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/sci/ - Science & Math

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5142384 No.5142384 [Reply] [Original]

During engineering calculations (on the loooow level mind you) the teacher wanted talk about and illustrate potential energy and gravity. He made a drawing of earth with a tunnel through and we had a funny chat about how a thing dropped in the tunnel would act.
Then I ask:

>does gravity travel at the speed of light too?
>u wot m8
>if the sun disappeared it would be dark on earth after 8 minutes but would earth go into free fall after the 8 minutes too or before?
>LOL ANUN DAT SUM PHILO SHIT RIGHT THUR

And they laughed.
Can anyone in here answer my question?

>> No.5142396

Gravity under most theories operates at the speed of light. Your question about freefall is poorly formed so I can't answer it.

>> No.5142412

>does gravity travel at the speed of light too?

wtf do you mean by travel? do you mean gravitons?


nobody knows shit about gravity, it's the least understood force. if you removed the sun the earth would just move relatively tangent to the parabolic path it took around the sun, no? it would happen instantly. i dont think even after 8 minutes that we would realize it was happening though

>> No.5142416

>>5142396
>>5142384

earth would leave its usual orbit at the same time it went dark. so yes, gravity travels at the speed of light

>> No.5142427

>>5142412
yes, they know
"Sir Isaac Newton thought that the speed of gravity was instantaneous, and Einstein assumed it traveled at the speed of light. Although scientists believe that Einstein was right, for nearly a century no one had been able to directly measure gravity's speed. However, on September 8, 2002, an international team of scientists did just that, using an experiment conceived by Sergei Kopeikin, professor of physics and astronomy at the University of Missouri-Columbia"

>> No.5142428

and im freeee(free fallin and i'm)eeeee(free fallin) freeee faaaalin fallin an i'm free fallin and i'm freeeeeee eeee(fallin and i'm)eeee free fallin fallin an im free fallin

>> No.5142440

>>5142412
>no one knows shit about gravity, here's the [incorrect] shit I know about gravity

>> No.5142453

In frame of general relativity:
a) the Earth is already in free fall
b) as gravity is weak in our solar system, linearized Einstein equations adequately describe it. And linearized system supports wave-like solutions, with propagation speed of c.

So you could say yes, gravity travels at speed of light. Tho in strong gravity the very term of propagation speed is incorrect.

>> No.5142459

>>5142427


that was one experiment that was non repeatable, had a wide range (0.8 - 1.2 * c) and heavily criticized.

>> No.5142498

No-one knows the speed of gravity, all tests have been roundabout ways of measuring the speed of light

>> No.5142520

The sun can't disappear, and GR doesn't deal in situations which it also forbids.

Exactly how it would play out would depend on how you moved the mass of the sun away, which would have to be done in a way respecting conservation of energy and momentum, and the speed of light limit.

There may also be a difference between GR's predictions and the observed result. While GR has generally agreed with experiment and observation, it's been very hard to properly test.

Gravitational waves, an inevitable result of changes in the gravitational field propagating at a limited speed and a major prediction of GR, have still not been observed.

>> No.5142593

From my understanding it is expected to travel at the speed of light, hence there should be gravitational waves, but these have yet to be observed.

>> No.5142606
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5142606

it'd probably happen at some speed, shy not light?

>> No.5142647

We don't even know if gravitons exist, OP. (gravitons are the theorized quanta -- discreet 'particles' -- of gravity) We know next to nothing about gravity because unlike the other fundamental forces (strong and weak interaction and electromagnetism) we haven't yet been able to look at gravity in terms of quantum mechanics.

>> No.5142653

>>5142606
Yeah, that's *more or less* what general relativity predicts (except for the sun "poofing out" part).
We have no clue if it's true or not though.

>> No.5142664

gravity doesn't really exist

>> No.5142674

>>5142664
just curvature in spacetime

>> No.5142693
File: 41 KB, 265x255, 41653fa4_f728_0d97.jpg [View same] [iqdb] [saucenao] [google]
5142693

>>5142606
Looks like earth will actually be pushed away.

>> No.5142719

>>5142412
>I don't know anything about X = nobody knows anything about X

Also,
>Refers to gravity as a force

>> No.5142800

>>5142719
Isnt gravity one of the four forces that govern the universe?

>> No.5142807

Changes in the gravitational field travels at the speed of light. But the field itself is present everywhere and such does not need to travel. If the sun disapeared it would take 8minutes to affect us since that is a change in the field.

>> No.5142808

>>5142800
there's 12

>> No.5142809

>>5142800
>four forces
>four
did you never learn about electroweak unification?

>> No.5142812

>>5142800

Yep. Gravity causes mass to accelerate. Therefore it is a force.

>> No.5142819

>>5142719
lol, 12 year old detected. Gravity is a fundamental force. And the guy you quoted is correct, we know very little of gravity.
>>5142800
Yes, don't listen to the other guy, he's troll or stupid. However, there MIGHT be more than four. For example, the Higgs field may be classified as a fundamental force, and could be synonymous with the hypothesized quintessence.
Basically it's like this:
--Theory Of Everything
----Quintessence (unconfirmed)
----Quantum Gravity
--------Gravity (General Relativity) (graviton) (effect: gravity)
--------Grand Unified Theory (Electrostrong Force)
------------Strong Nuclear Interaction (Quantum Chromoynamics) (gluon)
------------Electroweak Force
----------------Weak Nuclear Interaction (W boson and Z boson) (effect: beta decay)
----------------Electromagnetism (Quantum Electrodynamics) (photon) (effect: light)
--------------------Magnetism
--------------------Electricity
>>5142808
lol

>> No.5142824

>>5142809
I've never quite bought into it.

