# Gravitation due to acceleration

1. Nov 11, 2009

### dizam

The speed at which the earth is orbiting the sun increases it's mass due to relativity. Does centripetal force also increase the mass of the earth? Is the increase in mass enough to be measured? If so, then, the gravitation must also increase. Wouldn't galaxies also have more or less gravitation/mass due to their movement speeds?

Last edited: Nov 11, 2009
2. Nov 11, 2009

I like this and have similar questions. As long as we're measuring velocity of galaxies and stars, where is the reference point and how do we make sure that the reference point isn't moving in relation to the rest of the universe? Is there a way to measure velocity of the reference point and remove that from the rest of the equation? Why would or wouldn't centrifugal rotation add mass (I don't know what its called, but I'm thinking the answer is no due to the reactive force applied by the opposite side of the object. I think its kinda like putting both feet on a door and then pulling on the handle trying to open it, or pulling yourself up by your bootstraps...). HOWEVER, if centrifugal rotation does increase mass, or gravitational pull (either or), then would there be a measurable difference between the plane of rotation and the axis poles? Using the earth as an example, would there be more mass or gravitational pull at the equator than at the north or south pole? I'm inclined to say no, after all, one of the reasons that NASA launches from Florida is because of the inherent centrifugal force from the earth's rotation helps "throw" the rocket/ shuttle away from the earth.
However, lets say that this does happen. How fast would something have to rotate before the increase in mass/ gravitational pull become measurable? Relativistic speeds? As far as black holes go, this sure would help explain a lot. Lets say that a black hole rotates at near c for example. Now, how much of its pull is due to actual material mass and how much is due to relative mass? Is this possibly why x rays seem to escape from the poles of a black hole and not the rotational plane? Is it possible that black holes rotate fast enough that much of their pull is from the rotation and not from the compacted material? Is the material even more compacted due to rotation and length contraction? IF the massive gravitational pull DOES come from rotation, does that mean the black hole is rotating at c+ to capture light? Would a black hole not be a black hole if it didn't rotate fast enough? These questions will surely be shot down as soon as someone says no, rotation does not increase mass, no matter how fast something rotates, and then they prove it. And by prove, I don't mean throw math at me, I just mean point me to the person/ people/ theory that says it doesn't happen so that I can read it. I don't do so well with higher math.

3. Nov 11, 2009

### twofish-quant

It really doesn't. Way back in the 1950's, there was an author (it may have been George Gamow) that tried to explain the effects of relativity by talking about a speed object increasing it's mass. It turns out that this is not a good way of thinking about what is going on.

This is why thinking about relativity as "increasing mass" is not a good way of thinking about what's going on. It leads to all sorts of confusion. It turns out that when an object is traveling fast it *doesn't* increase it's gravitational pull, which is why the idea of fast objects increasing in mass is not an accurate description of what is going on.

4. Nov 11, 2009

### twofish-quant

I also like this because once people start asking questions like this it tells that they are thinking about what is going on, and it turns out that the reason things are confused is that thinking about a fast moving object as "increasing mass" is a bad way of thinking about what's going on.

What does happen is that relativity predicts is that as you get closer to the speed of light, it becomes harder and harder to accelerate. Someone back in the 1950's talked about this effect as "increasing mass" but it turns out to be not a great way of thinking about it because then you start asking whether this "increased mass" should result in more gravity, and it doesn't.

A better explanation is that one fact of the universe is that light in a vacuum is always moving at the speed of light. So no matter how fast you move toward a beam of light, it is always moving away from you at the same speed. So you can never catch up to it.

5. Nov 11, 2009

### dizam

Part of the energy used to propel the object closer to the speed of light is converted to mass right? Then wouldn't a planet gain mass as it gets closer to the speed of light? Also, could you give a more detailed explanation of the way we should be thinking about the energy/mass relationship? Thanks

6. Nov 11, 2009

### dizam

Or is the mass of the original object increased because of moving near/at the speed of light?

7. Nov 11, 2009

### dizam

8. Nov 11, 2009

### twofish-quant

No. As I mentioned if you think of things as gaining mass, then things become very confusing.

You start from the weird fact about the world which is that light in a vacuum always travels at the speed of light. You run toward a beam of light. It's still traveling away from you at the speed of light. Nothing to do with mass. It's a weird property of the universe.

So what happens is that the equation used to calculate energy changes to take into account this weird fact.

9. Nov 12, 2009

### qraal

No it's not Hawking radiation. It's produced when material is being accreted into a black hole, basically from infalling material crashing into other material that is piling up around the hole in what's called an Accretion Disk. The disk is also why the radiation is seen coming from the poles - the disk gets in the way of light being emitted in the same plane.

10. Nov 12, 2009

### granpa

longitudinal and transverse relativistic masses are different.

11. Nov 13, 2009