The Doomsday Argument

SPEAKER_1 (0:00)

This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (0:30)

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Matt O'Dowd (1:00)

Since the dawn of humanity, around 100 billion people have lived. How many will live in the future of our species? We might hope for a trillion times that if we colonize the galaxy.

But a simple statistical argument tells us that the doom of our species is much, much closer.

In some recent episodes we've explored the anthropic principle and seen how it can be used to explain the fact that both our planet and our universe seem very finely tuned to allow the development of life. Our planet and or universe can be rare and unlikely as long as there are enough other planets and or universes to stack the odds in favor of our existence. We touched on both the potential power and the potential misuse of this principle. Today we're going to push our luck and see how this controversial idea can be used to predict the physics of our universe and also to predict the imminent demise of the human race. To use the anthropic principle properly we need to use it carefully. We talked about weaving in the Copernican principle, which tells us that all else being equal we should expect to be in a pretty typical part of the universe. This is seemingly the opposite notion to the anthropic principle, but together we get something that we can use. Our existence selects for a particular type of potentially rare observer supporting environment, but we should expect to find ourselves in the most typical or likely type of environment that is consistent with our existence. In 1987, physicist Steven Weinberg used this form of anthropic reasoning to estimate the value of the cosmological constant. As a quick refresher, the cosmological constant defines the amount of dark energy, which is the stuff causing the expansion of the universe to accelerate. A number of theories in physics, from string theory to eternal inflation, predict that the cosmological constant could take on different values in different universes, and its value could be extremely high compared to this universe. Up to 120 orders of magnitude higher, according to the crudest predictions of quantum field theory. Getting a low cosmological constant is extremely unlikely according to these pictures.

But it's a good thing ours is low, because otherwise our universe would have blown itself up too quickly for stars and life to ever form. This is a good candidate for the anthropic principle. Perhaps we're just in one of the lucky universes with a low cosmological constant, because where else could we be? But if that's true, then we should be in the most common type of lucky universe. Under the assumption that lower cosmological constants are less likely than higher ones, Steven Weinberg reasoned that our universe should have the maximum cosmological constant.

SPEAKER_1 (4:00)

This episode is brought to you by Accenture. When your advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

SPEAKER_2 (4:30)

Experience a membership that backs what you're building with American Express Business Platinum. Enjoy complimentary access to the American Express Global Lounge Collection and apply to find out your welcome offer, which could be as high as 300,000 membership rewards points. American Express Business Platinum. There's nothing like it. Terms apply. Welcome offers vary and you may not be eligible for an offer. Learn more at americanexpress.com/business-platinum.

Matt O'Dowd (5:00)

A system that would allow galaxies to form and hence allow our existence. He got a value that was a factor of 10 higher than what would be observed 10 years later when dark energy was discovered. However, before that discovery even happened, Weinberg realized he'd made a mistake. He had assumed that we should be in the most common type of universe that allows our existence. But he realized that we should instead assume that we are the most common type of observer. He refined his estimate of the cosmological constant to be the value that most astronomers across the multiverse would observe. With a few assumptions, like that the number of astronomers in a given universe is proportional to the amount of mass that ends up forming galaxies in that universe. Weinberg calculated that the density of dark energy should most typically be observed to be around 5 to 10 times the density of matter. That was in the early to mid 90s, right before dark energy was actually discovered and found to be around 3 times that of matter. So he was off by a factor of 2, which considering the method and the fact that dark energy could potentially have spanned many many orders of magnitude is pretty amazing. Weinberg's refinement of the anthropic principle to talk about observers rather than environments is essentially using the self-sampling assumption, defined by philosopher Nick Bostrom as follows.