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What are the strong and weak anthropic principles? Do they resolve the fine-tuning problem?
Models for the big bang theory imply that the initial conditions of the universe would have had to be precisely right for habitable planets to form. Stephen Hawking argues that this need for fine-tuning indicates a problem with the big bang theory. He discusses anthropic principles in an effort to address the problem.
Read more to learn about the anthropic principles.
The Anthropic Principles
Hawking presents two different types of anthropic principles—weak and strong. He ultimately concludes that the approach of the anthropic principles doesn’t completely resolve the fine-tuning problem.
The Weak Anthropic Principle
The weak anthropic principle (WAP) states that you observe the universe having the right parameters to allow your existence because if it had different parameters that didn’t allow you to exist, you wouldn’t be here to observe it. Hawking acknowledges that the weak anthropic principle is true, but points out that it doesn’t really explain why the universe is the way it is.
(Shortform note: Although he doesn’t say so explicitly, Hawking’s dissatisfaction with the WAP may stem from the fact that it doesn’t satisfy his requirements for a theory. Recall Hawking’s earlier discussion of what makes a good theory: It should be consistent with past observations, and make testable predictions about future observations, such that it can be proven false if it doesn’t accurately describe reality. The weak anthropic principle is certainly consistent with past observations, since scientists have never observed a universe in which they cannot exist, but it makes no testable predictions, and it can’t be proven false. As such, it is more of a philosophical truism than a scientific theory.)
The Strong Anthropic Principle
The strong anthropic principle (SAP) postulates that either there are an infinite number of universes, each with slightly different parameters and laws of physics, or else our universe is infinitely large, with slightly different parameters and laws of physics in each region of the universe, such that the right conditions for human life have to occur somewhere. Naturally, human observers are only present where the conditions of the universe permit them to exist.
| Differentiating Between the Weak and Strong Anthropic Principles Hawking’s original presentation of the strong and weak anthropic principles makes it challenging to distinguish between the two, because he introduces the WAP with a suggestion that the universe is “large or infinite.” This is misleading, because the WAP doesn’t really depend on the size of the universe. Instead, the WAP merely points out that you observe the part of the universe where you exist having characteristics that support your existence because if it didn’t, you wouldn’t be there to observe it. Meanwhile, only the SAP requires that there be an infinite number of universes or places in the universe with different characteristics, such that at least one of them will inevitably support your existence. This distinction eventually comes out in Hawking’s discussion, but the way he initially defines the WAP and the SAP makes them sound almost identical. |
Hawking rejects the strong anthropic principle because it doesn’t fit with the scientific method. Specifically, one of the fundamental principles of science is that the laws of physics are uniform throughout the universe. For example, if you find that a certain theory works well on earth, but not on the moon, then you need to modify the theory to work everywhere. This rules out the version of the SAP that postulates the laws of physics are different in different parts of the universe.
Meanwhile, Hawking explains that for other universes to have different laws of physics, they must be completely separate from our own: It’s impossible to send matter, energy, or information back and forth. Since you can’t possibly observe them or interact with them, whether they exist or not is irrelevant.
| The SAP and the Gambler’s Fallacy Although Hawking doesn’t bring it up by name, his argument is essentially that the SAP commits a version of the “Gambler’s Fallacy.” Imagine a game where you pay a certain amount of money to roll a die, and if you roll a six, you win the pot of money. The first time you play, you know that the odds of rolling a six are one out of six (assuming the dice aren’t loaded). Thus, on average, you’d expect to win about once every six times you play. However, what if you’ve already played five rounds, and lost every time? Does that mean you’re guaranteed to win the next round? No, your odds of winning the next round are the same as ever: one out of six. The “Gambler’s Fallacy” is the idea that your past rolls somehow influence the probability of your next roll, such that they will average out in the end. Similarly, Hawking argues that other universes, if they exist, are completely independent of our own. Separate universes can’t interact, and thus influence each other, just like your past rolls of the dice don’t influence your future rolls. So the existence (or non-existence) of other universes ultimately has no effect on the probability of our universe being suitable for human life. |
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- The search for a theory that explains the history and evolution of our universe
- Stephen Hawking's discussions about time, space, dimensions, and quantum theory
- How time travel would theoretically work
