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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

#391

Post by Smaug123 »

Smaug123 wrote: post #361
msiyer wrote: post #353

Earlier quoted context omitted.

Physics does not magically become different outside the confines of a good school.

On the other hand, the Internet is black with the ink of people who think they understand physics but do not. Credentials do help you to distinguish someone who has a chance of knowing what they're talking about.

I mean, it's… true? My own (very highly upvoted) comment on the main thread of this article contains the text "I only took four lectures of General Relativity before I gave it up in favour of computability and logic", which is helpful context for anyone who reads it, because it lets the reader know in a fairly objective sense that I am familiar with some of the concepts of GR but am unlikely to understand everything in detail. You can't know whether someone on the Internet knows what they're talking about, but they can give you some of their context to help you decide. Admittedly "a good school" is not the most useful context, but it's better than no context.

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

#392

Post by criddell »

tarruda wrote: post #333
klunger wrote: post #322

Earlier quoted context omitted.

"This ultimately led [Einstein] to the realisation that gravity is best described and understood not as a physical external force like the other forces of nature but rather as a manifestation of the geometry and curvature of space-timeitself." This is in the introduction of the paper you shared...

As the parent comment says, everything in physics depends on how you observe ("... best described"). Like Sean Carroll said in the Veritassium video about the "many worlds" [1], there's no such such thing as "pressure", it is just the interaction of fluid molecules. For practical purposes though, it is best described as a scalar called "pressure". [1] https://youtu.be/kTXTPe3wahc?t=1043

I think I've read the same thing about temperature.

And now I'm wondering if it also applies to matter if matter is just an excitation of a quantum field.

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

#393

Post by lovegoblin »

Ancapistani wrote: post #102
Smaug123 wrote: post #82

Earlier quoted context omitted.

Yes, although you can't actually get anything with mass to travel at 1c (subject to our current understanding of physics).

As I understand it, the only thing that can travel at c are: * photons * spherical, massless cows in a vacuum

Anything massless travels at c, not just photons.

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

#394

Post by cestith »

sizt wrote: post #364
immmmmm wrote: post #343

Earlier quoted context omitted.

Ah sorry, i did 8 years of GR and strings.. I was replying to the headline: 'umm, yes and no...'

Physics is the study of making simple things complicated.

Physics is the study of tearing off the mask of simplicity to reveal the horrible complexity hiding underneath.

Once again, it's largely down to your frame of reference.

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

#395

Post by lovegoblin »

raducu wrote: post #283
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…

So, do gravitons exist or not?

Unknown. They are only hypothetical.

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

#396

Post by newobj »

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…

This is maybe one of those most disturbing things I've ever read. I'm going to be thinking about this for a WHILE.

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

#397

Post by quirkot »

Const-me wrote: post #360
jacobwilliamroy wrote: post #328

Earlier quoted context omitted.

Do we have to use this knowledge to toss nukes at each other? Surely there's something else we could be doing instead?

> Do we have to use this knowledge to toss nukes at each other? ICBMs were never used to toss nukes at people. Pretty sure their existence saved the world from WW3. > Surely there's something else we could be doing instead? The application most useful in everyway life is GPS.

ICBMs... the black knight and the white knight of the cold war

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

#398

Post by tsimionescu »

ben_w wrote: post #378
tsimionescu wrote: post #347

Earlier quoted context omitted.

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.

The thing is that this thought experiment is interesting, but it is wrong. It is true that it is not logically possible for a system of two objects to fall faster than both of its constituent parts, but that does not mean that it is a priori logically impossible for heavier objects to fall faster than lighter objects - the lighter object would slow down the heavier object somewhat, so the new "combined" object would fall slower than the heavier of the two objects, but it would fall faster than the lighter of them.

In fact, the fact that heavy and light objects fall at the same speed is a profoundly special property of the gravitational interaction. No other fundamental force behaves this way: in an electric field, objects with more charge will accelerate more quickly than objects with less charge (and the same is true for the weak and strong forces).

In fact, the acceleration of an object or particle in a field is proportional to its field-specific charge divided by its inertial mass. The really interesting thing about the gravitational interaction is that the "charge" associated with the gravitational field is exactly equal to the inertial mass of that object, so all objects accelerate at the same rate because of the gravitational field.

This truly special property of the gravitational field had no explanation until Einstein's theory of general relativity, which discarded the idea that the gravitational field is a field at all, and described the motions of objects only in terms of rectilinear movement in a curved space-time (the reason why inertial mass curves space time is still unexplained though - it seems to simply be a property of the universe).

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

#399

Post by tsimionescu »

sizt wrote: post #364
immmmmm wrote: post #343

Earlier quoted context omitted.

Ah sorry, i did 8 years of GR and strings.. I was replying to the headline: 'umm, yes and no...'

Physics is the study of making simple things complicated.

Is there any science other than physics more adept at describing extremely complex phenomena with extremely simple laws, with known, bounded error?

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

#400

Post by tsimionescu »

mocmoc wrote: post #381
mocmoc wrote: post #342

Earlier quoted context omitted.

No , is not a force

Before downvoting , prove Einstein wrong

This is like that silly discussion of the centrifugal force.

Forces are mathematical models, not fundamental properties of reality (we don't know what the fundamental properties of reality are).

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