Book: We Have No Idea

There are the known knowns, the known unknowns and the unknown unknowns, as Rumsfeld famously put it, or "we don't know what we do not know," as everyone else already knew.

This book is about the known unknowns, the mysteries of science like dark matter and dark energy, gravity, the speed of light and whether we're alone in the universe.  It's written for the layman, like me, using small words and lots of cartoons.  I'm anxious to dive into it and learn what I don't know.

Jorge Cham and Daniel Whiteson wrote:

Peter Higgs and several other particle physicists posited that if you add one more particle (the Higgs boson) and its field (the Higgs field) to the equations then mass as a particle label -- and why some particles [quarks] have more [mass] than others -- start to make sense.

Roughly, the theory goes like this: imagine a field that permeates the entire universe.  This field does something no other field does: rather than attracting or repelling anything, it [affects] inertial mass.  The more the field interacts with a particle, the more that particle has inertia, or has mass.  It goes one step further and suggests that the inertia generated by a particle interacting with this field is the particle's mass.  That's what it means to have mass.  Some particles feel this field very strongly; these particles have a lot of mass.  Other particles hardly feel this field so they have almost no mass.  According to the Higgs theory, that's what mass is.

This is HUGELY important.  Mass -- and gravity -- are therefore manifestations of this inertial field.  Dark matter is matter with high interactions with the inertial field but low reactions with everything else.  This explains the universe's mass paradox, the speed of light paradox, the age of the universe paradox....

HOLY CRAP.  This is central to any understanding of the nature of the universe. I never knew what the Higgs boson and field were all about before.

Quote :

Ralph H. Scheicher
Uppsala University

Starting from the Higgs field, is it obvious that the inertial mass should be equal to the gravitational mass? Equivalence of inertial and gravitational masses is something that follows from general relativity, if I understand correctly. But in particle physics, the Higgs field is responsible for giving the building blocks of matter their mass (okay, and plus whatever internal binding energy they posses ending up in m=E/c^2). Is it then in any way clear or obvious that this mass, which follows from the coupling strength to the Higgs field is the same as what goes into gravitational forces? Or is it an unresolved question, since no working quantum theory of gravitation exists yet (gravitation speculation?).


Hans Arnold Winther
University of Oxford

Yes the equivalence principle is the principle underlying General Relativity, or one can also say GR was constructed to reproduce this principle.
No, there is no known mathematical reason (like a consistency constraint) why all matter fields should couple universally to gravity. This is not the case for the other fundamental forces and this is not the case for the Higgs field itself, which is why different particles has different masses.
The principle of equivalence is open to experiments and the reason its so fundamental today is that it has stood the tests of these experiments. The University of Washington has an experiment (Eot-Wash) which tests the equivalence principle on different masses in the lab and there is also a 'mirror' on the moon (the Lunar Laser Ranging experiment) used to test it with the moon, earth and sun being the test-masses. See
i) http://www.npl.washington.edu/eotwash/EquivalencePrinciple
ii) http://en.wikipedia.org/wiki/Lunar_Laser_Ranging_experiment
The result of these precise experiments can (loosely speaking) be summarized as:
|mass(gravity)/mass(inertial) - 1| < 10^(-13)
which is a very tight constraint. Would be nice though if this principle followed 'from the math', but it would be even better if one of these experiments found a deviation from it as this signals new physics!


Frank Heile
Stanford University

The principle of equivalence is one of the assumptions on which general relativity is based. The Higgs Boson and the Higgs fields are predictions of the standard model of particle physics which incorporates special relativity. The Higgs field is only responsible for the rest mass of elementary Fermions in the standard model. Thus the Higgs is what gives electrons, quarks and other elementary Fermions their rest masses. The mass of the proton is mostly from the binding energy of the color force which is carried by gluons so it is not directly the result of the Higgs mechanism. Hoever, since special relativity is part of the standard model, and since E=mc^2, you could say that the standard model implies that rest mass from the Higgs mechanism and binding energy from the color force will both contribute equivalently to inertial rest mass of all particles.
However, gravitation and general relativity is not included in the standard model and therefore the standard model can say nothing about gravitational mass. Presumably whatever theory correctly combines gravitation with the standard model will be able to explain the equivalence of the inertial mass and the gravitational mass and we all anxiously await that unified theory!


