I like these kinds of visualizations for big things. Reminds me of "1 pixel wealth." https://mkorostoff.github.io/1-pixel-wealth/

This was awesome. It's strange to know that I already understood this (conceptually at least), yet seeing how slow it really is at this scale is confusing. Maybe that's not quite the right word. It makes my brain pause and go "that can't be right", but... It's right.

Brains are bad at these scales. Maybe mine is worse than average. I can't fully believe how impossibly far away so many things truly are.

On the topic of perfectly crafted depictions of scale in the Universe, I love this one: https://youtu.be/vcJHHU9upyE

Yes, I was often very confused as to why the speed of light shows up everywhere, until it was reframed for me in this way. The fact that light travels at the same speed regardless of your frame of reference becomes a little less mystifying.

It feels more intuitive to me when thinking about it as causality always unfolding around you at the same speed, no matter your own frame.

The constant c was not named for causality, but it is a nice coincidence.

It's not just Arvin Ash. That's actually fairly common terminology amongst physics educators nowadays. For starters: You'll find a lot of physics YouTube channels that say "speed of causality". It has even started to make its way into the astrophysics and physics textbooks in the last couple of years.

Thanks, I’ve never heard this and it’s quite profound. It’s always bothered me that there even is a top speed, and further that mass becomes infinite as it’s approached. But “speed of causality” makes these less strange.

"Light travels at the speed of causality". Why does light have that behaviour?

The question seems to me not why there is a speed of causality, but why the speed has this particular number. And it's not clear we know why that is, any more than we know why the proton mass is about 1836 times greater than the electron mass.

> Without even getting into math and physics, you can intuitively understand how infinitely fast causality would prevent time, and therefore everything else we know, from being possible.

Can you unbox this a little? I think I may just have Friday brain, but I'm having some difficulty convincing myself in the moment that infinite-speed causality development would prevent time.

And the other half of this is that brains are extremely complicated casual chains. Causality can traverse the diameter of the brain something like 10^8 times in the period it takes for light to be perceived.

So this seems like a better definition until you run into a problem, which you do pretty quickly: "casuality" isn't the easiest thing to define.

The best definition I think I've seen is to view the universe as a partially ordered set of events, meaning that you can only order events (in time) if they're within each other's cones of causality. Outside of that you cannot say which happened first. That's the partially ordered part.

But even that is incomplete and arguably even self-referential. What's a "cone of causality" (without relying on causality)?

Also, there's the issue of what exactly time is and whether events are time-symmetric or not. Many physicists seem to view time as an emergent rather than fundamental property of our Universe.

It also kind of is not, inasmuch as it doesn't argue that there's the possibility of a universe where a "slower" speed of light (whatever that might mean) would not permit the existence of humans. It merely argues that the speeds of the human-centric world have to be small fractions of c, whatever value c is, by dint of how the macroscopic relates to the subatomic.

In natural units, in all such theoretical universes, c is 1; and all that this argument really states is that humans and similar atom-based things have to move at very small fractions of 1.

If I had to write a “game of life” simulation that simultaneously calculated the effects of all events at each point in a 3D matrix over time, I would:

- make the matrix as rough as possible while still enabling interesting events (ie, try and maximize the Planck length)

- make the maximum speed at which events propagate across the matrix as slow as possible to save the CPU (ie, try to minimize the speed of light)

- limit the size of the simulated universe

But our Planck length is tiny and the universe is probably humonguous unless we’re being deliberately deceived by This Simulators.

So despite the suspicions aroused by the slow speed of light, we might live in the mother / “real” universe after all.

This is why somewhat realistic high-power fusion rockets like those portrayed in The Expanse require magnetic nozzles. The fusion plasma would never come into contact with the material the engine is made of or it would melt.

We could build a decent fusion pulse drive today if we had higher temperature more compact superconductors and super-efficient compact lasers that could fit in a spacecraft and ignite a strongly net-positive inertial confinement fusion pulse.

Our superconductors are almost good enough, but our high-power lasers are way too inefficient and bulky. We can't even make economically viable ICF on Earth with current lasers.

Yes, it takes forever to get anywhere at Warp 9. (-:

More seriously: Your very point is already made near to the beginning of the headlined article, in the book quote. You might want to read beyond the headline question, otherwise you're just repeating what the article already says.

It’s all a matter of perspective.

If you were to travel at nearly the speed of light, you could cross the universe in a matter of minutes. Of course an external viewer on say Earth would disagree and say it took billions of years, but who’s counting?

The size and age of the visible universe aren't that different in size though. I mean, they would have to be, the only reason they differ at all is because the universe expands.

Meanwhile we measure time in hundreds of millions of meters and space in nanoseconds. Something causes humans to be slow.

