Gravity

hackenslash

Well-Known Member
By the way, if you want absolutely the best experimental validation that our model of spacetime is orders of magnitude better than the luminiferous aether, you probably have one in your hand as you're reading this.

The global communications network that guarantees delivery of exactly the sort of communications were sharing here now, and especially if you're reading this on a mobile phone, is predicated on these ideas being correct to an extraordinary degree, the global positioning system. The clocks on board GPS satellites are corrected by a factor of approximately 38 microseconds per day to account for the relativistic effects of both the difference in altitude and the differential in gravitational time dilation and the effects of the satellites' high velocity. Without those corrections, it would be a matter of days before most of the planet couldn't find their own arses, as they'd drift out by about ten metres per day cumulatively.
 
You're talking about a luminiferous aether. This is precisely what the Michelson-Morley experiment falsified experimentally, and exactly what relativity answers, also experimentally validated by Eddington in 1919, and by observations of corrections to Newton's predictions for the precession of mercury's perihelion, and numerous experiments since, up to and including the detection of gravitational waves at LIGO.
Alight. I will check it out, along with some of the videos Nesslig suggested. Thanks guys.
 

Sparhafoc

Well-Known Member
Right, but air bubbles are attracted to each other, in accords with the inverse square law.

Actually, its simply the geometry of the surface of the liquid in which the bubbles float. Put water in a glass, for example, and the water curves up (the meniscus) where it is in contact with the glass sides - it makes a shallow half-pipe curve. When you float an object in that liquid, despite you not being able to see it, the curve of the liquid and the surface tension pulls the floating things (which are experiencing little friction) towards each other. This is compounded by other facts, such as that surface tension being depressed by the mass of the floating object (even a bubble) and the depression thereby sucking in close objects or in turn being pulled into the depression formed by those objects.

So while gravitation is affecting the bubbles, it's actually the world of surface tension which has greater dominion on this scale. As a human, you notice gravity but barely ever notice surface tension. If you were an insect, this would be reversed and your sensory world would be dominated more by surface tension than by gravity.


The same law that governs the attraction of bodies in space.

It's exactly not the same at all; space isn't a medium like liquid - it's absent a medium.


But bubble attraction is not really caused by a dent in space time is it? They are attracted to each other because there is a dense fluid pushing them together.

Well, gravity is depressing the fluid, and objects in the fluid are more strongly attracted to the more massive (contains more moles) liquid than to the
atmosphere, and the objects do create a depression in that surface because gravity is pulling those objects down against the resistance of the liquid.

But in space, objects aren't experiencing any resistance from the medium they're in because space is not a medium in that sense.

I realize that space is not a fluid, but it space also doesnt really have an apparent topology either, does it?

It doesn't at our level of apprehension, but gravitationally it has a topology. You can't see it, but there are troughs and peaks all the same.


So whats the point of thinking that it does?

Because it actually does, just not a visual topology as we'd be accustomed to imagine because it's not a thing.


Good point about the sun not being contained by a barrier like bubbles are. But actually I was thinking that their could be a barrier.

There's not: just a region of less dense plasma, else photons wouldn't be able to escape.


Or at least there could exist a "surface tension of space" that exists between space and the sun, which(for whatever reason) redirects light, giving an illiusion of a theorhetical bend in space time.

Again, space isn't a thing, so it's unable to possess attributes like these.

It's just an idea.

You won't hear any complaints from me. Having ideas like this, whether they're right or wrong, is about curiosity and wonder. Personally, I find bubbles to be an infinite source of speculative imagination for me. Everything about bubbles hints to me at tantalizing clues to understand a lot of the structure of nature - like for example the way a clump of bubbles together compress each other into hexagonal shapes. Absent any other pressures, bubbles will naturally form into spheres - the smallest possible surface area for the contained volume. Whereas multiple bubbles in contact form shared walls, gravitay forces now causes the bubbles to minimize the surface area incorporating that shared wall, and bubbles will therefore fall into the most efficient shape in that configuration which is a hexagon - look at 3 bubbles in contact with each other and their shared walls will always be at 120 degree angle from each other.

It's some deep shit - the structuralizing forces of the universe, the actual produced shape of the fabric of the universe, sitting right there in your bathtub.
 

Sparhafoc

Well-Known Member
Right but energy of light is electromagnetic. I thought electromagnetic energy was not effected by gravity..hmm..

