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Comment by arp242 | Hacker News Reader

To be honest I think this is almost a trick question, because you're combining two very different things (speed of light and performance of computers), and people are not very good at combining these sort of contexts intuitively (i.e. you need to do the calculations).

The other day I was watching The Twilight Zone, and one episode involves a crew "655 million miles" from Earth. I know what miles are, I know the rough layout and scale of the solar system, galaxy, and universe, but I had no idea how far away that actually is because it mixes these two units from very different contexts and intuitively it just doesn't make sense to me; is it still in the solar system? No idea. Converting it to AU showed it's about 7 AU, which put it in the appropriate context: a bit beyond Jupiter.

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I don't even know the scale of the earth, or even the state I live in, in a way that makes any physical or intuitive sense to me. It's almost purely a mathematical understanding.

It's always fun to see the repeated scale analogies as you build your way to enormous sizes and distances, starting with something familiar, zooming to something 100 or 1000 times bigger, doing it again and again. It engages our physical understanding while counting zeroes 2 or 3 at a time, but I don't think it actually gives us any physical understanding. It really is just counting zeroes.

I just don't think there's any chance you or anyone else has any idea how far Jupiter is except as a calculation.

>or even the state I live in

I can wake up in my hometown, drive west for 12 hours, and still be in the same state. I know how big my state is in those terms. How many miles is that? How many football fields is that? How many times does light cross the room during that travel distance?

Depends on your average speed.

The traffic can be that bad where I am too.

> I just don't think there's any chance you or anyone else has any idea how far Jupiter is except as a calculation.

I agree, it's very hard – almost impossible – to really "understand" things at scales that are so far outside of our human experience and intuition. However, "near Jupiter" does put things in to relative proportion, whereas "X million miles" doesn't.

> because you're combining two very different things (speed of light and performance of computers), and people are not very good at combining these sort of contexts

I'm not saying I like GPs puzzle, but I could kinda answer that question (my estimation was a bit higher, assuming GP's answer is exactly correct) only because he combined these contexts. Ask me, how many multiplications a computer can do in 30 ns and I'll say, well, I dunno, 10? Well, ok, I'm kinda used to log-times in μs, so maybe I'll say 100. I have no idea how much is 30 ns, but it sure seems a short time.

Also, I don't remember what is the speed of light. Probably, I can figure it out, if I try, but I can't just say from the top of my head.

What I can say instantly from the top of my head, though, is that individual computations are performed with a speed of light, literally, and that a single transistor in my PC is about a billion times smaller than my room, so assuming a single multiplication takes about a hundred of logic gate operations (again, I cannot really tell without thinking, but seems legit) I'll say: well, about 10 million, I guess? And it doesn't matter that I don't know how much time it'll take for light to travel across my room.

I agree.

I don’t think understanding the speed of light is something that can be considered intuitive by most people.

There is no generally observable way to understand the speed of light, it’s just instant to most people.

So without physics training, people just have no idea.

I always like the comparison of more computing power than it took to put a man on the moon!

Which used to be along the lines of a phone or car had more computing power than NASA in 1969.

But these days it’s probably a toothbrush ;-)

So it’s not as useful anymore.

I also have no idea of what level of computation converts to something practical. A million calculations a second means nothing on its own; what can that do? Draw a picture on the screen? Print a PDF?

So it has to convert to practical understanding.

Still, it’s a nerdy and fun comparison but one that isn’t understandable or relatable to the layperson.

The speed of light is a physics topic, but I’d argue it’s relevant to most people working in IT: it’s the maximum speed information can travel at.

Anything involving networks is very much affected by it. The internet runs on fiber, where the speed of light is a fundamental constant! Even electricity in copper obeys those same limitations.

Similarly, anything involving staying alive is affected by the distance from earth to the sun. Doesn't mean that it's relevant for anyone interested in staying alive to know it.

However unless you work in high frequency trading or are trying to layout components for a GPU you can generally ignore speed of light delays below a couple hundred miles. You might develop an understanding for the rough travel time at light speed from New York to San Francisco, or Frankfurt to Sydney, but that doesn't translate to a good understanding how long light takes from this end of the room to the other end.

