# Can gravity from other galaxies affect Earth, and how?

pegamin
Hello, I'm new at the forum. I'm a science-fiction writer. My question then concerns an imaginary, fictional situation. I'll try to be brief but it's not easy to explain:
Let's imagine that a character who lives on Earth is strongly attracted by the gravitational force of some other celestial body, say in the galaxy of Andromeda or even further away.
This character is so attracted by this extra-galactical gravity that, here on Earth, he/she simply floats in the air. Remember, this is for the sake of fiction.
Ok, can we imagine then that, for example at nights he floats (since Andromeda is "over his head") and at night he stays on surface (since Andromeda still attracts his body, but now the Earth is in the middle)?
I hope I'm explaining myself... thank you

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pegamin
pegamin
I know, it's coming up... but it's going to take a while, I even fear I might not be around to witness the event.
But in the meantime, my character's still suffering his gravity problem... and since his position according to Andromeda varies constantly (sometimes Planet Earth is between his body and the source of attraction), I imagine he would not float all the time?

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No, he wouldn't float all the time. At times he would be pushed down onto the ground at twice his normal weight, and at other times he would feel a force sideways in addition to the downwards force of Earth's gravity. It would be very disrupting.

pegamin and jim mcnamara
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No, he wouldn't float all the time. At times he would be pushed down onto the ground at twice his normal weight, and at other times he would feel a force sideways in addition to the downwards force of Earth's gravity. It would be very disrupting.

Most likely this person would never be pulled straight up or down but off to some angle. In fact I think that Andromeda can't be seen from the southern hemisphere, Cassiopeia can't and it's the constellation one can easily use to find Andromeda just off to one side. So if this protagonist lives in the south he will constantly feel like he has a lopsided, heavy bag on his back.

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pegamin and Drakkith
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If you're trying to stay anywhere near actual physics, this is not a very good mechanism to use.

pegamin
pegamin
Excellent! Thanks a lot for your useful answers, that actually trash my initial conception. Because yes, I'd like to stay somewhere near actual physics.
I mentioned Andromeda for no particular reason, but it could actually be a gravity pull from any other galaxy or celestial body, even farther away than Andromeda. Remember that this is just for the sake of fiction.
I was looking for a situation where poor Floyd (old English for "empty") would float (lopsided or straight up) at times, and stay on surface at other times, and these 2 situations being somewhat regular (predictable, 12 hours for each state) would have made writing easier.
Thanks to your answers, now I know this would be impossible, and if I ever wrote something along these premises I would get massacred by the scientific community.

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The notion of different people or objects experiencing the same gravity field differently is impossible, having a galaxy two and a half million light years away produce a 1g attraction even more so. But if you want to write a story about man who periodically becomes much heavier then so light he floats then just write that.

Unless you’re writing a hard-SF story where plausible science is itself central to the plot then you don’t have to worry about realistic explanations. You can throw out a crazy premise, leave it unexplained and have the focus of the story be the well thought out and interesting consequences.

Not being slavishly hard-SF does not detract from a story, it’s quite orthogonal to it.

pegamin and PeroK
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Most likely this person would never be pulled straight up or down but off to some angle. In fact I think that Andromeda can't be seen from the southern hemisphere, Cassiopeia can't and it's the constellation one can easily use to find Andromeda just off to one side. So if this protagonist lives in the south he will constantly feel like he has a lopsided, heavy bag on his back.
Someone might wonder, like me, what that would do to the wobble of the Earth spinning, and to the perturbation of the Earth's orbit in the long run.
While not a large, it is a continuous force of the person interacting with the earth.

pegamin
pegamin
Someone might wonder, like me, what that would do to the wobble of the Earth spinning, and to the perturbation of the Earth's orbit in the long run.
While not a large, it is a continuous force of the person interacting with the earth.
A very good point, Ryan.
So, following this completely un-scientific train of thought, let's consider your idea (that probably has enough juice in it as to open a dedicated thread).
Could we conceive then that: this not large but continuous force of the person interacting with the Earth indeed provokes a perturbation with the Earth's orbit, and ultimately with the galaxy's current course?
We know that in a relatively (by Universe's standards anyway) short time our beloved Milky Way will melt with Andromeda. Could an event such as imagined above ever prevent this crash, by changing the course of the Milky Way altogether?

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A very good point, Ryan.
So, following this completely un-scientific train of thought, let's consider your idea (that probably has enough juice in it as to open a dedicated thread).
Could we conceive then that: this not large but continuous force of the person interacting with the Earth indeed provokes a perturbation with the Earth's orbit, and ultimately with the galaxy's current course?
We know that in a relatively (by Universe's standards anyway) short time our beloved Milky Way will melt with Andromeda. Could an event such as imagined above ever prevent this crash, by changing the course of the Milky Way altogether?

