Gravitational Force!

Hi,

Just a question relating to cosmology, WAIT, don’t go, question may sound a bit strange, please bear with me…

It is known that the speed of light (SOL) can vary with different mediums, current knowledge states 299,792,458 meters per second is maximum speed in a vacuum and cannot be surpassed (currently)! Space is not a true vacuum from what I have been reading, its close but not a true vacuum.

But what is the speed of gravitational force? I mean if the Sun suddenly stopped shinning we would not see the effect for just over eight minutes, however if the gravitational force was suddenly switched off at the same instant, we would feel it almost immediately as our planet and all the other planets would continue in a straight-line into space (what a weird and frightening experience that would be!).
If that is the case then gravitational waves are somewhat faster then SOL (if there is such a thing). If gravitational waves have nothing to do with gravitational force it therefore follows that gravitational force is instantaneous (infinite speed) and thus breaks our long held theory that nothing can travel faster than SOL. Anyone else thought about this?
Oh boy my head is spinning :shock: , I’m off for lunch!

Clear skies
Mike

Hi,

Just a question relating to cosmology, WAIT, don’t go, question may sound a bit strange, please bear with me…

It is known that the speed of light (SOL) can vary with different mediums, current knowledge states 299,792,458 meters per second is maximum speed in a vacuum and cannot be surpassed (currently)! Space is not a true vacuum from what I have been reading, its close but not a true vacuum.

But what is the speed of gravitational force? I mean if the Sun suddenly stopped shinning we would not see the effect for just over eight minutes, however if the gravitational force was suddenly switched off at the same instant, we would feel it almost immediately as our planet and all the other planets would continue in a straight-line into space (what a weird and frightening experience that would be!).
If that is the case then gravitational waves are somewhat faster then SOL (if there is such a thing). If gravitational waves have nothing to do with gravitational force it therefore follows that gravitational force is instantaneous (infinite speed) and thus breaks our long held theory that nothing can travel faster than SOL. Anyone else thought about this?
Oh boy my head is spinning :shock: , I’m off for lunch!

Clear skies
Mike

I think this exact question came up before here

Anyway, gravity propagates a the speed of light. So if the sun disappeared, we would continue orbiting for 8 minutes before flying off on a tangent.

Yes, gravity travels at the speed of light. If anything travelled
faster than the SOL, we would see causality effects in that
signals would be transmitted outside the light cone, breaking causality.

The speed of light is set by interactions between photons and
virtual particles in space; these virtual particles are explained by
Heisenbergs Uncertainty principle: as the measurement of energy is
uncertain for small period (dE x dt ~< h), if we look closely at an area of
space for a given time period, particles temporarily appear and disappear.
The photons interaction with these is what 'slows' the photon to the
speed of light.

This has implications because of the Casimir effect: it is actually
possible to go _faster_ than c, but only by a very small amount:

math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/FTL.html

Gravity propogates at the speed of light only.

I understand that you can have two newtron stars closely orbiting each other. You then have a precise clock whuich you can use to measure the time taken for each newtron star to move in its orbit.

You can also measure the degredation in their orbits precisely due to their radiating away their orbital energy in the form of gravity. This is in accrodance with the predictions of relativity.

But apparently it turns out that only if gravity is propogating at the speed of light can you match the positions of the two newtron stars precisely.

Hope this makes sense

DB

You don't need to imagine the Sun disappearing in an instant, which is the way this is usually stated. The Sun's mass (and therefore gravitational attraction) is decreasing constantly as Hydrogen is converted to energy, therefore in theory this effect should be measurable.

You could have a probe (A) in close orbit around the Sun, with instruments sensitive enough to measure its orbital motion and hence the mass of the Sun (SmA) from Kepler's equations. It would then transmit this data to another probe (, in orbit some distance further out.

At the instant B receives this data, it does a similar calculation from its own motion, to obtain SmB, and compares the results. If the 2 masses are the same then you can deduce that gravity propagates at the same speed as the signal from A - i.e. the speed of light; otherwise if SmB is less than SmA this difference would equate to the mass converted by the Sun in the time taken for the signal to travel between A and B, and show that the effect of this had actually travelled instantaneously, or at least much faster than the signal.

I suspect that we're a long way from a being able to perform this experiment in practice

A few years ago there was an observation of Jupiter passing very close to or in front of a distant quasar which was supposed to demonstrate the finite speed of propagation of gravity, giving a value of between 0.8c and 1.2c, but there is disagreement about what can actually be deduced from it.

news.bbc.co.uk/1/hi/sci/tech/2238452.stm

There is another way of looking at it altogetther.

At the speed of light a clock stops dead in its tracks.
(Atomic clocks have even been witnesses slowing down on transatlic flights.)

Between the time the Sun disappears and an observer on Earth witnesses it NO TIME PASSES.

When we "observe" the Sun disappearing then THAT is when it disappears.

The speed of light can thus be thought of as "The speed of Time".

When we observe the light of an ancient galaxy we are seeing the NOW of that galaxy!!

So when NOW occurs differs from place to place.

"The universe is not just queerer than we imagine,
it is queerer than we CAN imagine"..J.B.S Haldane
(OLD meaning of the Q word!)

Peter.