It is affected by gravity, I think what you mean is that it's not attracted by gravity, but you're also aware that absent any external force, it will continue to travel in a straight line.

That straight line is warped under gravitational forces - but it's still a straight line (this is where it's confusing) from a photon's perspective; it's still going directly between point A and point B, it's just that the straight path between point A and point B is different. It's not the photon which is affected by gravity, it's the straight line the photon is taking which is affected by gravity.

I'm afraid you'll have to rely on someone else to exemplify this as I'm outside my knowledge and expertise from this point of analysis.
 

psikhrangkur

Active Member
It is affected by gravity, I think what you mean is that it's not attracted by gravity, but you're also aware that absent any external force, it will continue to travel in a straight line.

That straight line is warped under gravitational forces - but it's still a straight line (this is where it's confusing) from a photon's perspective; it's still going directly between point A and point B, it's just that the straight path between point A and point B is different. It's not the photon which is affected by gravity, it's the straight line the photon is taking which is affected by gravity.

I'm afraid you'll have to rely on someone else to exemplify this as I'm outside my knowledge and expertise from this point of analysis.
Is this mechanically different from point B moving to a different location? Can I say that you're in a mall, heading to Barnes & Noble because hack is there, and then you reroute to the food court because hack just texted you saying that he's there now, and all the while you were just heading straight to hack?
 

Sparhafoc

Well-Known Member
Is this mechanically different from point B moving to a different location?

It's in the same location, and the photon's traveling directly from point A to point B. It's just that the direct line from point A to point B conforms to the topology of spacetime. But as I said, I am at the limit of my ability to express ideas in such a way as to clarify rather than confuse, so I should leave it to someone else.

I notice some of my language in the above posts is lacking precision, but it was pre-coffee o'clock when I wrote it, so excuse the sloppiness! Hopefully Hack or Borg will clear it up!
 
Well, gravity is depressing the fluid, and objects in the fluid are more strongly attracted to the more massive (contains more moles) liquid than to the
atmosphere, and the objects do create a depression in that surface because gravity is pulling those objects down against the resistance of the liquid.

But in space, objects aren't experiencing any resistance from the medium they're in because space is not a medium in that sense.
Hmm..Ok so, Hackenslash referred me to the Michelson-Morley experiment and I found this video which explains it: It makes sense to me how this disproves the idea of space being a medium, or an "ether". Pretty awesome to me that these guys figured it out with just a few mirrors and a light (I am curious if such an experiment has been done in outer space).

I guess the problem I am having now is that if space is not a medium, then what exactly is it that is being curved by gravity? IS something there or not?

My tiny brain can barely understand such things! :(
 
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psikhrangkur

Active Member
It's in the same location, and the photon's traveling directly from point A to point B. It's just that the direct line from point A to point B conforms to the topology of spacetime. But as I said, I am at the limit of my ability to express ideas in such a way as to clarify rather than confuse, so I should leave it to someone else.

I notice some of my language in the above posts is lacking precision, but it was pre-coffee o'clock when I wrote it, so excuse the sloppiness! Hopefully Hack or Borg will clear it up!
Now I'm imagining a mountain or a valley appearing between a town and someone walking straight towards it.
 
It is affected by gravity, I think what you mean is that it's not attracted by gravity, but you're also aware that absent any external force, it will continue to travel in a straight line.
Thank you Sparhafoc, that is what I meant actually.

Or at least it would probably make more sense if that was what I meant. :p
 

hackenslash

Well-Known Member
(I am curious if such an experiment has been done in outer space).
yes, and more besides. Indeed, there's going to be an enormous iteration of this experiment in space if the pathfinder mission is a success. it will be called LISA, and it's a direct scaling up of that experiment. it's functionally identical to LIGO and the Michelson-Morley experiment, but rather than the 4 km arms LIGO has, it will have arms about 1 million km long, IIRC.

Unlike LIGO, which maintains two sites a long way apart to correct for local anomalies (it's so sensitive, it can pick up a heavy truck rolling by on the interstate some distance away), LISA only needs one site, because it doesn't have ground-based noise to account for.
I guess the problem I am having now is that if space is not a medium, then what exactly is it that is being curved by gravity? IS something is there or not?
The proper intuition can only ever come from shedding the notion that space is a thing in itself (eat me, Kant!). It's difficult, because there are things we intuit about space that aren't obviously true about time, such as being able to see extent while standing still. We can't see far in time (actually, we can, but this requires the same intuition), but we can literally look at the extent of space while standing still in space. We can't do that for time, because we don't appear ever to stand still in time.

