[mxplxxx]

The crisis of 21st century physics is this:

It has produced two different, and entirely irreconcilable, descriptions of what happens in the Universe:

1. Relativity Theory

2. Quantum Mechanics

They don't agree with each other.  Therefore there is something wrong about both of them.

Not necessarily. The various formulations of quantum physics sometimes appeared to disagree with one another but were found to be just different ways of expressing the same truth. Relativity is a theory of a continuum whereas quantum physics is a theory of discrete particles. i.e analogue versus digital. Think of a TV set where the same picture is produced by analogue and digital transmission types. Quantum theory may in fact be a simulation of relativity. Gravity in quantum theory is defined via the graviton particle whereas gravity in relativity is intrinsic in the geometry of the continuum.

[puppypower]

james  Muirhead asked the Naked Scientists:
   
What is the source of the force of gravity or what is it that reaches up from the Earth to pull the apple downward?
What do you think?

Mass attracts other mass, via gravity, and by doing so clumps in such a way to cause the shared space-time to contract. If we keep on adding mass to the clump, common space-time will continue to contract until it reaches a limit; singularity, which has a reference very similar to a speed of light reference; point-instant. This reference is where the gravitational potential of all the mass units are all at a minimum. If you add this all together, gravity simply reflects a movement of mass toward a common ground state, that is analogous to the speed of light reference.

[Bill S]

Release your marbles simultaneously from the top of the box.  They will fall to the bottom.  If you are being accelerated, their trajectories will be parallel, but if you are on the surface of a planet their trajectories will converge on the centre of the planet, so they will converge as they fall.

The first argument you use is one where you assume that the rockets center of mass (engine) is evenly distributed over the whole 'floor' of the rocket, right? I actually wonder there whether the rockets 'center of mass' won't be situated in a 'middle' of that floor too?

I was making no assumption about the centre of mass of the rocket.  To do so would be to introduce gravity to the part of the scenario that should relate to the acceleration, surely this would negate the equivalence argument because you would be comparing gravity to acceleration + gravity. 

[yor_on]

That's okay. I'm doing it. I started thinking about it in terms of where the 'center of mass' should be. Earth can be treated as if the center of its mass is inside its 'middle' approximately, that's also a reason to why the pebbles diverge as well as they 'accelerate', and it was those pebbles that made me start to wonder :) Because, assuming it to be correct then what it would say is that the 'center of mass' is 'everywhere' inside that rocket.
==

don't read me wrong Bill, it was one of the more interesting ideas I've seen, no matter its outcome, it made me wonder. Better add that even if it is this way the equivalence principle should stand, as it uses very small patches to describe the equivalence from, but generally speaking, I would still wonder a little :)

[RobC]

The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works.

Anybody like to speculate why it works?

[Bill S]

The following quote from Christopher Baird may help:

“The bottom line is that a uniform gravitational field is NOT a flat spacetime in the strict scientific sense since clocks tick at different rates at different locations in the uniform gravitational field. A uniform gravitational field may not look like a bar that is bent or a rubber sheet that is warped, but this just means that our analogies have limitations. It does not mean that there is no spacetime curvature.”

 “If you stand motionless in the field (not in free-fall), and let go of a ball, it will fall in a special direction - in the direction in which the gravitational field strength is changing (i.e. down).  Therefore, the existence of a special direction indicates that a uniform gravitational field is indeed a case of a curved spacetime. Again, it's not curved in the sense of a bar being bent to one side. It's curved in the scientific sense of spacetime behaving differently from one point to the next.

 Yes, gravity is a force. But it is a force that is more completely described by spacetime curvature and not Newton's law.”

[jeffreyH]

It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.

[Bill S]

Anybody like to speculate why it works?

I wondered if thoughts along those lines might arise from #1, but no one has picked up on it, so perhaps it’s a non-starter.

[RobC]

Quote from: PmbPhy on 27/12/2016 23:23:27The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works. Anybody like to speculate why it works?
                           

So many things in this weird world/universe that we know how it works but not why it works.

This last hundred years or so has been a most exceptional period of advancement, even rivaling the introduction of the printing press in the 15th century. We have achieved so much and we are on the brink of so much more.


