- The conversation is with Alex Filipenko, an astrophysicist and professor of astronomy from Berkeley.
- He was a member of the Supernova Cosmology Project and the High-Z Supernova Search Team, which discovered the accelerating expansion of the universe and the existence of dark energy.
- Dark energy is believed to be a repulsive force causing the universe to expand.
- The nature of dark energy is not fully understood, but it could be a constant density vacuum energy or a new energy field.
- There are hypotheses for different fields, such as quintessence, and even the possibility of a multiverse.
- The cause of the observed acceleration of the universe's expansion is still uncertain. Dark energy and dark matter may be analogous to Ptolemaic epicycles.

- The concept of retrograde motion was a mathematical scheme used to explain the apparent backward motion of planets in the sky.
- Fourier analysis, which involves representing periodic functions as a sum of sines and cosines, was developed based on this idea.
- Dark energy and dark matter are proposed explanations for certain data, but they may not be accurate.
- Copernicus proposed the heliocentric model of the solar system, but his observations did not significantly improve upon Ptolemy's geocentric model.
- Galileo's observations of Venus and Jupiter provided evidence for the heliocentric model.
- The development of scientific theories is based on data and observations, not just abstract thought experiments.
- Human civilization is unlikely to reach the heat death of the universe, but other celestial threats such as comets and the sun burning out could pose risks.
- Moving to other solar systems may be necessary for survival, but the long-term propagation of human civilization is uncertain.

- The oceans will evaporate within 1 to 2 billion years.
- Asteroid collisions could lead to mass extinctions, but early detection and deflection methods can be effective.
- Comets from the outer regions of the solar system pose a greater threat due to limited warning and unpredictable trajectories.
- Predicting and monitoring the movement of asteroids in the asteroid belt is important.
- Existential threats from stars exploding or the sun getting brighter are rare.
- Civilization-changing events like smaller asteroids or solar flares pose a greater danger.
- Astronomers play a crucial role in detecting and mitigating cosmic threats.
- Human-induced threats like pandemics or natural disasters on Earth are also significant concerns.

- Super volcanoes, such as the one under Yellowstone, could cause a significant drop in temperature and lead to mass extinction or the death of billions of humans.
- Humans have become dependent on technology and infrastructure, making us vulnerable to disasters that could disrupt these systems.
- Space exploration is seen as both a philosophical and practical endeavor, with the need to colonize other planets in order to ensure long-term survival.
- Elon Musk's timeline for colonizing Mars is seen as too optimistic, but his success with reusable rockets is seen as a game changer.
- Mars poses challenges for colonization due to its harsh environment and the need to build habitats and find sustainable sources of nutrition.
- Exoplanets have been discovered around many stars, with a significant number potentially being Earth-like and suitable for human habitation.

- Approximately 1% of stars have planets.
- It would take a rocket about a quarter of a million years to reach the nearest star at Earth's escape speed.
- Machines and robots are more likely to be able to travel long distances in space than humans.
- Artificial general intelligence could possess the qualities that make humans unique.
- Machines could be our evolutionary descendants and carry on our consciousness.
- Traveling close to the speed of light requires an unfathomable amount of energy.
- Wormholes and other theoretical concepts for faster-than-light travel are currently far-fetched or impossible based on current knowledge of physics.

- Non-rotating black holes cannot be traversed due to the singularity
- Rotating black holes offer theoretical ways to pass through and avoid singularity
- Charged black holes are unlikely to exist in nature
- Traversable wormholes in black holes are unlikely due to mass inflation instability
- Traversal of wormholes raises causality violation and grandfather paradox concerns
- Alcubierre drive is theoretical but faces practical difficulties and does not save time
- Extending human life or using suspended animation could make long journeys possible
- Reaching speeds of light or exceeding it requires infinite energy
- Possibility of intelligent life in the universe is uncertain, but our galaxy may not be teeming with it
- Human intelligence is not a natural evolutionary path and may not be a long-term advantage

