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Bitcoin is the clock

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In a decentralized world, Bitcoin has become latitude and its own unit of time.

The original title: "dry | COINS is the clock"

Original source: Ethereum enthusiasts

A clock with a clear light hung high in the air,

There is no right or wrong time to declare.

-- Robert Frost, My Familiarization with the Night (1928)

We still haven't figured out the great riddle of time. Time is but a concept; We don't even know if it really exists...

-- CPfford D. Simak, Shakespeare's Planet (1976)

As the saying goes, time is money. Conversely, money is also time: time is all the economic energy stored in the human body. However, the relationship between time and money is far more complex than it might seem at first glance. If money doesn't take time to make, money can't be money, at least not for long. To take it a step further, the thing that tracks the world of information is to track time.

When money goes digital, we have to agree on the definition of time, and that's the problem. You may think telling time is as simple as glancing at a nearby clock. If you're just doing routine work, that's fine. But when it comes to synchronizing a global, distributed network of people doing their own things, telling the time becomes a huge problem. If the clock can't be trusted, how do you get the right time? If your system spans galaxies, how do you establish the concept of time? How would you measure time in a world without clocks? What is time anyway?

To answer these questions, we need to learn more about the concept of time and how Bitcoin creates its own unit of time (block time, also known as block height). Questions we'll explore include: Why did timekeeping eventually become associated with bookkeeping? Why is there no absolute time in a decentralized system? How does Bitcoin use causality and unpredictability to define the present tense?

The chronograph has brought about great changes in civilization many times. As Lewis Mumford said in 1934: "The most important machine of the industrial age is not the steam engine, but the clock." Now the timer is once again driving a revolution in modern civilization: the most important machine of the information age is not the computer, but the clock. That clock is Bitcoin.

tracking

"The child learns to count things so that they can be counted as if they are all identical, either as a single object or as a group of objects."

-- David Eugene Smith, Teaching Elementary Mathematics (1900)

In general, there are two ways to keep track of objects: physical symbols and ledgers. You can use objects from the real world, such as shells, coins, or other tangible objects. You can also copy the state of the world by writing down what has happened on a piece of paper.

Imagine that you are a shepherd and want to make sure that every sheep returns home. You can put a collar on each sheep, and when one of them comes home, you take that collar off and hang it in the shed. Assume that each collar is attached to a separate peg, and as long as the peg is full, all the sheep are home. Of course, you can count them each time and make a list. However, before you start counting, you must make a new list to prevent double counting or missing.

Money is essentially a vehicle for tracking debt. Broadly speaking, the things that have served as money so far fall into two categories: physical objects and lists of information. More commonly, symbols and ledgers.

干货 | 比特币就是时钟（上）

It is important to understand the essential differences between the two types of currencies. Let me be clear here: physical symbolsdirectlyRepresenting the state of things; Books areindirectReflect the state of things. Both have advantages and disadvantages. For example, symbols are physical and distributed, while ledgers are informational and centralized. Symbols are intrinsically distrustful; ledgers are not.

In the digital world, we can only use ledgers (though there are plenty of gurus out there who would have you believe otherwise). This is the information world, not the material world. Even if you define a type of information as a "symbol," it is still a malleable piece of information written to a hard disk or some information storage medium, in the form of a record of information.

All digital information is essentially a ledger, and this is where the problem of "more than one dollar" comes in. Information never directly represents the state of the world. In addition, the movement of information also means that information can be copied. The information exists somewhere. To "move" the information, you have to copy it to another location and then delete the original information. In the physical world, there is no such problem, and movement is movement, not replication. The information world does not possess this characteristic. If you want to "move" information from table A to table B, you must copy the information from table A to table B. There is no other way.

Another way to think about it is "uniqueness." Physical symbols are unique combinations of atoms that cannot be easily copied. Pure information does not possess this characteristic. As long as you can read a message, you can copy it perfectly. Therefore, we can come to the conclusion that physical symbols are unique, while digital symbols are not. I even think the term "digital symbol" is questionable. Symbols can represent secret information, but never unique and unrepeatable information.

This characteristic difference suggests that information is virtually impossible to "hand over". Digital symbols cannot be transferred in the same way as physical symbols because you cannot be sure that the original owner has corrupted the information. Like all information, digital symbols can only be transmitted as ideas.

