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scienceclic also comes in an english edition. hands down the best visualization of general relativity I've seen.

https://www.youtube.com/watch?v=wrwgIjBUYVc

But under this point of view there's no force at all. Gravity doesn't exert a force on you, the movement under gravity is purely inertial. I know this is bonkers.

Here's the video that made me finally understand it https://www.youtube.com/watch?v=wrwgIjBUYVc (https://www.youtube.com/watch?v=twPaOtfpneo is another good video from the same author)

⬐ ijidak

Wow. Thank you so much for sharing these. These videos do make it clearer!!

It finally clicks!

Science Asylum is great, but I dislike the 90s educational show presentation format. An absolute gem I've stumbled across is ScienceClic:

https://www.youtube.com/c/ScienceClicEN

They have amazing presentations on Relativity, my favorite being:

"A new way to visualize General Relativity" - https://www.youtube.com/watch?v=wrwgIjBUYVc

"Hawking Radiation" - https://www.youtube.com/watch?v=isezfMo8kW

Another, while heavier and less accessible channel is Physics Explained:

https://www.youtube.com/c/PhysicsExplainedVideos

These are closer like lectures, but I enjoy them because they go into the histories of some of these experiments; and what the prevailing wisdom was around the time the experiments were performed:

"What is the Ultraviolet Catastrophe?" - https://www.youtube.com/watch?v=rCfPQLVzus4

⬐ 8note

As far as lecture-y YouTube series go, Sean Carrol put together "the biggest ideas in the universe" in early covid: https://www.youtube.com/playlist?list=PLrxfgDEc2NxZJcWcrxH3j...

https://m.youtube.com/watch?v=wrwgIjBUYVc

The rubber sheet analogy is criticised for this exact reason. Here’s a different visualisation that starts with what’s wrong with the rubber sheet: https://youtu.be/wrwgIjBUYVc

⬐ leephillips

That’s the best visualization of gravitation I’ve ever seen. Well done. I recommend it to anyone who’s been misled by the ubiquitous rubber sheet picture.

⬐ akomtu

That's clever. I'd suggest an improvement. When a sat has initial speed, show its small local reference frame, so we'd see that it always moves forward in its own reference frame, but the frame happens to be pulled to Earth.

⬐ rdtsc

Great video. Another one I like is "Why Gravity is NOT a Force" from Veritasium https://www.youtube.com/watch?v=XRr1kaXKBsU

⬐ drran

It uses gravity force to demonstrate that gravity force is not a force. ;-)

⬐ pomian

That's an amazing little video. thanks for posting. (Maybe that should go on to the main page, at some point)

⬐ jcun4128

Oh yeah I can see the 3D sinking inwards

that's a neat video haven't seen that slicing idea before

⬐ SavantIdiot

That's a great video. 7:50 is the important twist for me, and 10:25 really drives it home. I'll never forget that video now and it explains so much.

THere's still one "flaw" with this video: explaining that the grid "moves" is a little confusing. It doesn't move per-se, it .. evolves? ... over time. That's weird. I keep wanting to think the curves are static, but from t0->tn the grid pinches up. Yes, that's why they call it spacetime, but I have to stop and reset myself because how can the grid keep pinching up indefinitely but it doesn't it is just a concept. That is a stumbling block. 35 years after my last physics class...lol.

⬐ whydoineedthis

This is great. I hate the simplistic illustration that everyone relies on but really gives you no information. This video, while a bit more complicated, gives much more to mentally play with.

This is a good video explaining just that! https://www.youtube.com/watch?v=wrwgIjBUYVc

⬐ karmakaze

I don't know if I've seen this video before, but the way it describes and animates the concepts is the best one yet.

I have two thoughts that I haven't really seen discussed:

1. c isn't a limiting speed, it's the only speed through spacetime. Is there anything that isn't traveling at c through spacetime?

2. If everything is traveling at c through spacetime, then the all the spacetime dimensions are not in fact independent, so we live in 3 dimensions of spacetime. Given that why do we doubt or debate a holographic view of the universe? It's saying we live in 2D space and 1 time which is an interpretation of living in 3D spacetime.

⬐ jfengel

1. You're correct. It's most often phrased in terms of a spacetime interval, ds^2=dx^2-cdt^2, which is the "true", observer-independent distance between two events. You can divide by dt and rearrange to get a constant velocity c through spacetime. A different rearrangement lets you derive the gamma term from special relativity.

2. The dimensions are still independent. There are a total of five terms in the equation I just gave: ds (spacetime interval), dt (time), and dx (which is the three terms of space). You can express any one in terms of the others, leaving you with four independent variables.

