A Mass of Inertia
Source: New Scientist
February 3, 2001
London - What is this thing called mass? Pondering this
apparently simple question, two scientists have come up with a radical
theory that could explain the nature of inertia, abolish gravity and,
just possibly, lead to bizarre new forms of spacecraft propulsion.
Faced with the same question, you might answer that mass
is what makes a loaded shopping trolley hard to get moving -- its
inertia. Or, perhaps, that mass is what makes a bag of sugar or a
grand piano weigh something. Either way, the origin of mass is one of
nature's deepest mysteries.
Some particle physicists claim that a hypothetical
particle called the Higgs boson gives mass to subatomic particles such
as electrons. Late last year, hints that the Higgs really exists were
found at CERN, the European centre for particle physics near Geneva.
So, does the Higgs explain weight and inertia? The answer is probably
no.
Wait a minute. How can these physicists claim they have
discovered the origin of mass when their proposed mechanism fails to
explain the very things that make it what it is? Well, as Bill Clinton
might say, it all depends on what you mean by mass.
When these particle physicists speak of mass, they are
not thinking in terms of inertia or weight. Matter is a concentrated
form of energy. It can be changed into other forms of energy and other
forms of energy can be changed into matter -- an equivalence embodied
in Einstein's famous equation E = mc2. So in this sense, the mass of a
subatomic particle is a measure of the amount of energy needed to make
it. The Higgs can account for that, at least partly (see "Mass
delusion", p 25).
"But the Higgs mechanism does not explain why mass, or
its energy equivalent, resists motion or reacts to gravity," says
Bernard Haisch of the California Institute for Physics and
Astrophysics in Palo Alto. He believes instead that inertia and
gravity are manifestations of far more familiar effects. When you lift
that sack of potatoes or shove your shopping trolley, the forces you
feel might be plain old electricity and magnetism.
If the forces are familiar, their origin is anything
but. For in Haisch's view, they come out of the quantum vacuum. What
we think of as a vacuum is, according to quantum theory, a sea of
force fields. The best understood of all these fields is the
electromagnetic field, and it affects us constantly -- our bodies are
held together by electromagnetic forces, and light is an oscillation
in the electromagnetic field.
That these fields pop up in the vacuum is reflected by
Heisenberg's uncertainty principle, which states that the shorter the
length of time over which an energy measurement is made, the less
precise the result will be. So although the energy of the
electromagnetic field in the vacuum averages to zero over long periods
of time, it fluctuates wildly on very short timescales.
Rather than being empty, the vacuum is a choppy sea of
randomly fluctuating electromagnetic waves. We don't see or feel them
because they pop in and out of existence incredibly quickly, appearing
only for a split second. These fleeting apparitions are called virtual
photons.
But sometimes, virtual becomes real. Stephen Hawking
worked out that the powerful gravity of a black hole distorts this
quantum sea so much that when a virtual photon appears, it can break
free and escape into space, becoming real and visible just like an
ordinary photon. And a fundamental principle of Einstein's theory of
general relativity is that gravity is indistinguishable from
acceleration.
So if gravity can release photons from the vacuum, why
shouldn't acceleration do the same? In the mid-1970s, Paul Davies at
the University of Newcastle upon Tyne and Bill Unruh at the University
of British Columbia in Vancouver realised that an observer accelerated
through the quantum vacuum should be bathed in electromagnetic
radiation. The quantum vacuum becomes a real and detectable thing.
This idea hit Haisch in February 1991, when Alfonso
Rueda of California State University gave a talk about the
Davies-Unruh effect at Lockheed Martin's Solar and Astrophysics
Laboratory in Palo Alto. If an accelerated body sees radiation coming
at it from the front, Haisch thought, that radiation might apply a
retarding force. "I'm an astrophysicist," he says. "So I am used to
the idea that radiation -- for instance, sunlight - can exert a
pressure on bodies such as comet particles."
Rueda said he would do some calculations. Some months
later, he left a message on Haisch's answering machine in the middle
of the night. When Haisch played it back the next morning he heard an
excited Rueda saying, "I think I can derive Newton's second law."
According to Rueda, photons boosted out of the quantum
vacuum by an object's acceleration would bounce off electric charges
in the object. The result is a retarding force which is proportional
to the acceleration, as in Newton's second law, which defines inertial
mass as the ratio of the force acting on an object to the acceleration
produced. Haisch and Rueda, along with their colleague Harold Puthoff
of the Institute for Advanced Studies in Austin, Texas, published
their initial work in February 1994 (Physical Review A, vol 49, p
678).
This electromagnetic drag certainly sounds like inertia.
But do the calculations agree with the known inertial masses of
subatomic particles? Why are quarks heavier than electrons, even
though they have less charge? And why are the particles called muons
and taus heavier than electrons, even though they appear to be
identical in other ways? It might be because they are doing a
different kind of dance.
In deriving his result, Rueda adapted an old idea
proposed by quantum pioneers Louis-Victor de Broglie and Erwin
Schrsdinger. When low-energy photons bounce off electrons, they are
scattered as if the electron were a ball of charge with a finite size.