"... if you have an apple and I have an apple, we can exchange the apples and each of us will still have an apple. But if you have an idea and I have an idea, then after we have exchanged, we will each have two ideas.

-- Charles F. Brannan (1949)

Physical symbols (what we call physical bearer assets, or "cash") do not face this dilemma. In the real world, if you give me a coin, you lose the coin. There is no magic in the world that can duplicate this coin, and in order for me to get a coin, I must be given a coin. The laws of the physical world prevent more than one coin from being spent.

While multiple costs do exist in the non-digital world (the notorious fraudster George Parker once profitedby selling multiple homes, including landmarks like the Brooklyn Bridge), they require elaborate scams and gullible buyers. The digital world is different.

In the digital world, multiple costs are an inherent problem because we are always dealing with information. Anyone who has ever copied a file or used copy-and-paste knows that information can be copied perfectly, and that it is not bound to the medium in which it is stored. Say you have a digital photo. You can make a million copies of it, put some of those copies on a USB stick, and send it to thousands of people. A perfect copy is possible because a perfect fix can be used to eliminate all imperfections. Most importantly, the cost of copying is almost zero and there is no distinguishing between the copy and the original.

Again: information can only be duplicated. Digital information cannot be moved from A to B; it can only be copied from A to B. If the copy is successful, then the original at A is deleted, and it is called "moved". That's why the problem of multiple costs is so tricky. Without authority, no information can be transferred from A to B in A way that does not involve risk to counterparties. We must trust that the original will be deleted. A natural side effect of digital information is that we can't be sure how many copies exist and where they are.

Therefore, digital symbols can never be used as money. Symbol is difficult to copy because of its unique physical structure, so it has reliability. In the digital world, that advantage is gone. In the digital world, symbols are not to be trusted. Because of the inherent nature of information, the only viable form of digital currency is a ledger, not a symbol. So digital currencies have to face the issue of time.

Symbols are not temporal, ledgers are not

"What you can see is temporary, what you can't see is eternal.

-- Pap of Tarsus, Corinthians 4:18b

For physical symbols, it doesn't matter when the transaction took place (where your money came from). You either have money in your pocket or you don't. Spend it or not spend it. The only prerequisite for spending money is having money in your pocket. The rest is left to the laws of nature. In this sense, physical symbols are trustless and time-free.

In the case of ledgers, physical possession is less important. The person who keeps the books needs to make sure all the books are in good order. Features that were originally assigned by the laws of physics (you can't spend money that doesn't belong to you, and you can't spend the same money twice) must be enforced by man-made rules. It is these rules, not the laws of physics, that keep the books in order.

The shift from the laws of physics to man-made rules is the crux of the problem. The latter can be modified and broken, the former not. For example, you can't "fake" a physical gold coin; you have to dig it out of the ground. However, you can forge gold coins on paper. All you have to do is add an entry in the ledger saying you have some gold. If it were a central bank, a few keystrokes on a computer would have created trillions of dollars more. (Financiers call them "remortgages," "fractional reserve banking," and "quantitative easing" -- but don't get confused by the fancy terms, they're all essentially fake money.)

To ensure that the books, and those who manage them, are not being falsified, the books must be audited independently on a regular basis. The accounting of every entry in the ledger is not a big deal. Auditors should be able to examine past books to ensure that the books are reliable and valid. Without a reliable timestamp, we cannot verify the internal consistency of the ledger. We must establish a mechanism to determine the order of transactions.

Without an absolute concept of time, we cannot determine the order of transactions. If we cannot determine the order of transactions, we cannot be sure that the books were not falsified. How else can you prove how much money you have? Is there any other way to ensure that all the accounts are in order?

The distinction between symbols and ledgers highlights the need to track time. In the physical world, money is a timeless artifact that can be traded without supervision. In the digital world, marking currency requires a timestamp.

Centralized currency tokens

"Time engrave all marks and erases all marks."

- Yahia Lababidi (b. 1973)

The most common way to solve the problem of multiple costs (ensuring that each digital transfer occurs only once) is to create a centralized transaction list. Once you have a centralized list of transactions, you have a ledger that can act as a single source of truth. At this point, resolving multiple costs is as simple as going through the transaction list and making sure that each entry is correct. This is how PayPal, Venmo, Appay, and all the world's banks (including central banks) have solved the problem of multiple costs: the authorities.

