LAST month, I wrote about the challenge of explaining space-time. To give people a feeling for what I mean when I say that space-time is curved, sometimes I talk about a rubber sheet with a ball sitting on it. I point out that the ball causes the sheet to curve and the curvature of the sheet literally shapes where the ball will roll. I say we can think of this as an analogy for our local star – the sun – curving space-time around it. How is the sun able to do this? Because it has mass.
Increasingly, I think it is strange (and perhaps lucky for me) that no one ever follows up by asking: “But what is mass?” We science communicators can take it as a given that people have an intuition for the meaning of mass (that it is the amount of stuff available for gravity to pull on) and that our only task is to help people understand how it is distinct from weight (a measurement of the force that results from local gravity pulling on a massive object).
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But perhaps audiences should be unsatisfied with this. I certainly feel I could be doing a better job. So I checked an authoritative text: my favourite dictionary.
The first thing I learned is that my sense of “mass” isn’t listed first in the Oxford English Dictionary. That is reserved for the religious practice. Mass, in the physical sense, is labelled “n.2”. And within this entry, there are, unbelievably, nine multi-part definitions.
2a defines mass as: “A coherent body of matter of unspecified or indeterminate shape, and usually of relatively large bulk; a solid and distinct object occupying space.” My physicist’s intuition suggests this is a generalisation of definition 5b, which states: “Physics. The quantity of matter which a body contains, as measured by its acceleration under a given force or by the force exerted on it by a gravitational field; an entity possessing mass. Strictly distinguished from weight, although colloquially the two terms are often used interchangeably.”
I might modify this to say that mass is a statement of the quantity of matter which a body contains and that will experience acceleration due to any force applied to it. Isaac Newton’s second law even gives an equation that tells us that the amount of force an object is experiencing is proportional to how massive it is. But this raises some questions. Does mass exist outside of how an object is affected by forces? Where does mass come from? And what counts as matter?
Let’s start with a simple piece of information we feel confident about: an object’s mass is intrinsic. In other words, mass is a property the object carries around with it, wherever it goes and whatever it is interacting with. Of course, mass can change. I just finished eating an avocado, and now the leftover skin and pit are less massive and I am more massive because I transferred the flesh of the fruit into my body. This hints at another fundamental property: usually, mass is conserved. The avocado’s innards didn’t just disappear, they transferred to a different location, and my body will use a lot of this to generate energy that I will use to do things like write this column.
Even if I have convinced you that mass is inherent to matter, this raises the question of why the traditional definitions feel unsatisfactory and even tautological. Force is proportional to mass, and mass is a quantity whose measurement exists in relation to forces. Here, particle physics comes to our rescue.
Sort of. The nice thing about particle physics is that I can now say that a massive object, like an avocado, is made up of more fundamental constituents. There are compounds, which are made of molecules, which are made of atoms, which are made of protons, electrons and neutrons. Electrons are indivisible elementary particles, unlike protons and neutrons, which are composed of different combinations of up and down quarks, which, in turn, are elementary particles. So, an avocado gets a lot of its mass from the mass of electrons and the masses of up and down quarks. There is an additional element to mass: the binding energies that hold these particles together. Special relativity teaches us that mass and energy are fundamentally equivalent.
If elementary particles aren’t, in effect, made of other things, why do they have mass? The answer in the case of most particles in the standard model, our best description of nature at this scale, is related to the Higgs boson. The boson is one manifestation of the Higgs mechanism, which interacts with leptons like the electron as well as quarks in a way that leads to them developing the inherent property of mass. For now, I believe this is the best explanation of why objects have mass. But it is incomplete. We can’t show how the Higgs mechanism works for neutrinos, another elementary particle. As a theoretical physicist, this is a delight: massive problems like this are what we live for.
Chanda’s week
What I’m reading
Rosamund Bartlett’s translation of one of my favourite novels, Leo Tolstoy’s Anna Karenina.
What I’m watching
I recently caught up on every episode of reality TV show Vanderpump Rules so I too could understand Scandoval (a much-discussed relationship revelation in the series).
What I’m working on
I’ve started drafting my second book, The Edge of Space-Time, which gets its title from a textbook co-authored by Stephen Hawking.
Chanda Prescod-Weinstein is an assistant professor of physics and astronomy, and a core faculty member in women’s studies at the University of New Hampshire. Her research in theoretical physics focuses on cosmology, neutron stars and particles beyond the standard model