- The Fermi paradox questions why we haven't detected intelligent civilizations if they are common
- There is no clear evidence of extraterrestrial visitation on Earth
- If intelligent life exists, they could be billions of years ahead of us in technological advancement
- It is possible that they have already colonized the galaxy and are not interested in interacting with us
- The rarity of intelligent life may be the most likely explanation for the Fermi paradox

- Alex Filipenko, an astrophysicist and professor of astronomy, discusses dark energy and its role in the accelerating expansion of the universe.
- Dark energy could be a constant density vacuum energy or a new energy field, and there are hypotheses for different fields such as quintessence.
- The cause of the universe's expansion is still uncertain, and dark energy and dark matter may be analogous to Ptolemaic epicycles.
- The development of scientific theories is based on data and observations, not just abstract thought experiments.
- Human civilization faces threats such as comets, the sun burning out, asteroid collisions, and human-induced disasters.
- Space exploration and colonization of other planets are seen as important for long-term survival, but there are challenges to colonizing Mars.
- Machines and robots may be better suited for long-distance space travel, and artificial general intelligence could possess human-like qualities.
- Faster-than-light travel through wormholes or the Alcubierre drive are currently far-fetched or impossible.
- The possibility of intelligent life in the universe is uncertain, and the Fermi paradox questions why we haven't detected other civilizations.

- The possibility of finding life elsewhere in the universe, even in our lifetimes, is exciting but also potentially bad news if advanced life or a defunct civilization is discovered, as it suggests the "great filter" is ahead of us.
- The Fermi paradox suggests that there may be no other intelligent life out there, which would mean we are special and have made it through the filters.
- Broadcasting our presence to aliens is not a huge existential threat, as any aliens capable of coming to Earth and squashing us would already know about us.
- UFO sightings and the idea that aliens have started showing up since we developed nuclear weapons is intriguing, but there is no tangible evidence to support these claims.
- The scientific community should investigate UFO sightings, but without concrete evidence, it is difficult to take them seriously.
- The psychological tendency to ignore or dismiss strange phenomena, even when witnessed by credible individuals, suggests that if aliens were to show up, we would likely ignore their presence.

- The speaker discusses the conspiracy theory that the US government hides evidence of UFOs
- They believe there may be some legitimacy to protecting military secrets, but also see an opportunity to inspire people about science
- They quote Carl Sagan, saying that extraordinary claims require extraordinary evidence and welcome further investigation
- The speaker acknowledges that there may be conventional explanations for sightings, such as military or surveillance purposes
- They mention the possibility of slow-speed space travel but express skepticism about active aliens altering their trajectory in real-time
- The speaker encourages others to study UFOs if it is their calling, but personally found the evidence unconvincing and chose not to pursue it
- They believe that if aliens were to show up, there would be plenty of evidence for scientists to study
- The speaker mentions that they have not seen reports from hardcore scientists and astronomers about unexplained UFO sightings
- They discuss the difficulty for scientists in filtering through the large amount of misinformation and decoys in UFO research
- The speaker addresses the expanding universe and explains how the age of the universe is less than the radius of the observable universe due to the expansion of space.

- The expansion of space causes objects to appear to travel faster than the speed of light, but locally they are not exceeding the speed of light.
- The universe started out at a high density but was not a point, it was a hypersphere.
- The universe may be infinite in space, meaning it has no boundary.
- The concept of what happened before the Big Bang and the existence of other universes are hypotheses that are difficult to prove but are not unreasonable based on what we know about physics.

- The idea of the universe coming from nothing and inflating into its current state is based on well-tested physics, although it is not completely testable.
- There are different explanations for the creation of the universe, including quantum fluctuations violating the conservation of energy and the idea that energy isn't conserved in general relativity.
- There is currently no way to directly test what happened before the Big Bang or detect the presence of other universes.
- The lack of a verified quantum theory of gravity makes it difficult to answer questions about the beginning of the universe.
- Claims about information from before the Big Bang or the presence of other universes require extraordinary evidence, which has not been found yet.
- The Nobel Prize was awarded for the discovery of the accelerating expansion of the universe, not for explaining what causes the expansion.
- The expanding universe should be slowing down, but observations show that it is actually speeding up.
- The Nobel Prize has both positive and negative aspects, providing recognition and funding but also highlighting the limitations of individual recognition in team-based research.

