Can someone tell me the answer....

[CardyMow]

To my Y4 DS1's question? He came out from school yesterday, and apparently he and his friend were discusing ths after finishing their extension work in maths...If you subtract the speed of light from infinity, is the answer infinity? . I need to settle this argument between them by Monday!

Yes - infinity minus any finite
number is infinity (whichever infinity you're talking about but I assume they mean the vanilla variety)

there is some problem about a hotel with an infinate number of hotels en.wikipedia.org/wiki/Hilbert%27s_paradox_of_the_Grand_Hotel
which apparently helps explain the problem

OK so DS1 was right and his friend was wrong. Can anyone now tell me if it's normal for a not-quite-9yo to ponder questions like this??

Because I'm 29 and it's not something I've ever thought about!

Yes, quite within the normal range - loads of 7-9 year old boys (especially boys) get really fascinated by the idea of huge huge numbers, zillions, squillions etc for some reason and a good teacher will jump on the opportunity to show them some slightly more interesting bits of real mathematics (where "real"= completely unreal and bizarre).

Yes, it's normal (or I have similarly odd DCs)!

Anyone know if there is such a thing as negative infinity?

(ie: infinity is "everything", zero is nothing, but you can have negative numbers, so does it reach to negative infinity?)

I can't see why not, but what would such a concept be used for?

Vanilla variety? Thought that was ice cream, not infinity? Would now be a good time to point out that, embarrasing as it is, my 8yo in infinitely better at maths than I am, and your answers are slghtly confusing me?? ...

Onimolap - DS1 has asked me that question before as well. I also couldn't answer that!

Don't mind me I was just showing off being pedantic. There are different sizes of infinity in pure maths, but the non-exotic(vanilla) one that would be referred to in everyday speech is just the number you would get to if you counted 1 2 3 ... forever.
Stil don't know why children of that age get so fascinated by large numbers though - total mystery.
And don't have a good answer to the negatives question either.

this made me laugh as my almost 5 yr old is obsessed with big numbers and infinity at the moment. we had to go on wikipedia last night to look at some of the really large numbers and I was open mouthed at the complexity/algaebric (sp) formulas! his teacher told me that he had written the number 1 with a hundred '0''s after it and told her it was a googol!!

I'm so relieved to hear that other people have to have conversations like this with their kids - my ds is almost 7 and is constantly asking questions about huge numbers and what I think are quite abstract maths concepts. It doesn't help that I don't understand any of it.

I've sent him off with the book about the maths in the TV show Numbers. I didn't understand that either

mathematically there is a minus infinity.

but I wouldn't be able to prove it!

I love infinity, too - it's just the whole concept of something sooooo big there is noi way of calculating it!

so, yes, infinity - a number = infinity (unless you're subtracting infinity, in which case it's 0 )

There's a series of books called Murderous Maths aimed at upper Key Stage 2 I think, which you should be able to find in most libraries, which may be helpful in feeding some of the DCs mentioned on this thread. (I think the 5 year old may find them slightly advanced though!)

Yes there is a 'minus infinity' - when studying sequences etc they can tend to infinity or minus infinity or whatever :)

I love the hotel problem. I totally get infinity in a mathematical sense (I'm MathsMadMummy) it's when I try thinking about it physically it implodes my brain. I just CANNOT comprehend the idea of an infinite universe.

I asked my dh how to expian it and he said the trouble is infinity is such a huge number that what ever you take from it doesn't make any difference (although he also qualified this be saying infinity isn't like a normal number so normal rules of maths don't apply)

but he said it is like standing up and thinking how far is it to the sun, then taking another step closer - you are still X miles from the sun because the distances are so great

Also if anyone's interested, if you multiply infinity by a number you get... Wait for it... Infinity.

If you multiply infinity by a negative number, is the answer infinity or negative infinity?

And if you multiply negative infinity by a negative number, do you get infinity?

Um...

Probably

onimolap

yes, that's right - it acts in the same way as any positive and negative multiplication.

but it's always infinity!

normal rules apply -

0 x infinity = 0
infinity/0 = infinity (because you can't divide by 0!)
infinity/infinity = 1

But thank you, I will try to find the murderous maths books for DS1, I think he would enjoy them.

I wish they'd been around when I was a kid

my head hurts! I hope my DC's never ask these questions!

Infinity is a concept rather than a number.

Think how many even numbers there are...

Think how many odd numbers there are...

How many numbers would there be if you put all the even numbers together with all the odd nummbers?

What I found a bit weird in Group Theory was that even though the group of, say, even integers was half the size of the group of all integers, it's still infinite. Boggling.

But ooh I just love it :o