Three Roads To Quantum Gravity

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THREE ROADS TO QUANTUM GRAVITY
The lightest particle that can be created in this way is
called a pion. Using the basic laws of physics, including
Einstein’s special theory of relativity, one can work out a
simple prediction about the processes by which cosmic ray
protons and photons from the cosmic microwave background
interact to make pions. The prediction is that there
is a certain energy—called a threshold—above which this
is very likely to happen. A proton above this energy will
continue to interact in this way, losing energy each time,
until it is slowed down enough that its energy falls below
the threshold.
This works something like a 100% tax. Suppose there
were some income, say $1 billion, above which all income
would be taxed at 100%. Then no one would ever earn above
$1 billion a year, because 100% of their income above this
amount would be taxed. Our case is like a 100% tax on energy,
as all the energy that a cosmic ray proton may have
above the threshold will be removed, through processes that
produce pions by its interacting with the cosmic microwave
background.
This formula dictates that cosmic ray protons cannot hit
the earth with an energy greater than the threshold energy.
There is ample time in the protons’ journey for any additional
energy to be siphoned off in creating multiple pions.
I want to emphasize that this formula derives from the
well-tested laws of special relativity—the results should
therefore be very reliable. Thus, when this prediction was
proposed by three Russian physicists with the names of
Greisen, Zatsepin, and Kuzmin in the 1960s, it was very well
received in the scientific community. Researchers had no
reason to believe that cosmic ray protons would ever be seen
with energies greater than the threshold.
Convincing as it was, Greisen, Zatsepin, and Kuzmin’s
prediction turned out to be wrong. In the last several years,
many cosmic rays have been seen with energies greater than
the threshold. This startling piece of news has galvanized
scientists in the field. It is called the Ultra High Energy Cosmic
Ray, or UHECR, anomaly.