"It is a process of the problem is that the payee is difficult to test, before ever multiple spend a certain amount of money a holder. The usual solution is to introduce the trusted third party authority, or similar institutions like the mint, to examine every transaction, in order to prevent multiple payment... the solution of the problem is that the fate of the whole monetary system completely relies on the operators of mint, because each trade to after confirmation of the mint, the mint is like a bank."

-- Satoshi Nakamoto, White Paper

It is important to note that Nakamoto cannot make the information unreproducible. Every part of Bitcoin (source code, ledger, user's private key) can be copied. All this can be copied and tampered with. Mr Nakamoto, however, managed to create a system that rendered copies that violated the rules useless. The Bitcoin network implements complex mechanisms to determine which copies are useful and which are not. It is this mechanism that brings scarcity to the digital world. This mechanic is like a dance, and you need to control the rhythm.

Even centralized ledgers must adopt a uniform time-tracking approach to address multiple payments. When a transaction takes place, we must know who is making the transaction, how much is being made, and most importantly, when. In the information world, you cannot mark a currency without a timestamp.

"It is important to stress that, until Satoshi came up with the solution, decentralized ledgers were not possible because we had not been able to solve the problem of linking events to points in time in distributed systems."

-- Gregory Trubet-Skoy (2018)

Decentralized clocks

"Time takes everything away."

-- Aeschylus (525 -- 456 BC)

Time and order are very closely related. As Lespe Lamport put it in his 1978 paper Time, Clock, and Order of Events in Distributed Systems: "The concept of time is fundamental to the way we think. It stems from a more fundamental concept, the order in which events occur." In the absence of a central coordinated point of time, seemingly intuitive concepts like "first," "later," and "at the same time" have no reference. In Lamport's words: "The concept of 'first happening' defines that part of the sequence of events that is invariant in a distributed multi-process system."

In other words: If we can't put someone in charge of time (timing), how can we determine the order of events? If we don't have a central frame of reference, how can we get a reliable clock?

You might think that this problem would be easy to solve, since everyone could use their own clock. However, this only works if everyone's clock is on the right track (and everyone follows the rules). In a system where people have opposing, conflicting interests, everyone using their own clock would be a disaster. And, according to relativity, it doesn't span space.

Here's a thought experiment: If you lived in a world where everyone kept track of the sequence of events, how could you trick others? You can pretend that a transaction you sent today was actually yesterday (it was delayed for some reason) and that the money you spent today is still yours. Since every decentralized system communicates asynchronously by nature, the above scenario is more than a theoretical thought experiment. Messages do lag and the timestamp is inaccurate. Couple that with relativistic effects and the natural speed limit of the universe, and it's hard to discern the sequence of events without a centralized authority or observer.

"Who's there? Knock Knock." "Who is it? )

-- Classic homophonic peduncle

To better illustrate the point, let's look at a concrete example. Imagine that both you and your business partner have access to your company's bank account. You do business all over the world, so your bank account is in Switzerland, you live in New York and your business partner lives in Sydney. It's January 3rd and you're enjoying a weekend night at a hotel. It's already Monday morning at your business partner's, and she decides to use a debit card from her shared bank account to buy breakfast. The balance on the card at the time was $615. The local time was 8:21 a.m. Her breakfast cost $27.

In the meantime, you intend to pay for your accommodation with a separate debit card linked to the same bank account. At this point, the balance on the card is $615. The local time was 5:21 p.m. The accommodation is $599.

干货 | 比特币就是时钟（上）

In other words, you swiped your card at the same moment. What's going to happen? (Physicists, forgive me for using the phrase "at the same time," ignoring for a moment the effects of relativity and the fact that there is no absolute time in the universe. In addition, we also assume that the concept of synchronous events exists. Bitcoin itself is complicated enough!)

The central books of the bank probably received these two transactions one after the other, so one of you must be the lucky one and the unlucky one. If the central ledger receives both transactions at exactly the same instant (millisecond accuracy), the bank must decide who will spend the money.

