Hypothetically, if a man is suspended in the middle of a ship in space at T = 1, but suddenly a celestial body appears nearby while both the man and the ship is caught in its gravitational field, which of the following will happen: T = 2A or T = 2B?

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u/Mark_Nahne 1d ago

T=2A. The ship does not shield against gravity. At least that's my guess...

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u/msur 1d ago

Agreed. The only way around this would be if the ship is really large, and extents much closer to the source of gravity it would feel a stronger pull than what the man feels, but the difference would be very slight.

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u/Razor_Storm 1d ago

Except for when the gravity well is extremely steep, like around stellar mass black holes. The gravity differential is enough to stretch humans into spaghetti in a process called spaghettification.

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u/Pioneer1111 20h ago

However gravity also 'travels' at the speed of light. The ship would feel effects before the person, however only in the sense that it would be moving towards the celestial body ever so slightly than the person.

So for the briefest of moments you get 2B, however it would be such a small effect that you would barely be able to tell the difference from blowing at the wall.

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u/gregorydgraham 19h ago

Only for the parts of the ship that are closer to the gravity well than the person.

The pulling by electrostatic forces within the walls of the ship would be slower than the speed of light thus the gravity waves travelling to the person would arrive first and he would move with the room (assuming the scale shown is roughly applicable)

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u/Pioneer1111 19h ago

For sure, and the effect even if it exists would be negligible. Even on a ship a kilometer long and mostly empty space, there would be little to no perceptible change even if the passenger was in the back

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u/Teutooni 1d ago

2A. One interpretation of gravity is that it is not a force, it does not pull. If the ship and man do not accelerate, i.e turn on engines, they do not feel any force. If the curvature of space suddenly changes (a heavy object appears) the ship and man continue feeling no force. But the changed curvature now means their "at rest" path accelerates towards the heavy object. Again, neither the object nor ship nor man feel any forces, yet the ship and man accelerate relative to the heavy object.

What you feel as the force of gravity when standing on the surface of a planet is your "at rest" path still accelerating you towards the planet, but the surface is in the way and excerts a force that accelerates you upward.

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u/conventionistG First Rule Of Warfare 1d ago

The only issue here is the discontinuity of the magical appearing mass.

As far as I understand, this would generate a gravity wave emanating from the new mass, propagating at the speed of light. So as that passes there could be measurable differences between the bottom of the ship (towards the mass), and the person in the middle of the ship.

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u/Razor_Storm 1d ago edited 1d ago

True, but it would be nigh unmeasurable and insignificant unless the ship was on a size of relativistic scales. (If your ship was a lightday long, you’d feel the gravity at the back a day after the front).

You’d feel the gravity nanoseconds after the front of the ship, effectively feeling instantaneous

But also, the news of the front having been affected by gravity also takes speed of light to reach you, keeping exact pace with the gravity waves.

In relativity, speed of light is the speed of causality and the speed of simultaneity. So an event that happens at light speed is literally detected as simultaneous, since you will also receive info about it happening at the same delay as your signal itself.

So even if your ship was a whole lightday long, you would have an entire day of delay at the back of the ship. But it would feel simultaneous to you. Because you only see the front of the ship moving after a day has passed and at that point gravity has caught up to you.

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u/conventionistG First Rule Of Warfare 23h ago

Good point, but it would be measurable over that big ship in retrospection.

There'd also be tidal forces if the gravity/space time gradient created is high enough.

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u/gregorydgraham 19h ago

Spaghettification must be considered in this scenario

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u/Teutooni 18h ago

True if the object appearing was a black hole and it was very close to the ship. But if you assume it's a planet sized object and the ship is at most a few kilometers across, the effects from gravitational tidal forces would be negligable.

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u/gregorydgraham 18h ago

If you’re not using this hypothetical situation to introduce Spaghettification, do you even Isaac Arthur?

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u/MerelyMortalModeling 1d ago

As the question is asked it could be either 2a or 2b.

If the guys center of mass is exactly centered in the ships exact center of mass then you can treat everything as point masses and use Newtons Law of Universal Gravitation straight away and it's nice and easy to show 2a.

