Gravity

If gravity affects the direction of light but not it's speed, then why is gravity thought of as a bend in space time? Wouldn't it make more sense to treat objects in space as a negative mass, like air bubbles underwater being attracted to each other? Why cant the sun be an air bubble and space a fluid?

While I don't understand spacetime or whatever it is you just proposed, I'm not sure why we would expect gravity to have any noticeable effect on the speed of light. Isn't this a simple matter of inertia, and of light traveling so fast that gravity simply isn't enough to slow it down?

Gravity is so depressing: it really brings me down.

Led Zeppelin said:

If gravity affects the direction of light

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It's more that gravity affects the topography of the space that light is traveling through.

The standard visual explanation is to have some people hold a sheet tightly between them, then place something heavy enough in the centre to cause the sheet to dip. Then you roll something light, like a ping pong ball, across the surface of the sheet and note that the weight in the centre causing the sheet to dip also causes the ping pong ball to be drawn towards it, its path across the sheet changed. That visualization is meant to let you imagine how a photon (ping pong ball) can still travel in a direction (namely straight across the sheet) while having its path changed by the effect of gravity (the heavy object) with the added conceptual idea that spacetime itself is warped by gravitation.

Led Zeppelin said:

but not it's speed, then why is gravity thought of as a bend in space time?

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Because we're visual creatures who understand metaphors better when they relate to our own experiences.

Led Zeppelin said:

Wouldn't it make more sense to treat objects in space as a negative mass, like air bubbles underwater being attracted to each other? Why cant the sun be an air bubble and space a fluid?

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I can't see what that means. What's a negative mass, for example? How does a thing, which at this level of discussion must necessarily include some positive mass for it to be a thing, ever exist as a negative mass? Even antimatter is hypothesized to possess a positive mass.

Space can't be a fluid because a fluid is a particular state of molecules which are chemically bound together, i.e. covalent bonding. Space is not comprised of molecules.

The Sun can't be an air bubble because a bubble is a globule of one substance retained by a barrier of another substance, i.e. a soap bubble is air retained by a barrier of another substance. A sun is not a substance contained by a barrier, but rather various substances held together by its own mass. Gravity produces spheres from the coalescence of matter; a sun is held together by the gravity caused by the coalescence of matter, a bubble is not held together by gravity.

In the serie “How the universe works” they talk about gravity maybe check that its interesting and you will learn a lot.

Led Zeppelin said:

If gravity affects the direction of light but not it's speed, then why is gravity thought of as a bend in space time? Wouldn't it make more sense to treat objects in space as a negative mass, like air bubbles underwater being attracted to each other?

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Gravity doesn't actually affect the direction of light (caveat at the end_, it curves the space the light is moving through.

it's one of those things that requires the generation of a particular intuition that's counter to one of the most fundamental definitions you have in your head from your earliest education, and it's all to do with how we teach geometry, which desperately needs updating to circumvent the issue you're having.

In particular, we give a definition for the shortest distance between two points that is, not to put too fine a point on it, misleadingly incorrect.

So what is the shortest distance between two points? The conventional answer is the one from Euclidean geometry, and it's the stock answer that everybody not educated in physics beyond high school will recite without thinking. Hardly surprising, because it's intuitive, and it took a completely different kind of thinking to pin down why it was wrong.

I don't know how much study you've done of geometry, or if you've ever done any study on conic sections, but it's fairly intuitive to see that a circle is special case of an ellipse. In reality, a straight line as it appears in Euclidean geometry is also a special case of something else, a geodesic. The answer to the question, then, is that the shortest distance between two points - regardless of specific geometry; Euclidean, hyperbolic, spherical - is a geodesic.

So, the light is still travelling the same path, not being bent at all, but the space it's moving through is curved by the presence of mass, and the light is still travelling a 'straight line' but it's doing it through curved space.

That caveat I glossed over at the beginning is that gravity does, in fact, bend the path of light due to the mass inherent in its energy, but this is functionally irrelevant to the question because it's negligible, and isn't the subject of the question. You wouldn't be able to measure this bending with any technological means or technical cunning we currently possess.

Led Zeppelin said:

Why cant the sun be an air bubble and space a fluid?

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Well, space can't be a fluid because, if it were, the names 'Michelson' and 'Morley' would be among the most vaunted physicists in history, rather than being associated with the greatest null result in the history of physics. Had space been a fluid, they'd have detected changes in the speed of light dependent on the motion of the source or observer, yet they measure the speed for light to be the same in both the direction of travel and orthogonal to it.

Edit: I should add a caveat here: Michelson and Morley ARE among the most vaunted physicists in history, and rightly so. They were awesome experimentalists, and did a lot of hugely important work.

That Dirac, though. He was a cunt.

Greetings,

hackenslash said:

Well, space can't be a fluid because, if it were, the names 'Michelson' and 'Morley' would be among the most vaunted physicists in history, rather than being associated with the greatest null result in the history of physics. Had space been a fluid, they'd have detected changes in the speed of light dependent on the motion of the source or observer, yet they measure the speed for light to be the same in both the direction of travel and orthogonal to it.

