The unbearable lightness of being... Gravity.

[FarmyardPants]

Bedazzled said:

No it wouldn't, go read up on inertia. Does an astronaut suddenly 'fly off' out of Earth orbit when he goes on EVA, because his mass is less than the rocket...?

Quite.

To be in orbit around something is (according to GR) to be travelling at a constant velocity through a distorted spacetime. Velocity is a vector, I hear you say, and yes, it is - the orbiter is travelling in a straight line as far as it is concerned. It feels no force of acceleration, and acceleration is a requirement of changing velocity(*). Experiments performed by the orbiter behave exactly the same way as if it was stationary in deep space, or travelling at a constant million km/h through space. If the spacecraft had no windows, GR says that there is no experiment that could be performed on the spacecraft that could differentiate between these scenarios (or to be pedantic, no "point" experiment - if the spacecraft was very large then the inhabitants would be able to detect the presence of a gravitational gradient, eg by using atomic clocks or maybe by detecting tension in a wire between two masses).

(*)From an external frame, eg from a few million km away where the distortion of ST is reduced, we see the body as travelling round and round and therefore accelerating / changing velocity.

Another thought experiment is to allow yourself to freefall towards a large body with a pendulum in your hand. It would just float with you. Only when you fire your thrusters to maintain a constant distance from the body would the pendulum have any tension in the string and swing towards the body. You are now accelerating to maintain your position, you feel the force of acceleration and compared to a traveller without thrusters who is "stationary" (see above) you are accelerating away from him. The body is somehow "sucking in" spacetime and you have to fight against it to maintain your position in erm...spacetime? That doesn't make much sense. You are maintaining your position in space, you are pretty certain about that, so what is it that is being sucked in? It's the point in space that existed in that location a short time ago. Spacetime is being distorted.

[R300will]

Bedazzled said:

R300will said:

The mass of the sun may not have changed but the mass of the earth has if its vanished and been replaced by the people living on it. They will have much less mass than the earth therefore the Sun will have a different gravitational pull on them and since their velocity would be the earths orbiting velocity (assuming it just vanished) then their orbital path would change causing them to fly off.

No it wouldn't, go read up on inertia. Does an astronaut suddenly 'fly off' out of Earth orbit when he goes on EVA, because his mass is less than the rocket...?

No because he/she is attached to the Space rocket which is flying at a certain velocity and distance necessary to maintain a stable orbit. When the astronaut goes off with a jetpack on they would have to keep within a certain speed range of the orbiting rocket in order to stay in the same orbit.

If they stopped moving then they would fall back to earth and if they sped up too much they would fly off out of the orbit. Something to do with angular momentum i believe

So if the earth just popped and vanished then the mass of the people wouldnt be anywhere near equal to the mass of the earth so their velocity (or angular momentum) will be too much so they will not follow the original earths orbit and will probably fly off instead.

This is how i understand it anyway but if you're adamant that i'm wrong i'll ask a friend studying physics for clarification.

[FarmyardPants]

Time to phone a friend :-)

The spaceship isn't "flying" as in using any energy, it is just floating just as the EVA astronaut is. If he dropped his screwdriver would it fly off? I think not.

[R300will]

FarmyardPants said:

Time to phone a friend :-)

The spaceship isn't "flying" as in using any energy, it is just floating just as the EVA astronaut is. If he dropped his screwdriver would it fly off? I think not.

It would probably fall back to earth. Friend will be phoned

[RegMolehusband]

I have a question. Let's say we're sitting on Pluto with a light meter pointed at the Sun and our newly invented gravitational force measuring device also pointed at the Sun. Then as result of some cataclysmic event the Sun ceases to exist.

It would take about 328 minutes for this fact to be recorded via the light meter.

Now the killer question, is the nature of gravity such that the gravitational force meter would immediately drop to zero, and we would notice Pluto and the planets simultaneously heading off at a tangent?

If so then I may well have had an Einstein-like insight revealing a faster-than-light phenomenon that lays the foundation for exploring the stars. A posthumous knighthood will do. Thank you.

[R300will]

RegMolehusband said:

I have a question. Let's say we're sitting on Pluto with a light meter pointed at the Sun and our newly invented gravitational force measuring device also pointed at the Sun. Then as result of some cataclysmic event the Sun ceases to exist.

It would take about 328 minutes for this fact to be recorded via the light meter.

Now the killer question, is the nature of gravity such that the gravitational force meter would immediately drop to zero, and we would notice Pluto and the planets simultaneously heading off at a tangent?

If so then I may well have had an Einstein-like insight revealing a faster-than-light phenomenon that lays the foundation for exploring the stars. A posthumous knighthood will do. Thank you.

Cool idea but i think as gravitational force can only be transmitted as fast as the speed of light then it would take the full 328 mins for you to notice the sun vanishing and the planets starting to sod off.

