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Comment by AndrewOMartin | Hacker News Reader

This article mentions that the speed of light seems fast to humans living daily lives, but is not so fast on an astronomical scale.

The best demonstration of this I've ever seen is on "If the Moon Were Only 1 Pixel - A tediously accurate map of the solar system" [1].

If you've not seen it before, I recommend opening it, using your mouse wheel to scroll from the beginning (near the Sun) to Earth. It should take about a minute, but there's some commentary on the way. Then, to save your mouse and your finger some work, try clicking the icon in the bottom right hand corner to auto-scroll the map at the speed of light.

[1] https://joshworth.com/dev/pixelspace/pixelspace_solarsystem....

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

Shameless plug: during lockdown I did a whole series of these, called Spacewalks: https://youtube.com/playlist?list=PLul2c76M6HJySkSXYMoLXW9VC...

These videos were super fun to make and kept me sane when I found myself with far too much free time and a bunch of world news to avoid. I never did the fifth (and final) walk but it's only about 100 meters long so I hope one day to do it in person (if I ever end up with that much free time again).

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

It’s always crazy to me how empty most of space is, and that yet despite that gravity is strong enough on large timescales to pull a lot of stuff together. Then I get concerned as to why EVERYTHING wasn’t pulled together and it’s literally just this little bit of angular momentum conserved across vast distances and added together from all these little particles that is keeping everything apart just enough for us to exist.

Well, the other forces have something to say about it too. If you compress stuff hard enough then the other forces will violently blow it apart.

Nah gravity would win (see: black holes)

Something to do with the universe expanding faster than gravity could ever hope to pull things together right?

Also we don’t know why the universe is expanding. Not just expanding but accelerating. Why is spacetime itself stretching and what is doing it?

Yeah yeah dark matter blah blah, but it’s not a concise enough answer.

Dark matter doesn't make the universe expand. It is massive, and thus it brings things together.

Dark energy, on the other hand, is the postulated cause of the expansion of the universe.

Edit: not a Physicist!

> but is not so fast on an astronomical scale.

This is something I find incredibly counter-intuitive. At the photon’s reference frame (speed of light), time stops. In our reference frame, I’d expect some kind of “divide by zero” error in nature, resulting in the infinite speed of light. But it’s not infinite. It’s just… some constant.

It's the maximum speed through space and time. The speed of light through time is just 1s/s. Einstein says space and time are equivalent, so as your velocity through space approaches c, your velocity through time approaches zero and vice-versa.

The photon has zero velocity through time because its velocity through space is c. In its reference frame, no time passes between creation and destruction, but from all other reference frames it appears to be moving through space in discrete time but with high velocity.

It has to do with photons not having rest mass. If something with rest mass were to go at the speed of light, then you run to the numeric problems.

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.

I thought light in a vacuum was (outside of some corner cases) the fastest you could get in this universe? That seems pretty fast on an astronomical scale to me. I mean it’s probably just that we have extremely short lives on an astronomical scale.

The speed of light is actually the speed of causality. No matter, or more importantly information can go faster than that. This has some real consequences even on human scales.

For example, if you've ever opened an electronic device and seen squiggly traces everywhere, it's because of light speed[0]. When signals get fast enough, you have to make sure the traces have the same length to within a fraction of a millimeter. Otherwise, the signals on two traces arrive at different times and nothing works.

[0](electrons move at ~c, more or less)

There's a significant delay in satellite communications, on the order of milliseconds. It takes several seconds to reach the moon. 20 minutes to Mars, a few hours to Jupiter. Famously 8 minutes to the sun. The nearest galaxy is 1.5 million years away.

Consider how stupefyingly large our solar system is. Then consider how insanely huge the galaxy is: 90,000ly. It takes ninety thousand years for a photon to go from one edge of the galaxy to another. Then compared to the universe at large, our galaxy is essentially a single point.

Light is extremely slow and the universe is vast beyond our meat brain's ability to comprehend.

>[0](electrons move at ~c, more or less)

Pedantic: electrons (in a wire) move at relatively ordinary speeds. Wikipedia says this [0]:

>The drift velocity in a 2 mm diameter copper wire in 1 ampere current is approximately 8 cm per hour. AC voltages cause no net movement. The electrons oscillate back and forth in response to the alternating electric field

Think of it more like a long row of billiard balls. You hit the first one, it moves a little, then it hits the next one, that one moves a little etc. Except the "hit" is just getting closer to the next electron to make it move (like if they were magnets).

At least that's my understanding of it.

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Maybe "hit" is the correct word? Isn't it the same forces at play that stop my hand from going through the table?

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[0] https://en.wikipedia.org/wiki/Speed_of_electricity

You are correct, electrons (aka current) through a wire move at relatively low speeds at room temperature because of wire resistance. Changes of voltage however, like if you flip a data pin from 0V to 5V move at the speed of light.

So switching circuits need to take into account "transmission line" effects.

The this is only the drift velocity. Electrons are still orbiting the nuclei at relativistic speeds. Plenty of room at the bottom...

I don’t think you understood my point? Why did you type all of that, when it’s basically what everyone else has said. I know about max speed of information and causality and have for decades. I was talking that light in a vacuum speed is basically “fast” since nothing is fast, in both and absolute and a relative sense, at least in my take :)

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