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

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

#371

Post by msiyer »

immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

Perspective is everything.

A simple example to understand how perspective can alter reality is to imagine being a point floating in a 3-D space where you are able to see the X-Y-Z axes. Now, imagine tracing (0,0,0) when you are at (500,500,500). Trivial.

Now, trace (0,0,10^100). That is a huge Z-line you would say. This is the side view.

Now, move to (0,0,10^100) from (500,500,500). This is the top view. What do you see?

Another example: When viewed from perpendicular to X-Y plane, a circular motion on X-Y plane looks properly circular.

When the same motion is viewed standing far away on the X-axis, the motion resembles an oscillation.

Same motion, different perspectives, seemingly different results.

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

#372

Post by taliesinb »

immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

A fun little physical experiment: a density gradient of the water in a fish tank (caused say by putting sugar in the water) will cause a laser to bend: https://www.youtube.com/watch?v=DhNZP2KgMLw

How this can be connected to gravity I do not exactly know. I suppose the change in the refractive index of the water is akin to g_{00} component of the Riemann metric, since it produces a compensating change in the wave vector at that point. Whether you could imagine GR consistently in terms of changes in the local speed of light a.k.a. changes in the permittivity of free space, again, I do not know.

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

#373

Post by nsonha »

the_arun wrote: post #318
derex wrote: post #228

Earlier quoted context omitted.

g is not gravity, it’s a constant that quantifies the force that gravity impresses on a mass in Newtonian mechanics

https://en.wikipedia.org/wiki/Gravitational_acceleration Isn't "G" (uppercase G) is gravitational constant and Lower case "g" is acceleration?

yes but they have nothing to do with your assessment "gravity is acceleration". "Forces cause acceleration" is more accurate, there are 4 kinds of forces so gravity is just one cause.

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

#375

Post by pa7x1 »

marta_morena_28 wrote: post #175
gabia wrote: post #159

Earlier quoted context omitted.

The effect of a spacetime transformation isn't just to redefine straight lines along which particles move. It means measurements (e.g. lengths, areas, time intervals) are different depending on where you are in the spacetime. The are forces don't come with these "extra" effects - an EM field doesn't stretch and contract space. However, there are a lot of parallels between electromagnetism and relativity! Quite often…

The main difference though is that gravitation (probably) doesn't have a mass of its own, while EM fields do. Plus, matter does not react to EM fields in the way it does towards gravitation. I.e. light is not "pulled" by EM fields. However, these are technicalities. If all matter reacted to EM fields the same way it would to gravitation, would that make EM fields no force either? Or put another way, gravitation act u…

> The main difference though is that gravitation (probably) doesn't have a mass of its own, while EM fields do.

Both gravity and EM fields have energy which is what couples to the gravitational field. Neither of the fields has mass, though.

> If all matter reacted to EM fields the same way it would to gravitation, would that make EM fields no force either? Or put another way, gravitation act universally on all particles, while EM fields do not. That necessarily has consequences when it comes to relativity. However it seems odd to argue that general relativity would exclude gravitation from being a force. If it acted only on a subset of particles, it would likely be in the same position as EM fields, and suddenly become a force again?

This is very well thought. Indeed, the equivalence principle, the fact that gravity couples to everything in exactly the same way (and that includes gravity itself as per the previous clarification) lurks behind our ability to reinterpret gravity in a geometric fashion. After all, if something didn't interact with gravity in the same way as everything else we could establish an experiment to differentiate if a spaceship is accelerating or stationary under a gravitational field (see Einstein's mental experiment) by measuring how that thing behaves. And that same fact would stop us from interpreting gravity as curvature of spacetime itself.

To your last point, speaking of forces is probably antiquated anyway, although still in use partly for historical reasons partly abuse of terminology. Preferably we should use the term "interactions", after all some of the "forces" do not result in push or pull as we usually understand a force in Newtonian mechanics but in things like color change. Others, like the gravitational "force" can be expressed entirely as spacetime geometry. But discussing semantics is quite pointless so as long as everyone understands in what way the term "force" is an abuse of terminology it's OK to keep using it.

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

#377

Post by htiainen »

jiggawatts wrote: post #256
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…

Some interesting alternative viewpoints I've seen: Matter continuously destroys spacetime at its location, sucking in the fabric of the universe. The Universe isn't expanding; matter is shrinking. Light isn't redshifted on the way to us, it's just that our sensors are getting smaller relative to the unmodified wavelength of light.

> The Universe isn't expanding; matter is shrinking.

Is there any way to distinguish these? Surely it depends on who's point of view you're looking from

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

#378

Post by ben_w »

tsimionescu wrote: post #347
ben_w wrote: post #337

Earlier quoted context omitted.

Heavy objects are made of many light objects[0], so no, they would still accelerate at the same rate. [0] with the exception of light, which is very light[1], but made of light particles rather than light particles. [1] Homographs ahoy

There are heavy objects that are not made of lighter objects as far as we can tell, such as the Higgs boson or quarks. As far as we currently know, they should also fall at the same speed near the earth.

Fair point — I was just rehashing a very simple thought experiment that showed why the acceleration should be the same for heavy and light objects alike, but you are totally correct, it isn’t really ok for me to count all bradyons as equivalent.

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

#379

Post by edna314 »

immmmmm wrote: post #315

Physicist here, gravity is a force, just a different one. Also, like everything else in physics: it depends how you observe it. For instance, electromagnetism comes from the curvature of a U(1) bundle over space time, the (local) U(1) symmetry yields electromagnetic interactions. For gravity the symmetry is the (local) Pointcaré (SO(1,3) + translations) symmetry and curvature of spacetime itself. Also gravity on Eart…

I was always wondering if there is a way of distinguishing a straight line in a curved space from a curved line in a flat space, given that you cannot 'look' at space time from the outside. Would that be possible?

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

#380

Post by shireboy »

willis936 wrote: post #35
shireboy wrote: post #8

I saw this and the related Veritasium video, but am still scratching my head about something. Does this mean that away from all gravity a body does not experience any time? ie a person on a spacecraft stopped in intergalactic space would not age relative to persons on planets?

It's not that the absence of gravitational fields slows time up relative to observers in more massive reference frames, it's that the presence of gravitational fields speeds up time relative to observers in other reference frames. You would age about the same as you would in microgravity. You could get closer and closer approximations by going into Earth orbit, solar orbit, and galactic orbit. Each approximation has…

I understand, but the corollary of time appearing to speed up close to gravity well (article explains this is not a field per se) is that it would appear to slow down further away. Is there a minimum speed of time relative to earth?
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