[RegMolehusband]

R300will said:

Cool idea but i think as gravitational force can only be transmitted as fast as the speed of light then it would take the full 328 mins for you to notice the sun vanishing and the planets starting to sod off.

How do you know that gravitational force can only be transmitted at the speed of light. Why wouldn't it be similar to a force field around a magnet? OnOnOn.....OFF!

[R300will]

RegMolehusband said:

R300will said:

Cool idea but i think as gravitational force can only be transmitted as fast as the speed of light then it would take the full 328 mins for you to notice the sun vanishing and the planets starting to sod off.

How do you know that gravitational force can only be transmitted at the speed of light. Why wouldn't it be similar to a force field around a magnet? OnOnOn.....OFF!

Don't know for sure but i think its right that forces can only be transmitted at the speed of light.

[FarmyardPants]

RegMolehusband said:

How do you know that gravitational force can only be transmitted at the speed of light. Why wouldn't it be similar to a force field around a magnet? OnOnOn.....OFF!

Magnetism travels at the speed of electromagnetic waves - the SoL. Time and space are interchangeable in GR calculations I believe, one light second being the unit of distance used to convert. I think if the Sun disappeared it would not be deemed to have happened on Pluto or anywhere else until the effect can be measured. In other words, on Pluto, it hasn't actually happened in that frame yet. You have to remember that time is not an absolute.

The idea that something doesn't happen until you see it happen is key to special relativity.

[RegMolehusband]

But as GV said in his first post "That means fundamentally we have to look at gravity as a 'different' type of field to those we understand today". Whilst it is closely related to electro-magnetic fields it has different characteristics. e.g no negative pole. So it will behave differently from a magnetic field.

[R300will]

RegMolehusband said:

But as GV said in his first post "That means fundamentally we have to look at gravity as a 'different' type of field to those we understand today". Whilst it is closely related to electro-magnetic fields it has different characteristics. e.g no negative pole. So it will behave differently from a magnetic field.

I mentioned this earlier about the negative gravity. My thoughts are (and they're most likely widly wrong which could be why it was ignored rather than answered) This negative energy stuff thats causing the universe to expand faster and faster etc, could that be a form of anti gravity?

[Gene Vincent]

R300will said:

RegMolehusband said:

But as GV said in his first post "That means fundamentally we have to look at gravity as a 'different' type of field to those we understand today". Whilst it is closely related to electro-magnetic fields it has different characteristics. e.g no negative pole. So it will behave differently from a magnetic field.

I mentioned this earlier about the negative gravity. My thoughts are (and they're most likely widly wrong which could be why it was ignored rather than answered) This negative energy stuff thats causing the universe to expand faster and faster etc, could that be a form of anti gravity?

To date there has been no evidence of -ve gravity, every time we think we might have observed it, we later find just another energy source overcoming the very weak Gravity effect.

The expansion of the Cosmos is very well explained in the 'spontaneous' production of empty space where the 'gravity' in the empty space is so weak that even the exotic production of space itself is sufficient to overcome it.

All fields are both +ve and -ve in their probabilities, we may have to accept that the field that describes gravity may well be a true 0 to +ve one, the maths, and more importantly, the geometry of that field does not fit readily into the model we have, there is (I think) something in the correlation of the extreme weakness and the place of that '0'.

To try to explain this perhaps there is a correlation between the root value of gravities '0' and the extreme -ve probability, but every attempt at that has failed.

[Gene Vincent]

moreflaps said:

Gene Vincent said:

moreflaps said:

Err, bit of a physics slip there eh GV?

Only for the purposes of illustration, the full paragraph was designed to convey an image of how a photon conveys its information without corruption from others looking right across their line of sight.

If you can think of a better way to explain that then please do so.

moreflaps said:

Isn't a simpler explanation that gravity is the projection of a higher order dimension onto the dimensions of Minkowski space time resulting in it not being flat ...

We've not got to explanations, we are still at the 'conditions' stage... and there are other hypotheses.

Plus, you don't need further dimensions for Minkowski spacetime to incorporate gravity.

Just placing a planet in Minkowski spacetime as per SR results in the trajectories of inertial observers not bending towards that planet, so any falling observer will not be inertial!

You need another spacetime not a dimension, the inertial trajectories of which are falling trajectories.

You need a different spacetime for each and every arrangement of matter.

Falling into the planet will then just be following an inertial path, and that needs no force to be present.

This is why gravity is not a force, but rather the result of following the contours of spacetime and the shape of spacetime depends on the matter present.

No dimensions added or needed.

But we will get to explanations that do have them.

(edit... syntax!)

Edited by Gene Vincent on Thursday 13th September 12:05

There is no need for waves to interfere with the direction of propagation of other waves. You don't have to invoke photons like particles with discrete trajectories -just look at waves in the ocean! The key is that the propagation of a wave is self sustaining...

