I think it was numberphile, or maybe vsauce, who did a video on infinities. It was really interesting. I learnt a lot, then forgot it all.
this post was submitted on 02 Oct 2025
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Ah yes, I remember my eyes glazing over as things got too complicated to fit through my thick skull
The first one, because people will die at a slower rate.
The second one, because the density will cause the trolley to slow down sooner, versus the first one where it will be able to pick up speed again between each person. Also, more time to save people down the rail with my handy rope cutting knife.
You've misunderstood "some infinities are bigger than others." Both of these infinities are the same size. You can show this since each person on the bottom track can be assigned a person from the top track at 1 to 1 ratio. An example of infinities that are different sizes are all whole numbers and all decimal numbers. You cannot assign a whole number to every decimal number.
Matt parker does a good video on this. I can't remember the exact title but if you search "is infinite $20 notes worth more than infinite $1 notes" you should find it.
The bottom rail represents the real numbers, which are "every decimal number".
Natural numbers < whole numbers < rational numbers < real numbers
actually you can show that the naturals, integers and rationals all have the the same size.
for example, to show that there are as many naturals as integers (which you do by making a 1-to-1 mapping (more specifically a bijection, i.e. every natural maps to a unique integer and every integer maps to a unique natural) between them), you can say that every natural, n, maps to (n+1)/2 if it is odd and -n/2 if it is even. so 0 and 1 map to themselves, 2 maps to -1, 3 maps to 2, 4 maps to -2, and so on. this maps every natural number to an integer, and vice-versa. therefore, the cardinality (size) of the naturals and the integers are the same.
you can do something similar for the rationals (if you want to try your hand at proving this yourself, it can be made a lot easier by noting that if you can find a function that maps every natural to a unique rational (an injection), and another function that maps every rational to a unique natural, you can use those construct a bijection between the naturals and rationals. this is called the schröder-bernstein theorem).
it turns out that you cannot do this kind of mapping between the naturals (or any other set of that cardinality) and the reals. i won't recite it here, but cantor's diagonal argument is a quite elegant proof of this fact.
now, this raises a question: is there anything between the naturals (and friends) and the reals? it turns out that we don't actually know. this is called the continuum hypothesis
There are more reals than naturals, they do not match up 1 to 1, for exactly the reason you mentioned. Maybe you misread the meme?
Yeah, but if you can line up the elements of a set as shown in the bottom track, then they're, at most, aleph 0.
I don't think we should take the visuals of the hypothetical shit post literally.
If they say there's one guy for every real number, let them
It's just an illustration... how else would you draw it?
First, I start moving people to hotel rooms...
I mean, the bottom. The trolley simply would stop, get gunked up by all the guts and the sheer amount of bodies so close together. Checkmate tolley.
How do we know it's an accurate illustration? They might have jacked up the trolley with monster truck wheels or something.
either way infinite people die, just not getting involved
Getting killed by a train is apparently just an inevitably in this universe. Either choice is just the grand conductors plan.
Bottom. Train will stall/derail faster.
This is why it is important to only hire union trolley operators. They are trained to stop the trolley.
like the infinite monkeys with typewritters, universal limits to the rescue. Trolley's are slow. Each bump makes them slower. Some of the people in the discrete line will have long lives until an excruciatingly painful death from dehydration.
I do what I always do: run to the trolley, then jump up and pull the emergency stop because I hate false dilemmas.
Correct, because if we ignore some important facts you could also have infinite time to stop the trolley. Checkmate, false dilemma creators.
It'll make it through maybe 3 infinities before derailing. Go bottom, end it faster.
- I lay some extra track so the train runs over the perverts that come up with these "dilemmas" instead. Problem solved. 👍
The second one. It'll be a bit rough, but overall should be a smoother ride for the occupants.
Some infinities are bigger than other infinities
Is this actually true?
Many eons ago when I was in college, I worked with a guy who was a math major. He was a bit of a show boat know it all and I honestly think he believed that he was never wrong. This post reminded me of him because he and I had a debate / discussion on this topic and I came away from that feeling like he he was right and I was too dumb to understand why he was right.
He was arguing that if two sets are both infinite, then they are the same size (i.e. infinity, infinite). From a strictly logical perspective, it seemed to me that even if two sets were infinite, it seems like one could still be larger than the other (or maybe the better way of phrasing it was that one grew faster than the other) and I used the example of even integers versus all integers. He called me an idiot and honestly, I've always just assumed I was wrong -- he was a math major at a mid-ranked state school after all, how could he be wrong?
Thoughts?
Hilbert's Paradox of the Grand Hotel seems to be the thought experiment with which you were engaged with your math associate. There are countable and uncountable infinities. Integers and skip counted integers are both countable and infinite. Real numbers are uncountable and infinite. There are sets that are more uncountable than others. That uncountability is denoted by aleph number. Uncountable means can't be mapped to the natural numbers (1, 2, 3...). Infinite means a list with all the elements can't be created.