Jean-René Cudell
University of Liège

I want to point out that gravity does not couple to the "gravitational mass" but rather to the energy-momentum tensor, i.e. 16 numbers. So there is really no relation between "gravitational mass" and "inertial mass", except in Newtonian physics. This is why photons (with zero inertial mass) are affected by gravity.
Now, the standard model does ot include gravity, but one can build the standard model and include classical gravity (field theory in curved spacetime) and that leads to fascinating effects like Hawking radiation and the Unrih effect (see the book on Winitsky's page for a wonderful account: (https://sites.google.com/site/winitzki/). This is where gravitation and the standard model can meet.


Emanuele Fiandrini
INFN - Istituto Nazionale di Fisica Nucleare

Ralph,
inertial and gravitational masses are two things that in principle are completely different and distinct.
What the diffference is between the two? Inertial mass measures the "inertia", that is the tendency of a body to oppose to any variation of its velocity, while the gravitational mass is the coupling strength to the gravitational field; the gravitational mass plays the same role as the electric charge for elm interactions, the color charge for strong interactions and the particle flavour for weak interactions.
Quantitatively (let do a completely classical argument): inertial mass M_i is the the mass in Newton's law F = M_i x a; gravitational mass M_g is the coupling strength to gravitational field in the gravitational Newton's law F_g = (GM/R^2)X M_g. Then, M_i x a = F_g = (GM/R^2)X M_g. The quantity is parenthesis ( ) is the gravitational field (say "G") and M_g is the "gravitational charge", so that one can write: F_g = M_g x G, as one writes, for instance, for the electric field M_i x a = q x E.
What makes the gravity so special is that we find *experimentally* that M_i = M_g, so that all the bodies move exactly in the same way in a gravitational field (ie have the same acceleration), irrespective of their "mass". We don't know why M_i = M_g.
The equivalence principle or the GR covariance principle says exactly this.
Now coming to your question: the underlying gauge symmetries that describe the fundamental interactions require the fundamental fields to be massless. Higgs field is supposed to give a mass to fundamental particles, as quarks, leptons and gauge fields through some mechanism (the Higgs mechanism of spontaneous symmetry breaking). This term is the one appearing in the motion's equation of the field particle, that is M_i (ie it is M_i in the classical limit). If now we put the particle in a gravitational field, then it will feel a "force" given by "the gravitational charge" times the gravitational field. In principle there is no reason why the gravitational charge and the inertial mass should be equal.
Maybe we are still missing something fundamental (ie a quantistic theory of gravitation).

Source

Though deceptively light-hearted and modest, this book raises some truly profound and mind-boggling questions. Alas, there are no answers, just hints.

• Assuming the speed of light is constant -- which the authors do but I do not -- we can see 13.8 billion years in every direction, without seeing the edge of the universe. This means the universe is both older than 13.8 byo and at one point expanded faster than the speed of light. Remember stars did not form at the beginning of the universe.
  
• "there are parts of the universe that are only now starting to feel one another gravitationally because the effects of gravity are also limited by the speed of light" -- the authors toss off this statement without elaboration or proof. I'm still pondering its implications and whether it must be true. For instance we know photons, which are massless and travel at C, are bent by gravity. Would this be true if gravity could not get to the photon before the photon entered the gravitational field?
  
• The authors contend that if the universe is infinite, then the Big Bang could have occurred everywhere at the same time. There would be no need for a point singularity at the start. I'm having a hard time wrapping my head around that. If the universe is infinite, why does it need a beginning?

In the last chapter, the authors take a stab at the Drake Equation. They conclude there's too much uncertainty, too many unknowns for any meaningful predictions but their gut guess is that we're alone in the (visible) universe.