- The Nobel Prize is only given to a handful of people, even though large teams of brilliant individuals contribute to experimental fields.
- The Nobel Prize for Physics is not mentioned in Alfred Nobel's will, it is just a tradition.
- The author's team was invited to participate in Nobel Week, but the Nobel Prize itself does not recognize all team members.
- The author believes that science is now done by big teams and the Nobel Prize should be reevaluated to reflect this.
- The Breakthrough Prize in Fundamental Physics recognizes larger teams, but the Nobel Prize still only recognizes a few individuals.
- There is controversy and disagreement over who should receive the Nobel Prize in certain cases.
- The author is unsure if they were one of the three deserving individuals for the Nobel Prize they received.
- The author preferred the team dynamic and culture of the second team they worked with.

- The Nobel Prize sometimes waits for deserving individuals to die before awarding the prize or doesn't give it at all when it's unclear who the recipients should be.
- Jocelyn Bell discovered pulsars but her advisor, Anthony Hewish, received credit as well. She did not let this affect her career and won many other prizes.
- Vera Rubin, one of the discoverers of dark matter, never received the Nobel Prize but was recognized in other ways.
- The unfairness and tension of prize decisions make them more special.
- The lack of diversity in Nobel Prize recipients gives the impression that science is still dominated by white men.
- The Breakthrough Prize and other similar prizes recognize teams and emphasize the contributions of all members.
- Credit assignment is a complex issue, and many important contributors are often forgotten.
- Supernovae are exploding stars that synthesize heavy elements and provide raw material for new stars and life.
- Supernovae can be used as beacons to measure distances in the universe.
- There are different types of supernovae depending on the mass and characteristics of the star.

- Possibility of finding life elsewhere in the universe is exciting but could suggest a "great filter" ahead of us
- Fermi paradox suggests we may be special and have made it through filters
- Broadcasting our presence to aliens is not a big threat
- UFO sightings lack tangible evidence, should be investigated but not taken seriously without proof
- Tendency to ignore or dismiss strange phenomena suggests we may ignore aliens if they show up
- Conspiracy theory of US government hiding evidence of UFOs discussed, potential for inspiring people about science
- Speaker acknowledges conventional explanations for sightings, skeptical about active aliens altering trajectory
- Encourages study of UFOs if interested, but personally found evidence unconvincing
- Difficulty for scientists in filtering through misinformation and decoys in UFO research
- Expansion of space causes objects to appear to travel faster than light, but locally they are not exceeding light speed
- Universe started as a hypersphere, may be infinite in space with no boundary
- Hypotheses about what happened before the Big Bang and existence of other universes difficult to prove but not unreasonable based on physics
- Lack of direct testing for what happened before the Big Bang or presence of other universes
- Lack of verified quantum theory of gravity makes it difficult to answer questions about the beginning of the universe
- Nobel Prize awarded for discovery of accelerating expansion of the universe, not for explaining the cause
- Nobel Prize has positive and negative aspects, recognition and funding but limitations in recognizing team contributions
- Controversy and disagreement over Nobel Prize recipients
- Lack of diversity in Nobel Prize recipients suggests science is still dominated by white men
- Breakthrough Prize and similar prizes recognize teams and emphasize contributions of all members
- Credit assignment is complex, many important contributors often forgotten
- Supernovae are exploding stars that synthesize heavy elements and provide raw material for new stars and life
- Supernovae can be used as beacons to measure distances in the universe
- Different types of supernovae depending on star mass and characteristics.

- Type 1a supernovae are used to study the expansion of the universe
- They come from white dwarf stars, which are incredibly dense
- The fusion reactions inside the white dwarf cause a thermonuclear runaway, leading to an explosion
- Not all type 1a supernovae are the same, they have different luminosities and decline/rise in brightness at different rates
- By calibrating and measuring nearby type 1a supernovae, their power scale and uncertainties can be determined
- Thousands of type 1a supernovae are now available for data analysis
- These explosions can happen in our galaxy, but would not pose an existential threat unless they are within 10 light years
- The explosions give birth to expanding gases that mix with other gases and eventually form new stars and planets
- The elements in our DNA and our bodies come from nuclear reactions in stars and the explosions contribute to the enrichment of gases in the galaxy over time.

