Since naturally occurring uranium (U-238) has a half life of 4.5 billion years, then it means half of the uranium on earth has decayed into lead by now. But why only half, and why that specific half? What was special about the particles that did decay? Were they different in some way?
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It doesn’t, it decays with random chance. After 4.5 billion years a single uranium-238 atom has a 50% chance to have decayed.
There are so many atoms in a sample of uranium, that after 4.5 billion years, almost exactly half of them have decayed. It’s just a matter of statistics.
For any given sample, if you could count every atom, it may vary from sample to sample how many have decayed, but there’s so many that it’s just a rounding error.
Imagine you have a big bag of popcorn kernels. Every minute, each kernel has a tiny chance to pop. After a certain time, let’s say an hour, you notice that about half the kernels have popped. That’s similar to the “half-life” of radioactive materials.
Now, for the uranium:
Predictable but Random: Even though the popcorn pops at a predictable rate (half in an hour), you can’t tell which specific kernels will pop first. It’s the same with radioactive atoms; we know half will decay after a certain time, but we can’t say which ones.
Nothing Special About Them: There’s nothing different about the popcorn kernels that popped first. Similarly, the uranium atoms that decayed weren’t special or different.
Always a Chance: Even after an hour, the remaining popcorn kernels can still pop. In the same way, the remaining uranium atoms will keep decaying, but it’ll take another 4.5 billion years for half of those to turn into lead.
So, it’s all about probability. Like rolling dice or flipping a coin, but on a much bigger and longer scale!
Every single atom of uranium 238 has at any time a very small chance of decaying. That rate of decay just happens to coincide with half of it decaying every 4.5 billion years. The uranium that hasn’t decayed has essentially just been “lucky” so far, and probability dictates how that distribution is met.
Noting is special, all atoms are identical and have not memory
Half-life is the mathematical result if the probability of something happening at each moment in time is constant. It does not matter if a radioactive isotope was formed 1 billion years ago or 10 minutes ago. The probability if it decaying within the next minute is the same. Atoms do not age.
Let’s assume you have 1000 atoms and each has a 5% chance of decaying each minute. That is equal to 95% remaining.
After 1 minute 1000 * 0.95 =950 atoms likely remain. After two minutes it will be 950 *0.95 = 1000 * 0.95 * 0.95= 1000* 0.95^(2) = 902.5 lets round it up to 903 atoms. after n minutes you have 1000* 0.95^(n) atoms left
If you continue that you get the table at the end of the post.
The number of atoms is half to 500 after approximately 13.5 minutes. It is halved again to 250 at 27 minutes which is 13.5 minutes after the first halving. Halved again to 125 after 41.5 minutes, again 13.5 minutes later. If we did not round the number it would continue forever.
The time it takes for half of the atoms is independent of the number of atoms you start with, 1000 is just a constant, and half remains when 0.95^(n)=1/2. If you solve that equation you get n=13.51.
A U-238 atoms have a very low chance of decaying in each moment in time. It has no memory so the chance does not change over time. The result is that it takes a very long time for half of them to decay.
If you put $100 into a bank account and get 5% yearly interest you can calculate the time it takes to double. The double time is the same idea as half-life, the different ei just that one is for grown and the other for reduction each moment in time
0 1000
1 950
2 903
3 858
4 815
5 774
6 735
7 698
8 663
9 630
10 599
11 569
12 541
13 514
14 488
15 464
16 441
17 419
18 398
19 378
20 359
21 341
22 324
23 308
24 293
25 278
26 264
27 251
28 238
29 226
30 215
31 204
32 194
33 184
34 175
35 166
36 158
37 150
38 143
39 136
40 129
41 123
42 117
43 111
44 105
45 100
46 95
47 90
48 86
49 82
50 78
51 74
52 70
53 67
The main thing you’re missing is the impossibly huge number of radioisotopes in a sample. In grams of uranium you’d have about 10 with 21 zeros after it nuclei. At that point, it’s not about individual atoms, you’re safely in the realm of statistics.
If each atom has a certain probability to decay, and that probability is truly random and independent of all the other atoms, then the sample as a whole will exhibit exponential decay. You can calculate the time by which half of the atoms will have decayed, and you have no idea which ones, but probabilities are basically facts when you have a billion trillion of something.