I would dearly like to hear more of the why rather than the how.

[evan_au]

It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.

This should be possible - the surface would be a straight line (at least in regions small enough where you could consider gravity to be constant).

You need to calculate the frictional/aerodynamic/magnetic induction energy losses of the ball rolling at the intended speed.

• Then place the surface at an angle that just compensates for all of these losses.
• If you start the ball rolling at the correct speed, it will keep rolling at this speed.
• It would be less complicated if the experiment were done in a vacuum (air pressure increases much more rapidly than Earth's gravity), and if the ball were non-conductive and non-magnetic (magnetic field intensity varies dramatically at different points on Earth's surface).

How would this help the study of gravity?

[timey]

The source of gravity is mass. To be precise its active gravitational mass. The mass generates a gravitational field and the field exerts a force on whatever is in the field. The mechanism to do this is currently unknown. All we know is how it works, not why it works.

Anybody like to speculate why it works?

I think that what one should really be asking about is why it is that open space, which doesn't even comprise of thin air, is actually curved...

Q: What is curving?
A: There is nothing there to be curved.

Then one can look to gravity and ask if there is something about gravity that could cause a body of mass, or light to move in a curve.

Different values of M cause differing curves...
But any value of m in relation to M follows the same curve, therefore, although M is the cause of gravity, it can only be the g-field that is causing the curve.

When describing an acceleration or deceleration - acceleration is simply a phenomenon where it takes a lesser amount of time to cover the same distance.  And a deceleration is a phenomenon where it takes a longer amount of time to cover same distance.
If you draw a line describing a linear deceleration followed by a linear acceleration between 2 bodies of M, the line will be a curve.

Therefore there lies the possibility that space itself is flat, and that gravitation is a time dilation phenomenon within the g-field itself that is causing the geodesics that m and light must travel, and as a result space is only appearing to be curved.

[geordief]

When describing an acceleration or deceleration - acceleration is simply a phenomenon where it takes a lesser amount of time to cover the same distance.  And a deceleration is a phenomenon where it takes a longer amount of time to cover same distance.
If you draw a line describing a linear deceleration followed by a linear acceleration between 2 bodies of M, the line will be a curve.

Therefore there lies the possibility that space itself is flat, and that gravitation is a time dilation phenomenon within the g-field itself that is causing the geodesics that m and light must travel, and as a result space is only appearing to be curved.

If this hypothetical**   time dilation effect is increased without limit I take it that we approach a (relative)  velocity of c for any massive object.

Does the value of c act as an  impediment to the otherwise seeming possibility that succeeding events could happen  simultaneously (in an absolute way)?

And does this value of c act as a practical  impediment to the ideas that seem to go around of "tears in the fabric of space time" ,wormholes etc  ?

I may not have made myself clear(because my  reasoning is faulty no doubt) but perhaps you understand the (fuzzy?)  point I am trying to make....






** I  am not making  out time dilation  itself to be hypothetical but I think you yourself introduced it as a relevant factor in a hypothetical schema.(I have bolded  and underlined  where I think you are introducing this as speculation )

[timey]

Well that is a bit on the fuzzy side, but I think I understand what you are getting at.

However, this hypothetical time dilation of the g-field is decelerated at h from M, and can be considered as an aether type scenario.

The speed of light in open space will be being held relative to longer seconds, but in the reference frame of the longer seconds, the speed of light will be 299 792 458 metres per second of that reference frame.
Similarly, when approaching a black hole, the speed of light will be held relative to shorter seconds, but in the reference frame of the shorter seconds the speed of light will be 299 792 458 metres per second.

Now all that remains is to add GR time dilation and SR time dilation for mass, but not the length contraction/dilation of space associated with SR, for the reason that this effect is already taken care of via the g-field as a temporal measurement rather than a spatial measurement.

Now because it is the g-field that is inherent with the 'hypothetical' time dilation of the g-field, we can say that GR time dilation for m in relation to the g-field is gravity potential energy related, and is the exact opposite to the hypothetical g-field time dilation with respect to M, whereas GR time dilation is positive value, and the g-field time dilation is negative value.

