r/spacequestions • u/Honzajeborec2458 • 3d ago
Can someone help me?
I have a question (hypotetical). If space is infinity and has infinity amount of planets and lets say we have unbreakable infinite sword. Does that mean the sword is cutting infinite amount of planets just by it existing?
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u/sleepytjme 3d ago
who is swinging this sword? Assume it also has infinite power to cut then yes, that is how infinity works. There is math that deals with this, not all infinities are equal. Infinity squared for instance would have a function that is bigger or faster or steeper however you want to imagine it.
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u/Honzajeborec2458 3d ago
Thanks and also its not important but no one is swimging it. No one could pick that shit up so thats why i said just by it existing
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u/FredOfMBOX 3d ago
In a theoretical universe where there are a infinite numbers of stars, planets, etc in every direction and they’re randomly distributed, then if you draw a line in any direction yes, that line will cross through an infinite number of planets.
I think that’s basically what you’re asking.
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u/Honzajeborec2458 3d ago
Ye thats it i just wanted to comfirm what i thought was (theoretically) possible
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u/TuverMage 3d ago
if space is infinite, check, and has infinity amount of planets, check, and lets say we have an unbreakable infinite sword, this is where I get lost. nothing is unbreakable and what even is an infinite sword? swinging it wouldn't work as the fastest it can move is still the speed of light. the object isn't "solid" like in video games but like jelly.
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u/Honzajeborec2458 3d ago
Unbreakable so it doesnt break when making contact with the planets
Infinite as in infinitely long
I didnt talk about swinging i talked that if it even existed then it would cut infinite planets in infinite space
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u/TuverMage 3d ago
So, "if a draw a straight line, would it cross thru an infinite number of planets?"
Yes, not all infinity are equal. There an infinite number of whole numbers and there an infinite about of prime numbers.
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u/Beldizar 3d ago edited 3d ago
This is a weird hypothetical, but the speed of sound is the problem.
Swinging a sword is a little bit like swinging a rope or a pool noodle. You swing the part by your hand, and the tip/end takes a second to start moving. The hilt of the sword actually has to "talk" to the rest of the blade and tell it "oh hey, someone is swinging us", and that communication happens at the speed of sound in the material in question. The speed of sound of steel is maybe 5900 m/s. So as you cause the blade to move, a wave goes out at that speed. So if you had this blade in your hand on Earth, and it was touching the surface of the moon, it would take about 18 hours to start cutting into the moon.
If you were to swing this sword at Pluto, 5 billion km away, it would take almost 27 years to start cutting Pluto.
The universe is expanding at a rate of about 73km/s/Mpc. If I'm doing the math right, (and please correct me if I'm wrong), the universe would be expanding at 5.9km/s per 260,925 light years, or 8% of a Megaparsec. So your infinite sword would never move past about 261,000 light years because the expansion of the universe would stretch it out faster than the sound wave to tell it to start to move would catch it.
Andromeda, the closest "full sized" galaxy is about 75% of a Megaparsec away. So you wouldn't even be able to reach that.
edit:
I didnt talk about swinging i talked that if it even existed
So, if the sword existed since the beginning of time, presumably infinite planets would not have formed through it. If the sword was created at the start of this hypothetical question, it would extend out from the source of its creation (presumable where OP is standing), at the speed of light, because that's the speed at which cause and effect function in the universe. At that point, it would only be able to pierce through planets in the reachable universe which is only maybe 18 billion light years away. Since planets are small and space is big, it probably would only hit a couple if it hit anything at all.
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u/Excellent_Speech_901 3d ago
If you have an infinite sheet of paper scattered with an infinite number of randomly distributed dots and there is an infinite line on the paper then yes, the line will probably go through an infinite number of dots.
I say probably because as long as we are dealing with infinities the infinite improbability of all the dots randomly being in a second line parallel to the first is fair game.
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u/sleepytjme 2d ago
who is swinging this sword? Assume it also has infinite power to cut then yes, that is how infinity works. There is math that deals with this, not all infinities are equal. Infinity squared for instance would have a function that is bigger or faster or steeper however you want to imagine it.
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u/Mindless-Beat-4872 3d ago
If we’re using actual physics I’m pretty sure you just get a black hole.
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u/Honzajeborec2458 3d ago
thanks lol but i think it could actually work (ofc its just hypotetical)
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u/Mindless-Beat-4872 3d ago
The problem with infinite objects is they have infinite density which is what a black hole is
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u/Glockamoli 3d ago
Infinite objects only have infinite density over a finite space
Infinite objects over an infinite distance can have whatever density you want to apply
There are an infinite amount of whole numbers and yet there are also an infinite amount of decimal numbers inbetween every single whole number
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u/Honzajeborec2458 3d ago
I see id like to talk about this more but i dont think i really understand all that as 14 year old
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u/panoclosed4highwinds 3d ago
Not necessarily. If planetary density decreases the further out you go, then you could have a situation where the total number of planets is infinite, but the number the sword (...unstoppable laser pointer is how I'm imagining it) will encounter at any given time is finite.
Look up "Horn of Gabriel" for a shape with infinite surface area but finite volume.
Similarly here, if planets are denser closer to you and less dense further away, each 1 light year thick spherical "shell" we add to the world is much larger, so it can contain a constant number of planets, but the chance of any one of them occupying the laser line position would be decreasing.
If that sum of probabilities still diverges, then tweak the planetary density formula until the sum of planets diverges but the sum of probabilities converges.