xkcd #3174: Bridge Clearance

10 months ago by xkcd bot to c/xkcd

Title text:

A lot of the highway department's budget goes to adjusting the sign whenever the moon passes directly overhead.

Transcript:

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Source: https://xkcd.com/3174/

explainxkcd for #3174

probable_possum 60 points 10 months ago

True. But wind sheer in the lower atmosphere could be an issue. Sometimes the moon gets in the way, too.

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Hamartiogonic 31 points 10 months ago

Not just the moon. You can’t just drive here with a 46 billion light year tall lorry without crashing into some stars, galaxies, black holes and what not.

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PixelatedSaturn 19 points 10 months ago

Really?

Can we calculate this? Let's do specs first. Let's say we only drive until the next overpass, which is likely in the next few km , let's say 5. And we drive 40kmh so for 450 second the lory is swinging around the universe trying to hit anything. Would it?

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skibidi 45 points 10 months ago

Well.. Earth's rotation would mean that the top of the lorry would be moving at 3.3 million light years per second ... Or you know, about 100 trillion times the speed of light.

That might break some things.

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Cocodapuf 14 points 10 months ago

At this point I'm actually curious about the mass of the lorry.

I wonder if we could estimate the mass of a lorry, 2.5 meters wide, 20 m long, 46 billion light years tall. Let's assume it's filled with jars of peanut butter.

At that size, it could well be the most massive object in the universe.

Now that I think about it, it could also be too tall, possibly a tipping hazard around turns.

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Snazz 11 points 10 months ago

Unless the lorry was driving over the exact geographic north or south pole.

Side note: the tallest lorry where the top doesn’t move faster than the speed of light at the equator is 3.8 light hours tall, which is weird to think about because the top doesn’t start moving until well after the bottom has reached it’s destination.

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MintyFresh 6 points 10 months ago

Some hydrogen definitely, probably a lil helium

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Hamartiogonic 6 points 10 months ago

The mass of the universe is not distributed evenly, so it gets really complex. However, as a semi-qualitative assessment, I can say that the vast majority of the universe is just empty space, so you should be fine for the most part. However, the longer you allow the top of the lorry to scrape the edge of the observable universe, the more likely it is to hit something.

Think of it like throwing darts. The more you throw, the more likely it is for you to hit the bullseye. If you keep on driving your lorry for an hour or two, the top has already swept across an enormous arc and probably plowed through multiple galaxies along the way. Keep on sweeping and eventually you’ll smack into something.

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Natanael 2 points 10 months ago

There's actually math for this where you look at the distribution and distances of mass and calculate probability of hitting something based on distance (created to calculate things like how far you can see in the woods)

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chiliedogg 5 points 10 months ago

The earth is also rotating and orbiting the sun, which is rotating around the galaxy, which is itself moving.

Top of that lorry is gonna be whipping around at relativistic speeds.

Heck, just hitting a potholes will make it move millions of lightyears.

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PixelatedSaturn 2 points 10 months ago

Yep, but universe is so empty.

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hansolo 2 points 10 months ago

You're onto the right answer. At some point:

the middle of the lory would be the center of gravity of the lory, and no longer have any grip on the road. Making it a space lory with the lower parts dragging in the atmosphere.
the top of the lory would be so tall that if it was driving, the change in angle of a fraction of a degree would make the top of the lory travel faster than light to keep up. This might happen sooner than running into other stars. If the lorry is 1 light year tall and goes over bump that changes its angle by only 1 degree, the top of the lory would move 1.6 x 10^11^km within a few seconds. So the lory might sheer apart, or would sway like a skyscraper in wind with each molecule moving at the speed of light all over the place as the truck bumps around on a road where even minute variations would make the top move wildly.

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PixelatedSaturn 3 points 10 months ago

That is absolutely true, but the intention of the question is different. The lorry is imaginary - imagilorry.

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logicbomb 18 points 10 months ago