Fundamental laws - Astronomical reach

Friday, Jan 31, 2014

 Unraveling the Mysteries of the Invisible Universe (with science writer Simon Mitton, Princeton University Press, 2013). Photo by Denise ApplewhiteWe live in a world of very small things (atoms) and very large things (stars, galaxies). How can the same laws of nature describe such different objects? Two people who have given the matter some thought are Adam Burrows and Jeremiah Ostriker, both professors of astrophysical sciences at Princeton University.

Their new paper, "Astronomical reach of fundamental physics," published this week in the Proceedings of the National Academy of Sciences, explains how fundamental physical laws can describe objects that are as small as an atom or as massive as a galaxy. This exercise illustrates the unifying power of physics and the profound connections between the small and the large in the cosmos we inhabit. The fundamental laws of nature, the researchers say, have amazing consequences.

Professors Ostriker and Burrows spoke with interviewer Catherine Zandonella of the Office of the Dean for Research at Princeton University.

List to the interview in this podcast or read the transcript below.

Download the podcast (.mp3) for later listening.



What made you decide to look at the astronomical reach of fundamental physics?

Adam Burrows' interests range from the theory of supernova explosions to the atmospheres of extra solar planets, brown dwarfs, and high-energy astrophysics. (Photo by Keren Fedida)Adam Burrows: One of the things you notice when you are doing general physics is that things are connected in ways that people don't always recognize. You study quantum mechanics or relativity; you are focused on various aspects of the small or the fast, etc.

But when you do astrophysics, you look at these things fairly broadly, and you incorporate the disparate realms of physics that you find in the laboratory, and apply it, in as many ways as you can, in the large.

What you find is that those things involved with the very small — such as quantum mechanics and atoms and molecules and nuclei — and those that deal with the very large things, and even those with which we are familiar, having to do with gravity and the gravitational attractions, when combined together, actually can inform just about everything in the Universe.

This particular paper was motivated by many things, but one thing at least in my mind was to bring together the simple arguments that show that there's a unity to science, and that the things that the people in the physics department might study — whether involved with Planck's constant or the speed of light, or the charge of the electron, things that involve fairly small objects — actually can explain in some detail the largest things in the Universe: stars in particular, but also galaxies, clusters of galaxies.

Life itself in principle can be explained in simpler terms, and it is that simplicity that underlies the complexity around us that we wanted to articulate as best we could.

Jeremiah Ostriker: I agree with all of that but would give quite a different answer.

If you take books that were written in the first half of the 20th century, about science, they often gave back-of the envelope arguments — simple arguments to explain how many atoms there were in a star, how many atoms there would be in the Universe. And people were used to thinking in very simple order-of-magnitude ways, and that informed physics, and that informed science. (Enrico) Fermi was famous for asking questions of his students, so they could give really simple answers.

It has gone out of fashion. Right now the students think that the answers only come out of gigantic calculations on supercomputers.

The details may come from that, but the essential elements have to be simple. All masses have to be related to one other: they have to be so many protons, or so many electrons.

And so we thought it would be useful to go over again the early work that people have done on how you can understand things in a very, very simple way, and also to update it with new things that have been discovered.

So, why are most galaxies between here and here, there are none bigger than this, and none smaller than that? There must be some good reason for that, and if you think about it, there are good reasons for it.

Burrows: I like that answer, too.

What made the two of you decide to look at this together?

Ostriker: We have both done calculations in the past. I'm not sure either of us has published them.

Burrows: No, that is true.

Ostriker: So we talked about it, and then we thought it would be fun to write a paper together.

Burrows:  These sorts of questions having to do with astronomical objects and the basic physical underpinnings can be addressed, and there is a whole tradition of making this connection.

And in this paper we wanted to bring together many of these arguments, update them, and provide them to the cognizant audience that might be interested in them. In fact, we found quite a bit of interest.

Ostriker: It is interesting that stars can only exist in a certain range. They cannot be less than this or more than that. Planets, ditto: if you make a planet bigger it starts to burn and becomes a star — it is not a planet anymore. If you take a galaxy and try to make it bigger, it becomes a cluster of galaxies, not a galaxy. If you try to make it smaller than that, it seems to blow itself apart.

To understand why things are as they are, is what the Greeks understood as science and what we still do, and to bring these arguments together seemed valuable.

What does it tell us about what the Universe is? What reality is? What we are?

Burrows: One of the things that it tells us is that in fact the Universe is quantifiable. There are natural laws that apply in the small and the large. There is a detailed understanding of the mechanism of the Universe.

That sounds a little grand, but everywhere we look we can explain with physical principles. Whether they are applied to small objects or large objects, we can apply these principles to determine many of their properties and understand them in great detail.

The Universe and the world follow natural laws and are explainable and are quantifiable on all scales.

Ostriker: So, if you go out at night with your child or grandchild and you see the stars, and your child asks why are they that brightness? Why are they bright enough for us to see? Why aren't they so bright that they burn out our eyes? Why isn't the Earth 100 times bigger in size? These are the questions that bright kids might ask.

Burrows: I hope that bright kids will eventually read these articles, because there is a whole tradition of doing these simple studies to connect things. And it is "connection" that is the important word in all of this.

People sometimes have lost the connectivity of the various sciences. They stovepipe things into chemistry, and biology, and aspects of physics, but what an astrophysicist does and should attempt to do at all times is to integrate these different disciplines to solve the problems that he or she encounters.

And you can do so. You can bring the statistical physics and quantum mechanics and relativity and gravitational physics together, as the Universe does effortlessly, to explain things that wouldn't have otherwise been explained, but with simple arguments.

It is the simplicity of the basic arguments that underlie many of these objects that we wanted to articulate and communicate.

Ostriker: (Subrahmanyan) Chandrasekar, who was my teacher, said there is a maximum mass for a white dwarf, a type of star. Well, nobody has ever found one higher than that mass, so he was right, there is a simple argument for it.

So, in many of these cases, the understanding was sufficiently good that people were able to make predictions, and then, all the coincidences that the observer would notice, you could understand them.

And that's the magic.

Burrows: The "Chandrasekar mass" to which Jerry is referring can be derived in terms of Avogadro's number, which we associate with chemistry; Planck's constant, which we associate with quantum mechanics and the systematics of the small; the speed of light; and Newton's gravitational constant.

You bring these things together, you shake appropriately, and you can explain this particular phenomenon, and see that Chandrasekar was perfectly right. But you can see where it comes from, fundamentally, at the nexus of many of the great tributaries of physics over the last 100 years.

Do you think it will be possible to explain different aspects of life?

Ostriker: Biology has tended to be an observational science and deriving things from first principles has not been possible in the past but I hate to predict the future on that.

Burrows: It may well be that we have enough physical knowledge, but biology is so complex, and we have to unravel the complexity.

Think of all the progress that has been made on the genome and the connectome and all of the big data, bioinformatic revolutions that people hear about. This is an indication that people are starting to come to grips with the complexity that is inherent in life.

What do you think these findings mean for the average person?

Ostriker: If we had wanted to write this for high school students, I think we could have. You can do it all with high-school math. You need elementary math but not more.

Burrows: This is a way of reaching across what has been a divide, to encourage the view that science is an integrated and broad enterprise, and everyone contributes.

The paper, "Astronomical reach of fundamental physics," by Adam S. Burrows and Jeremiah P. Ostriker, was published in the Proceedings of the National Academy of Sciences (Early Edition) on January 29, 2014. doi: 10.1073/pnas.1318003111. PubMed ID24477692.