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Er, ... space-time may be a superfluid.

Kindest regards,

James

Even if it were a superfluid, that's not a fluid!

Incidentally, people say they remember where they were when something momentous happened, but the first time I ever encountered the existence of superfluids was a day I will never forget. I couldn't process how I'd spent - at that time - around 27 years alive and had no idea such things could exist in reality. I was buzzing my balls off for days. I still feel the urge to clap like a child whenever I watch footage.

Dragan Glas said:

Greetings,


Er, ... space-time may be a superfluid.

Kindest regards,

James

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As my cogent cohort has so coherently contrived, that's not a fluid, and also space isn't spacetime.

Greetings,

OhhhhhhhhHHHHH, you spoilt it!

Kindest regards,

James

To me, this youtube channel is among the best that explains relativity in the simplest manner.

Here are the key videos that are most relevant to your question

I discovered that channel a few weeks ago. Truly excellent content. It's a fine line between simple enough and misleading, and they hit it right in the pocket.

Sparhafoc said:

It's more that gravity affects the topography of the space that light is traveling through.

The standard visual explanation is to have some people hold a sheet tightly between them, then place something heavy enough in the centre to cause the sheet to dip. Then you roll something light, like a ping pong ball, across the surface of the sheet and note that the weight in the centre causing the sheet to dip also causes the ping pong ball to be drawn towards it, its path across the sheet changed. That visualization is meant to let you imagine how a photon (ping pong ball) can still travel in a direction (namely straight across the sheet) while having its path changed by the effect of gravity (the heavy object) with the added conceptual idea that spacetime itself is warped by gravitation.




Because we're visual creatures who understand metaphors better when they relate to our own experiences.




I can't see what that means. What's a negative mass, for example? How does a thing, which at this level of discussion must necessarily include some positive mass for it to be a thing, ever exist as a negative mass? Even antimatter is hypothesized to possess a positive mass.

Space can't be a fluid because a fluid is a particular state of molecules which are chemically bound together, i.e. covalent bonding. Space is not comprised of molecules.

The Sun can't be an air bubble because a bubble is a globule of one substance retained by a barrier of another substance, i.e. a soap bubble is air retained by a barrier of another substance. A sun is not a substance contained by a barrier, but rather various substances held together by its own mass. Gravity produces spheres from the coalescence of matter; a sun is held together by the gravity caused by the coalescence of matter, a bubble is not held together by gravity.

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Right, but air bubbles are attracted to each other, in accords with the inverse square law. The same law that governs the attraction of bodies in space. 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.

I realize that space is not a fluid, but it space also doesnt really have an apparent topology either, does it? So whats the point of thinking that it does? 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. 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.

It's just an idea.

hackenslash said:

That caveat I glossed over at the beginning is that gravity does, in fact, bend the path of light due to the mass inherent in its energy, but this is functionally irrelevant to the question because it's negligible, and isn't the subject of the question. You wouldn't be able to measure this bending with any technological means or technical cunning we currently possess.

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

psikhrangkur said:

While I don't understand spacetime or whatever it is you just proposed, I'm not sure why we would expect gravity to have any noticeable effect on the speed of light. Isn't this a simple matter of inertia, and of light traveling so fast that gravity simply isn't enough to slow it down?

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Well, I mean its not obvious that gravity would not affect the speed of light..I mean, things move sometimes because of gravity, dont they?

Led Zeppelin said:

Well, I mean its not obvious that gravity would not affect the speed of light..I mean, things move sometimes because of gravity, dont they?

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Thats because light has both the properties of a wave and a particle, without the downsides of being a particle or a wave.

Led Zeppelin said:

Right, but air bubbles are attracted to each other, in accords with the inverse square law. The same law that governs the attraction of bodies in space. 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.

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By electrostatic forces.

Led Zeppelin said:

I realize that space is not a fluid, but it space also doesnt really have an apparent topology either, does it? So whats the point of thinking that it does?

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Its apparent topology is flat.

Led Zeppelin said:

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. 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.

It's just an idea.

Click to expand...

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.

It's a fine idea, had by many fine minds over the history of physics and astronomy, but it is, as are many beautiful theories, slain by an ugly fact. Actually, they're some of the most beautiful facts in physics, IMO, but I have proclivities in that regard, so I'm not really objective.

Led Zeppelin said:

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

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It absolutely is. When light travels close to a body with mass, we see a reduction in energy, wherein energy has been sapped from the light, lowering its energy and stretching its wavelength. This is gravitational redshift.

Led Zeppelin said:

Well, I mean its not obvious that gravity would not affect the speed of light..I mean, things move sometimes because of gravity, dont they?

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Gravity doesn't affect the speed of light, but it does affect the space through which the light travels, stretching its wavelength. In the case of a photon, its energy is expressed as its wavelength. As the light travels through the gravity well, it's moving through a frame in which time is running slower, wherein it's locally travelling at c. It's always travelling locally at c, according to ALL observers. As it comes out of the gravity well, it will be measured to have lost energy while travelling through the gravity well, measured as a reduction in frequency overall, corresponding to a stretching of its wavelength, exactly as Einstein predicts.