Sean Carroll


American Physicist, Theoretical Cosmologist, Senior Research Associate in the Department of Physics at the California Institute of Technology, and Author

Author Quotes

Your memory isn?t as good as you think. When you remember an event in the past, your brain uses a very similar technique to imagining the future. The process is less like ?replaying a video? than ?putting on a play from a script.? If the script is wrong for whatever reason, you can have a false memory that is just as vivid as a true one. Eyewitness testimony, it turns out, is one of the least reliable forms of evidence allowed into courtrooms.

The multiverse, by itself, doesn't offer an explanation for every cosmological fine-tuning problem. If a parameter needs to be smaller than a certain value for life to exist, there's no anthropic reason for it to be much smaller than that value. We therefore have a prediction: anthropically-selected parameters should be of the same order of magnitude as the largest value compatible with the existence of life. Indeed, this prediction was successfully made by Steven Weinberg for the vacuum energy, over a decade before it was actually discovered.

This is not a universe that is advancing toward a goal; it is one that is caught in the grip of an unbreakable pattern.

The particular aspect of time that I'm interested in is the arrow of time: the fact that the past is different from the future. We remember the past but we don't remember the future. There are irreversible processes. There are things that happen, like you turn an egg into an omelet, but you can't turn an omelet into an egg.

This leaves us with 73% of the universe in an even more mysterious form ? "dark energy." Once the expansion of the universe was discovered, Einstein's original motivation for introducing the cosmological constant evaporated. But the idea didn't go away, and physicists later realized that this parameter had a very natural interpretation ? the energy density of empty space, or "vacuum energy" for short. In 1998 two groups of astronomers made a surprising discovery: the universe is not only expanding, but accelerating ? distant galaxies are moving away from us faster and faster over time.[4] This is contrary to our expectation that the gravitational pull between galaxies should slow the expansion down. The most straightforward explanation for this acceleration is to posit dark energy ? a smooth, persistent form of energy that isn't localized into particles, but is spread throughout space. Vacuum energy, or Einstein's cosmological constant, is the simplest candidate for dark energy; it features a density that is strictly constant, unchanging through space or time. But more complicated models are possible, and cosmologists are currently working hard to test the hypothesis that the dark energy density is truly a constant. If it is, we can predict the future of the universe ? it will expand forever, gradually cooling and diluting away until nothing is left but empty space.

The past and future are equally real. This isn?t completely accepted, but it should be. Intuitively we think that the ?now? is real, while the past is fixed and in the books, and the future hasn?t yet occurred. But physics teaches us something remarkable: every event in the past and future is implicit in the current moment. This is hard to see in our everyday lives, since we?re nowhere close to knowing everything about the universe at any moment, nor will we ever be ? but the equations don?t lie. As Einstein put it, ?It appears therefore more natural to think of physical reality as a four dimensional existence, instead of, as hitherto, the evolution of a three dimensional existence.?

Time exists. Might as well get this common question out of the way. Of course time exists ? otherwise how would we set our alarm clocks? Time organizes the universe into an ordered series of moments, and thank goodness; what a mess it would be if reality were complete different from moment to moment. The real question is whether or not time is fundamental, or perhaps emergent. We used to think that ?temperature? was a basic category of nature, but now we know it emerges from the motion of atoms. When it comes to whether time is fundamental, the answer is: nobody knows. My bet is ?yes,? but we?ll need to understand quantum gravity much better before we can say for sure.

The point, I take it, is to understand how nature works. Part of that is knowing how to do calculations, but another part is asking deep questions about what it all means. That?s what got me interested in science, anyway. And part of that task is understanding the foundational aspects of our physical picture of the world, digging deeply into issues that go well beyond merely being able to calculate things. It?s a shame that so many physicists don?t see how good philosophy of science can contribute to this quest. The universe is much bigger than we are and stranger than we tend to imagine, and I for one welcome all the help we can get in trying to figure it out.

We are part of the universe that has developed a remarkable ability: We can hold an image of the world in our minds. We are matter contemplating itself.

The problem with "creation from nothing" is that it conjures an image of a pre-existing "nothingness" out of which the universe spontaneously appeared ? not at all what is actually involved in this idea. Partly this is because, as human beings embedded in a universe with an arrow of time, we can't help but try to explain events in terms of earlier events, even when the event we are trying to explain is explicitly stated to be the earliest one. It would be more accurate to characterize these models by saying "there was a time such that there was no earlier time." To make sense of this, it is helpful to think of the present state of the universe and work backwards, rather than succumbing to the temptation to place our imaginations "before" the universe came into being. The beginning cosmologies posit that our mental journey backwards in time will ultimately reach a point past which the concept of "time" is no longer applicable. Alternatively, imagine a universe that collapsed into a Big Crunch, so that there was a future end point to time. We aren't tempted to say that such a universe "transformed into nothing"; it simply has a final moment of its existence. What actually happens at such a boundary point depends, of course, on the correct quantum theory of gravity.

When society puts some small fraction of its wealth into asking and answering big questions, it reminds us all of the curiosity we have about our universe. And that leads to all sorts of good places.