So what would happen if there were no banks? Who decides the order of trade? What if it wasn't just the two of you, but hundreds or even thousands of people at the same time? What if you don't trust these people? What if someone wants to be evil, such as turning back their clocks and pretending their trades were minutes ago?

"We needed a time-dependent tool to create an accepted ordering and maintain a unique historical record without relying on any central coordinator."

-- Giacomo Zucco, Discovering Bitcoin (2019)

This is why all previous attempts at digital cash have relied on centralised registries. That is, we have to trust someone in order to make the right decisions about the order of things. We need a centralized participant to act as a centralized clock.

Bitcoin's solution to this problem is to redefine time. Bitcoin measures time in blocks, not seconds.

Take blocks as units of time

Emperor interest contention, is the style of time;

To expose the lies and let the truth come out;

The years mark, to the past seal;

Hold the night, wake the dawn;

6. Punish the wicked until they repent;

"-- Shakespeare, The Disgrace of Lucrece (1709)

All clocks rely on a regular process, known as a "tick." The ticking sound of a grandfather clock is essentially indistinguishes it from the buzzing sound of modern quartz and atomic clocks. Specifically, we measure seconds or minutes by the number of times a part of a clock swings or vibrates.

Large pendulum clocks have long pendulums that swing clearly. Smaller, more specialized clocks require special parts. The frequency at which a clock vibrates (the "tick" frequency) depends on the context in which it is being used.

Most clocks vibrate at a fixed rate to tell the time accurately. However, some clocks vibrate at a variable rate. A metronome, for example, can be programmed to vibrate at a preset frequency and, once set, to beat time at a constant frequency. Bitcoin ticks at different intervals because of the probabilistic nature of its internal mechanisms. But the purpose of both is the same: to make music so that the dance can go on.

Isn't the Bitcoin in front of you a clock? Satoshi Nakamoto did suggest that the entire Bitcoin network is like a clock, in his words, a distributed timestamp server.

In this paper, we propose a solution to the multi-payment problem by using a point-to-point distributed timestamp server to generate computational proofs for the sequence of occurrence of a group of transactions.

-- Satoshi Nakamoto, Bitcoin: A Peer-to-Peer Electronic Cash System (2009)

Judging from the bibliography at the end of the Bitcoin white paper, it's clear that timestamps are a fundamental issue. Of the 8 references, 3 are related to timestamps:

· "How to Time-stamp a Digital Document", S. Haber, W.S. Stornetta (1991)

· On how to improve the efficiency and reliability of digital timestamps, D. Bayer, S. Haber, W.S. Stornetta (1992)

· "How to design a secure timestamp service with minimal trust", H. Massias, X.S. Avila, J.-J. Quisquater (1999)

As Haber and Stornetta described in 1991, a digital timestamp is a date that uses a computational process to make it impossible for a user (or an adversary) to change a digital document backwards or forwards. Unlike paper documents, digital documents are easily tampered with and do not necessarily leave any obvious signs on the physical medium. In the digital world, falsification and tampering can reach an unassailable level.

Given the malleability of information, adding a timestamp to a digital document is a very complex process. Intuitive solutions simply don't work. In the case of a text document, simply adding a date to the end of the document is of no use, since anyone (including yourself) can easily change the date. What's more, you can fake dates in the first place.

Time is a causal chain

At the extreme, the world is nothing more than a network of relationships.

"-- Tim Berners-Lee, Weaving the World Wide Web (1999)

Faking dates is a widespread problem, and not just in the digital world. In a kidnapping case, for example, the kidnapper needs a way to prove when the kidnap took place.

干货 | 比特币就是时钟（下）

Time to prove

This works because newspapers are hard to forge and easy to verify. The kidnappers could not have predicted the front page news and faked the photos weeks in advance, as the newspaper's front page covered the previous day's events. Thus, the issue date of the newspaper held by the hostage in the photo is proof that the hostage is alive.

This approach highlights an important concept: causation. The arrow of time reflects the cause and effect of events. Without cause and effect, there is no time. In the cyberworld, the hash function is crucial to solving the timestamp problem because it introduces causality. Without a document, we cannot generate a cryptographic hash, so there is a cause-and-effect relationship between the document and the hash: the data must be present before the hash can be generated. In other words, without the computational irreversibility of one-way functions, there would be no causal relationship in the cyberworld.