At high energies, electromagnetic and weak forces are indistinguishable, therefore, they are the same force. But at low energies, they are different forces. What?

I understand the unification of electric and magnetic forces. That's explained quite straightforwardly and easily demonstrated. But I've never heard anyone explain the motivation for or reasoning behind electroweak unification other than, "We like to unify things. We found a way to rationalize a particular case, so they can still act like they're unrelated forces except in particularly messy experiments where the distinction is blurred, but we can say they're unified. Isn't that nice? We deserve a pat on the back and more funding."

>> No.5142825

>>5142812
General relativity does not consider gravity a force, and does not consider being in freefall as acceleration. Anything that prevents you from being in freefall is acceleration.

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

>> No.5142852

>>5142824
Electroweak unification is part of the Standard Model.

The Standard Model doesn't account for gravitation and hasn't been proved mathematically self-consistent.

Electroweak unification doesn't work without the Higgs mechanism, which is still speculative. (yes, even after the recent suggestive evidence from the LHC)

>> No.5142856

>>5142852
As is Electrostrong. It's Quantum Gravity (or whatever you want to call it) that has yet to be integrated.

>> No.5142861

>>5142824
They are different forces at low energy, but they still become one at high energy, and can be put into formula together. It's all a matter of trying to discover a single formula that combines all fundamental forces (theory of everything).

>> No.5142870

>>5142856
The Standard Model is not a GUT. GUTs generally have to include the Standard Model.

>> No.5142877

>>5142870
Oh. Why isn't it 'officially' integrated then, seeing as we've got them all in terms of QM and in the form of GUT?

>> No.5142896

>>5142861
It's not a meaningful simplification, though. Electromagnetic and weak forces involve 4 bosons (W+, W-, Z0, photon), the electroweak involves 5 (W+, W-, W0, B0, Higgs).

Arguably, this isn't increased complexity, because the Higgs explains mass, which would otherwise need to be explained or included, but it isn't reduced complexity either.

I just don't see a motivation for preferring unification.

>> No.5142901

>>5142877
The Standard Model is a standard model, a framework believed to fit all experimental evidence from accelerators to date. There are multiple competing GUTs which each (at least in theory) include the Standard Model.

>> No.5142911

>>5142896
Unification seeks to ultimately unify ALL forces. Electromagnetism, strong interaction, weak interaction, gravity, Higgs field/quintessence, and any others that may exist. Electroweak and electrostrong are just steps along that path.

>> No.5142916

>>5142901
Ah, I didn't realize there's still multiple GUTs. I thought it was down to one GUT and we're just trying to incorporate gravity now. If and when it is confirmed which GUT is correct, it will be included in the Standard Model?

>> No.5142923

>>5142916
I would assume so. They might rename it, though. I'm not really all that familiar with the history and principles of the evolution of the Standard Model, and whether they were still calling it by that name when it was significantly different.

>> No.5142941

>>5142923
I see. Thanks.

>> No.5143121

>>5142520
You have never worked with General Relativity, have you? Most work done in the field has to do with non-real situations.

Also, General Relativity is more than capable of describing a system that starts out with massive, moving bodies (i.e, the solar system without the sun).

>> No.5143199

>>5143121
>Also, General Relativity is more than capable of describing a system that starts out with massive, moving bodies (i.e, the solar system without the sun).
Nobody said it wasn't, but that's different from a system in which the sun is instantaneously removed.

The question is about what happens with the sun's gravitational field if the sun is just suddenly gone.

If you remove the sun very quickly, there's going to be a gravitational wave, the features of which would vary depending on how the sun is removed. If you remove it by means other than moving its mass away at sublight speed, the gravitational wave is going to be a singularity. The math breaks down and fails to predict meaningful effects.

>> No.5143238

gravity can be detected therefore the presence/absence of gravity (caused by the presence/absence of mass) can transmit information. therefore it can't travel faster than light, so the earth would orbit the non-existent sun for as long as we still recieved its light. the information about the suns disappearence caused by a change in our orbit cannot reach us before the last rays of light.

>> No.5143251

>>5143199
You are correct, I retract my objection.

>> No.5144230

proposed experiment
>set a small rock in orbit around an asteroid, 1 light second away (or something along those lines)
>plant explosives all over the asteroid
>boom, blow it up
>observe to see if the rock orbiting the asteroid changes trajectory the very moment the asteroid blows or whether it takes a complete second
>if it takes a complete second it would confirm that gravitational waves move at the speed of light. if it takes less then a second then thats something we need to know

>> No.5144270

>>5144230
so many things wrong with that experiment

>> No.5144272

>>5144230
wouldn't work. the asteroid chunks still have mass and are (roughly) still in the same location that the asteroid occupied prior to the explosion.
maybe if you set a small object, some sort of manmade satellite equipped with sensors to measure the exact moment of change in trajectory, and annihilated the larger object with a large amount of antimatter, you could get a decent reading. the one problem with that is the whole "large amount of antimatter" thing.

>> No.5144321

>>5144272
you just have to take regular matter and make the electrons go backwards with magnets. i do it all the time. my whole house is powered with antimatter.

>> No.5145571

a better way to say it is that compression or expansion of spacetime travels at the speed of light.