Question for physics guru

Cosimo · 2 Feb 2011 at 15:30

With so many physics questions today it must be homework day.

salami1212 · 2 Feb 2011 at 15:31

simulatorman said:
With so many physics questions today it must be homework day.

homework was never interesting

DAnDan · 2 Feb 2011 at 15:32

salami1212 said:
Well to be fair, no one has measured the speed of gravity. What your saying though, is that if for some strange reason the sun disappeared, the earth would shoot off 8 minutes later?

I should have said if the bottom box was travelling at the speed of light left and right, what would happen to the top box?

Yeah, the theory is that we would not feel the effect on the change from the sun's gravity until around 8 minutes.

If the bottom box was moving that quickly, the amount that the top box fell would defilitely be so small that the top box would still be able to sit on top of it.

But to answer your original question, yes things do fall instantly, but they start off slowly and accelerate with time.

Inquisitor · 2 Feb 2011 at 15:41

salami1212 said:
Well to be fair, no one has measured the speed of gravity. What your saying though, is that if for some strange reason the sun disappeared, the earth would shoot off 8 minutes later?

Well, for it to happen any sooner would at least violate relativity.

salami1212 said:
I should have said if the bottom box was travelling at the speed of light left and right, what would happen to the top box?

That makes the question invalid and unphysical.

Amleto · 2 Feb 2011 at 15:44

the box will fall much slower if it's foggy.

stopper · 2 Feb 2011 at 17:01

salami1212 said:
Also, if somethings so heavy it gravitates faster then light, is it there?

When reaching the speed of light, time itself slows down.
So if some thing is falling that fast it is about to take over light, time will simply slow it down to just shy of light speed, because it's a physical impossibility to pass light.
Imagine a train hurtling along at light speed, then some one inside the train walks forward to the front, if they passed light speed they would cease to exist, how can they be there and their reflection/light not be? So time slows down to allow them to move forward.

So nothing can gravitate faster than light

ZeniTimes · 2 Feb 2011 at 17:03

salami1212 said:
Also, if somethings so heavy it gravitates faster then light, is it there?

I don't know, is your brain still there?

dazzerd · 2 Feb 2011 at 17:07

Hhhmm from college......

Unless you beleive in gravitons and the like (which most scientists don't) then gravity is an effect of geometry, i.e. mass effecting the curvature of space. To say it travels at x speed is misleading.

ZeniTimes · 2 Feb 2011 at 17:13

Gravity is a force that is generated between to objects that have mass. The speed of light is related to mass via e=mc^2 hence they are realted.

Inquisitor · 2 Feb 2011 at 17:14

dazzerd said:
Indeed; popular understanding if I remember from college is that gravity is simply an effect of geometry to say it travels at the speed of something is misleading.

But changes in that geometry cannot propagate instantaneously.

ZeniTimes · 2 Feb 2011 at 17:16

oh year plus the formula for the gravitation force between to bodies has the speed of light in it.. hence they are related

dazzerd · 2 Feb 2011 at 17:20

I changed my original post....

Anyway yes your probably right which would mean that the "speed" of it depends on the mass of the body in question. But its still being talked about in the wrong terms.

Light cannot escape from a blackhole (in most circumstances) this doesn't mean that gravity travels faster than light at best it means that the force of gravity in the case of the black hole curves space up so much that it cant escape.

ZeniTimes · 2 Feb 2011 at 17:25

gravity exerts a measurable force on photons (light), similar to a lens bending the light. The force of gravity is related the mass of the photon and the mass of the black hole (black hole being many many times more massive). So yeah the force of gravity from a black hole is so great that not even photons can escape its grasp

ZeniTimes · 2 Feb 2011 at 17:26

Here's an example of how gravity can effect light

http://lmgtfy.com/?q=gravitational+lensing

Inquisitor · 2 Feb 2011 at 17:29

ZeniTimes said:
gravity exerts a measurable force on photons (light), similar to a lens bending the light. The force of gravity is related the mass of the photon and the mass of the black hole (black hole being many many times more massive). So yeah the force of gravity from a black hole is so great that not even photons can escape its grasp

Photons are massless

ZeniTimes · 2 Feb 2011 at 17:30

Inquisitor said:
Photons are massless

prove it

Inquisitor · 2 Feb 2011 at 17:33

ZeniTimes said:
prove it

Well the entirety of modern physics is built upon that assumption (and by extension, all other sciences too), so…

SlyReaper · 2 Feb 2011 at 17:36

Greebo said:
How? Gravity is a force and hence is acceleration whereas the speed of light is a velocity. How can the two be the same?

They mean that gravitational fields propagate at the speed of light. For example, we're 8 light minutes away from the sun. If the sun spontaneously popped out of existence, we'd continue orbiting the point where it was for 8 minutes.

Calabi · 2 Feb 2011 at 17:39

Isnt this already proven with the card shot in half thing.

Here.

http://www.artsology.com/edgerton_photography.php

Magical floating cards?

Pate · 2 Feb 2011 at 17:46

SlyReaper said:
They mean that gravitational fields propagate at the speed of light. For example, we're 8 light minutes away from the sun. If the sun spontaneously popped out of existence, we'd continue orbiting the point where it was for 8 minutes.

Out of curiosity, what would happen then?