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For example, if I wanted to "collect" important bitcoins is there any way to track a coin rather than a series of transactions? If I wanted one of the bitcoins from the infamous "Bitcoin pizza" purchase, for example, is there any way to do so?
Perhaps the question I should be asking is more like "Do bitcoins themselves even exist or just the records of transactions?"
Here is a possible schema describing transactions from two addresses A and B that contain initially 50 BTC each:
(A) 50 btc --tx1-----------------------> 25 btc (C)
\
`-> 25 btc (D) --.
\
tx3--> 50 btc (*)
/
.-> 25 btc (E) --'
/
(B) 50 btc --tx2-----------------------> 25 btc (F)
There are three transactions here, and they mention only values: those transactions are stored in the block chain like:
tx1: (input on A: 50BTC; output on D: 25BTC, output on C: 25BTC)tx2: (input on B: 50BTC; output on E: 25BTC, output on F: 25BTC)tx3: (input on D: 25BTC, input on E: 25BTC; output on *: 50BTC)Only global amounts are involved, not individual satoshis. Could you tell where each of the satoshis in * came from? A or B? Just impossible.
Conclusion: There is no such thing as individual satoshis, they don't have identities in the Bitcoin framework.
(And it's the same, or even worse, for bitcoins. An individual bitcoin is not well defined. It might be a grouping of smaller amounts in the same place (wallet), or at the same address, or in the same transaction, but at a given point in time.)
It is however possible to track money flows between addresses, which may “dilute” into other flows, as illustrated in this answer.
I've been playing with Gavin's "bitcointools" (again) to track what happened to the famous "pizza" Bitcoins.
It turns out that rather than being rare collector's items, the 10,000 BTC exchanged for two pizzas have spread out to over a million different Bitcoin addresses since buying the pizzas, not counting the dilute fragments that ended up in transaction fees.
As of block 166149, on Feb 10th 2012, 1,037,382 addresses have held the pizza coins since they were spent. 174,584 of those still hold pizza coins (the others may or may not be empty addresses, since it's possible to have two different transactions output to the same address without the coins becoming mixed together. In other words it's possible to have "pizza coins" and "non pizza coins" in the same address and still be able to tell which are which. Of the approximately 600k addresses which are currently holding funds, around 29% of them contain pizza coins.
The shortest chain of transactions from the pizza purchase to currently held pizza coins has length 9, and the longest has length 6,723.
There is no currently unredeemed output which contains 100% pure pizza coins. All the pizza coins have been diluted somewhat with other coins. The purest remaining are these 100 BTC which are 90.7276% pure pizza coin, and just 11 transactions separated from the pizza transaction. (The most recent 9 transactions each only have 1 input, so it's easy to follow it back that far. In the next transaction back the pizza coins came from both the last but one input, worth 4223 BTC, and 5th from last input, worth 5777. Together those 2 inputs make up the full 100% pure 10k BTC pizza coins. The other inputs in this big transaction, worth 1022 BTC in total are the only non pizza coins introduced. 10000 * 100 / 11022 = 90.7276%, the purity).
I also checked my wallet to see if I was holding any pizza coins. 8 of my addresses are holding pizza coins, but very diluted. My 'strongest' has a purity of 1.1760%, and is a tiny withdrawal I made from MtGox in October 2011.
Finally, here's some data showing the month-by-month spread of the pizza coins:
pizza payment of 10000.00000000 BTC to 17SkEw2md5avVNyYgj6RiXuQKNwkXaxFyQ
is in block 57043
-block --date-- --total-- ---used--%total- -active---%total---%used-
58815 May 2010 4 2 (50.00%) 4 100.00% 200.00%
63561 Jun 2010 4 2 (50.00%) 0 0.00% 0.00%
71436 Jul 2010 26 8 (30.77%) 24 92.31% 300.00%
77452 Aug 2010 34 10 (29.41%) 13 38.24% 130.00%
82997 Sep 2010 138 37 (26.81%) 106 76.81% 286.49%
88892 Oct 2010 2,016 265 (13.14%) 1,898 94.15% 716.23%
94801 Nov 2010 6,295 577 ( 9.17%) 4,428 70.34% 767.42%
100409 Dec 2010 7,510 835 (11.12%) 1,464 19.49% 175.33%
-block --date-- --total-- ---used--%total- -active---%total---%used-
105570 Jan 2011 13,271 1,540 (11.60%) 6,150 46.34% 399.35%
111136 Feb 2011 19,055 2,632 (13.81%) 6,423 33.71% 244.03%
116038 Mar 2011 42,027 5,605 (13.34%) 24,341 57.92% 434.27%
121126 Apr 2011 69,544 9,085 (13.06%) 29,727 42.75% 327.21%
127865 May 2011 123,997 26,032 (20.99%) 57,911 46.70% 222.46%
134121 Jun 2011 249,138 54,047 (21.69%) 132,348 53.12% 244.88%
139035 Jul 2011 382,845 93,928 (24.53%) 146,156 38.18% 155.60%
143408 Aug 2011 494,140 108,340 (21.92%) 126,134 25.53% 116.42%
147565 Sep 2011 589,942 119,491 (20.25%) 111,178 18.85% 93.04%
151314 Oct 2011 680,787 131,370 (19.30%) 105,320 15.47% 80.17%
155451 Nov 2011 780,292 142,302 (18.24%) 114,013 14.61% 80.12%
160036 Dec 2011 885,172 156,321 (17.66%) 120,631 13.63% 77.17%
-block --date-- --total-- ---used--%total- -active---%total---%used-
164780 Jan 2012 998,087 170,082 (17.04%) 136,985 13.72% 80.54%
166149 Feb 2012 1,037,382 174,584 (16.83%) 54,346 5.24% 31.13%
Edit: I just found these 100% pure allinvain coins - undiluted after 24 hops from when 25k BTC was stolen from his computer.