When adding SR time dilation for mass to this picture, we can observe that because the speed of light is held relative to the negative value, that the speed that m is travelling at will be a higher percentage of the speed of light in that reference frame and the SR time dilation effects that m will experience will escalate.
In this manner, the m cannot exceed the speed of light of the reference frame it is travelling through.

Within this set up there lies the possibility to warp the slower time of the g-field reference frames in order that m will experience its travel path through space at a faster rate by regulating SR time dilation effects, and get 'there', wherever that may be quicker, but that is a far more advanced conversation

[evan_au]

Q: What is curving?

One way to look at the curvature of space is to add up the angles inside a triangle.

• In high-school geometry (as developed by Euclid), the sum of the internal angles of a triangle is exactly 180 degrees.
• On the surface of the Earth (or in a closed universe, or near a black hole), the sum of these angles is slightly larger than 180 degrees
• In an open universe (or on the surface of a horse-saddle), the sum of these angles is slightly less than 180 degrees.

So grab your protractor, and head off to the nearest black hole...


See: https://en.wikipedia.org/wiki/Non-Euclidean_geometry#Uncommon_properties

[timey]

Head off to the nearest black hole?

"Why whatever do you mean?"
(chuckle)

Should my feelings now be hurt I wonder...

But I believe you are missing the fact that if it takes a constant velocity a longer amount of time to cover a distance, this will cause the appearance of a greater distance. ie: a curve.
But this curve will only be an appearance because the distance remains a constant straight line of 180 degrees in the face of the longer time value...

...and without a horse saddle, dependent on the gait, less than 180 degrees can be quite lumpy, unless you're riding a nice wide cob!

[jeffreyH]

It should be possible to create a surface curved in such a way that a ball rolling along it under the influence of gravity will have a constant velocity. The creation of such a surface will be useful in the study of gravitation.

This should be possible - the surface would be a straight line (at least in regions small enough where you could consider gravity to be constant).

You need to calculate the frictional/aerodynamic/magnetic induction energy losses of the ball rolling at the intended speed.

• Then place the surface at an angle that just compensates for all of these losses.
• If you start the ball rolling at the correct speed, it will keep rolling at this speed.
• It would be less complicated if the experiment were done in a vacuum (air pressure increases much more rapidly than Earth's gravity), and if the ball were non-conductive and non-magnetic (magnetic field intensity varies dramatically at different points on Earth's surface).

How would this help the study of gravity?

I wrote that on 9th Feb and you expect me to remember my train of thought? I have a kitchen nightmare going on here. I will get back to you on this point.

[Colin2B]

 

One way to look at the curvature of space is to add up the angles inside a triangle.....

I agree, but would also add the quote from Bill:

 

….....it's not curved in the sense of a bar being bent to one side. It's curved in the scientific sense of spacetime behaving differently from one point to the next.

So if we measure distance and time, near a mass, and find that the intervals (spacings) change in such a way that we can describe them as being on a curve, then it's reasonable to say that spacetime curves. It's only a description of what we observe.

 
We can take Evan's example of curvature even further Bill. If you look at a globe you see parallel lines of latitude. Someone might mistakenly think that if they stood on the intersection of a latitude and longitude and set off in a straight line along the surface, that they would follow the line of latitude. In fact they would follow a great circle which curves down to meet the equator.
If we were to imagine the latitude direction to be time – and we are always travelling through time like it or not – then the curvature would pull us towards the equator. Of course, we don't think of ourselves as travelling through time, so it seems to us that we are falling, or being pulled down, a line of longitude directly towards the equator.

 
Of course the real problem is that most physics is descriptive of what we observe, rather than trying to assign a cause. Every time we dig deeper to find a root cause we have to find words to describe that deeper layer and that description can only be in terms of a yet deeper layer. So even if someone finds a graviton say, the next question will be “how does it cause the attraction?”.

[jeffreyH]

It's turtles all the way down.

[jeffreyH]

At the moment I am looking at the inertia tensor when I get the rare half hour free. Since inertia and gravity appear to be related then I may have something useful soon.

[syhprum]

Gravitons or space time warping I think that we must use double think like waves and particles and agree that both are correct.