I think that’s mostly theoretical. There’s a lot of hardware on long connections that adds latency. The signal also may take fairly large detours (in some cases, it may even hop via a geosynchronous satellite. That adds about 70.000km to a trip, and another 70.000 for the return signal. For most users that is a thing of the past, though)

For example, when I ping microsoft.com, I get something around 100ms (Aside: what does that time mean? Round-trip or one way? I read a few man pages for ping, but couldn’t find out)

There also is the weirdness that, when pinging apple.com or google.com I get about 6 ms. They must be playing tricks with DNS.

It mainly acts as a lower bound. If your pings return 5ns, you know that answer cannot possible be correct. But I agree that's a rather constructed use case.

I think it captures something magical - that computers can do so much while a photon is in transit. This is something that to us is the definition of instantaneous (intuitively - not actually).

How is it very different? The speed of light is based on a length and so is a second. Clocks per second and light speed are directly related to distance.

Agree but to be fair I wouldn't have known the answer either if they said 30ns.

Then again, I do know as a dev that unless it's happening every frame (ie for games), as long as you're not introducing non-scalable complexity, the computation time of a bit of reasoning is often negligible.

The real question is, "how many Twilight Zone episodes away from ubiquitous quantum computing are we?" (is joke)

Units definitely matter but anything outside of our day to day experience is difficult to communicate widely. I think most people don't know more than Jupiter is actually really far away and really really big compared to the Earth. 7 AU means nothing to my brain just like 655 million miles. It's a computed unit based on mean distance from the center of the Earth to the center of the Sun; I also have no idea how far we are from the Sun because my brain just experiences a hot circle in the sky. The only reason these things seem to make sense is because we have seen really vague maps of the Solar System. Other falsely but widely-accepted measures include using state sizes to describe other regions. People readily accept it despite never having been to said state or potentially any other state ... or even across their own state.

I think these are accepted because of a false confidence in understanding (think Dunning-Kruger) based on the fact that we have some information.

Personal anecdote: A friend flew from the East coast to Arizona and asked if we could just drive to the other side of the state to see the Grand Canyon in the evening. I had to explain that it's 6 hours one-way despite being in the same state. Had I instead said "Arizona is the size of 22 Connecticuts," the idea of how long it would take to get there would still not make any sense.

PS: I said "vague map of the Solar System" because generally nothing is always the correct scale. Either the planetary distances are the correct scale from each other or the planets are the correct scale from each other for pragmatic reasons. Sometimes, none of those things are scaled properly. Some animations attempt to show things correctly but that's still difficult to comprehend because of the non-linear velocities required to keep the attention of the viewer.

To me, 655 million miles means literally nothing to me beyond "really really far." Probably further than the moon I think? Definitely closer than the nearest non-sun star.

7 AU has at least some meaning. Further than the sun. Probably closer than Jupiter if I had to guess? Definitely closer than Pluto. Would take on the order of 100 minutes for light to travel that distance. We could send a space vehicle there in a single-digit number of years.

My brain can't comprehend either number in the context of my morning commute, but at least it can comprehend 7 AU in the context that that number was given. I can't tell you how long it would take to walk that distance, but that doesn't matter since nobody will ever walk that distance. What I am able to do is make inferences about the implications of that distance (assuming no fiction/sci-fi stuff): we could have a conversation with a few back and forth messages over the course of the day, and they aren't getting home any time soon but they probably won't die of old age.

That's the point being made about combining contexts.

What units do you think you know the scale of the solar system in? The speed of light is approximately 300 million meters per second, so call it 300 thousand kilometres per second, or very roughly 200 thousand miles.

Also, if I were asking that sort of question in an interview I would fine with a candidate getting a conversion wrong as long as they explain their thought process, and I’d help them correct the error early. I’m this sort of question is t meant to be a trick, it’s about whether you can connect different variables which may not appear connected at first glance.

> it's about

It's about absolutely nothing at all. You can project whatever you want on pointless questions like that. Whatever you think you're measuring in a candidate by asking them, well, you're not, lol.

> What units do you think you know the scale of the solar system in?

> if I were asking that sort of question in an interview

The impression I had was that the question was asked in a social context: over coffee, beers, or something like that.

> What units do you think you know the scale of the solar system in?

AU, as I mentioned: https://en.wikipedia.org/wiki/Astronomical_unit

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