Edit 2: I don't mean to imply that the allinvain coins haven't been thoroughly looted. They have touched 755,796 different addresses since being stolen and are currently sitting in 109,235 different addresses, including 8 from my own personal wallet. The exact same 8 as have pizza coins in them, it turns out.
This is actually similar to the "Can a bitcoin be counterfeited?" question.
The answer is no, because as you seemed to hint at, there is no such thing as a bitcoin. There is simply a transaction list, that has consensus around it, where no wallets are ever allowed to go negative.
The simplest way to understand why the answer is "no" is to understand a very basic concept about Bitcoin transactions:
Each transaction (except generating transactions) has one or more "inputs" that pull in Bitcoins. And each transaction has one or more "outputs" that offer those Bitcoins.
In the transaction itself, all the Bitcoins pulled in form a pile. There is no way to track a particular input to a particular output, nor is that meaningful.
Imagine a transaction pulls in 25 Bitcoins from one place and 25 Bitcoins from another place and then sends out 10 Bitcoins to each of five different places. What does it mean to ask which input the coins in a particular output came from?
Bitcoins are more like account balances than pennies. If you put 10 pennies in a pile and give 1 out, you can ask which penny they got. But if you deposit two $10 checks and then withdraw $5, which check did that $5 come from? It's not even a meaningful question.
You might want to have a look at the concepts of "taint" and "colored coins". You can start here:
Taint: https://bitcointalk.org/index.php?topic=92416.0
Colored Coins: https://bitcointalk.org/index.php?topic=106373.0
If you know the TX number, you can go to blockchain.info, search the TX number, click "View Tree Chart" and a tree chart will show up and you can click on the yellow/orange circles. That is how I found out a bitcoin I sent to an exchange ended up plying ping-pong with SatoshiDice.
A "bitcoin" is an individual output of a transaction. It is a "coin", because it is not possible to spend only a part of it and somehow leave the rest resting in your wallet, hanging off the transaction it came from.
A transaction can be viewes as this: take these "bitcoins", melt them, mint new ones according to the specifications provided by me and distribute them as I am instructing. Each transaction destroys the bitcoins that are its inputs and produces bitcoins that are its outputs. The "transaction fees" are like "unused raw material" which returns back to the "ground" and can only be extracted back as a coin by mining.
This is similar to real physical coins - the cashier can't take a half of your 50 cent coin when your bill says you only owe 25 cents, he only can take the whole 50 cent coin and give you a 25 coin back to get rid of the excess payment of yours. Similarly if you have only 50 BTC coin on your account, and you want to pay 5 BTC, you (or, more specifically, your BTC client) must "melt" the 50 BTC coin to create a 5 BTC and a 45 BTC coin before it can give the 5 BTC to the destination address you specified. And this is also why you have to specify a destination for the 45 BTC coin as well (otherwise the 45 BTC will turn into "raw unmined bitcoin material" which is not directly usable and must be mined).
So the answer is "yes". A bitcoin starts its existence at an output of a transaction and continues its existence until it enters an input of a transaction. When the bitcoin entered another transaction, it no longer exists; the outputs of that another transaction are completely different bitcoins.
Wow crazy to look back 12 years later after ordinals and see how wrong you all were. https://decrypt.co/resources/what-are-ordinals-a-beginners-guide-to-bitcoin-nfts how could you have known though right?
