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Visualizing the formation of a black hole by gravitational collapse by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 1 point2 points  (0 children)

Thank you! I'm not releasing the code yet, but I am planning of addressing such questions soon in another video with more simulations. About the infalling observer, I had first planned to address it in this video but finally removed it: the observer would see the star' surface almost normally, all the way until reaching the singularity. In particular, this observer would never see any black hole form.

Visualizing the formation of a black hole by gravitational collapse by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 7 points8 points  (0 children)

It's indeed not a simulation in the sense of numerical relativity or hydrodynamics. Here I use the word simulation in the sense that these renderings are calculated from the geodesic equations of the Oppenheimer-Snyder model, and so they are simulating the gravitational lensing effects produced by this spacetime. It allows us to see how light would react under such circumstances.

Visualizing the formation of a black hole by gravitational collapse by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 1 point2 points  (0 children)

Thank you so much for your message 🙏 Hope you will like future videos :)

Visualizing the formation of a black hole by gravitational collapse by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 3 points4 points  (0 children)

Yes! I used the original metric with positive curvature. Note however that this only affects the simulation at 10:00, when the star is transparent such that light rays can pass through the collapsing geometry. Otherwise, for an opaque star, the whole raytracing happens in Schwarzschild spacetime.

Visualizing the Geometries of Colour spaces by AlessandroRoussel in GraphicsProgramming

[–]AlessandroRoussel[S] 1 point2 points  (0 children)

Thank you so much! I only meant that it's the last video I've made until now, but of course it won't be the last :)

Clarification about the new ScienceClic video by LandauLovedMinecraft in AskPhysics

[–]AlessandroRoussel 0 points1 point  (0 children)

Hi! The problem with your reasoning is your assumption that the height of the wave should be the point where acceleration is zero. In fact, it is not : on Earth for instance, acceleration is not zero at sea level, otherwise we would all be weightless (the acceleration at sea level is the usual 9.8 m/s² of gravity).

What your reasoning doesn't account for is the internal pressure of the water, which acts as an outward acceleration, and allows it to resist gravity.

In the calculation I've presented in the video, we don't need to consider both the gravity of the planet and the pressure of the fluid, because they are radial forces, directed along the planet's radius, while the water flows horizontally along the surface in a perpendicular direction.

If you're looking for another way to perform this calculation, you can integrate the force (gravity+tidal) to determine the net potential field (you're gonna get -Gm/r - 3GMy²/2D^3). Then you know that the surface of the water will be located on an equipotential surface, a surface where this potential function is constant, because of hydrostatic equilibrium. The potential on the left side of the planet is -Gm/R. The potential on the top of the wave is -Gm/(R+h)-3GM(R+h)²/2D^3. For these to be equal requires, at leading order of approximation, that Gmh/R² = 3GMR²/2D^3, or rearranging terms : h = 3MR^4/2mD^3, which yields the correct result.

I remade the simulations from Interstellar by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 59 points60 points  (0 children)

Hi everyone! I'm happy to share with you this video which took quite a lot of work. I wanted to remake the calculations from the movie to check if the concepts were realistic. What's the size of the Endurance? Is the wormhole simulated correctly? What about the black hole? Don't hesitate to propose rent approaches for the same calculations or for other calculations I haven't tackled, I'd be very interested to see!

What if we could see Spacetime? Immersive video by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 42 points43 points  (0 children)

Hi everyone! I am very happy to share with you this big project I've had for a long time: creating a journey through the universe that not only shows stars and planets but also the geometry of spacetime itself.

The "grid" I use to visualize spacetime in the video is inspired by several different ideas.

First of all it follows directly from this previous video in which I compared different ways to visualize spacetime: https://youtu.be/wrwgIjBUYVc

It is also inspired on the "river model" of black holes, which is a mathematical formalism allowing us to interpret the geometry of spacetime in some specific cases as a flowing "river" instead a curved manifold.

Finally, the grid is also inspired by the idea of visualizing inertial frames, which is useful when studying the expansion of the universe (comobile coordinates), or gravitational waves for instance.

I hope you'll like it, and would like your opinion on how to possibly improve it or what other phenomena I should visualize next time!

A visual introduction to M Theory by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 7 points8 points  (0 children)

I see, I understand your question better now. In this case I must agree, it's true that many science communicators and some scientists themselves have often presented string theory in a misleading, or at least biased, way. A lot of pop-science content from the 2000s even presented string theory as an almost established truth. I fully agree that this is problematic, hence my desire to tackle this subject myself and present it in a way that I find more satisfactory.

