The astrophysicist and social media phenom Katie Mack is ready to tell
you about the fate of the universe.
"I don’t think I’m a gloomy person,”
Katie Mack said. She just likes
thinking about the end — the annihilation of Earth, the solar system, our
galaxy and especially the universe. Apocalyptic topics that can put even these
uncertain times into perspective. “The destruction of the whole universe:
There’s nothing bigger and more dramatic than that,” she said.
Change is in the nature of her career. As she began her undergraduate studies
at the California Institute of Technology, cosmologists were processing the
1998 discovery that some mysterious entity called “dark energy” was pushing
galaxies apart from one another. While working toward her Ph.D. at Princeton
University, the first results from the Wilkinson Microwave Anisotropy Probe
(WMAP) came out, providing “our first really detailed accounting of the
contents of the universe,” she said. “Since WMAP was partly led by people at
Princeton, it was a big part of life there, and hugely exciting; I felt like I
was right at the ground floor on some of the most exciting discoveries in
cosmology.”
In her academic career — she’s currently on the faculty at North Carolina
State University — she investigates the nature of dark matter, the physics of
the early universe, the evolution of galaxies and the nature of black holes.
But she’s most widely known as a science communicator and social media star.
On Twitter, her
@AstroKatie account has over
350,000 followers. And her upcoming book, The End of Everything
(Astrophysically Speaking), tells the stories of true-life astronomical
apocalypses. (In an ironic twist, the release of her book about cosmic
cataclysms had to be delayed until August because of the coronavirus
pandemic.)
Quanta Magazine caught up with her via videoconference during her time as a
Simons Emmy Noether Fellow at the Perimeter Institute in the spring. An edited
and condensed version of the conversation follows.
Why is it important to study the universe, even at a time when there are
more urgent problems that require the attention of scientists?
Well, it’s not like every person on earth can suddenly become an
epidemiologist or a doctor or a respiratory expert and put all of their
energy into that. We can’t all just drop everything and become different
people. I know that there are a lot of physicists trying to do some kind of
disease modeling, trying to put their skills toward that effort. And I don’t
know if anything will really come of that or not — but I think that there’s
a limited scope for that sort of respecialization.
But in terms of “why think about the universe when the world is difficult
and things are happening?” — Well, why have the arts, why have music? Why
literature? It’s part of the human condition to be curious and to want to
understand the universe in every possible way. Part of what makes us human
is those kinds of questions, that kind of curiosity, that kind of joy. And I
think that we can’t — we can’t just shear all that away because things are
bad.
Your book looks at how astrophysical objects — including the entire
universe — come to an end. What made you want to investigate this?
I think it’s just — it’s the biggest, most dramatic thing that you can
think of. The destruction of the whole universe: There’s nothing bigger and
more dramatic than that. I like those big questions. I like things that are
kind of hard to imagine, but that could have consequences that are just
impossibly huge. Those are a lot of fun.
In your book, you talk about your influences, and you mention Stephen
Hawking in particular. How did his work affect you?
I read A Brief History of Time, and I saw some documentaries about Hawking.
And I was just so fascinated by all these mind-bending things — like black
holes and the Big Bang and time travel — you know, really mind-boggling
topics. And just the idea that these are things you can actually study and
learn something about, and understand in a quantitative, concrete way — I
thought that was amazing.
I did meet him, actually; I guess I was probably 15. Hawking was giving a
talk at Caltech. And my mom took me and a friend of mine to go see the
lecture. As we were leaving the lecture theater, somehow we ended up going
the same direction that Hawking was. So we sort of ran into him on the walk
to the car. And I was too shy to say anything. But my friend went up to him
and said: “My friend would like to say something to you.” And so I went and
I said something like, “I really admire your work.” And he said, “Thank you
very much.” So that was my first encounter with him.
Many of us now interact with scientists on social media. It’s probably safe
to say that more people read your tweets than will ever take a college-level
physics course. What is it like to be a scientist in the public eye?
I am keenly aware of the responsibility that comes with having a platform,
and having a voice, and then having an influence on people. And the more
visibility you have, the more responsibility you have to ensure that what
you’re saying is responsible and not harmful in whatever way. I’m very aware
of that, and I worry about that all the time.
I’ve also had a few people tell me that — that I inspired them one way or
another. And that’s been very affecting, certainly. And a little scary. And
I’ve been told by some people that seeing a youngish woman in this field has
given them the confidence, as women, to approach it themselves. I’ve heard
that from a few young women in high school or college.
Let’s talk about the long-term fate of the universe. For many years, if you
picked up a cosmology textbook, it would list three possible fates — that it
might expand forever; that it might collapse in a “big crunch”; or, in a
sort of borderline case, that it might expand forever but just barely. How
have those options changed?
