# Using Gravity to obtain relativistic speeds

1. Apr 21, 2010

### M_Navas

Is it possible??

I am not familiar with the idea due to not having studied anything about it yet, but I am curious

I mean it is theorized that you need unfathomably large amounts of energy (some sources say infinite amounts of energy) and gravity is, in a sense, an unlimited amount of energy so why couldn't it be used to obtain relativistic (near light speed) and maybe even light speed travel.

This would have to mean that there is no limit to how fast an object can be accelerated by a bigger body with higher gravity, I am not quite sure how that would work because there needs to be a limit to how fast something can go right? So does the increase in speed result in an increase in mass until it gets to the point where the accelerating object then has the SAME amount of mass and gravity as the object which accelerated it??

2. Apr 21, 2010

### Nabeshin

A massive object can only accelerate a stationary body to the maximum of the massive object's escape velocity. For earth, ~11km/s. For the sun, ~600km/s. Compact objects such as white dwarfs or neutron stars have escape velocities as appreciable fractions of the speed of light.

So, gravity is no an infinite energy source or anything ridiculous like that. Plus, if you let an object fall towards the sun to pick up 600km/s it's either a) going to hit the sun or b) going to lose all the energy as it travels away. Of course, there's gravitational slingshotting but that's something else altogether.

In sum: No.

3. Apr 21, 2010

### M_Navas

not sure how it is "ridiculous" but thats besides the point...

I am not implying it just falls down and goes back up with its increased velocity so gravitational "slingshotting" must be precisely what I am talking about...

also, if an item enters Earth's atmosphere at the exact velocity which is required to escape the atmosphere, will gravity simply have no effect on it??

4. Apr 21, 2010

### pallidin

Inertia might be the key term here, Navas, as you are asking for changes in acceleration.

5. Apr 21, 2010

### FeDeX_LaTeX

Gravitational slingshotting means that the object would simply come to rest in the middle, correct?

6. Apr 21, 2010

### Nabeshin

You can read about it here:
http://en.wikipedia.org/wiki/Gravity_assist

Definitely not an infinite source of energy and it would take a ridiculously long amount of time and ingenuity to accelerate something to relativistic speeds using these maneuvers. This makes me think it wasn't exactly what you were after, although I could be wrong.

No, gravity will pull the object down faster and so when it hits the surface it will hit with v>vescape. The reasoning for what I said in my first post is that is the maximum energy that gravity gives the falling body. In the example you mentioned, the item already had some initial velocity apart from what the earth gave it, so it is not surprising that it ends up going faster.

I'm having difficulty explaining because I'm not sure precisely what your question is, I suppose. Pointed questions are definitely helpful, so I don't have to try to explain all of (what I know) about gravitation!

7. Apr 21, 2010

### mate0

gravitational slingshotting does not increase the speed at which an object is traveling, it just changes the direction.
the only object that could conceivably accelerate an object up to relativistic speed would be a black hole (inside the event horizon), because the escape velocity here is above the speed of light, however we can't see what happens inside the event horizon of a black hole for the same reason.

8. Apr 21, 2010

### M_Navas

I really do not have a specific question here, I am just trying to get some sort of "spark" to jumpstart my brain...I was just thinking though, even if it WOULD take hundreds of years to obtain relativistic speeds by using the gravities of the giant bodies in and around the solar system, wouldn't it be worth it to at least give it a SHOT and test some theories?? Say it takes 100 years (being hypothetical here) to obtain half the speed of light, we can use a Tardigrade as a means of study to see how such high speeds affect aging...induce it into a coma that lasts X amount of years so that it "wakes up" when the craft nears the relativistic speeds and then over X amount of years to slow down the craft using the gravity of objects in the solar system and or a complex series of parachutes that can last through the entering of an atmosphere of say Jupiter to slow the ship down and then bring it back out at a speed much much slower...or instead of using a Tardigrade (which is the only animal which can survive for a length of time in a vacuum) use some bacteria that is frozen and thaws after X amount of years and then undergoes mitosis and splits at a relatively slow rate and then we can tell how the speed affected time of the bacteria by seeing how many times it has split upon bringing it back...

If any of this is completely hopeless and utterly pointless please let me know, but also explain why because I do not have a background with any of this as I am still in high school (but that doesn't mean I know nothing about physics and such, I have taught myself a bit in the past few years and am trying to delve deeper into the pools of knowledge for physics and astrophysics at least) so all the information you guys think is even partly relevant is appreciated.

9. Apr 22, 2010

### Janus

Staff Emeritus
That depends on the reference point. While it does not increase the speed relative to the body around which it is doing the sling shot, it can do so relative to another reference. For example, the Voyager probes used Jupiter to increase their speed relative to the Sun to Solar escape velocity. It works like an elastic collision where the probe borrows orbital velocity from the planet.

10. Apr 22, 2010

### Janus

Staff Emeritus
Here's the thing. When you use the masses in the Solar system to boost an object's speed, you can do so only in increments of maybe a few km/sec at a time. In would take multiple passes to get to get to even a small fraction of c. Long before that, you would reach escape velocity from the Sun and the object would leave the Solar system.

11. Apr 22, 2010

### mate0

yes, however this would not be useful in trying to accelerate a slowmoving object to near the speed of light, because you would need another body that is already traveling at around that speed.

12. Apr 22, 2010

### Staff: Mentor

In principle you could do this with a pair of black holes which were moving towards each other provided that the black holes were sufficiently massive. Alternatively, a single sufficiently massive black hole which was already traveling at relativistic velocities could do it.

Share this great discussion with others via Reddit, Google+, Twitter, or Facebook