But in very high-energy interactions, the electrons
behave more as if they are point-like. So de Broglie and Schrsdinger
proposed that an electron is actually a point-like charge which
jitters about randomly within a certain volume.
This can account for both kinds of behaviour: at high
energies, the interaction is fast and the electron appears frozen in
place; at low energies, it is slow, and the electron has time to
jiggle about so much that it appears to be a fuzzy sphere.
Haisch and Rueda believe that de Broglie and
Schrsdinger's idea was on the right lines. The electron's jitter could
be caused by virtual photons in the quantum vacuum, just like the
Brownian motion of a dust particle bombarded by molecules in the air.
"Random battering by the jittery vacuum smears out the electron," says
Haisch.
This is important because Haisch and Rueda suspect that
their inertia-producing mechanism occurs at a resonant frequency.
Photons in the quantum vacuum with the same frequency as the jitter
are much more likely to bounce off a particle, so they dominate its
inertia.
They speculate that muons and taus may be some kind of
excited state of the electron, with a correspondingly higher resonance
frequency. That would probably mean a greater mass, as there are more
high-frequency vacuum photons to bounce off. Quarks might also be
resonating in a different way from electrons.
"If we knew what caused the resonance we would probably
be able to explain the ratio of the various quarks' rest masses to the
electron rest mass," says Haisch. The cause of such excitations might
lie in string theory, which treats particles as tiny vibrating
strings, but this is only conjecture.
If inertial mass is an electromagnetic effect, why does
the neutrino appear to have some mass, even though it doesn't feel
electromagnetic forces? This might be easier to explain. The
electromagnetic field is not the only field in the vacuum. There are
two other force fields: the weak nuclear force and the strong nuclear
force. Both could make contributions to mass in a similar way to the
electromagnetic field.
Neutrinos only feel the weak force, which could explain
their small mass. Quarks feel the strong nuclear force, and that could
affect their mass. It is even possible that strong-force fluctuations
in the vacuum dominate the masses of quarks and gluons. As these
contributions are much harder to work out than the electromagnetic
ones, no one has attempted them yet.
Vacuum-packed
So much for inertia. But what about the force holding
you to the floor? Can the vacuum account for gravitational mass too?
The idea of linking gravity with the quantum vacuum was suggested by
Russian physicist Andrei Sakharov in 1968 and has been developed
recently by Puthoff. Haisch and Rueda's latest project is to connect
this idea with their work on inertia.
It's still highly speculative, but they think they can
explain away gravity as an effect of electromagnetic forces.
Oscillating charges in a chunk of matter affect the charged virtual
particles in the vacuum. This polarised vacuum then exerts a force on
the charges in another chunk of matter. In this rather tortuous manner
the two chunks of matter attract each other. "This might explain why
gravity is so weak," says Haisch. "One mass does not pull directly on
another mass but only through the intermediary of the vacuum."
Einstein's theory of general relativity already explains
gravity beautifully in terms of the warping of space-time by matter,
so this "geometrical" description ought to be compatible with the
quantum-vacuum picture. Haisch points out that the curvature of space
can only be inferred from the bending of the paths of light rays. But
the polarised vacuum would bend light paths, just as a piece of glass
does when light enters or leaves it.
"The warpage of space might be equivalent to a variation
in the refractive index of the vacuum," Haisch conjectures. "In this
way, all the mathematics of general relativity could stay, intact,
since space-time would look as if it were warped." And all the strange
predictions of general relativity, such as black holes and
gravitational waves, would be manifestations of this polarised vacuum.
If they can get their idea to work, Haisch and Rueda
will have a theory of quantum gravity -- the long-sought marriage of
Einstein's general relativity with quantum mechanics. It would finally
allow physicists to understand the first moments after the big bang,
and the crushing singularity at the core of a black hole.
That just leaves rest mass, the kind of mass that's
equivalent to energy. According to Haisch, the Higgs might not be
needed to explain rest mass at all. The inherent energy in a particle
may be a result of its jittering motion, the buffeting caused by
virtual particles in the vacuum.
"A massless particle may pick up energy from it, hence
acquiring what we think of as rest mass," he says. If this were the
case, all three facets of mass would be different aspects of the
battering of the quantum vacuum. "It would be a tidy package."
It may be that there is no explanation for inertial and
gravitational mass. They may just come hand in hand with rest mass.
This is what many particle physicists believe. "Some people think
Haisch and Rueda are on the right track, others think they are on a
wild goose chase," says Paul Wesson, an astrophysicist at the
University of Waterloo in Ontario, Canada.
But if gravitational and inertial mass do emerge from
the vacuum, perhaps we could take control of them. It might be
possible to cancel mass, creating an inertia-less drive that could
accelerate a spaceship to nearly the speed of light in the blink of an
eye.
To do this we would have to exclude quantum fluctuations
from a region where there is matter -- blow a bubble in the vacuum.
Haisch doesn't know if that is possible. "Nature does not abhor a
vacuum," he says. "However, it may abhor a vacuum in the vacuum."
This article appeared in the February 3 issue of New
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by Marcus Chown
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