Two sets with infinitely many things are the same size when you can describe a one to one mapping from one set to the other.
For example, the counting numbers are the same size as the counting numbers except for 7. To go from the former set to the latter set, we can map 1-6 to themselves, and then for every counting number 7 or larger, add one. To reverse, just do the opposite.
Likewise, we can map the counting numbers to only the even counting numbers by doubling the value or each one as our mapping. There is a first even number, and a 73rd even number, and a 123,456,789,012th even number.
By contrast, imagine I claim to have a map from the counting numbers to all the real numbers between 0 and 1 (including 0 but not 1). You can find a number that isn't in my mapping. Line all the numbers in my mapping up in the order they map from the counting numbers, so there's a first real number, a second, a third, and so on. To find a number that doesn't appear in my mapping anywhere, take the first digit to the right of the decimal from the first number, the second digit from the second number, the third digit from the third number, and so on. Once you have assembled this new (infinitely long) number, change every single digit to something different. You could add 1 to each digit, or change them at random, or anything else.
This new number can't be the first number in my mapping because the first digit won't match anymore. Nor can it be the second number, because the second digit doesn't match the second number. It can't be the third or the fourth, or any of them, because it is always different somewhere. You may also notice that this isn't just one number you've constructed that isn't anywhere in the mapping - in fact it's a whole infinite family of numbers that are still missing, no matter what order I put any of the numbers in, and no matter how clever my mapping seems.
The set of real numbers between 0 and 1 truly is bigger than the set of counting numbers, and it isn't close, despite both being infinitely large.
It is true! Someone much more studied on this than me could provide a better explanation, but instead of "size" it's called cardinality. From what I understand your example of even integers versus all integers would still be the same size, since they can both be mapped to the natural numbers and are therefore countable, but something like real numbers would have a higher cardinality than integers, as real numbers are uncountable and infinite. I think you can have different cardinalities within uncountable infinities too, but that's where my knowledge stops.
It's pretty well settled mathematics that infinities are "the same size" if you can draw any kind of 1-to-1 mapping function between the two sets. If it's 1-to-1, then every member of set A is paired off with a member of B, and there are no elements left over on either side.
In the example with even integers y versus all integers x, you can define the relation x <--> y = 2*x. So the two sets "have the same size".
But the real numbers are provably larger than any of the integer sets. Meaning every possible mapping function leaves some reals leftover.
Weeeell... not really. It's pretty well settled mathematics that "cardinality" and "amount" happen to coinciden when it comes to finite sets and we use it interchangeably but that's because we know they're not the same thing. When speaking with the regular folk, saying "some infinities are bigger than others" is kinda misleading. Would be like saying "Did you know squares are circles?" and then constructing a metric space with the taxi metric. Sure it's "true" but it's still bullshit.
Change the numbers to rubber balls with pictures of ducks or trains and different iconography. You can now intuit that both sets are the same size.
I side with you, though the experts call me stupid for it too.
if for all n < infinity, one set is double the size of another then it is still double the size at n = infinity.
I know it seems intuitive but assuming that a property holds for n=infinity because it holds for all n<infinity would literally break math and it really doesn't make much sense when you think about it more than a minute. Here's an easy counterexample: n is finite.
You're not stupid for it. Since it makes sense.
However, due to the way we "calculate" the sizes of infinite sets, you are wrong.
Even integers and all integers are the same infinity.
But reals are "bigger" than integers.
The top one, because time is still a factor.
Sure, infinite people will die either way, but that is only after infinite time.
Tankies hate this one weird trick.
Yeah, but in the bottom one, the people are packed infinitely dense, which will probably cause the train to derail, saving infinitely more people.
what if the trolleys got a cow catcher
I ignore the question and go to the IT and maintenance teams to put a series of blocks, physical and communication-system-based, between the maths and philosophy departments. Attempts to breach containment will be met with deadly force.
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I pull the lever, if the cart goes over the real numbers it will instantly kill an infinite amount of people and continue killing an infinite amount of people for every moment for the rest of existence.
If I pull the lever a finite amount of people will die instantly and slowly over time tending twords infinity but due to the linear nature of movement it would never actually reach Infinity even if there are an infinite number of people tied to the track a finite amount is all that could ever die.
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In the top one you will never actually kill an infinite number of people, just approach it linearly. The bottom one will kill an infinite amount of people in finite time.
Edit: assuming constant speed of the train.
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Bottom.
Killing one person for every real number implies there’s a way to count all real numbers one by one. This is a contradiction, because real numbers are uncountable. By principle of explosion, I’m Superman, which means I can stop the train by my super powers. QED
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you know, I'm not sure you can have an uncountably infinite number of people. so whatever that abomination is I'll send the trolley down its way. it's probably an SCP.
What about a time loop where only one person dies, but infinite times?
Considering that there's a small but non zero chance of surviving getting ran over by a train some of them are gonna survive this and since there are infinite people that will result in infinite train-proof people spawning machine
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