- The piece discusses the formation of the solar system and the evolution of life on Earth
- It emphasizes the awe-inspiring nature of this story and the power of science to understand it
- The role of supernovas in the creation of heavy elements is highlighted
- The author mentions Richard Feynman as a mentor and describes his intuitive understanding of nature
- The author also discusses the importance of being able to explain science to the general public
- The author shares memories of Feynman and the impact he had on their teaching style
- The importance of asking questions and encouraging curiosity is emphasized.

- Meaning in life is subjective and varies from person to person
- Setting goals and accomplishing them can give a sense of meaning
- Changing the world is not necessary for a meaningful life
- Helping others often gives a sense of satisfaction and meaning
- The ability to explore and understand the universe gives potential for meaning
- The universe creating intelligent beings like humans is fascinating
- The existence of a multiverse suggests that our universe is rare and interesting
- Earth is not special, there are likely other planets conducive to life
- The conversation with Alex Filippenko was an honor
- Sponsorship acknowledgments and closing remarks

- Type 1a supernovae are used to study the expansion of the universe
- They come from dense white dwarf stars and result from thermonuclear runaway
- Type 1a supernovae vary in luminosity and brightness changes
- Measuring nearby supernovae helps determine their power scale and uncertainties
- Thousands of type 1a supernovae are available for analysis
- Explosions can occur in our galaxy but are not a threat unless within 10 light years
- Supernova explosions contribute to the formation of new stars and planets
- Supernovae enrich gases in the galaxy over time and play a role in the creation of heavy elements
- The article discusses the formation of the solar system and evolution of life on Earth
- Richard Feynman is mentioned as a mentor and the importance of explaining science to the public is discussed
- Memories of Feynman and his impact on teaching style are shared
- The importance of curiosity and asking questions is emphasized
- Meaning in life is subjective and can be found through setting goals, helping others, and exploring the universe
- The existence of a multiverse suggests the rarity and interest of our universe
- Earth is not unique, and there are likely other planets suitable for life
- The author expresses gratitude for a conversation with Alex Filippenko
- The text ends with sponsorship acknowledgments and closing remarks.

Alex Filippenko is an astrophysicist and professor of astronomy at Berkeley. Please support this podcast by checking out our sponsors:
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EPISODE LINKS:
Alex's Website: https://astro.berkeley.edu/people/alex-filippenko/

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OUTLINE:
0:00 - Introduction
2:08 - Universe expansion
3:32 - Dark energy
11:00 - Scientific revolutions
22:50 - Asteroid hitting Earth
26:22 - Giant solar flares and the power grid
33:22 - Elon Musk and space exploration
38:13 - Exoplanets
45:35 - Traveling close to the speed of light
47:45 - Traveling faster than the speed of light
56:11 - Intelligent life in the universe
59:46 - Fermi Paradox
1:09:24 - Finding alien life would be bad news
1:14:20 - UFO sightings
1:27:30 - Universe expansion speed
1:32:14 - The universe is infinite
1:36:30 - What happened before the Big Bang?
1:40:46 - Roger Penrose
1:44:20 - Nobel Prize for the accelerating universe
2:05:55 - Supernova
2:17:19 - The greatest story ever told
2:21:16 - Richard Feynman
2:28:09 - Meaning of life

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The following is a conversation with Alex Filipenko, an astrophysicist and professor of astronomy from Berkeley.

He was a member of both the Supernova Cosmology Project and the High-Z Supernova Search Team, which used observations of the extragalactic supernova to discover that the universe is accelerating and that this implies the existence of dark energy.

This discovery resulted in the 2011 Nobel Prize for physics.

Outside of his groundbreaking research, he's a great science communicator and is 1 of the most widely admired educators in the world.

I really enjoyed this conversation and I'm sure Alex will be back again in the future.

Quick mention of each sponsor, followed by some thoughts related to the episode.

Neuro, the maker of functional sugar-free gum and mints that I use to give my brain a quick caffeine boost.

