A Couple of Questions About Gravity

[YellowSubmarine]

But if the zero-point energy or virtual particles or so-called gravitons or whatever responsible for gravity are not of wavelike nature and not limited to the speed of light, that doesn't mean going back in time. Time dilation of a massive object traveling at 90% lightspeed or so doesn't really necessitate the colorful SR description that time itself has slowed down, at least from a mechanistic point of view, but just that its physical processes, like beta decay or even how fast an electron orbits, are slowed.

But if you could send a message instantaneously, it would mean that you can send a message back in time.

Imagine two starships A and B that depart from Earth in opposite directions with speed of exactly 0.866c. After 8 seconds, starship A sends an instantaneous message to starship B and starship B immediately sends a reply. The question is, when will starship A receive the reply?

The answer is 6 seconds before it sent it.

Edit: I'm sorry, I quickly checked again, and according to at least one person with understanding of relativity, this thought experiment is wrong for the reason that a real tachyon would suffer some effects due to time dilation that would cause the paradox not to occur in this case. I have next to no idea how that would apply to the supposed instantaneous gravitational propagation, but since it's not about tachyons my point might possibly still stand.

 

[Bill Morris]

Okay, YS, I'll play. Remember that I opined that it's not really time itself but physical processes that are slowed, and that perhaps calling it time is just being colorful to attract students to physics courses.

Captain Archer and Empress Sato are together in a swimming pool uniformly 3 feet deep. At the bell, they wade in opposite directions at 86.6% maximum wading speed for 8 seconds. Archer turns and tosses a water-polo ball to Sato. The ball travels at five times maximum wading speed, and Sato catches it--not before but a split-socond after it was thrown. Without the contrivance, the magic sort of disappears, doesn't it?

If you pee while riding a motorcycle, yes, then you can expect that to go backwards, but a clock slowed by air resistance isn't going to run backwards if you get a gale-force tailwind that lets you break the previous limit. Nor should you expect an atomic clock slowed by traveling at 0.9 c to run backwards if you break the lightspeed barrier, simply because it's not time itself that was being slowed.

 

[YellowSubmarine]

That's not really a fair comparison, because, I'm not certain about Archer, but I would have trouble swimming away from Empress Sato, actually I would have trouble swimming in any direction other than towards her.

I don't think that the idea that it's only physical processes that slow down can ever work, in particular it doesn't quite work when you have different relative ageing of the two travellers and you try to jump across frames of reference - in the relative swimming pool Sato ages slower in Archer's frame, Archer ages slower in Sato's frame, and should Archer turn back because he couldn't resist the temptation, due to the change of the inertial frame, he'd find that Sato has actually aged faster, and if the pool was too long, she might already be too old to be attractive. Should Sato turn back, she'd reach Archer when she's still young and he's the one who's gotten older.

Now, I'm not sure what instantaneous should mean, but whatever instantaneous means, they would experience a paradoxical situation at the point of the U-turn. If the Archer's superluminal ball doesn't reach its destination “too soon” like in my previous post, but instead arrives right on time – when Sato has aged exactly as Archer when he threw the ball – they would experience a time jump when Archer makes the U-turn, because the ageing of both would be at the same speed on their communication channel. In particular, Sato would experience a communication hole – a huge period in which she didn't receive anything from Archer, and everything she sent to him was received at the exact same instance.

Everything to the point where Archer remembers how hot Sato is and turns is symmetrical. The communication should either arrive “back in time” for both, or it should arrive at the right moment for both*, and if it does the latter, Sato would have to wait during the communication blackout, which I'm sure she wouldn't like very much and she'll probably ditch Archer. Not to mention that Archer would be killed instantly by the radiation from her communication because all Sato's fury arrived on his communicator compressed to an instant.

* Well, there's a third option, faster than light but not instantaneous, but that would have the same hole, only smaller, I think...

 

[MontgomeryScott]

1. Gravitational interactions are bound by the speed of light. Gravity is not instantaneous. (In fact, in a relativistic universe, the very concept of "instantaneous" is meaningless.)

Has that ever been tested, and how? If the Sun suddenly vanished, would it take 8 minutes before Earth's orbit is affected? That's kinda weird.

Yes. It would take 8 minutes before the Earth's orbit is affected. But that's not what's weird, what's weird is that the Earth is always falling towards the actual position of the Sun, not towards the observed position of the Sun. If it were the latter, it would eventually fall out of the solar system.


http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html
https://secure.wikimedia.org/wikipedia/en/wiki/Speed_of_gravity

The actual position of the sun determines by the Earth's momentum in orbit around it; so this balances out on both sides, and thus the earth's orbit is determined by the average. Meanwhile the observed position is always skewed towards one side. So it makes sense.

Meanwhile about "seeing gravity," I think he's talking aout gravity telescopes-- and yes, these do exist.

 

[Stiletto]

My pancreas attracts every other pancreas in the universe
With a force proportional
To the product of their masses.
And inversely proportional
To the distance between them
--Weird Al Yankovic

 

[BlackFire3]

there's something i've been wondering about earths gravitational forces. how would gravity change if the earth never rotated (orbit around the sun yes, but not spin) i mean since the earth rotates the spinning would cause a minor counter-force to the gravity pulling you down. i was wondering if gravity would increase if the earths rotation wasn't there to act as a force to lessen it.

 

[ThankQ]

As I understand gravity, it is related to mass. Not velocity, torque, momentum... Whatever "motion" properties are in play.

Silly 2:00 AM thought: Gravity is well known to be significantly weaker than the other three fundie forces. While my personal opinion is that gravitation does propagate at the SoL, how about this:? What if gravity is equally strong as EM and the strong/weak nuclear forces. The PERCEIVED weakness come from that, in some manner, gravitational forces are moving at speeds faster than the SoL. That is to say that gravity "uses up" most of its force by booking it across the cosmos at Warp 10. ("Warp 10" was just for Star Trek fun). Fuck the Higgs, gravity loses its energy in the same way a sprinter would vs. a marathon runner. Not, by any means, a perfect analogy, but I think the idea comes across.

 

[B.J.]

there's something i've been wondering about earths gravitational forces. how would gravity change if the earth never rotated (orbit around the sun yes, but not spin) i mean since the earth rotates the spinning would cause a minor counter-force to the gravity pulling you down. i was wondering if gravity would increase if the earths rotation wasn't there to act as a force to lessen it.

I think you can answer your own question just from observation of objects at the poles. If the centrifugal force created by the Earth's spinning was anything appreciable, you'd see a huge difference between what you feel at the equator versus what you feel at the poles.

 

[YellowSubmarine]

there's something i've been wondering about earths gravitational forces. how would gravity change if the earth never rotated (orbit around the sun yes, but not spin) i mean since the earth rotates the spinning would cause a minor counter-force to the gravity pulling you down. i was wondering if gravity would increase if the earths rotation wasn't there to act as a force to lessen it.

Earth rotates with 0.4651 km/s at the equator. Earth escape velocity is 11.2 km/s. Rotation speed is only a fraction of the speed that would cancel out Earth's gravity. Let's see how much outwards acceleration it will give you...

The formula is acceleration = velocity² / radius, Earth's radius is 6378.1370 km, so we have:

(0.4651 km/s)² / 6378.1370 km = 0.033916 m/s²

That's 0.35% of Earth's gravity, so the change you experience is negligent.