around the Sun, the Sun's orbit around the center of the galaxy, our galaxy's headlong rush towards wherever we're going... – jamesqf Jan 12 '17 at 5:00
• @user20310 To be even more exact, we can only feel a difference between accelerations. If all of your body is accelerated equally (as in free fall in a vacuum), you don't feel a thing. When sitting on a chair, you experience zero net acceleration with regards to the ground (since it's being cancelled out by the chair pushing back at you), but you feel the full force of gravity against the full force of the chair pushing back at you. You could say that what we really feel is the compression or tension that results from differential forces (though that's not the entire truth either). – Luaan Jan 12 '17 at 8:50
• I thought this was going to be a question along the lines of: "How did Jim not die instantly since at 0.5 c he's encountering ~10^14 Hydrogen atoms / second, each at 145.25 MeV?" – imallett Jan 13 '17 at 8:11

The very short answer is that the acceleration from the ship's engines is likely to be quite small. Note that this is an ionic drive that accelerates continually rather than a rocket that expends all of its fuel in a g-force inducing blaze of glory.

[On Computer Screen]
Failing systems: Life support. Fusion reactor. Ionic Drive

Assuming that the ship goes from .5 of lightspeed to .55 of lightspeed in the two years that Jim's awake, the relative rate of acceleration while he's out of contact with the ship's deck would be substantially below .5kph. In the couple of minutes he was outside, he would travel less than the length of a swimming pool (relative to the ship), a hardly noticeable amount.

On top of that, the ship has a "gravity drive" that appears to be able to affect the gravity within the ship. It's possible (if not downright likely) that this drive is calibrated to stop spacewalkers from experiencing the effects of the accelerating engines by pulling them forward at much the same rate as the engines are pulling them back.

• I think the question is more complicated than this. Unfortunately, due to the speeds involved, we're going to need to introduce relativistic frames. If we use your numbers, then because of the way relativistic velocities add, he would see the ship accelerating at 1.171km/h/s away from him. Outside for two minutes, he would see a drift of 2362 metres, well enough to take him away from the ship. – A Simmons Jan 12 '17 at 12:05
• @A.Simmons - At .5 of c, relativity effects would be less than 16%. Additionally, as he's moving at .5 of c already, the relative difference in speeds between him and the ship accelerating away from him would be negligible, something like 0.500000001kph – Valorum Jan 12 '17 at 12:11
• Your answer makes no sense, how can an acceleration be measured in kilometres per hour? – theonlygusti Jan 12 '17 at 13:13
• @user20310 - Note that Jen doesn't float gently to the floor when the drive turns on. She drops like a stone. – Valorum Jan 12 '17 at 14:45
• @User20310 - It's also rather clearer in the original script that the 'Gravity Drive' (and gravity manipulation in general) are what made interstellar flight possible. – Valorum Jan 12 '17 at 14:47

We are hurtling through space at an insane speed at this moment (30 kilometers per second riding on the back of the earth); did you notice it?

No, because you can't feel speed, you can only feel acceleration. "The wind in your face" makes you think you can feel speed, as do the bumps in the road but that's not the speed itself.

Again, think of travelling in a plane (no wind, few bumps); feels like you're barely moving because youre barely accelerating

In answer to your second question; yes, if the engines are firing there is acceleration, and if he's outside the ship it should be accelating away from him. That said; an alternative kind of engine, the ion drive is designed for long low burns rather than our current space rockets which are short and powerful, this can be far more fuel efficent. If thats the case perhaps it was accelerating away from him, just not by very much. This is consistent with the fact that the main engines seem to provide negligible artificial gravity, almost all artificial gravity coming from the gravity drive, so acceleration is far less the 1g

