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

#1

Post by dmitrybrant » Thu, Sep 10, 2026, 1:58 AM UTC

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. Here is the first version of it!

Show HN: What if the speed of light was 5 km/h?
rivendell.dmitrybrant.com

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

#2

Post by skulk » Thu, Sep 10, 2026, 2:34 AM UTC

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

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

#3

Post by dragonfax » Thu, Sep 10, 2026, 2:36 AM UTC

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.

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

#4

Post by bananaflag » Thu, Sep 10, 2026, 2:39 AM UTC

Btw there is also this

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

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

#5

Post by wging » Thu, Sep 10, 2026, 2:41 AM UTC

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

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

#6

Post by gryfft » Thu, Sep 10, 2026, 2:45 AM UTC

It would be nice to add controls for mobile.

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

#7

Post by Kranar » Thu, Sep 10, 2026, 2:46 AM UTC

dragonfax wrote: post #3

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.

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.

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

#8

Post by scotty79 » Thu, Sep 10, 2026, 2:48 AM UTC

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

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

#9

Post by tomjakubowski » Thu, Sep 10, 2026, 2:51 AM UTC

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

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

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

#10

Post by eggn00dles » Thu, Sep 10, 2026, 2:56 AM UTC

dragonfax wrote: post #3

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.

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.