BetterHelp, an online therapy with a licensed professional.

Masterclass, online courses that I enjoy from some of the most amazing humans in history, and Cash App, the app I use to send money to friends.

Please check out these sponsors in the description to get a discount and to support this podcast.

As a Side note, let me say that as we talk about in this conversation, the objects that populate the universe are both awe-inspiring and terrifying in their capacity to create and to destroy us.

Solar flares and asteroids lurking in the darkness of space threaten our humble, fragile existence here on Earth.

In the chaos, tension, conflict, and social division of 2020, it's easy to forget just how lucky we humans are to be here.

And with a bit of hard work, maybe 1 day, we'll venture out towards the stars.

If you enjoy this thing, subscribe on YouTube, review it with Fat Stars on Apple Podcasts, follow us on Spotify, support on Patreon, or connect with me on Twitter at Lex Friedman.

And now here's my conversation with Alex Filippenko.

Let's start by talking about the biggest possible thing, the universe.

Will the universe expand forever or collapse on itself?

That's 1 of the big questions of cosmology.

And of course, we have evidence that the matter density is sufficiently low that the universe will expand forever.

But not only that, there's this weird repulsive effect, we call it dark energy for want of a better term, and it appears to be accelerating the expansion of the universe.

So if that continues, the universe will expand forever, but it need not necessarily continue.

It could reverse sign, in which case the universe could, in principle, collapse at some point in the far, far future.

So like, in terms of investment advice, if you were to give me, and then to bet all my money on 1 or the other, where does your intuition currently lie?

Well, right now I would say that it would expand forever because I think that the dark energy is likely to be just quantum fluctuations of the vacuum.

The vacuum 0 energy state is not a state of 0 energy.

That is the ground state is a state of some elevated energy which has a repulsive effect to it.

And that will never go away because it's not something that changes with time.

So if the universe is accelerating now, it will forever continue to do so.

LR – And yet, I mean, you so effortlessly mentioned dark energy.

Do we have any understanding of what the heck that thing is?

AC Well, not really, but we're getting progressively better observational constraints.

So, you know, different theories of what it might be predict different sorts of behavior for the evolution of the universe.

And we've been measuring the evolution of the universe now, and the data appear to agree with the predictions of a constant density vacuum energy, a zero-point energy.

But 1 can't prove that that's what it is because 1 would have to show that the measured numbers agree with the predictions to an arbitrary number of decimal places.

And of course, even if you've got 8, 9, 10, 12 decimal places, what if in the 13th 1, the measurements significantly differ from the prediction?

Then the dark energy isn't this vacuum state, ground state energy of the vacuum.

And so, then it could be some sort of a field, some sort of a new energy, a little bit like light, like electromagnetism, but very different from light that fills space.

And that type of energy could in principle change in the distant future.

It could become gravitationally attractive for all we know.

There is a historical precedent to that, and that is that the inflation with which the universe began when the universe was just a tiny blink of an eye old, a trillionth of a trillionth of a trillionth of a second, the universe went whoosh, it exponentially expanded.

That dark energy-like substance, we call it the inflaton, that which inflated the universe, later decayed into more or less normal gravitationally attractive matter.

So the exponential early expansion of the universe did transition to a deceleration, which then dominated the universe for about 9000000000 years, and now this small amount of dark energy started causing an acceleration about 5000000000 years ago.

And whether that will continue or not is something that we'd like to answer, but I don't know that we will anytime soon.

LRS So there could be this interesting field that we don't yet understand that's morphing over time, that's changing the way the universe is expanding.

I mean, It's funny that you were thinking through this rigorously like an experimentalist.

But what about the fundamental physics of dark energy?

Is there any understanding of what the heck it is?

Or is this the kind of the God of the gaps or the field of the gaps?

So like there must be something there because of what we're observing.

I'm very much a person who believes that there's always a cause.

There are no miracles of a supernatural nature, okay?

So, I mean, there are 2 broad categories.

Either it's the vacuum zero-point energy, or it's some sort of a new energy field that pervades the universe.

So if it turns out that it's 1 of these new fields, and there are many, many possibilities, They go by the name of quintessence and things like that.