干货 | 比特币就是时钟（下）

There was A before there was B

With causality, we can create a chain of events that link together. Thus, a secure digital timestamp scheme can make history in a digital world where time did not originally exist.

Causality determines the chronological order of events. If an event was triggered by some previous event and caused some later event, the event's place in history is determined and will not change.

-- Bayer, Haber, Stornetta (1992)

There is no doubt that causation is crucial to economic calculations. Since a ledger is actually a visualization of economic calculations between multiple partners, causality is also critical to each ledger.

We need a system where all participants agree on a single historical record... Our proposed solution is based on a timestamp server.

-- Satoshi Nakamoto (2009)

What's interesting is that all the components that make Bitcoin work already exist. Back in 1991, Haber and Stornetta introduced two schemes that "effectively prevent fake timestamps." One is a scheme that relies on trusted third parties, and the other is a more complex "distributed trust" scheme that does not rely on trusted third parties. The authors even discover the inherent problems behind the causal chain of trust events and the conditions required to rewrite history. In other words, "the only way to succeed in evil is to be prepared with a time-stamp chain long enough that even the most skeptical challenger can't suspect it." Today, Bitcoin has a similar attack vector, the 51% attack (see the next section).

A year later, Bayer, Haber, and Stornetta built on their previous work by proposing to use a "Merkel tree" instead of a simple linked list to link all the events. Merkle tree is a simple and efficient data structure, which can calculate a certain hash value from multiple hash values. From a timestamp perspective, this means that one unit of time can hold multiple events. In addition, the authors propose an improvement on the distributed trust model they proposed in 1991, in which a "world championship" was held to determine a single "winner", who would publish the calculated hash in a public place (like a newspaper). Sound familiar?

We can see that newspapers are an excellent example of how to think better about the second property of time: unpredictability.

Causality and unpredictability

Time is not a reality; it is a noema or a metron...

-- Antiphon the Wise, On Truth (3rd century AD)

Causality, while important, is far from enough. Unpredictability is also essential to the passage of time. In the physical world, we describe the passage of time by observing natural processes. We observe that entropy is increasing and we call it the arrow of time. Although for the most part the laws of nature seem to have nothing to do with the arrow of time, some things are actually irrevocable. As the saying goes, a broken mirror is hard to round.

Similarly, the digital world requires an entropy function to create the arrow of time. SHA256 hashes and cryptographic signatures are not unbreakable, but like a broken mirror almost impossible to reassemble, SHA256 and cryptographic signatures are almost unbreakable.

If there is no entropy increase, we can change the timestamp at will. For example, the order of Fibonacci numbers is causal, but not entropy-enhancing. In the Fibonacci sequence, each number is created by adding the first two numbers together. So the Fibonacci sequence is a causal chain. However, the Fibonacci sequence cannot be used to tell time because it is completely predictable. It's like saying a kidnapper can't use a photo of a hostage with a calendar to prove he's alive. We can't use things that are predictable as proof of time. We can only use things that can't be predicted in advance, like the front page of the day's newspaper.

Bitcoin's unpredictability is achieved through transactions and proof of work. Just as no one can predict what will appear in tomorrow's newspaper, no one can predict what the next bitcoin block will look like. You can't predict what transactions will be packaged in a block, because you can't predict what transactions will be broadcast in the future. More importantly, you can't predict who will solve the current proof-of-work puzzle and what the resulting solution will be.

Unlike newspapers, proof of work is directly related to events that have already occurred. Proof of work is not just a record of the event, but the event itself. It is this direct probability-based connection that eliminates the need for trust in proof of work. The only way to find a valid proof of work is to make a lot of guesses, each of which takes a small amount of time. The number of guesses required to find a solution is probabilistic and thus constitutes the time chain of Bitcoin.

Using the causal order of hashing chains and the unpredictability of proof of work, the Bitcoin Network provides a mechanism to create an undisputed history of events. If there is no causal relationship, we cannot distinguish between events. Without unpredictability, the causal order would be meaningless.

As far back as 1992, Bayer, Haber, and Stornetta gave a clear explanation of the kidnappers' practice: "To be certain that a document was created after a certain time, it must record events that occurred but could not be anticipated in advance."

