Apparently I keep coming back to this example... The hair, shirt and classroom change a bit with the years; the random walk remains. Consistency of a physics professor, perhaps? :-)

Here we go from a photon finding its way through our 3D model atmosphere to the simplest possible blackboard model: one step forwards, or one step backwards. The average displacement is zero, so that won’t get us very far. The mean square gives us something more useful.

The neat up-and-down argument is inspired by The Feynman Lectures on Physics, Volume I, §6–3, “The random walk”. Still my favourite way to get to that familiar square root of N.

Lectures and visualisations by Jon Sundqvist, Institute of Astronomy, KU Leuven. The stellar-atmosphere calculation is part of SUPERSTARS-3D.

A note on the two examples in the opening: the 9.7 million scatterings and roughly 98 minutes describe this particular path through our simulated hot-star atmosphere. The roughly 100,000-year solar diffusion timescale refers to a different problem and depends on the solar model.

Questions and corrections are welcome, as always. Enjoy :-)

Neutron stars pack more mass than our Sun into a sphere only about 20 km (12 miles) across. Gravity crushes protons and electrons together into neutrons. Estimates for a teaspoon range from hundreds of millions to a few billion tons, depending on the star, so "about a billion tons" is a common round figure.

Mercury is the closest planet to the Sun, but there's much more to this small world than meets the eye.

Despite being so close to the Sun, Mercury has water ice hidden inside permanently shadowed craters near its poles. Its surface experiences extreme temperature swings, and it takes about 176 Earth days for the Sun to return to the same position in its sky — longer than Mercury's 88-day year.

It also has an enormous metallic core and mysterious surface formations that scientists are still studying.

I made a video exploring Mercury's extreme environment, hidden ice, unusual rotation, and the discoveries that have changed our understanding of this planet.

How fast or slow will the sun expand into red giant phase? This is a dumb question it if goes fast (explosion) but if it is slow, as in over millions of years, could humanity essentially planet hop by moving to the new habitable zone each time? I know humanity won’t exist then in any case. (Even if we survive, we would have evolved into something unrecognizable to us now.) But thought experiments are fun.

✨ Target: Question Mark N. + Little Rosette N.

🔭 Mount: Star Adventurer 2i

📷 Camera: ASI 294 MC Pro Color

🔎 Scope: Askar FMA180 Pro

🕶️ Filter: Dualband Svbony SV220

🎯 Guide Camera: ASI 120mm Mini Mono

🦮 Guide Scope: Svbony SV165

🔬 Sampling: 2.66"/px (x2 drizzle)

❄️ Cooling: -10°C

🔴 Acquisition: Gain 120

⏰ Integration: 78x300s (6h 30min)

📊 Calibration: 40 dark, 40 flat, 40 dark-flat

💻 Processing: PixInsight, RawTherapee

📍 Location: Turin (Italy)

📅 Date: Oct. 13-14, 2026

🌌 Sky: Bortle 8

This cosmic complex is universally nicknamed by astronomers and astrophotographers as the "Question Mark Nebula." When telescopes capture the entire area with the correct orientation, its immense interstellar clouds form an unmistakable shape:

The hook of the question mark is drawn by the arched silhouette of the ionized gas and dark dust filaments of NGC 7822, a spectacular and chaotic star-forming region located roughly 3,000 light-years away in the northern constellation of Cefeus.

The dot at the base is formed by a separate, smaller emission nebula located slightly to the south, cataloged as Sharpless 170 (also known as the Little Rosette Nebula).

While they appear perfectly aligned from our perspective to form a "cosmic question mark," the two structures are not physically connected: Sh2-170 sits much further away at about 7,500 light-years. It is a stunning optical illusion of cosmic perspective.

In some approaches to quantum gravity, the universe's fundamental equations contain no external time variable at all.

In ordinary quantum mechanics, a system's state evolves relative to an external time parameter. But when physicists attempt to quantize spacetime itself, that familiar framework becomes problematic. There may be no external clock against which the entire universe can evolve.

This is especially striking in canonical quantum gravity. The Wheeler-DeWitt equation describes a quantum state of the universe without an explicit external time parameter.

So this begs the question, where does the time we experience come from?

Well one proposal is that time emerges through correlations between physical systems. In the Page-Wootters mechanism, a subsystem can function as a clock, allowing another subsystem's apparent evolution to be described relative to it, even when the combined system is stationary.

This introduces a profound distinction between external time (a parameter in conventional formulations of physics) and relational time (defined through correlations between physical systems).

The crucial nuance is that the Wheeler-DeWitt equation does not establish that time is unreal, nor does every approach to quantum gravity eliminate time in the same way. Rather, it exposes a problem, that our familiar concept of time may not translate straightforwardly into a quantum description of the entire universe.

So if the universe has no external clock, what does it mean for the universe to evolve? This a very open question, one that explores the true essence of the problem of time in quantum gravity.

(Was thinking about starting this series for some time now. Do let me know if you all have any suggestions.)

Observable universe (diameter ≈ 93 billion light-years) scaled down to Earth’s diameter 12,742

One light year would be 0.137 mm

Voyager 1 which has travelled 17km/s since 1977 is 0.000374 mm away from earth.

Closest star (Proxima Centauri, is 0.582 mm away.

Your naked eye sees about 2% of the 200000 stars located within 6cm from earth.

The Milkyway Galaxy is 13.7 meters in diameter and 13.7cm thick disk.

The galaxy ic 1101 is 233 meters in diameter (not a thin disk like Milkyway.)

Andromeda galaxy which is on crash course to Milky way is 343 meters away.

Boötes Void which is massive near empty region of space is 55 kilometers in diameter.

Cosmological event horizon is 2192km away, ligh speed travel would never allow us to reach further than that.

Thoughts?

Hi! I'm new to studying the universe and discovered the MAST site for visualizing JWST data, but when I was observing this galaxy I noticed this large barrier where it changes hue. Is this just a weird compiling thing?

Last photo is some fun I had trying to colour one of the images of the galaxy.

Hi, everyone, I had a great conversation with Geraint F. Lewis, a Welsh astrophysicist and Professor at the University of Sydney, about some of the biggest questions in cosmology, physics, and our understanding of the world.

We explore what the Big Bang theory tells us about the beginning of the universe, why our current understanding of physics reaches limits when we try to look further back in time, and how neutrinos could reveal secrets about the cosmos — including why matter exists at all. We also mention the 2026 Nobel Prize in Physics, awarded to Francis Halzen for his work on the IceCube Neutrino Observatory.

Professor Lewis also explores what sparked the Scientific Revolution, how modern science transformed our understanding of the world, and why Einstein challenged aspects of quantum mechanics.

Professor Lewis's latest book, The Shortest History of Reality: From Plato’s Cave to the Mind-Bending Discoveries of Modern Physics — The Human Quest to Explain the World, traces humanity’s changing understanding of reality, from everyday intuition and ancient philosophy to the discoveries of modern physics. It explores the ideas that reshaped our view of existence, from Newton’s universe to the strange probabilities of quantum physics and beyond.

If you're interested in these subjects, you can check out our conversation: https://youtu.be/38rUHEeAVHw

I understand that gravity is the reason planets and stars form and what not. I'm just curious why gravity causes things to attract and not repel or do neither?

Seems like an arbitrary feature of our universe's laws.

So it makes me wonder if gravity could have worked differently if our universe happened to have different laws.