[ 3 / biz / cgl / ck / diy / fa / ic / jp / lit / sci / vr / vt ] [ index / top / reports ] [ become a patron ] [ status ]
2023-11: Warosu is now out of extended maintenance.

/sci/ - Science & Math

View post   
File: 14 KB, 200x250, newtonapple[1].gif [View same] [iqdb] [saucenao] [google]
5018147 No.5018147 [Reply] [Original]

If the acceleration of an object in a gravitational field is irrespective of mass, what happens to objects travelling at just below the speed of light when their movement is in the same direction as the acceleration they experience from the field? What stops them from accelerating to superluminal speeds?

>> No.5018170

>>5018147

>objects

>> No.5018171

I would guess that this is still true for the objects own time. Good question though.

>> No.5018181

gravity isn't a special actor if you look at it pragmatically. The energy and momentum of the object would increase just the same as if there was a rocket engine mounted to its back, altogether preventing it from gaining superluminal speeds.

>> No.5018180

the short answer:

When an object is travelling at a speed really really close to c, it will become increasingly difficult to accelerate it. Basically, such an object will not accelerate at 9.8m/s^2.

>> No.5018183

>>5018180
Apologies, I was under the impression that acceleration through gravity was irrespective of mass or velocity, which is how it can bend light.

>> No.5018199

>>5018183
Mass increases as you approach c. Mass basically becomes infinite at c.

>> No.5018220

>>5018181

Gravity is special because gravity can accelerate objects without depleting itself. Foreign mass may alter a field, but it won't deplete it.

>> No.5018222

>>5018199
sure, but gravity accelerates things independent of mass (because it's actually an inertial pseudoforce due to curvature of spacetime).

yah, this is an interesting question. I think you can work out from conservation laws what must happen in order for conservation to hold, but figuring it out from a gravitational perspective I think you'd have to actually do the GR math.

>> No.5018320
File: 247 KB, 762x390, brainexplode.png [View same] [iqdb] [saucenao] [google]
5018320

10/10 question OP.

>> No.5018321

>>5018220
doesn't change the fact that it won't accelerate a mass above c.

>> No.5018323

Newtonian gravity can't really handle this.

You need the Rindler metric.

http://en.wikipedia.org/wiki/Rindler_coordinates

>> No.5018529

>>5018321
But why? Saying it can't because it can't is religious style bullshit.

>> No.5018554

>>5018199
>Mass increases as you approach c. Mass basically becomes infinite at c.
That's quite misleading, since the rest mass remains the same. Your energy and momentum approach infinity as you approach c.

>> No.5018559

You cannot use Newtonian mechanics to treat something about how things moving close to the speed of light behaves. You need general relativity. In GR it is still true that things fall equally fast irrespective of mass (that is one of its main points), but what does matter is how fast an object is going. The effect of gravity is no longer just "acceleration = 9.8 m/s²", but instead "a falling object follows geodesics through the curved spacetime". These geodesics will never accelerate an object to superluminal speeds simply because of the local causality structure inherent in GR, i.e. you cannot have a geodesic starting as timelike (subluminal) and then suddenly becoming spacelike (superluminal). This follows from the basic result that the tangent vector is preserved along geodesics, really.

>> No.5018570

Newton's gravitational model is outdated; to effectively understand the motion or gravity of anything travelling at relativistic speeds, you have to use the Theories of General Relativity and Special Relativity to find the answers to these things.

tl;dr Newton old, Einstein new