Speed of Light and Gravity Wells

jallenecs · Jul 3, 2013

I know the bit about how, as a body approaches the speed of light, relative time slows, and mass approaches infinity. But I'm not trained in this field, so I don't understand one part. Increased mass equals increased gravity. As the mass increases, does it start to create its own gravity well? And let's pretend the moving body is already inside a gravity well? Let's say a ship inside a planet's gravity field, or in significant proximity to a star. How would those two gravity wells act on one another? Basically, what happens if I get a ship up to something like 50 or 60% of light speed, inside a star system? What if it's in a LaGrange point, or in near-planet orbit?

Pretend that my ship traveling at the 50% light speed decided to fly between the Earth and the moon. How would its passage affect the two bodies? What about between the Earth and another planet, like Venus or Mars? Through an asteroid field?

That's assuming it forms a gravity well at all. I was a music major, which doesn't really prepare you for these heavy physics questions.

WeaselFire · Jul 5, 2013

Paging Dr. Hawking...

Jeff

King Neptune · Jul 5, 2013

Fifty percent of c wouldn't do much. Remember the Lorenz transformation and apply it. Since it is a moving body the effect would change, and its initial mass was relatively small. At .5c the relativistic mass would be the original mass divided by .86, or 1.15 times the original mass.

If you want a large effect, then have it run at 0.999999999c. That would get the mass up to enough to make a difference.

paulcosca · Jul 5, 2013

If the technology was sufficient that such speeds were possible, then you'd probably have pretty strict regulations on when and where you could use those speeds. Much like boating on a lake, where there are "no wake zones"

Whenever I encounter that kind of travel in fiction, it always occurs to me that there would be about a million things that could go wrong any time you do it. There's no time for evasive maneuvers or course correction. No logical way to detect things coming up on you. If there's a big asteroid a million miles away, you'd hit it in a matter of seconds with such force that you'd be lucky if there was visible dust left.

Those are just my two cents, and I apologize that they aren't entirely on topic about the specific issues dealing with gravity. Hopefully King Neptune's comments helped in what you were looking for

spice chai · Jul 6, 2013

And correct me if I'm wrong, King Neptune, but wouldn't the energy consumption be impossibly high to produce enough relativistic mass to perturb the Earth Moon system? For example, if you wanted your ship to have a gravity well equal to the moon's, wouldn't you need to burn an amount of antimatter fuel at least equal to the mass of the entire moon?

In other words, no ship is ever going to have enough relativistic mass to perturb an orbital system?

benbradley · Jul 6, 2013

jallenecs said:
I know the bit about how, as a body approaches the speed of light, relative time slows, and mass approaches infinity. But I'm not trained in this field, so I don't understand one part. Increased mass equals increased gravity. As the mass increases, does it start to create its own gravity well?

I don't think so, and if it does, any gravity effects won't be the most significant or noticeable thing.

And let's pretend the moving body is already inside a gravity well? Let's say a ship inside a planet's gravity field, or in significant proximity to a star. How would those two gravity wells act on one another? Basically, what happens if I get a ship up to something like 50 or 60% of light speed, inside a star system? What if it's in a LaGrange point, or in near-planet orbit?

Pretend that my ship traveling at the 50% light speed decided to fly between the Earth and the moon. How would its passage affect the two bodies? What about between the Earth and another planet, like Venus or Mars? Through an asteroid field?

Presuming you could do this, it would have no effect, because the ship's mass is insignificant compared to planets or moons. And as King Neptune was alluding to, the effect is nonlinear. To double its mass, the ship would have to go 87 percent of lightspeed (from this online relativistic calculator: http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/tdil.html), and the mass is still insignificant.

But if there's a human being aboard, you can't get to 50 percent lightspeed within a solar system without killing all aboard, because the acceleration would be so many g's it would squash everyone.

It takes a year of acceleration at 1g (the force we feel on Earth) to get to a significant fraction of light, and by then you're way outside of the inner planets, and perhaps even past Pluto.

And yes, it takes a HUGE amount of energy to get even a small spaceship to relativistic speed.

That's assuming it forms a gravity well at all. I was a music major, which doesn't really prepare you for these heavy physics questions.

RichardGarfinkle · Jul 7, 2013

The answer is kind of daft. Yes its relativistic mass increases its gravity, but it's going by so freaking fast that it won't have much affect unless it gets very close to an object in which case gravity is the least of aanyone's worries.

Gravity is a very weak force. It gets weaker quickly the farther away the objects are from each other. Most of its affects are long term and happen to objects moving very slow relative to each other. An object's relativistic mass is only present for objects to which it has a high relative velocity. As a result high speec means high gravity for too brief a time to have much effect.

King Neptune · Jul 8, 2013

spice chai said:
And correct me if I'm wrong, King Neptune, but wouldn't the energy consumption be impossibly high to produce enough relativistic mass to perturb the Earth Moon system? For example, if you wanted your ship to have a gravity well equal to the moon's, wouldn't you need to burn an amount of antimatter fuel at least equal to the mass of the entire moon?

In other words, no ship is ever going to have enough relativistic mass to perturb an orbital system?

Yes, it would take as much energy input (roughly) as the mass equivalent (you can calculate backwards using the Lorenz transformation.). There are no practical ways with today's technology to get a spaceship near the speed of light.

jallenecs · Jul 9, 2013

thanks guys!!!! I got very lucky and, through a friend of a friend, who put me in contact with an astrophysicist at Jodrell Bank. He told me pretty much exactly the same thing you guys did.

I love science fiction junkies. They're so on top of this stuff!

Michael Davis · Jul 11, 2013

Mass and gravity are different. In the world of the large (standard physics) objects (like humans) possess mass, other objects (like earth) exhibit a pull (attraction) based on their mass. Einstein theorized (and later confirmed) we sense gravity as a result of the affect it has on space. For example a large object (like earth) creates a "well" (curvature) in space with in turn draws other object inward. In the world of the small (quantum level) sub atomic particles are responsible for mass (via Higgs boson particles) and gravity (by way of graviton particles).

Regarding your other question, all bodies (even a ship) cause space curvature and thus gravity even on the earth or moon. For example, before astrophysics learned to use stellar "winking" (when planets fly between us and stars) they'd detect planets revolving around a sun by the wobble in its movement as the planet circled the star. That's right, a tiny planet even affects an object a million times larger by its gravity. Ain't science amazing (g).

Speed of Light and Gravity Wells

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