Yet another Hacker News

Show HN: What if the speed of light was 5 km/h?

35 points dmitrybrant 1 hour ago 22 comments

I've always wanted to make a visualization where the speed of light was scaled down to human speeds, so that we could intuit relativistic effects with everyday objects. And here is the first version of it!

skulk 56 minutes ago | parent

in the real world if the speed of light was 5 km, we'd probably be a lot smaller!

dragonfax 54 minutes ago | parent

Well I'm not a physicist, but we all travel through time at the speed of light. With a much smaller speed of light, I think relativistic speeds would be much easier to achieve. I.e. you could age noticeably differently than your family on a solo car drive to get groceries.

Kranar 44 minutes ago | parent

The more accurate statement is that all objects have the same speed in spacetime. Time is just one dimension of spacetime and objects do not all travel along that dimension with the same speed. For example, an object travelling through space at close to the speed of light will have a speed along the time dimension close to zero.

Having said that... this is mostly a slogan and makes for good Youtube videos, but it falls apart the moment you subject it to even the most basic scrutiny. For one... it doesn't make sense to talk about a speed through time since speed is literally a measure of change with respect to time, so a change in time with respect to time doesn't make any sense.

But as a slogan, it captures the idea that motion through space and the rate of your own clock are tightly linked together.

pixl97 31 minutes ago | parent

>it doesn't make sense to talk about a speed through time

Only if you're talking about other people's clocks and not your own. Your own clock is one second per second, but someone else's may be slower or faster relative.

Kranar 25 minutes ago | parent

Once again, just subject your slogan to some basic scrutiny. Your own clock is one second per second, which is a pure number, 1... that's not a speed.

So then you say take someone else's clock and compare it to your own clock, but that is still just a pure number. A speed is not a number, it has units like meters per second. A ratio of two clocks has no units at all, the units cancel. Whatever number you get, 3 or 9, it's just a factor by which two clocks disagree. You can absolutely interpret this ratio as a rate, but that doesn't make it a speed.

And a speed is exactly what the slogan needs, because the diagram/geometry it's using involves a right angled triangle where on one side of the triangle is your speed through space, and on the other side of the triangle is your speed through time, and the hypotenuse of the triangle is fixed at c (since every object travels through spacetime at this fixed speed).

For that to mean anything both sides have to be the same kind of quantity. You can't put meters per second on one side and a bare number on the other and call it a triangle. Nor can you patch it by multiplying the ratio by c to give it units, because the thing you get then grows as you speed up instead of shrinking. Either it has the right units and the wrong behavior, or the right behavior and no units at all. There's no version of this geometry that produces a consistent picture.

bilekas 15 minutes ago | parent

Are you trying to tell me time is relative?

eggn00dles 34 minutes ago | parent

well the meter and the kilogram themselves are either completely or partially defined by the speed of light. its a fundamental constant that a large portion of nature depends on, so i doubt such surgical changes could be realized.

bananaflag 51 minutes ago | parent

Btw there is also this

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

wging 49 minutes ago | parent

See also https://gamelab.mit.edu/games/a-slower-speed-of-light/, a 2012 game that did the same thing

vunderba 31 minutes ago | parent

There's a great demo video of it as well.

https://youtu.be/uu7jA8EHi_0

Some of the details in the paper are super cool. Since the game pulls "c" down to near walking speed, you receive photons at different rates depending on their direction relative to your motion (relativistic aberration / searchlight effect) resulting in neat visual shifts.

I don't usually download games, but I might have to make an exception just to try it out.

gryfft 45 minutes ago | parent

It would be nice to add controls for mobile.

scotty79 42 minutes ago | parent

Next step, taking into account effects of acceleration according to general relativity.

tomjakubowski 39 minutes ago | parent

MIT Game Lab made a take on this back in 2012.

https://gamelab.mit.edu/games/a-slower-speed-of-light/

chamomeal 11 minutes ago | parent

Loved that game!! Blew my mind in my 1st year physics classes

tristanj 32 minutes ago | parent

Velocity raptor is a free game that accurately simulates the effects of special relativity. It slows down the speed of light to 3m/s and you solve puzzles as a cute dinosaur. https://www.testtubegames.com/velocityraptor.html

There is also a helpful FAQ which explains all the strange things happening, such as length contraction, time dilation, and color shifting: https://testtubegames.com/srel101.html

fuzzfactor 29 minutes ago | parent

Can't see your website but I would think everything you see would have so much latency it would be worse than today's internet with dialup using 2GB of RAM on an Atom CPU ;)

zrail 29 minutes ago | parent

The first few Discworld books explore this a little bit too. I think pterry dropped the concept after that, I don't really remember it later in the series.

the-mitr 19 minutes ago | parent

In the Three Body Problem Trilogy by Liu Cixin some of the more advanced civilisations have the ability to change the speed of light locally with very interesting consequences for the that local space

divyekapoor 14 minutes ago | parent

Pedantic note: The speed of light in glass, optical fiber and water is different from vacuum (much lower). On a certain scale, humanity also knows how to locally alter the speed of light though not to this extent.

walrus01 11 minutes ago | parent

It's not actually that the speed of light in a singlemode fiber optic cable is slower, but that the photons aren't travelling in a direct path down the fiber like water down a pipe. They're bouncing off the interior walls of it at like a 45 degree angle billions/trillions of times before it reaches the other end of the cable.

Basically imagine like if you wanted to walk in a straight line between two points at both ends of a empty, long, narrow rectangular warehouse, and instead of walking in a straight line from A to B, you bounced off each wall at an angle (like a pool ball ricocheting off the bumpers) to get from one end to the other. Making your cumulative distance travelled on foot much greater.

aaronbrethorst 8 minutes ago | parent

I'm being vague to avoid spoilers, but I'll just say that it serves for a very interesting meditation on the concept of loss, and has stuck with me for the 5+ years since I read the trilogy.

dares2573 18 minutes ago | parent

Then we will never be able to see aliens again.