In fact, most of the measurable effects of relativity are time effects. If you understand the result of one process (not really an experiment, but it did by this process provide experimental validation of a specific fact about relativity), you'd immediately see why. That process is the voyage of the Cassini-Huygens mission.

Here's the scenario:

Mission leaves Earth and heads for the inner solar system. All the mission trajectories are calculated using the Newtonian formulation for gravity. Why? Because it's a lot easier. Exact solutions to General Relativity are as rare as rocking-horse turds (Einstein himself thought exact solutions would never be found, but he was rapidly disabused of this notion by Karl Schwarzschild, who devised the metric for black holes). In any event, after executing four gravitational slingshot manoeuvres (two around Venus, one around Earth and one around Jupiter), and a journey through space of something in the order of 15×10^11 kilometres, inserted itself into Saturn's orbit within twenty metres of its intended target. That's a fairly spectacular demonstration of just how piddling the disagreements are between Newton and Einstein on the kinds of length scales we're accustomed to dealing with.

Again, some of that discrepancy is due to gravitational (General) relativistic effects, and some due to velocity related (Special) relativistic effects.

Spacetime curvature in both case is massively more apparent in the time direction, which is why we can measure discrepancies readily at mundane velocities and distances. For example, by taking three caesium clocks and syncing them, then putting two on planes to fky around the world in opposite directions and then bring them back together. Short distance, small speeds, yet measurable differences in the time readout on the three clocks when they were brought back together.

And it isn't the clocks. Caesium clocks are the most accurate, reliable clocks we have. All our modern international timings are based on these clocks (one of the clocks in the international network was one of the clocks in the experiment), and they're extraordinarily accurate and run at rates predictable well down into the decimal expansion, yet these rates are violated when they're in motion.

So the answer to your question, then, is that what's curving is, mostly, time.
My tiny brain can barely understand such things! :(
You're doing fine. You're asking the right kinds of question, and that's the purest form of philosophy.
 

hackenslash

Well-Known Member
This is worth a shufty:
Physicists in Germany have performed the most precise Michelson-Morley experiment to date, confirming that the speed of light is the same in all directions. The experiment, which involves rotating two optical cavities, is about 10 times more precise than previous experiments – and a hundred million times more precise than Michelson and Morley’s 1887 measurement.

I'll add that "Lorentz invariance" means 'constant for all observers'. This because, to get the proper result from two observers in different inertial frames, you take their measurements and apply Lorentz transformations. 'Proper' here means 'as measured by an observer freefalling between the two inertial frames' and is the shorthand for 'real' spacetime (for a given value of 'real').
 

hackenslash

Well-Known Member
Incidentally, and while this might seem somewhat self-serving, if you're really interested in developing an intuition for relativity, I can't recommend a better place to start than here:

The Idiot's Guide to Relativity

You should find that very approachable, and there is no math in there that you're not entirely familiar with, albeit some of it looks slightly unfamiliar until you read the post.
 

Sparhafoc

Well-Known Member
I guess the problem I am having now is that if space is not a medium, then what exactly is it that is being curved by gravity? IS something is there or not?

On Earth, any apparently open space is actually full of gas. So space is like that, only absent all the gas! :D

It's one of our chauvanisms - an expectation based solely on having evolved within a biosphere characterized by things we come to see as 'normal'. When you think about it, it's our little pocket of solid and gases that's weird - to any order of approximation, the universe consists mostly of very little at all - great expanses of nothing (well, a few particles scattered here and there) with occasional smudges of concentrated matter. All courtesy of gravity, otherwise the universe would be a pretty boring, lonely place.
 

hackenslash

Well-Known Member
On Earth, any apparently open space is actually full of gas. So space is like that, only absent all the gas! :D

It's one of our chauvanisms - an expectation based solely on having evolved within a biosphere characterized by things we come to see as 'normal'. When you think about it, it's our little pocket of solid and gases that's weird - to any order of approximation, the universe consists mostly of very little at all - great expanses of nothing (well, a few particles scattered here and there) with occasional smudges of concentrated matter. All courtesy of gravity, otherwise the universe would be a pretty boring, lonely place.
Indeed, and here we're only talking about relativity.

allan adams.png
 

21st Demon Lord

Active Member
 
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