It was 256bits point rather than mine. You can calculate the effect on the Earth by considering the force the man is exerting. When he's facing a certain direction his weight is doubled, when the Earth spins so that he is pointing the other way he weighs nothing. Therefore on one half of the planet he exerts zero force and on the other he exerts a downward force, how much:

F = m * a

If he masses a healthy 75kg then in his heavier phase F = 75 * 20m/s = 1500 Newtons. Given that this is the max force he exerts we can half it to get an average force on the Earth in the direction of Andromeda as 750 Newtons. What effect is that going to have? Well let's use the equation again and plug in the mass of the Earth to find out how much the average acceleration on Earth will be:

a = F / m

A = 750 / 6e24 = 1.25e-22 m/2

So, on average, the man is accelerating the Earth in the direction of andromeda at a trillionth of an angstrom per second. If this starts when he is ~15 and he lives to 90 that's ~2.5 billion seconds of living like this. In that time he would have imparted a velocity of a third of a picometer a second. I doubt that would even be measurable.

If he lived for 7.5 billion years, so long that the Earth has been absorbed by the sun, he would have imparted 30 micrometers of velocity on the planet. Again, I doubt that would even be measurable.

pegamin
Wow. Thank you for your rapid feedback, that leaves me rather speechless...

You always give me a lot to think about. As for my story, for the moment I think I'm going to stick to your previous advice; "Not being slavishly hard-SF does not detract from a story, it’s quite orthogonal to it".
So just for the sake of fable, and although that may get me some scorn from scientifically-inclined readers, I think I may allow this guy to change the course of the MW. (thanks to 256bits, who provided the idea).

Hey, it's a musical comedy after all, so I guess everything's allowed.

Thank you again.

256bits
...You can calculate the effect on the Earth by considering the force the man is exerting. When he's facing a certain direction his weight is doubled, when the Earth spins so that he is pointing the other way he weighs nothing. Therefore on one half of the planet he exerts zero force and on the other he exerts a downward force, how much:
...
I think that is a constant force toward Andromeda any time of day. It is just the direction of Andromeda that changes relative to the surface of Earth.

...
If he masses a healthy 75kg then in his heavier phase F = 75 * 20m/s = 1500 Newtons. ...If he lived for 7.5 billion years...

Are we including the water in his blood plasma and bladder? If so when does it stop getting included? If we include his breath we would have ~10 million liters, (~12 tons) per year adding to the effects. Would be less if he stays in closed rooms and recycles his air. The molecules would escape Earth's gravity when they were in the upper atmosphere. That would have serious consequences for the atmosphere in a trillion years.
Once each of the molecules takes off toward Andromeda they accelerate at 1 g. That becomes a stream of relativistic cosmic rays within a year. They will do some damage when they get to Andromeda.

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I think that is a constant force toward Andromeda any time of day. It is just the direction of Andromeda that changes relative to the surface of Earth.

Yes but at points when Andromeda is above the man he no longer exerts a force on Earth, he floats as the gravity is canceled out. He exerts a force when the Earth is between Andromeda and him.

Are we including the water in his blood plasma and bladder? If so when does it stop getting included? If we include his breath we would have ~10 million liters, (~12 tons) per year adding to the effects. Would be less if he stays in closed rooms and recycles his air. The molecules would escape Earth's gravity when they were in the upper atmosphere. That would have serious consequences for the atmosphere in a trillion years.
Once each of the molecules takes off toward Andromeda they accelerate at 1 g. That becomes a stream of relativistic cosmic rays within a year. They will do some damage when they get to Andromeda.

It's not my setting but I assume that the effect rapidly dissipates once a part of him is severed from the majority. It's total fantasy anyway so the mechanism by how him and not-him are differentiated by this magic andromeda facing gravity wand can be waved away as easily as the general mechanism of said wand.

Yes but at points when Andromeda is above the man he no longer exerts a force on Earth, he floats as the gravity is canceled out. He exerts a force when the Earth is between Andromeda and him.

You are currently interacting with Earth via gravity. Your shoes or chair are pushing down to cancel that force. When Andromeda is directly overhead the man still exerts the gravitational pull on planet Earth but now his shoes/chair are not interacting. The effect remains a 750 Newton acceleration of Earth and the solar system.

..It's not my setting but I assume that the effect rapidly dissipates once a part of him is severed from the majority. ...

That is unfortunate. You could use the byproducts for a lot of aerospace applications.

If you cultured his stem cells and grew a fleshy tumor more massive than him would it be pulled instead? An 800 N engine with only a nutrient bath for propellant cost would be much more efficient than the best ion engines. There are a lot of possibilities for Oort cloud and interstellar exploration that could be done in the direction of Andromeda. You could time the launch and start toward the sun for other solar system exploration. Once the craft was at cruising speed you could dissolve it and grow a new tumor for the next mission.

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