Those are the three possibilities that make sense in a universe without a
cosmological constant, or without dark energy. But what we learned in the
late ’90s was that there is something that is causing the expansion to
accelerate. So it’s very hard to see how a big crunch would happen.
Is this where the idea of the “heat death” of the universe comes in?
Yes. The heat death of the universe is the end state of a universe that’s
ruled by accelerated expansion forever. Every gravitationally bound system —
galaxies, clusters of galaxies — gets more and more isolated from one
another. And then each one ends up alone, and everything else gets carried
farther and farther away such that they lose contact. So in the ultimate
future, if we’re heading toward a heat death, our little group of galaxies,
the Local Group, will be isolated. We won’t be able to see other galaxies at
some point. We won’t even see evidence of the Big Bang, because we won’t see
anything else that’s out there. And as that carries on, eventually star
formation halts, because there’s no new material being brought in. The stars
you have burn out. A lot of things fall into black holes, then the black
holes evaporate. Particles decay. And if you leave that alone long enough,
eventually you get a universe where the only thing that’s left is a few
strange particles and some radiation.
And in some sense this is the end of time?
From a physics perspective, if you define the arrow of time to be the
direction of increasing entropy, once you reach the heat death, the arrow of
time ceases to exist. And if there’s no arrow of time, I don’t know what the
point of time is anymore.
In your book you mention other possible fates, like the “big rip.”
If you have a kind of dark energy where the energy density is not constant,
but is increasing over time, then — then you get this, this thing called
phantom dark energy. And that leads to this horrific destruction of the
universe in a finite time. Dark energy will start to overwhelm the
gravitational binding of every galaxy. And the dark energy in this room is
going to start overwhelming the binding of the stuff in this room. It starts
to pull apart things that should not otherwise be affected by the expansion
of space. So space itself is destroyed, basically.
How seriously do physicists take this scenario?
I think that people don’t take it seriously just because it’s hard to
envisage a fundamental theory that would make that happen and be consistent
with other things we assume to be true about the universe. But in terms of
the data, we can’t rule it out, and we may never be able to rule it out.
As terrifying as the big rip is, vacuum decay is perhaps even more
terrifying.
Vacuum decay is my favorite, for sure, for a few reasons. One, because it
is a very dramatic kind of idea. But also because it seems to come right out
of left field.
Vacuum decay is the idea that the universe that we are living in is not
fully stable. We know that when the universe started, it was in this very
hot, dense state. And we know that the laws of physics change with the
ambient temperature, the ambient energy. We see that in particle colliders.
We see that if you have a collision of high enough energies, then the laws
of physics are a little bit different. And so, in the very early universe —
the first tiny few microseconds or whatever it was — it went through a
series of transitions. And after one of those transitions, we ended up in a
universe that has — that has electromagnetism and the weak nuclear force and
the strong nuclear force and gravity. It created the laws of physics that we
see today.
And if that’s true, then some disturbance of our current universe could end
up sort of kicking the laws of nature into a different state in which the
laws of physics are different, with different fundamental particles and
fields. If that happens, then at the point in space where that occurs, you
get a bubble of this “true” vacuum, this other vacuum state of the universe,
forming. And that bubble would expand at about the speed of light throughout
the universe and destroy everything it encounters.
Do physicists take the idea of vacuum decay more seriously or less
seriously than the big rip?
Much more seriously, because it doesn’t violate any fundamental principles
we know of. However, I would say that most physicists, although we take it
very seriously, we mostly don’t believe it’s going to happen. And the reason
for that is that the way you get to vacuum decay as a possibility is to say
that the Standard Model of particle physics — our current understanding of
how particle physics works — is the whole story. [Editor’s note: Most
physicists believe that the Standard Model is not the whole story.] So a lot
of people would say, you know, this is an interesting problem and we do take
it very seriously — but we think it’s not going to happen.
And from our point of view, the decay would be instantaneous?
It’s painless; you don’t see it coming. You don’t notice when it happens,
because you can’t feel it. You can’t see it. And then you don’t exist
anymore. It’s technically inconsequential, right? It’s hard to think of it
as being too tragic, in that sense.
If the universe is fleeting, how is all of the stuff that we do worth
doing?
That’s a huge thing that I’ve wrestled with in the course of writing this
book, and I don’t think I came to a solid conclusion. It’s different from a
personal death, because people think about their own death and they think,
well, I’ll live on in some way through my children or my great works, or
just the impact I had on the people around me. There will be some legacy to
my existence in some way. But if it’s the whole cosmos that’s ending, that
is no longer true. I think there’s a point at which you did not matter. And
I don’t think we have the emotional or philosophical tools to wrestle with
that.
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