The holographic view is a different phenomenon, derived from a basic integration of QM with GR (semiclassical gravity). It relies on the way black holes are complete information traps, so that only the surface area (and not the volume) are meaningful. (All of the information in it has to pass through the surface and nothing inside matters.) It also requires that energy be quantized; otherwise you get the dimension back via the infinite degrees of freedom of each particle.

So yes, you do get 3D spacetime, but only by completely rearranging everything. It doesn't pop directly out of general relativity. GR still requires 4 numbers to place any event.

⬐ karmakaze

Thanks for the short explanation that I can look further into. After rethinking it, it makes sense.

Could it be handwavingly said that a single observer only experiences spacetime in 3D?

⬐ ohiovr

This is definitely the smartest video I've seen in years, perhaps ever.

Questions I have: If the apple were traveling back in time would it essentially be falling away from the earth instead of falling into it? Would traveling forwards in time like George did in The Time Machine 1960 film (or the unnamed protagonist of the book), would essentially get crushed by ramming himself into the earth? I like these notions because it helps relieve me of grandfather paradoxes of time travel and make me dream of travel to the stars without having to forever say goodbye. Also like the fact that it doesn't require an extra incomprehensible dimension like in the case of warp bubbles.

⬐ jfengel

Yes, an object moving backwards in time would fall upwards.

An object moving forwards in time faster than the speed of light would crush everything, not just itself into the earth. Its energy would go to infinity. That's why "warp bubbles" always require exotic matter with negative energy. Negative energy does a lot of things that run counter to how we expect spacetime to behave, which is what leads us to think that it probably doesn't exist (or we'd have observed it by now).

⬐ ohiovr

Wouldn't the rate of time reversal play a part in to the pressure exerted to the ground? If time were going at 1 second per second forwards we experience an acceleration of 9.8 meters per second per second, if we went 2 seconds per second would we experience an acceleration of 19.6 meters per second per second?

⬐ jfengel

You might, depending on what "rate" meant.

The reverse case is a simple matter of swapping the T coordinate, t->-t, for the whole universe. Making a local version of it, such that you're going "faster" than the rest of the universe, leaves a boundary condition whose properties are undefined.

⬐ ohiovr

For something sci fi how about a kind of gradient?

My favorite gravity explanation video:

https://youtu.be/wrwgIjBUYVc

⬐ anotheryou

Also good :)

With a slingshot maneuver, why does the the direction of the inertia change though? If you zoom out far enough the lines are mostly parallel with a "dip" where there as mass. Why can an object enter parallel to a line, be dragged with the line towards the mass, miss it and exit on a path not aligned with a line? https://i.imgur.com/qj8fafd.png

⬐ lxmorj

You can only slingshot in a direction another body is orbiting (to gain velocity). You're harvesting it's momentum, basically!

⬐ lxmorj

The more I try to read about this the more unsure I am I truly understand it, so perhaps not...

⬐ anotheryou

Not sure if the term itself restricts it to an accelerating maneuver, but of course you can do the reverse and slow down.

⬐ OnlyOneCannolo

That's a screenshot of an animation where the grid is moving. The lines don't capture the object. They just nudge it a bit. Kind of like a ball rolling across a treadmill.

Also that image doesn't show the slight twist in the grid caused by the earth's rotation. Another part of the video does though.

⬐ Zanni

Brilliant. The rubber sheet model of warped space has always bothered me.

That's linked at the bottom of the article.

Another good video to visualize GR is this by ScienceClic: https://www.youtube.com/watch?v=wrwgIjBUYVc

In another comment zestyping posted this video which also has spacetime contracting, in a sense: https://www.youtube.com/watch?v=wrwgIjBUYVc

⬐ GuB-42

It is actually that video that helped me!

The thing is, I watched the French version and I didn't know an English version existed, that's why I didn't post it here, French speakers are, I believe, a minority.

And BTW, the French channel has an 8 part explanation of the maths behind general relativity that is the best I have ever seen. It is on a level above most pop science video since it actually shows the equations, tensors, etc... but the explanations are actually quite accessible.

https://www.youtube.com/watch?v=sg2BBldgKpo

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!

⬐ elteto

What a fantastic video.

⬐ xt00

Thats a great video

⬐ mensetmanusman

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.

⬐ anomaloustho

I think you might be getting at MOND here, but so far, some other observations seem to indicate that the lack of observable mass are actually clumps of some type of matter. Because not all galaxies diverge from the math. Many do, but not all.

The inconsistency points towards an actual type of matter as opposed to systematic error.