Scientists say Albert Einstein’s principle of the constancy of
the speed of light holds up under extremely tight scrutiny, a
finding that rules out certain theories predicting extra
dimensions and a “frothy” fabric of space.

The finding also demonstrates that basic ground- and space-
based observations of the highest-energy gamma-rays, a form of
electromagnetic energy like light, can provide insight into
the very nature of time, matter, energy and space at scales
extremely far below the subatomic level — something few
scientists thought possible.

Dr. Floyd Stecker of NASA’s Goddard Space Flight Center in
Greenbelt, Md., discussed the implications of these findings
in a recent issue of Astroparticle Physics. His work was based
partly on an earlier collaboration with Nobel laureate Sheldon
Glashow of Boston University.

“What Einstein worked out with pencil and paper nearly a
century ago continues to hold up to scientific scrutiny,” said
Stecker. “High-energy observations of cosmic gamma-rays don’t
rule out the possibility of extra dimensions and the concept
of quantum gravity, but they do place some strict constraints
on how scientists can go about finding such phenomena.”

Einstein stated that space and time were actually two aspects
of a single entity called space-time, a four-dimensional
concept. This is the foundation to his theories of special and
general relativity. For example, general relativity posits
that the force of gravity is the result of mass distorting
space-time, like a bowling ball on a mattress.

General relativity is the theory of gravity on a large scale;
quantum mechanics is the theory of the atom and subatomic
particles on a very small scale. Quantum mechanics does not
describe gravity but rather the other three fundamental
forces: electromagnetism (light), strong forces (binding
atomic nuclei), and weak forces (seen in radioactivity).

Scientists have long hoped to meld these theories into one
“theory of everything” to describe all aspects of nature.
These unifying theories — such as quantum gravity or string
theory — may involve the invocation of extra dimensions of
space and also violations of Einstein’s special relativity,
such as the speed of light being the maximum attainable
velocity for all objects.

Stecker’s work involves concepts called the uncertainty
principle and Lorentz invariance. The uncertainty principle,
derived from quantum mechanics, implies that at the subatomic
level virtual particles, also called quantum fluctuations, pop
in and out of existence. Many scientists say space-time itself
is made up of quantum fluctuations which, when viewed up
close, resemble a froth or “quantum foam.” Some scientists
think a quantum foam of space-time can slow the passage of
light. The foam would slow higher-energy light particles, or
photons — such as X-rays and gamma-rays — more than lower-
energy photons of visible light or radio waves.

A fundamental variation in light speed would violate Lorentz
invariance, the basic principle of special relativity. Such a
violation could be a clue to unification theories. Scientists
have hoped to find Lorentz invariance violations by studying
gamma-rays coming from the farthest reaches of the visible
universe, where the quantum foam of space may act to slow
light traveling to us for billions of years.

Stecker looked much closer to home to find that Lorentz
invariance is not being violated. He analyzed gamma-rays from
two relatively nearby galaxies about half a billion light-
years away with supermassive black holes at their centers,
named Mkn 421 and Mkn 501. Some of these galaxies’ gamma-rays
collide with infrared photons in the universe. These
collisions result in the destruction of the gamma-ray and
infrared photons, as their energy is converted into mass in
the form of electrons and positively charged antimatter-
electrons (called positrons), according to Einstein’s famous
formula E=mc 2.

Stecker and Glashow have pointed out that evidence of the
annihilation of the highest-energy gamma-rays, obtained from
direct observations of Mkn 421 and Mkn 501, demonstrates
clearly that Lorentz invariance is alive and well and not
being violated. If Lorentz invariance were violated, the
gamma-rays would pass right through the extragalactic infrared
fog with insufficient energy to cause annihilation.

“The implication is if Lorentz invariance is violated, it is
at such a small level — less than one part in a thousand
trillion — it is beyond the ability of our present technology
to find,” Stecker said. “These results may also be telling us
the correct form of string theory or quantum gravity must obey
the principle of Lorentz invariance.”

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