• We're also accelerating, actually, towards the center of the Earth. ;) I leave it as an exercise for the reader to figure out the magnitude of our acceleration. – jpmc26 Jan 12 '17 at 3:39
• @jpmc26 No, we are not accelerating as velocity does not change. We are being attracted towards the center of the Earth, but that force is compensated by the one exerted on our feet (or our ass, if you are sitting) by the floor/chair you are on, with a net result of 0 (no force = no acceleration = no change of speed). All of the above does not apply if you are in an elevator or in free fall right now. – SJuan76 Jan 12 '17 at 9:37
• @SJuan76: you're neglecting the Earth's rotation. – Harry Johnston Jan 12 '17 at 9:55
• @HarryJohnston I am at the North Pole. Now seriously, Earth rotation is only a minimal componentn. – SJuan76 Jan 12 '17 at 10:01
• @SJuan76 Velocity is a vector quantity. That means that changes in direction are also acceleration. Moving in a circle is a well understood motion, and it requires a rather significant acceleration vector pointing toward the center of rotation. Moving in an ellipse is similar, but the acceleration vector wouldn't be a constant magnitude. – jpmc26 Jan 12 '17 at 14:10

Narrative magic. Writers rarely worry about numbers.

The bigger question is How was he not cooked through

He is moving through interstellar space at .5 c outside the ship.

Interstellar space is not perfectly empty. There is a thin mist of atoms, mostly hydrogen.

There's about 1 atom of hydrogen per cubic cm.

While outside the hull then as long as he's not in the shadow of the ship he's sweeping a path through space hitting the atoms in that volume of space.

Lets assume that the area of the forward facing surface of his suit exposed as he moves through space is about .5 meters square.

If he spends 30 minutes outside the hull then he'll have traveled about 15 light minutes through space.

So he'll have passed through about 135 km^3 of space at .5 c

So we're talking about hitting 1.35×10^17 hydrogen atoms at .5c

That is a very small quantity of material but it's going very very very fast.

The that's 20 megajoule of kinetic energy over 30 minutes.

At those speeds the atoms will either rip right through him as radiation or stop in his suit or inside him and produce heat.

http://www.wolframalpha.com/input/?i=kinetic+energy++of+1.35%C3%9710%5E17+hydrogen+atoms+at+.5c

Imagine lighting a fire and burning about a liter and a half of petrol over about half an hour with someone suspended a few inches above it.

They'd probably be a tad crispy.

Even if the ship isn't accelerating he's still screwed

There's a much bigger problem as well:

http://www.wolframalpha.com/input/?i=velocity+of+75+kg+with+20+megajoules+of+kinetic+energy

Being hit by those hydrogen atoms doesn't just heat him up and possible give him cancer, it also slows him down by about .4 meters per second assuming he weighs about 75kg.

after 30 minutes he'd be traveling something like 730 meters per second slower than the ship and is screwed.

These numbers are of course all approximate and making some naive assumptions like perfect transfer of momentum and nothing zipping through him without stopping etc but it should be in around the right ballpark.

Many scifi stories dealing with travel faster than about .1c tends to run into similar problems because it's hard to comprehend just how fast large fractions of light speed really are. These are speeds where a pebble hits with the energy of a nuclear bomb.

• I'm pretty sure that the ship was protected by the shield so it would make sense that the shield would extend to protect space walkers. – Erik Jan 12 '17 at 17:22
• The ship has a gigantic magnetic ramscoop. – Valorum Jan 12 '17 at 17:26
• Actually, the atoms that "rip right through him" aren't the ones that you want to worry about. It's the ones that stop within (or at least are slowed down by) his body tissues that will do all of the harm. – Solomon Slow Jan 12 '17 at 18:35

Regardless of whether you apply Newtonian or Relativistic systems to this, the ship should accelerate away from the space walker if he's not anchored to the ship.

Before he leaves the ship, both he and the ship are not moving relative to each other because they are both accelerating.

Once he leaves the ship, he's no longer accelerating as part of the ship. It will accelerate away from him.

It doesn't matter whether we're at relativistic speeds relative to some other object ( like Earth ). To the ship and the space walker, their relative speed starts as zero ( same in Newtonian and Relativistic mechanics ). In the absence of any acceleration they'd stay in the same relative position in space.

But with one accelerating and the other not, their relative motion changes.

As a result the space walker sees the ship accelerate away, and the ship sees the space walker fall far behind it.

Some interaction between ship and space walker would be required to prevent this. A physical line, some sort of magnetic field, something. Without such an interaction, the walker is in trouble.