干货 | 比特币就是时钟（下）

Press proof

It is this combination of causality and unpredictability that allows us to artificially define "now" in a digital world that has no notion of time. As Bayer, Haber and Stornetta said in a 1991 paper: "request timestamp of the client's order as well as submit to the hash value is unpredictable. Therefore, if our client requests before signing certificate contains sequence of bits, just know certificate timestamp later than these requests... certificate must be included in the document before the bits of this requirement can also indicate the time from another direction, order, because the timestamp mechanism unable to issue certificates in advance, unless it has a request at the moment."

All the components are here. The genius of Satoshi Nakamoto was to put all these components together, eliminating the need for a timestamp mechanism.

Time to prove

The cause is hidden, but the effect is known.

"-- Ovid, Metamorphosis (8 A.D.)

Let's recap: To spend money in the digital world, we have to rely on ledgers. For the books to be reliable, we need to know the order of transactions. To determine the order, we must use a timestamp. Therefore, if we want to create trustless currency in the digital world, we must remove any entity that creates and manages the timestamp, as well as the single entity responsible for timing.

Genius like Satoshi Nakamoto found a solution: "In order to implement a point-to-point distributed timestamp server, we need to use a proof-of-work system, similar to Adam Back's Hashcash."

The reason we need to use proof-of-work systems is because we need something native to the digital world. Once you understand the informational nature of the digital world, it's clear that computing is all we have. If your world is made up of data, there will be data manipulation.

Proof of work is a peer-to-peer mechanism because it requires no trust. The proof of work is trust-free because it is isolated from any external input, such as a clock or a newspaper. It depends on one thing only: computation cannot be done without the input of work. In our world, generating work takes energy and time.

A bridge to time

I knew I was possessed.

We ran across the bridge -- the burning bridge

The flames raged behind them,

We stand on the brink of death,

Honey, you and I are against the world.

"-- Kate Bush, Burning Bridge (1985)

Without proof of work, we are bound to have problems with the information input mechanism, because the physical world and the information world will always be separated. Sheep are not counted when they are marked on a list, a map is not comparable to a real territory, and the news in a newspaper is not necessarily an actual event. Similarly, even if you use a real-world clock to create a timestamp, that doesn't mean the actual time is the same.

Frankly, we cannot believe that data represent reality, unless it is the reality inherent in the data itself. The genius of Bitcoin's difficult-modifiable proof of work is that it creates its own reality, as well as space and time.

Proof of work is a direct link between the digital and physical worlds. Only this connection is established in an untrusted manner. Everything else depends on external input.

The difficulty of creating bitcoins is adjusted to maintain the connection between Bitcoin time and human time. Like clockwork, the Bitcoin system recalibrates the mining difficulty every time 2,016 blocks are mined. The difficulty adjustment is designed to keep the average block time down to 10 minutes, thus establishing a stable connection between the physical and information worlds. As a result, the timing of Bitcoin's clock needs to be readjusted according to human perception of time. A purely clock-based difficulty adjustment is not feasible because it would completely cut bitcoin off from the human world. The goal of the difficulty adjustment is to prevent people from going too fast (or too slow).

As Einstein taught us, time is not absolute. There is no such thing as universal time. Time is relative; simultaneity does not exist. By this fact alone, all timestamps (especially those that span distant space) are inherently unreliable, even if there is no opposition between participants. (Which, by the way, is why GPS satellite timestamps have to be constantly adjusted.)

For Bitcoin, the fact that the human timestamp is imprecise is not very important. Nor does it matter that there is no absolute frame of reference to begin with. The timestamp needs only to be accurate enough that the calculated block time based on a base of 2,016 blocks is reliable. To ensure this, a block's "physical world" timestamp will only be accepted if the following two criteria are met:

1. The timestamp must be greater than the median timestamp of the previous 11 blocks.

2. The timestamp must be less than the network adjustment time plus two hours. (The "network adjustment time" is the median timestamp returned by all the nodes you are connected to.)

