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Gravity is not a force – free-fall parabolas are straight lines in spacetime

timhutton.github.io

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Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#241

Post by leptons »

GuB-42 wrote: post #88

My enlightening moment about general relativity: apples do not fall on the ground, instead, the earth is inflating, and the inflation of the earth is accelerating at 9.8 m/s^2. Eventually, the ground catches the apple. Of course, you are going to tell me that the earth is not inflating, obviously, because it is still the same size after so many years. But here is the trick: the earth is inflating at the same rate as…

I'm not sure how that hypothesis works out with things like black holes and gravitational lenses, friction, and a lot of other established physics. Somehow I figure people that believe in the expansion hypothesis will have some kind of workaround for those.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#242

Post by pdonis »

umvi wrote: post #232
pdonis wrote: post #217

Earlier quoted context omitted.

> From the point of view of a photon, no time elapses between its the origin and destination endpoints. No, this is not correct. The correct statement is that the concept of "elapsed time" does not apply to a photon; it only applies to timelike worldlines, not null worldlines. To put it another way, if your statement were true, it would mean that the origin and destination events were the same point in spacetime. But…

It's impossible for an object with mass to go the speed of light. But you can observe what happens as you approach it: Let's say I put you in a spaceship and accelerate you to 50% the speed of light toward the sun. From an inertial viewer's perspective you are travelling toward the sun at half the speed of light and it takes you ~16 minutes to crash into the sun. But from your perspective it only took ~14 minutes to…

> you can observe what happens as you approach it

Yes, but you cannot extrapolate from this to say that the time lapse for a photon would be zero. A photon is not the limit of objects with mass going closer and closer to the speed of light, because "closer and closer to the speed of light" is frame-dependent, but a photon's speed being c is not. I can find an inertial frame in which each of your objects is at rest, and in that frame, you are the one who is "close to c" (in the opposite direction). But that doesn't mean your elapsed time approaches zero. By contrast, it is impossible to find any frame in which a photon is at rest. The two types of objects are fundamentally different.

In more technical language, the action of Lorentz transformations on photons is fundamentally different from their action on timelike objects. So it is simply not valid to view photons as some sort of limit "as speed approaches c" of timelike objects.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#243

Post by pixl97 »

mensetmanusman wrote: post #165
mrfusion wrote: post #20

Earlier quoted context omitted.

Are we flat in the time dimension? Or what is our time size?

We don’t know, because we don’t know if the past or future are objectively real, or if we ride the wave so to speak.

Eh, that view doesn't make sense since every particle has it's own light cone, hence it's own time cone.

PBS spacetime did an episode about this recently.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#244

Post by pdonis »

willis936 wrote: post #230
pdonis wrote: post #219

Earlier quoted context omitted.

> I think the shell theorem says that the gravitational forces cancel if you are on the inside of a hollow sphere and all mass is on the surface. No, it's stronger than that. It says that any spherically symmetric distribution of matter outside a certain radius exerts no "gravitational force" on anything inside that radius, whether there is matter inside that radius or not.

What if my distance to one point on the shell was zero? Would I not feel infinite acceleration towards the massive point on the shell that I was infinitely close to?

> What if my distance to one point on the shell was zero?

Then you are not inside the shell, you are on it. The shell theorem only applies if you are inside the shell.

> Would I not feel infinite acceleration towards the massive point on the shell that I was infinitely close to?

In General Relativity, matter is not viewed as point particles. It is viewed as a continuum, described by the stress-energy tensor. This is one of those cases where the difference shows up.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#245

Post by mensetmanusman »

zestyping wrote: post #119

This video really cleared up a lot of misunderstandings for me: https://www.youtube.com/watch?v=wrwgIjBUYVc I had never visualized it this way in my head before, and it all makes a lot more sense now. Highly recommend!

Great video.

Reminded me of the arguments still going on about ‘whether this is true or not’ in the sense that the mathematics is painting this more accurate picture than what Newton’s math painted, but the math can’t explain most of the universe’s lack of observable mass/energy, so there might be some higher level of mathematics that describes a different but ‘more true’ state of events.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#246

Post by bawana »

but the calculation of time dilation due to a gravitational force results in a DIFFERENT amount than time dilation due to accelerating in a spaceship at one G. So the equivalence principle would fail- I can tell if I am accelerating due to gravity or if I am accelerating due to motion by measuring time dilation. Without doing the math you can know this is true by looking at the age of a person on the ground on earth and the age of a person in a spaceship accelerating at 1 G. The spaceman will experience greater time dilation (age slower) as his speed approaches that of light. In a spaceship people will age slower. But both people will only feel 1 G.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#247

Post by lopmotr »

ummonk wrote: post #213
Smaug123 wrote: post #14

I heard an interesting question at one point: "how come, when you throw a ball up on Earth, the parabola is so strongly curved? Spacetime is nearly flat, so how can a straight line become such a steep parabola?" I'll answer this question as I understand it, but I only took four lectures of General Relativity before I gave it up in favour of computability and logic, so if there is a more intuitive and/or less wrong an…

Great answer! It’s also why I despise the popular portrayal of space time curvature. It looks at space in isolation rather than space time as a whole, and provides no intuition as to why objects traveling at different speeds follow different trajectories. FWIW I think that in general it is better to just teach people that gravity is an acceleration in classical spacetime (as opposed to a force or curvature). It is si…

I think treating gravity as an acceleration is too abstract for many high-school students. For all the on-the-Earth problems, you'd have to analyze them in an accelerating reference frame. That means Newton's laws and the equations of motion don't apply. You'd have to use special linear acceleration equations. An inertial reference frame is simpler and more general. Gravity fits that OK if you consider it to be a force distributed in proportion to mass.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#249

Post by lopmotr »

j1vms wrote: post #54

Gravity is not a force. The surface of the Earth is moving up to the object in free-fall at an acceleration of 9.8 m/s^2. The force pushing the surface, and the pressurized atmospheric shell, upward is a result of the processes occurring within the Earth (likely, in particular, those within the the core).

What direction is the earth accelerating in Australia? It's not deforming.

Re: Gravity is not a force – free-fall parabolas are straight lines in spacetime

#250

Post by pixl97 »

xwdv wrote: post #114
GuB-42 wrote: post #88

My enlightening moment about general relativity: apples do not fall on the ground, instead, the earth is inflating, and the inflation of the earth is accelerating at 9.8 m/s^2. Eventually, the ground catches the apple. Of course, you are going to tell me that the earth is not inflating, obviously, because it is still the same size after so many years. But here is the trick: the earth is inflating at the same rate as…

Does this mean some sort of artificial gravity may be possible after all by causing some kind of inflation?

Gravity is just acceleration in that sense.
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