Which isn't that surprising, life is basically a diffusion process gone haywire and while we're more efficient than just a big rock being pushed by small particles we still rely on statistical physics to push molecules around and it takes a while for those statistics to average out.

> Light, on the other hand, takes billions of years to cross the visible universe

Right, and that's just the visible universe. The full extent of the universe is much larger -- I think that the most cautious lower bound estimate is that it's 250x larger. It could be 10^10x larger, or even infinite/unbounded. In such a vastness, the speed limits for light and baryonic matter are perplexingly slow.

If photo started a stopwatch and proceeded to travel 100T light years, the stopwatch would still read zero. That is pretty fast imo. It all depends on perspective.

> Light, on the other hand, takes billions of years to cross the visible universe.

Well... That's to be expected. It's right there on the definition.

on first glance it seems like an interesting take, but then you realize (as someone else already pointed out) that the fastest thing in the universe is not fast, and therefore nothing is fast? a little more thought should make you realize this is a poorly formed take. Also, worth repeating, please read the article before posting. It may be that your insight or critique is present and discussed in the article already.

I get what you are saying at, but viewed in a another way, you just said that the fastest thing in the universe is really slow.

So this touches on the anthropic principle, which is to say that if the Universe (and the constants within it) were other than what they were, we wouldn't be able to exist to contemplate it.

The speed of light being "slow" in cosmic terms is almost necessary for our existence in that we need a relatively long period of relative stability in order to evolve into sentient life. And that becomes a whole lot harder if, say, the Milky Way was only one light day across.

^^ this

Direct link to Part 2: https://profmattstrassler.com/2024/10/03/why-is-the-speed-of...

If you break up "mv^2" into its constituent dimensions you get m(d/t)^2 = m(d^2)(t^-2). Now the so-called kinetic energy of the object only "manifests itself" whenever there is a change in velocity of the object in question. Well, the derivative of velocity is an acceleration, so the object in acceleration would be represented as mdt^-2, aka "a force". Hence the energy of the system is simply that force acting over some distance d.

As to the internal/intrinsic energy of a given object, think of it as "hidden potential energy". It is essentially the energy that was required to turn photons into the matter that you, and I, and everything else are made of! The equation itself is mc^2 simply because that is what you get when you rearrange and simplify the experimentally-verified equations which it was drawn from. Likewise, for c is nothing more than the measured value of the speed of light in vacuum for any observer. Of course the choice of units is completely arbitrary. Whether you state it in miles per hour, kilometers per second, or whatever, the ratio remains constant.

The mv^2 isn't a geometrical property - it's because the kinetic energy can also be thought of as the integral of an object's momentum with respect to time.

Well, depends on your viewpoint, I guess. When you really get into the details, it turns out we don't "really understand" anything.

We are just making more and more detailed observations and then creating mathematical models of these behaviors. For example, we observe that space is curved around mass. We can model that and it helps us understand what's going on, so it's useful.

We don't, however, understand what exactly is curved and what is this empty space that curves.

It is not a fallacy. In special relativity the important concept is the frame of reference. For photons there is not no inertial frame of reference where photon will be stationary (relative or absolute means nothing actually).

The whole concept of measurement (which you will need to describe something as stationary) depends on an observer travelling at speed velocity less than speed of light. If you Try to assign a inertial frame of reference to a photon you will break laws of special relativity

- photons always travel in speed of light from perspective of any inertial observer whether it is moving or at rest (verified by experiment)

- relativity prohibited mass less particles (like photon) to be stationary.

E = mc^2 doesn't apply in photon and apply only on stationary objects. The complete equation is

E^2 = (p^2 c^2) + (m c^2)^2

Where m is the rest mass. In photon case the second terms vanishes and the energy is merely the first term.

-Time and space behave differently for photons. In relativity, time dilation and length contraction become extreme at light speed. A photon does not experience the passage of time in the way objects with mass do. Therefore, trying to define a reference frame for a photon would lead to contradictions in our understanding of spacetime.

So in general it doesn't mean much absolute vs relative on photon case.

There is no absolute reference frame that would distinguish "absolutely stationary", as you call it, from "relatively at rest". There is only relative motion. Photons are "always in motion" in that there does not and cannot exist any massive object with respect to which a photon is at rest.

Generally, "equivocation" is the fallacy of using the same word with different meanings in an argument.

https://en.wikipedia.org/wiki/Equivocation

I read the same article and I didn't find "absolutely stationary" in the article. My physics is rusty but even from my recollection of high school physics there is no such thing. It's clear that the word stationary means zero velocity, and first year high school physics taught that any measurement of velocity must be done in a reference frame. The idea that a photon is always in motion is true regardless of which reference frame you pick; it does not imply there are objects that are in absolute rest.