The reason why science and religion are actually incompatible is that, in the real world, they reach incompatible conclusions. It?s worth noting that this incompatibility is perfectly evident to any fair-minded person who cares to look. Different religions make very different claims, but they typically end up saying things like ?God made the universe in six days? or ?Jesus died and was resurrected? or ?Moses parted the red sea? or ?dead souls are reincarnated in accordance with their karmic burden.? And science says: none of that is true. So there you go, incompatibility.

While the Big Bang model ? the picture of a universe expanding from a hot, dense state over the course of billions of years ? is firmly established, the Big Bang itself ? the hypothetical singular moment of infinite density at the very beginning ? remains mysterious. Cosmologists sometimes talk about the Big Bang, especially in popular-level presentations, in ways that convey more certainty than is really warranted, so it is worth our time to separate what we know from what we may guess.

The strength of the electromagnetic interaction, for example, is fixed by a number called the fine-structure constant, a famous quantity in physics that is numerically close to 1/137.

While we don't claim to understand the absolute beginning of the universe, by the time one second has elapsed we enter the realm of empirical testability. That's the era of primordial nucleosynthesis, when protons and neutrons were being converted into helium and other light elements. The theory of nucleosynthesis makes precise predictions for the relative abundance of these elements, which have passed observational muster with flying colors, providing impressive evidence in favor of the Big Bang model. Another important test comes from the cosmic microwave background (CMB), the relic radiation left over from the moment the primordial plasma cooled off and became transparent, about 380,000 years after the Big Bang. Together, observations of primordial element abundances and the CMB provide not only evidence in favor of the basic cosmological picture, but stringent constraints on the parameters describing the composition of our universe.

The success of primordial nucleosynthesis gives us confidence that we understand what the universe was doing about one second after the Big Bang, but anything before that is necessarily speculative. Even the formulation "one second after the Big Bang" should really be interpreted as "one second after what would be the moment of infinite curvature in the most straightforward extrapolation to earlier times." But there are different degrees of speculation.

You live in the past. About 80 milliseconds in the past, to be precise. Use one hand to touch your nose, and the other to touch one of your feet, at exactly the same time. You will experience them as simultaneous acts. But that?s mysterious ? clearly it takes more time for the signal to travel up your nerves from your feet to your brain than from your nose. The reconciliation is simple: our conscious experience takes time to assemble, and your brain waits for all the relevant input before it experiences the ?now.? Experiments have shown that the lag between things happening and us experiencing them is about 80 milliseconds.

The world is not magic ? and that?s the most magical thing about it.

The world keeps happening, in accordance with its rules; it's up to us to make sense of it and give it value.

Then we compare the predicted abundance of such a WIMP with the actual abundance of dark matter.

There are (at least) two popular mechanisms to obtain a multiverse. One is the many-worlds or Everett interpretation of quantum mechanics; I won't discuss this idea here, because the various "branches of the wave function" describing different worlds all share the same basic laws of physics. The other kind of multiverse is in some sense more prosaic, in that it simply posits regions of space-time outside our observable horizon, in which conditions are very different ? including, in principle and often in practice, the parameters specifying the laws of physics, such as the mass of the neutron or the vacuum energy. This latter scenario has garnered a great deal of attention in recent years, in part because it seems to be a natural outcome of two powerful ideas that were originally pursued for other reasons: inflationary cosmology, and superstring theory. Inflation uses the fact that dark energy makes the universe accelerate, but posits an initially small region of space filled with a temporary form of super-dark-energy at an enormously high density. This causes this small region to grow to fantastic size, before the dark energy ultimately decays. In many versions of the theory, the decay isn't complete, and at least some region is always undergoing ultra-fast inflationary expansion.[18] From string theory we get the idea of a "landscape" of possible vacuum states. A "vacuum state" is simply a configuration of empty space with an associated set of physical laws. That is, what we think of as spacetime comes in a variety of phases, much like water can be in solid, liquid, or gaseous forms. In string theory there seems to be a mind-boggling number of possible phases (over 10500), each characterized by different physical constants, including the set of elementary particles and the number of macroscopic dimensions of space.

There are actually three points I try to hit here. The first is that the laws of physics underlying everyday life are completely understood. There is an enormous amount that we don?t know about how the world works, but we actually do know the basic rules underlying atoms and their interactions ? enough to rule out telekinesis, life after death, and so on. The second point is that those laws are dysteleological ? they describe a universe without intrinsic meaning or purpose, just one that moves from moment to moment. The third point ? the important one, and the most subtle ? is that the absence of meaning ?out there in the universe? does not mean that people can?t live meaningful lives. Far from it. It simply means that whatever meaning our lives might have must be created by us, not given to us by the natural or supernatural world. There is one world that exists, but many ways to talk about; many stories we can imagine telling about that world and our place within it, without succumbing to the temptation to ignore the laws of nature. That?s the hard part of living life in a natural world, and we need to summon the courage to face up to the challenge.

There is no reason, within anything we currently understand about the ultimate structure of reality, to think of the existence and persistence and regularity of the universe as things that require external explanation. Indeed, for most scientists, adding on another layer of metaphysical structure in order to purportedly explain these nomological facts is an unnecessary complication. This brings us to the status of God as a scientific hypothesis.

There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.

They then build detectors that patiently wait for the faint signal of a dark-matter particle passing through and perturbing a nucleus.

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American Physicist, Theoretical Cosmologist, Senior Research Associate in the Department of Physics at the California Institute of Technology, and Author