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Comment by chongli | Hacker News Reader

The speed of light is not fast. It's really, really slow! That is, slow relative to the size of the universe and the timespan of the universe, relative to human scales of size and timespan.

Humans can travel around the world (our domain) in a matter of hours (on rockets, our fastest mode of travel thus far). Similarly, the fastest waves can cross the ocean in a matter of hours. Light, on the other hand, takes billions of years to cross the visible universe. It's downright glacial at those scales!

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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.

I did read the article and it is not making the same point I’m making. It takes the position that humans are extremely slow, relative to the speed of light. I’m taking a different position: that humans are extremely fast, relative to the domain in which we operate (the earth, our cities, our neighbourhoods, our households).

The paragraph beginning "And yet c is also slow." in the article is pretty much what you said.

If you now want to make the point that humans are fast on human-centric scales, which you did not really make above, you enter a whole other discussion that involves biology, and humans not really being very fast at all compared to some other creatures. You have, after all, to introduce non-human entities, rockets, to show examples of humans being "fast".

And really fast rockets aren't examples of humans being fast, as they are good examples of humans becoming dead, from the accelerations involved for starters. Take a human out of a rocket system, and it can go much faster. Human-ridden rockets are in fact slow, too, even on human-centric scales, compared to the things that are extremely fast in the human-centric world. So that argument falls down.

Which leads to part 2, pointed out in a top-level comment by Animats, at https://profmattstrassler.com/2024/10/03/why-is-the-speed-of... which goes on to explain that humans are by necessity slow.

There are even biological discussions of the same idea: why humans are slower than, say, houseflies. We're slower than c for physics reasons, and we're slower than things on our own scale for biological reasons. We actually are not extremely fast. We aren't as fast as our machines, nor even as fast as some other creatures.

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?

Actually, you couldn't cross it in a matter of minutes. In fact, you would never even reach the edge of the visible universe. This is because the edge of the visible universe is expanding away from us at faster than the speed of light.

I would really like to hear from an actual physicist on this question since both of you seem correct for one of the reference frames and the only way I see to reconcile the two is with weird singularities like "the particle reaches the edge of the visible universe in infinite time according to the rest observer". (And I don't think that is right either.)

The edge of the visible universe functions for us like a cosmic event horizon. Similar to the event horizon around a black hole. A particle leaving earth at light speed can never reach or go beyond that horizon. Even in infinite time. That's assuming the universe continues to expand. If it starts to contract then, yeah, the horizon is going to crash in on us.

What does this look like from the photon's frame of reference, then? Or is it nonsensical to try to describe things from a frame of reference at C?

This is probably a huge reasoning error, but wouldn’t you “expand away” with the expanding away part of the universe at the same rate, sort of riding along with the expansion given that c is the same in the point of reference (the expanding away faster than c part)?

n.b. I obviously lack the vocubulary to communicate properly about this, help needed!

Again, this is a matter of perspective. The amazing long lived Earth observer would see the universe expand out of view, and you along with it wouldn't they?

Yes, after billions of years you would move outside of the sight horizon of the long-lived observer on earth and disappear from view. For you, the traveler, this would happen in mere minutes. But you wouldn't have crossed the universe in that time because the edge of the visible universe is constantly expanding away from us faster than we can travel to catch up with it. Even if we travel at the speed of light.

I remember reading several years ago that there are celestial bodies that we will NEVER be able to see precisely because of this.

Lawrence Krauss has given a talk that mentions something similar. He says that we live in a good time because we can still see "everything" around us. At some point in the far future, any observers won't be able to determine many things about the universe because the "stuff" in it will be too far away to observe.

https://youtube.com/watch?v=7ImvlS8PLIo @ 50:57

Even wilder to me is that one day, anyone living in the Milky Way will look up at the sky and be unable to see any other galaxies. If it weren’t for archaeological evidence they would never have any way of knowing about the existence of those other galaxies.

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.

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.

I think in the past I would’ve readily accepted this explanation but now it seems to me like a just-so story.

Bacteria can exhibit doubling times on the scale of tens of minutes. We know of trees that live for thousands of years.

On the other hand, we know of chemical reactions that can propagate significantly faster that the speed of sound (high explosives) and nuclear reactions that propagate even faster that this. At the other end of the scale, we have mildly radioactive elements with half-lives measured in billions of years.

This is all to say that everything is relative and no matter what constants you choose for the universe, they’re going to seem arbitrary.

> 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.

Are galaxies and stuff out in that 250x or is it just empty space?

It's not observable by us, so we don't know. The figure is (to simplify) a deduction from what we can see, the observable universe, and the fact that we don't see any of the consequences that there would be in the observable bit if the entire universe were smaller.

* https://doi.org/10.1111/j.1745-3933.2011.01040.x

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.

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.

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.

^^ this

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