But there are many categories of those sorts of fields.

We try with data to rule them out by comparing the actual measurements with the predictions.

And some have been ruled out, but many, many others remain to be tested.

And the data just have to become a lot better before we can rule out most of them and become reasonably convinced that this is a vacuum energy.

LRS So there is hypotheses for different fields, like with names and stuff like that?

Generically, quintessence, like the Aristotelian fifth essence, but there are many, many versions of quintessence.

There's even ideas that this isn't something from within this dark energy, but rather there are a bunch of, say, bubble universes surrounding our universe.

And this whole idea of the multiverse is not some crazy madman-type idea anymore.

It's real card-carrying physicists are seriously considering this possibility of a multiverse.

And some types of multiverses could have a bunch of bubbles on the outside, which gravitationally act outward on our bubble because gravity or gravitons, the quantum particle that is thought to carry gravity is thought to traverse the bulk, the space between these different little bubble membranes and stuff.

And so, it's conceivable that these other universes are pulling outward on us.

That's not a favored explanation right now, but really nothing has been ruled out, no class of models has been ruled out completely.

Certain examples within classes of models have been ruled out, But in general, I think we still have really a lot to learn about what's causing this observed acceleration of the expansion of the universe, be it dark energy or some forces from the outside, Or perhaps, I guess it's conceivable that, and sometimes I wake up in the middle of the night screaming, that dark energy, that which causes the acceleration, and dark matter, that which causes galaxies and clusters of galaxies to be bound gravitationally even though there's not enough visible matter to do so.

Maybe these are our 20th and 21st century Ptolemaic epicycles.

So Ptolemy had a geocentric and Aristotelian view of the world, everything goes around Earth.

But in order to explain the backward motion of planets among the stars that happens every year or 2 or sometimes several times a year for Mercury and Venus, you needed the planets to go around in little circles called epicycles, which themselves then went around Earth.

And in this part of the epicycle where the planet is going in the direction opposite to the direction of the overall epicycle, it can appear in projection to be going backward among the stars, so-called retrograde motion.

And it was a brilliant mathematical scheme.

In fact, he could have added epicycles on top of epicycles and reproduce the observed positions of planets to arbitrary accuracy.

And this is really the beginning of what we now call Fourier analysis, right?

Any periodic function can be represented by a sum of sines and cosines of different periods, amplitudes, and phases.

So it could have worked arbitrarily well, but other data show that, in fact, Earth is going around the sun.

So are dark energy and dark matter just these band-aids that we now have to try to explain the data, but they're just completely wrong.

And as a scientist, I have to be open to that possibility as an open-minded scientist.

How do you put yourself in the mindset of somebody that, or a majority of the scientific community, or a majority of people believe that the Earth, everything rotates around Earth,

the mindset of somebody that, or a majority of the scientific community, or a majority

of people believe that the Earth, everything

rotates around Earth, how do you put yourself in that mindset and then take a leap to propose a model that the sun is in fact at the center of the solar system?

I mean, so that puts us back in the shoes of Copernicus, right?

500 years ago, where he had this philosophical preference for the Sun being the dominant body in what we now call the solar system.

The observational evidence in terms of the measured positions of planets was not better explained by the heliocentric Sun-centered system.

It's just that Copernicus saw that the Sun is the source of all our light and heat.

And He knew from other studies that it's far away.

So the fact that it appears as big as the moon means it's actually way, way bigger, because even at that time, it was known that the sun is much farther away than the moon.

But the observed positions of planets at the time in the early to mid 16th century under the heliocentric system was not a better match, at least not a significantly better match than Ptolemy's system, which was quite accurate and lasted 1,500 years.

That's so fascinating to think that the philosophical predispositions that you bring to the table are essential.

So like you have to have a young person come along that has a weird infatuation with the sun.

That almost philosophically is, however their upbringing is, they're more ready for whatever the simpler answer is.

It's sad from an individual descendant of ape perspective because then that means like me, you as a scientist, you're stuck with whatever the heck philosophies you brought to the table, and you might be almost completely unable to think outside this particular box you've built.