As for the distortion of space time to explain gravity that is fine and what I was alluding to, the question is, how is space time bent? Is the bending explained by our experiencing the projection of higher dimensions (that we, as 3D beings cannot ever perceive) so that real space is (say) 4 dimensional but that we experience/see only 3 dimensions. Thus we think we travel intertially in geodesics in 3D but which are actually geodesics in 4 dimensions so that our path in 3D becomes distorted... The distortion we call gravity and we then make equations to describe a 'force' that may not exist at all...

By analogy, what about how a 2D being would experience interaction with a 3D object that distorts the 2D space they live in...

Cheers

You say there is 'no need' yet in the macro world we that they do all the time from the waves of a dropped stone in the sea to radio interference.

We have to explain this doesn't happen in the micro world.

Saying there is 'no need' simply isn't good enough.

Spacetime is warped through the presence of energy/mass being present within it or acting upon it.

Other dimensions are largely matrices or tensors for calculation, I say largely as Time is a discrete finite point dimension and many of the others used are similar but considered infinite, but much of that is a bit of mathematical artifice.

[Lost_BMW]

Just, belatedly read the thread - work pressures minimising time for things that make me think - and disappointed to see it just stopped, 'thought there'd be more to come?

So, posting by way of a bump hoping it will be a thread resurrection and that GV can tolerate the (few) regular naysayers...

[Gene Vincent]

I've simply allowed some posters the space to post their take (as mine seems so repugnant at times) on this difficult to explain subject.

I was hoping for some here to be forthcoming, but it is more satisfying for some to be a 'sniper' than to poke your head above the parapet and make a contribution it seems.

But I'll take this on another step during the week-end whilst revving up for the Grand Prix.

[Lost_BMW]

Gene Vincent said:

I've simply allowed some posters the space to post their take (as mine seems so repugnant at times) on this difficult to explain subject.

I was hoping for some here to be forthcoming, but it is more satisfying for some to be a 'sniper' than to poke your head above the parapet and make a contribution it seems.

But I'll take this on another step during the week-end whilst revving up for the Grand Prix.

I shouldn't let the (regular crew of) snipers put you off. They rarely have anything interesting to offer.

The reason we rather selfishly leave it to you and (most of us) don't post anything except questions is because few of us (I'd guess/from looking at profiles) are professional or trained theoreticians, cosmologists, astrophysicists etc. We're pretty much relying on you on this! Sorry.

But it is all very interesting and has led me at least to wander off and read much more widely. As for the antis and the whingers, not those with deeper questions or an alternative view to express/check out but the "I'll take issue with Gene Vincent whatever he says because he won't say who he is.." types the rest of us can easily ignore their noise. Or they could sod off and try to score points somewhere else; just a thought!

[Gene Vincent]

In the light of recent developments in Field theory we should take a little time to consider the speed of the application or action of gravity.

Newton contended that gravity acted instantaneously and until the beginning 19th century this view held considerable sway.

Then for a while the speed of gravity was though not to be infinite but about 7 million times faster.

Then Einstein brought the matter to a 'close' by his work clearly stating that Gravity was no faster than the propagation of light.

Since that time we have adhered to this with only occasional tests of the truth of this.

The tests have all confirmed that Einstein was right...

Well, sort of.

There is no doubt that Einstein was right, but in the same papers that confirmed his 'correctness' contained the seed of the possible fall in this much vaunted assertion.

Einstein was the prime mover of Field theory and in recent months we have to realise that Field theory may just hold a greater truth about the 'speed' of gravity.

Gravity really does 'act' at the speed of light and both GR and SR remain intact, but its application is instantaneous.

This is because the Quantum Field that we are at present defining for what we call gravity, is mono-polar. It is part of mass and being positive in nature at all times there is no delay in the application as all the rest of the attractors are all running positive too, gravity gets the jump on everything. Thankfully it is very weak and this initial advantage is overcome readily by the other forces.

It also helps us understand the London Dispersion Force which previously, and rather mysteriously, allowed non-polar atoms to interact as if they were charged.

The most extraordinary 'possible' outcome of this new understanding is that it may just hold the clue as to the nature of the Fine Structure Constant, by 'perhaps' acting as a form of 'synchromesh' between interaction... but... BUT... why this would appear 'like' a particle that behaves 'like' it is composed of 12 Quarks is at the moment totally beyond my present understanding.

[mattnunn]

So Gene, are you saying anti gravity is impossible?

[Jinx]

So gravity acts in a similar way to a liquid - whilst pushed at one end it pushes at the other instantaneously - though doesn't break SR or GR.....
But liquids cannot be compressed hence why hydraulics work but can the same be said about gravity?

[Gene Vincent]

mattnunn said:

So Gene, are you saying anti gravity is impossible?

You can overcome gravity because it is so weak, but there is no actual -ve gravity.