• This is a good overview of the situation, but doesn't answer the question of why he doesn't head backwards relative to the ship's forward acceleration. – Valorum Jan 12 '17 at 15:50
• He doesn't because the ship's writers didn't bother with real physics. I did offer a possible wild idea ( magnetic field ), but real world - that guy is dead because even if he tries to touch the ship again before it passes beyond him, it's moving and he isn't so "splat" is a good word for what will happen. – StephenG Jan 12 '17 at 16:01
• @StephenG Why "splat" and why not "silent groping at a departing object as he floats aimlessly into wild space" ? Or is there a part of the ship behind him that he would be hit by as it accelerates without him? (I haven't seen the movie yet) – TylerH Jan 12 '17 at 16:03
• @TylerH - Well, he's tethered to the ship so he's not just going to float off. – Valorum Jan 12 '17 at 16:15
• @TylerH - When he went for his initial spacewalk an automatic tether snaked out of the wall and attached itself to him before he was able to leave the airlock. I don't recall it detaching when he turned off his magnet boots. – Valorum Jan 12 '17 at 16:42

My knowledge of the movie is limited to the trailers, but there's a few things we can glean from the ships design.

• The ship has a helical structure firmly attached to the central spine
• The helical structure does not appear to have any points of articulation
• The ship spins
• There aren't obvious reaction mass tanks on the design

So, from this, we make the following assumptions

• The engine is a high efficiency, low-thrust drive
• The helical structure is a centrifuge/habitation area

And from those, we can further deduce

• Gravity comes completely from the ships spin, as a high-thrust engine would push you into the wall and not the floor

This is a very long way of saying the ship doesn't accelerate very quickly. Millimeters per second^2, tops.

So how does he not drift away from the ship when it's traveling at .5c? Because speed != acceleration. There's no space-wind to blow him back, so the ship is gonna pull away at a few mm/s/s - something he'd have to wait outside for a few hours to even notice. Relativistic effects don't even come into it, since they're moving at pretty much the same speed and have the exact same frame of reference.

This is ignoring any fancy handwavium gravity fields to keep him in place.

• Sure, but if you have gravity control, Occam's Razor suggests that the answer to any question about why "gravity doesn't x" is going to be "because we can control it" – Valorum Jan 12 '17 at 19:40
• @Valorum True, but if you have a magical gravity generator reliable enough to (presumably) function on a centuries long voyage, why bother making your ship a big, spindly, and fragile centrifuge? Just make the ship a big brick with engines on one end and crew on the other... All this being said Why bother with artificial gravity anyway? All the passengers are in cryo, so they don't need it. A couple months of zero-g while they figure out landing zones for a colony isn't going to hurt much. Hell, it'd probably be the most fun they ever have. – UIDAlexD Jan 12 '17 at 20:03
• A better question is "why make the ship like a luxury cruise liner if the passengers are off to a colony world of hardship and a life of quiet desperation?" – Valorum Jan 12 '17 at 20:09
• @Valorum Given how well thought out everything else seems, I'm guessing Quantum Mechanics/ Nanomachines/ PopSci garbage written by someone with no scientific background. Begs the ultimate question, Why bother watching this movie? – UIDAlexD Jan 12 '17 at 20:13
• @Valorum Can't they, when landing, use the cruise liner as a temphome? Until they've built something equivalent in situ... – Vincent Vancalbergh Jan 12 '17 at 22:00

The engine is a combination of ION drive and cold fusion. It accelerates the ship constantly. At half trip, the ship will turn 180 degrees and start decelerate for the last half of the journey. The artificial gravity is a combination of centrifuge rotation and engine slow acceleration. That is why the floor is at an angle relative to "level ground". The movie is a masterpiece of science! Almost zero faults. The only one could maybe be the fact they reach Arcturus faster than expected , assuming that the ship is in constant acceleration (thus 0,5c speed was not reached since beginning).The last problem is magnetic shielding, the real issue of interstellar travel! You really need tremendous power to build and sustain such a shield! Imagine that earth magnetic field is not enough to keep everything out and still it moves very slow comparingly.

• This does not specifically answer the question asked. – Gallifreyan Apr 7 '17 at 5:13

The answer is: The ship would eventually accelerate away from him, but slowly since they are both traveling at the same speed and he is subject to minuscule drag in the void of space.

As @Valorum stated there is very little good science behind this movie.

• This doesn't seem to offer anything else that hasn't already been said. – Valorum Jan 14 '17 at 21:32

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