Right, this is why I'm saying that, as an objective scientist, 1 needs to have an open mind to crazy-sounding new ideas.

And even Copernicus was very much a man of his time and dedicated his work to the Pope.

The Sun was a little bit off center it turns out, and a slightly off center circle looks like a slightly eccentric elliptical orbit.

So then when Kepler, in fact, showed that the orbits are actually in general ellipses, not circles, the reason that he needed Tuco Brahe's really great data to show that distinction was that a slightly off-center circle is not much different from a slightly eccentric ellipse.

And so, there wasn't much difference between Kepler's view and Copernicus's view, and Kepler needed the better data, to Cobras data.

And so that's, again, a great example of science and observations and experiments working together with hypotheses and they kind of bounce off each other.

They play off of each other and you continually need more observations.

And it wasn't until Galileo's work around 1610, that actual evidence for the heliocentric hypothesis emerged.

It came in the form of Venus, the planet Venus, going through all of the possible phases from new to crescent to quarter to gibbous to full to waning gibbous, third quarter waning crescent, and then new again.

It turns out in the Ptolemaic system with Venus between Earth and the Sun, but always roughly in the direction of the Sun, you could only get the new and crescent phases of Venus.

But the observations showed a full set of phases.

And moreover, when Venus was gibbous or full, that meant it was on the far side of the Sun.

That meant it was farther from Earth than when it's crescent, so it should appear smaller, and indeed it did.

So that was the nail in the coffin in a sense.

And then Galileo's other great observation was that Jupiter has moons going around it, the 4 Galilean satellites.

And even though Jupiter moves through space, so too do the moons go with it.

So first of all, Earth is not the only thing that has other things going around it.

And secondly, Earth could be moving as Jupiter does, and things would move with it.

We wouldn't fly off the surface and our moon wouldn't be left behind and all this kind of stuff.

So that was a big breakthrough as well, but it wasn't as definitive in my opinion as the phases of Venus.

Perhaps I'm revealing my ignorance, but I didn't realize how much data they were working with.

So there's, so it wasn't Einstein or Freud thinking in theories.

It was a lot of data and you're playing with it and seeing how to make sense of it.

So it isn't just coming up with completely abstract thought experiments.

It was based in part on Galileo's observations of balls rolling down inclined planes, supposedly falling off the leaning tower of Pisa, but that's probably apocryphal.

In any case, the Inquisition actually did, or the Roman Catholic Church, did history a favor, not that I'm condoning them, but they placed Galileo under house arrest.

And that gave Galileo time to publish, to assemble and publish the results of his experiments that he had done decades earlier.

It's not clear he would have had time to do that, you know, had he not been under house arrest.

And so, Newton, of course, very much used Galileo's observations.

Let me ask the old Russian overly philosophical question about death.

So we're talking about the expanding universe.

How do you think human civilization will come to an end if we avoid the near-term issues we're having?

Do you think we have a shot at reaching the heat death of the universe?

Yeah, so we're gonna leave out the anthropogenic

Which I actually think are greater than the celestial causes.

So, no way will we as humans reach the heat death of the universe.

I mean, it's conceivable that machines, which I think will be our evolutionary descendants, might reach that, although even they will have less and less energy with which to work as time progresses, because eventually even the lowest mass stars burn out, although it takes them trillions of years to do so.

So the point is that certainly on Earth, there are other celestial threats, existential threats, comets, exploding stars, the sun burning out.

So we will definitely need to move away from our solar system to other solar systems.

And then the question is, can they keep on propagating to other planetary systems sufficiently long?

In our own solar system, the sun burning out is not the immediate existential threat.

That'll happen in about 5 billion years when it becomes a red giant.

Although I should hasten to add that within the next 1 or 2 billion years, the sun will have brightened enough that unless there are compensatory atmospheric changes, the oceans will evaporate away.

And you need much less carbon dioxide for the temperatures to be maintained roughly at their present temperature, and plants wouldn't like that very much.

So you can't lower the carbon dioxide content too much.

So within 1 or 2000000000 years, probably the oceans will evaporate away.