In other words, difficulty tuning is about keeping time constant, not safety, difficulty, or energy consumption constant. The design is ingenious because good money must have a high cost of time, not energy. It's not enough to create absolute scarcity if money is only associated with energy, because once we get better at energy production, we can create more money. Time is the only thing that cannot be regenerated. As Jpian Simon says, time is the ultimate resource. Because of this, Bitcoin is the ultimate form of money, because the number of Bitcoins in circulation is directly related to the ultimate resource of the universe (time).

Difficulty adjustment is crucial. Without difficulty adjustments, Bitcoin's internal clock would run faster and faster as more miners joined the network or miners became more efficient. We will soon run into coordination problems. Once the block-out time falls below a certain threshold (say, 50 milliseconds), the system cannot agree on a shared state, even in theory. It takes about 66 milliseconds for light to travel from one side of the Earth to the other. So even if our computers and routers were perfect, we would be at our wit's end: confronted with two events, there would be no way to determine the order. If we don't periodically adjust the timing of bitcoin blocks, we're in a dead end -- we have to solve the coordination problem faster than the speed of light. Time is also a source of cryptographic instability problems (see Chapter 1). Cryptography works by exploiting an asymmetry of time: it takes only an instant to build a cryptographic wall, but a long time to break it, unless you have a key.

So, in a sense, proof of work and difficulty adjustments are artificially slowing down time, at least from the Bitcoin network's point of view. In other words, Bitcoin is forcefully controlling the internal rhythm, using low frequencies to provide sufficient buffer space for communication delays between peers. Bitcoin's internal clock is reset every time 2,016 blocks are mined. As a result, only one active block is dug every 10 minutes on average.

From the outside, Bitcoin is about aggregating asynchronous messages that are broadcast across the globe into a parallel universe. This parallel universe has its own rules, and a space-time view. From the perspective of the Bitcoin network, transactions in the exchange pool have no temporal properties. Only transactions that are packaged into a valid block are given a time: the block number of the exchange.

干货 | 比特币就是时钟（下）

Bitcoins v0.01 ALPHA (2009).

The brilliance of this solution is beyond words. Once you can define your own time, it's easy to identify the sequence of events. As a result, people can easily agree on what happened, in what order, and who owes whom what.

The difficulty adjustment ensures that Bitcoin's internal metronome hits a constant beat. It is the conductor of the Bitcoin orchestra that can keep the music alive.

But what makes you think that "workload" is ultimately something to rely on? The answer is threefold. First of all, because it takes work to calculate; Generating workload takes time; In our case, work -- guessing random numbers -- is hard to cheat.

Probability based time

Time forks endlessly into innumerable futures.

"-- Borges, Garden of Forking Paths (1958)

Finding a valid nonce for a bitcoin block is a guessing game, akin to rolling a die, flipping a coin or roulette. Essentially, you're looking for an astronomical number. Every guess gets you no closer to the answer. Guess right or try again.

Every time you flip a coin, you have a 50% chance of getting heads or tails -- even if you've flipped it 20 times before and you've all come up heads. Similarly, during bitcoin mining, the probability of a valid block appearing every second is about 0.16%. It doesn't matter when the last block was found. The wait time before finding the next block is always the same: 10 minutes or so.

As a result, each tick of the Bitcoin clock is unpredictable. Compared to the clocks we humans use, the Bitcoin clock seems crude and inaccurate. As Gregory Trubet-Skoy said, "It doesn't matter whether the clock is accurate or not. What matters is that everyone has the same clock and that the state of the whole chain is explicitly related to the timing of the clock." While Bitcoin's clock is probabilistic, it's not an illusion.

Time is an illusion. This is especially true at lunchtime.

"-- Douglas Adams, The Hitchhiker's Guide to the Galaxy (1979)

In the Bitcoin system, however, "now" is definitely an illusion. With no central authority in the network, strange things can happen. Unlikely, but what if two people find a valid block at the same time? (Again, apologies to all physicists.) In other words, two clocks in different locations ticked simultaneously. The content of the two blocks is likely to be different. They have different histories, but they are all equally valid.

This is the so-called chain SPPT (Chain SPPT), is a natural occurrence in the operation of the Nakamoto Consensus. Like a flock of migrating birds, now split into two lines, now join together. Over time, nodes in the Bitcoin network eventually develop a shared history, thanks to the probabilistic nature of guesswork.