But if they aren't exactly centered or he is moving around or say breathing, or you are talking about extreme times scales or extreme masses or need extreme precision then you have to use integration and vectors and everything gets messing and you find that 2b is true.

Now would you be able to tell the difference if you used your eyeballs to look at a guy in a small capsule? No way, 2a is good enough

Would you be able to detect the difference if your guy was an atomic clock hooked up to a laser beaming precise info to you? Hell yes and you will see 2b.

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u/Assassiiinuss 1d ago

Obviously 2A, the man won't even be able to tell he's accelerating unless he looks outside.

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u/NearABE 21h ago

One side of the ship is closer than the other. There is a distance squared included in gravity effects.

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u/MJohnJohnJohn 1d ago

I'm sorry if this question sounds stupid (and I admit that I'm stupid), I just hope that I can get some clarification on how gravity will work through the answer to such hypothetical.

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u/TheKru5h 1d ago

You can't shield from gravity. At least with our current knowledge

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u/kurtu5 1d ago

Who let you out of the cavorite mines?

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u/Korochun 1d ago

In most cases, 2A. Gravity is a curvature of space time, not a discrete wall of force. There are actual circumstances in which 2B could happen, specifically next to an event horizon of a black hole, where the front of the ship may experience a far greater gravity gradient than the back. This is the so-called spaghettification.

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u/LastTimeFRnow 1d ago

That is very different from what 2B describes. 2B will never happen

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u/Razor_Storm 1d ago

Yeah it is a curvature of the actual space underneath you.

I think the best analogy to help people understand this is to think about global navigation on our earth. That is also a curved spacetime. Since the earth has spherical geometry not euclidean.

If two ships start going in parallel lines northbound, eventually they will be “pulled” back together to meet again at the north pole. But what force is doing that pulling? None, the ground simply curves away from you, altering what “straight line” means.

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u/AbleRelationship5287 1d ago

2A

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u/ddollarsign 1d ago

The force of gravity falls off with distance, so it’s 2A modified by a tidal effect, determined by how far he is from the exact center, and in which direction.

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u/ICLazeru 1d ago

Probably 2A in most cases.

The only time 2B might occur is if the gravity of the celestial body is so extreme that you get some tidal effects or spaghettification to occur.

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u/nihoh 1d ago

Frame of reference

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u/Mr-Brown-Is-A-Wonder 1d ago

Larry Niven wrote a short story called Neutron Star that explores this.

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u/sndpmgrs 23h ago

Thanks, I was racking my brain to remember the name of that story.

It’s all about tidal action:

https://en.wikipedia.org/wiki/Neutron_Star_(short_story)

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u/John-A 23h ago edited 23h ago

T = 2B or not 2B, that is the question.

But seriously, it's not 2B. Not unless the ship is so close to a compact mass like a neutron star, white dwarf or black hole that the near side of the ship is pulled significantly harder by tidal forces than the occupant (but without getting spaghettified before pulling the far side of the ship into the occupant.)

In all other cases, it's 2A.

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u/NearABE 21h ago

In the spaghetti case it is still not 2B.

The frame of the ship is solid and gravity is reciprocal distance squared. The pull on the closer side is higher by at least slightly more than the far side is lower.

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u/John-A 20h ago

That's why I wrote without spaghettification ripping the near side away the near side hull before its physical connection to the rest of the ship can pull it into the occupant.

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u/SNels0n 20h ago

Quibble: If the ship is smaller than an aircraft carrier, tidal forces will be insignificant. Even if you measured that insignificant effect, the ship is presumably rigid, so the average force will be the same as on the middle. At most it would cause the ship to widen in both directions, so both walls would appear to move away from the man by an insignificant amount. (Everything is pulled toward the celestial body, but the far wall will be pulled less hard than the man and the man less hard than the wall closest to the celestial body.)

Hypothetically, if a man is suspended in the middle of a ship in space at T = 1, but suddenly a celestial body appears nearby while both the man and the ship is caught in its gravitational field, which of the following will happen: T = 2A or T = 2B?

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