But on a sooner timescale than that, I would say an asteroid collision leading to a potential mass extinction, or at least an extinction of complex beings such as ourselves that require quite special conditions unlike cockroaches and amoebas to survive.

1 of these civilization changing asteroids is only 1 kilometer or so in diameter and bigger, and a true mass extinction event is 10 kilometers or larger.

Now, it's true that we can find and track the orbits of asteroids that might be headed toward Earth.

And if we find them 50 or 100 years before they impact us, then clever applied physicists and engineers can figure out ways to deflect them.

But at some point, some comet will come in from the deep freeze of the solar system.

And there we have very little warning, months to a year.

Oh, the deep freeze is sort of out beyond Neptune.

There's this thing called the Kuiper Belt.

And it consists of a bunch of dirty ice balls or icy dirt balls.

It's the source of the comets that occasionally come close to the sun.

And then there's a even bigger area called the scattered disk, which is sort of a big donut surrounding the solar system way out there from which other comets come.

And then there's the Oort cloud, W-O-O-R-T, after Jan Oort, a Dutch astrophysicist.

And it's the better part of a light year away from the sun, so a good fraction of the distance to the nearest star, but that's like a trillion or 10 trillion comet-like objects that occasionally get disturbed by a passing star or whatever, and most of them go flying out of the solar system, but some go toward the Sun and they come in with little warning.

By the time we can see them, they're only a year or 2 away from us.

And moreover, not only is it hard to determine their trajectories sufficiently accurately to know whether they'll hit a tiny thing like Earth, but outgassing from the comet of gases when the ices sublimate, that outgassing can change the trajectory just because of conservation of momentum, right?

Gases go out in 1 direction, the object moves in the other direction.

And so, since we can't predict how much outgassing there will be and in exactly what direction, because these things are tumbling and rotating and stuff, it's hard to predict the trajectory with sufficient accuracy to know that it will hit.

And you certainly don't want to deflect a comet that would have missed, but you thought it was gonna hit and end up having it hit.

That would be like the ultimate Charlie Brown, you know, goat instead of trying to be the hero, right?

What would you do if, it seemed like in a matter of months that there is some non-zero probability, maybe a high probability that there would be a collision?

So from a scientific perspective, from an engineering perspective, I imagine you would actually be in the room of people deciding what to do.

Because if you only have a few months, that's not much time in which to deflect it.

Early detection and early action are key.

Because when it's far away, you only have to deflect it by a tiny little angle.

And then by the time it reaches us, the perpendicular motion is big enough to, you know, to miss Earth.

All you need is 1 radius or 1 diameter of the Earth, right?

That actually means that all you would need to do is slow it down so it arrives 4 minutes later, or speed it up so it arrives 4 minutes earlier, and Earth will have moved through 1 radius in that time.

So it doesn't take much, but you can imagine if a thing is about to hit you, You have to deflect it 90 degrees or more, right?

And you have to slow it down or speed it up a lot, if that's what you're trying to do to it.

And so decades is sufficient time, but months is not sufficient time.

So at that point, I would think the name of the game would be to try to predict where it would hit.

And if it's in a heavily populated region, try to start an orderly evacuation, perhaps.

But that might cause just so much panic that I'm, I mean, how would you do it with New York City or Los Angeles or something like that, right?

I might have a different opinion a year ago.

I'm a bit disheartened by, you know, in the movies, there's always extreme competence from the government.

Right, but we expect extreme incompetence, if anything, right?

But sort of from a medical perspective, I think you're saying there, and a scientific 1, it's almost better to get better and better, maybe telescopes and data collection, to be able to predict the movement of these things, or like come up with totally new technologies that you can imagine actually sending out like probes out there to be able to sort of almost have little finger sensors throughout our solar system to be able to detect stuff.

Yeah, Monitoring the asteroid belt is very important.

And 99% of the so-called near earth objects ultimately come from the asteroid belt.

And so there we can track the trajectories.

And even if there's, you know, a close encounter between 2 asteroids, which deflects 1 of them toward Earth, it's unlikely to be on a collision course with Earth in the immediate future.

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Alex Filippenko: Supernovae, Dark Energy, Aliens & the Expanding Universe | Lex Fridman Podcast #137