The Nakamoto Consensus simply requires that the correct history be on the heaviest chain (i.e., the chain that contains the most proof of work). So if we have two histories, A and B, some miners are going to write history on A, and some miners are going to write history on B. Once the miners on one side find the next valid block, the miners on the other side accept that they are mining in the wrong history and move on to the heaviest chain (by definition, the chain that represents what actually happened). In Bitcoin, history is written by the winners.

The payee needs to have evidence that, for each transaction, the vast majority of nodes agree that they received the transaction first... When there are multiple payment objects in the same transaction, only one is valid. The payee must wait an hour or so before he is convinced that the transaction is valid. At that point, the network will be able to resolve all possible multi-cost races.

-- Satoshi Nakamoto (2009)

The above paragraph reveals the secret of the distributed coordination problem. Nakamoto is through this way to solve the above mentioned "at the same time payment" problem, but once and for all. To hell with relativity!

Because of the probabilistic nature of the Bitcoin clock, the "now" (what we call the top of the chain) is always uncertain. The past (blocks below the top of the chain) is always certain.

To understand more, we have to go back even further.

-- Gordon Clark, A Christian Perspective on People and Things (1951)

As a result, for some peers, the Bitcoin clock sometimes goes back a step or two. If one of your top chains happens to lose to the other, your clock will go backwards and forwards, overwriting the history you thought was correct. If your clock is probabilistic, so is your memory of history.

Tick tock tock -- what time is it?

Tick, tick, tick... It's stopped at C619.

Is that true? Is it going to be slow?

Be it or not: 8 is not wrong before 9.

This clock is wrong. Sometimes it goes backwards.

Punctuality is the devil, centralization is not!

The clock is ticking, ticking, ticking, ticking, ticking.

There's no good in messing around. Tick to the next block.

-- A poem about bitcoin and time (2020)

conclusion

Time remains one of physics's biggest mysteries, raising questions about the very definition of physics itself.

-- Jorge Cham and Daniel Whiteson, "We Know Nothing: A Guide to an Unknown Universe" (2017)

Tracking things in the information world means tracking a series of events, and therefore tracking time. Tracking time requires an agreement on the present (a point in time that forever connects the past with the future). In the Bitcoin system, "now" is the end of the heaviest proof-of-work chain.

The two most important components of the temporal structure are causality and unpredictable events. Causality is used to define the past, and unpredictable events are used to construct the future. If the order of events is predictable, it is possible to skip. If there is no connection between events, it is easy to change the past. Because the Bitcoin system defines internal time, it is extremely difficult to cheat. If one wants to cheat, one must rewrite the past or foresee the future. Bitcoin's time chain protects against both.

If we look at bitcoin in terms of time, it should be clear that "blockchain" (a data structure that connects events through causality) is not a major innovation, or even a new idea. We can see this if we look at the previous literature on timestamps.

A blockchain is just a chain of blocks of data.

Peter Todd -

The innovation of Satoshi Nakamoto was how the parties could independently agree on the history of events without centralised co-ordination. Satoshi Nakamoto found a way to implement a decentralized timestamp mechanism. This mechanism (a) does not rely on timestamp agencies or servers, (b) does not require newspapers or any other physical media as evidence, and (c) ensures that the travel time is essentially constant, even when running in environments with faster CPU clock times.

Timing requires causality, unpredictability, and coordination. In Bitcoin, causality is provided by one-way functions, namely, cryptographic hash functions and digital signatures that lie at the heart of the Bitcoin protocol. Unpredictability is achieved through proof-of-work puzzles interacting with nodes: you can't predict in advance what the other nodes are doing, and you can't predict in advance what the solution of the proof-of-work puzzle will be. The coordination is achieved through difficulty adjustments, and the secret is to link Bitcoin time to human time. Only by connecting the physical world with the information world can we reach consensus on time relying solely on data.

Bitcoin is timeNot only in one way. Bitcoin's smallest unit, satoshi, is time, because it's money; Bitcoin's network is also time, because it's a decentralized clock. Bitcoin has these magical properties because of the tireless running of the Bitcoin clock. Otherwise, the whole Bitcoin system will fall apart. And that's why this incredible piece of Internet money can benefit everyone.