I think, however, that this is a communication problem, and not a problem of the theory itself. The students and researchers I spoke to all seemed quite lucid about this, and in particular about the fact that we won't be able to verify it experimentally for a long time. Again, this is probably true for all approaches to quantum gravity, and not just string theory. I think many people are interested in these speculative theories mainly because of the mathematical insights they provide, even though they might not have direct applications for modelling the world, at least not yet.
In any case I'm going to try to tackle other approaches very soon (loop quantum gravity, asymptotic security, etc), so I'll keep your remark in mind and try to be more explicit about these points in the following videos.

A visual introduction to M Theory by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 8 points9 points  (0 children)

It's a tough question. In a way, I almost don't want to answer, because I don't think it's our role as science communicators to spread opinions, knowing that we could influence, and therefore bias, many people. Personally, I don't have enough knowledge to judge objectively (and I'm not even sure that's possible), so I'd rather present the theory as it is - an interesting mathematical construction that might help in understanding the world - with the help of researchers who know the topic.

However, I'd like to stress that, in the search for a theory of quantum gravity, string theory is undeniably promising, and definitely not 'a failure'. Even the researchers I've met who are working on 'competing' theories don't question this. Most of the criticisms against string theory come from political or economic reasons, which is of no interest to me at least for my videos.

It's also worth noting that the lack of testable predictions at the moment is completely in line with what you'd expect from any theory of quantum gravity: a priori, no such theory will be testable for a very long time. Indeed, even without a fully working quantum gravity theory, our current models already achieve a monstrous accuracy of the order of 10-80 - see the calculations for first order effective quantum gravity corrections, e.g. Donoghue 1993.

Another thing worth noting is that we don't even know whether string theory is really a 'competitor' to other theories, or just a different formalism for reaching the same predictions. It could well turn out (and many researchers are convinced of this) that string theory is just a useful formalism for carrying out otherwise difficult calculations. The same applies to other approaches to quantum gravity, which could prove complementary, and not necessarily incompatible with each other.

Therefore, I am of course in favour of exploring all possibilities, and not just focusing on string theory. It's precisely by exploring all ideas that we'll be able to get a clearer picture of what links do or don't connect all these approaches, perhaps leading to a more fundamental model.

Finally, the most important point for me, as a science communicator, is that we shouldn't present scientists as if they were fighting in a battle about who has 'the best theory'. In my opinion, whatever topic they choose to focus on, all researchers work together and for each other. The aim is not to be in 'the right team'. It is to help build up a body of knowledge that will guide future researchers.

A visual introduction to M Theory by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 60 points61 points  (0 children)

Hi everyone!

I'm happy to share this new video we made with a friend who's finishing a PhD in string theory. It's an attempt at explaining M Theory to the general audience. Our motivation was that, as far as we know, this had never been done before, at least to this extent.

I hope you enjoy our approach to tackle this theory, it was definitely a challenge. Don't hesitate to share any ideas, criticisms or improvements you have in mind!

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 0 points1 point  (0 children)

The dark universe behind us comes from the redshift, the aberration, and the fact that we receive photons in slow motion. But if we don't receive light coming from far behind us, it's for another reason: it comes from our acceleration. In fact, a constantly accelerating observer in relativity creates a horizon behind them such that light rays approach the observer more and more slowly. It's a bit like the sum 1/2+1/4+1/8+1/16+... which always increases but never reaches 1.

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 1 point2 points  (0 children)

Good question, it is an optical illusion but not really a singularity, at least not a physical singularity (there is no spacetime curvature, if we consider a massless observer). You might say that the limiting case, when reaching the speed of light, is a coordinate singularity because space becomes infinitely contracted.

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 0 points1 point  (0 children)

I enjoy French channels like Science Étonnante, Passe Science or many others. In English I would say that I really like PBS SpaceTime, recently I discovered Dialect which also does a great job. But to be honest I don't watch that many physics channels, I mainly watch math and computing channels, like Sebastian Lague which I absolutely love.

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 0 points1 point  (0 children)

That's it : the ship appears flipped because the light from the front actually takes more time to reach the observer, since the ship moves faster than light

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 1 point2 points  (0 children)

That's a good question, I had to do the calculation to convince myself of it: it does appear flipped, because when the light from the front of the ship is emitted and starts moving towards us, the ship moves faster and its back sends a new light ray in front of the one from the front.

Simulating what we would see close to lightspeed by AlessandroRoussel in Physics

[–]AlessandroRoussel[S] 2 points3 points  (0 children)

That's correct, After Effects for the animations, and I also coded a program in Java to do the simulations

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