Once you have half of them left, you still have to wait just as long for half of what’s left to decay. You’re starting with half as many, but you’ll get half the decays because there are half as many, so the half life is the same.
Because the larger the set of random events, the more predictable the outcome of their total is. If you were to flip a coin, and get a penny every time you got heads, and lose a penny every time you got tails, the longer you repeated this bet, the more likely you’ll be at or near zero pennies.
One kilogram of pure Unranium 238 has about 2,529,240,000,000,000,000,000,000 molecules in it. Each one of those molecules has a tiny chance of decaying, in any given second. But that tiny chance, repeated that many times, is going to start averaging out pretty quickly.
As for why they measure decay in terms of ‘half-life’, it’s because that, too, is because after decaying away half the Uranium, you’ll be left with 1,264,620,000,000,000,000,000,000 molecules of U238, and 1,264,620,000,000,000,000,000,000 molecules of something else. That process will continue until all of the moleules have decayed into a stable isotope.
I have 16 coins. I am going to flip all of them. When they come up tails, they die.
Round 1: HTTHHTTTHTHHTHHT (8 died, 8 lived)
Round 2: THHTTTHH (4 died, 4 lived)
Round 3: HTHT (2 died, 2 lived)
Round 4: HT (1 died, 1 lived)
Half life = 1 round
You can do this with other random generators too. For example you could roll 6-sided dice and all the ones that land on a 1 die. Each round 1/6 will die. You can work out that the half life is 2.59 rounds so on average every 2.59 rounds, half the dice will have died.
Radioactive particles are like this, over some time period, some randomly decay. The half-life is the time it takes for half to have randomly die.
To add to other explanations: you don’t realise how many atoms there are. Think a trillion dollars is a lot or unimaginably wealthy? A trillion coal atoms weights 2*10^-11 grams or 0,02 nanograms. If were to be on a coal atom in the middle of this tiny spec you’d see nothing but coal in every direction forever. And yet the amount is nearly impersivable to us.
Same thing goes for the passage of time. Sure for us 4,5 billion years is a lot due to our lifespans. Is it the same from an atoms perspective? We aren’t even sure protons are stable.
Radioactive particles constantly play the lottery of quantum mechanics (or physics in general). When they hit their jackpot, they decay. This means two things: decays happen randomly, and decays happen independent of other particles.
The result is an exponential decay, which can be formulated into a halftime. You can also reformulate the exponential decay into a thirdtime or decitime or whatever else is convenient.
It’s statistics – pure and simple. Let’s start with coins. We’re going to toss a LOT of coins. Every time a coin comes up tails, we’ll throw it away. So – take a REALLY BIG bag full of a LOT of identical coins. Toss every one of them once. On average, (very close to) half of them will be tails, so we’ll throw away (very close to) half. But it was a REALLY BIG bag of coins, so we still have a BIG bag left.
Take the ones that are left, and toss all of them again once. On average, (very close to) half of them will be tails, so we throw away (very close to) half. But it was a BIG bag of coins, so we still have quite a few left. And we can pretty much keep going until we run out of coins, and statistics will mean that, every time, (very close to) half of what we have left will come up tails.
The half-life of our coins in this experiment is, literally, “one toss”. After each toss, half the remaining coins have gone. We have absolutely no way of knowing ahead of times which coins will go and which will stay, because all the coins are the same, the coins don’t care; every single toss could come down either way, with equal probability. And the probabilities don’t change with more tosses – the chance for any individual coin, at any individual toss, is always 50%.
Radioactivity is just like that. Whatever may be ACTUALLY happening under the covers of reality, atomic decay happens just like our coin tosses, except that, rather than single tosses, we’re talking about periods of time. if you toss a single atom of U-238 – i.e. watch it for 4.5 billion years – there is a 50% chance that it will decay at some point during that period, and throw itself away. If it doesn’t, and you toss it again – watch it for another 4.5 billion years – there is still (only) a 50% chance that it will decay during that time. So if we have a LOT of U-238 atoms, and we watch them for any given period of 4.5 billion years, at the end of it half of them will have decayed and half of them won’t. Which is precisely the definition of radioactive half-life. But – there’s absolutely nothing special about individual atoms, and we have no way of knowing which ones will decay and which won’t.
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