Time changes might cancel some cosmic calculations
Posted: April 1, 2002

Coupling two well known but previously uncombined scientific concepts might render moot certain prevailing assumptions used in studying the physics of fast moving particles and the universe in which they move.

Physicists have long understood but never considered the interplay between the finite accuracy of clocks and how travelling at speeds approaching the speed of light distorts time, according to Dr. Richard Lieu, an associate physics professor at The University of Alabama in Huntsville (UAH).

His 'timely' analysis is the topic of a research paper scheduled for publication in the April 1 print edition of "Astrophysical Journal Letters."

"Time is a basic parameter of all of our experiments," said Lieu. "The energy of a moving particle, its frequency, is measured in cycles per second. And Planck time controls the accuracy of our clocks. Our clocks can't be more accurate than Planck time."

In physics, the smallest measureable interval of time is Planck time, five times ten to the minus forty fourth seconds. (Forty four zeros on the right hand side of the decimal point, then a five.) This represents the limit of how accurate our clocks can be.

But perceived time depends on an object's speed relative to the clock doing the measurements. For someone riding along with the cosmic ray, time would seem to move normally, while clocks in a stationary laboratory would appear to move much slower. From the cosmic ray's frame of reference, the laboratory clock's Planck time -- the limit of its accuracy -- would get longer. From the cosmic ray's frame of reference, the laboratory clock becomes less accurate.

"This means that when it comes to using our laboratory clock to measure the 'time' of a fast moving particle, the inaccuracies involved will be much larger than the Planck time itself," said Lieu.

An observer standing in a laboratory or floating in a spacecraft might detect an object moving at incredible speed, then try to decipher how much energy that particle carried. The problem, says Lieu, is that an observer in a lab can't accurately gauge how fast or slow time is flowing on that fast-moving object, and this will affect the way we can measure the properties of the object, including its energy.

"The faster the relative motion (the greater the difference between the speeds of two objects), the more inaccurately you are able to determine the conditions of the particle," he said.

Some of the problems astrophysicists might face due to this inaccuracy are seen in the mystery of the most powerful cosmic rays ever detected. Cosmic rays are presumed to be the nuclei of atoms, stripped of their electrons and accelerated to speeds near the speed of light.

"The speed of these cosmic rays is very close to the speed of light," said Lieu. "They are the fastest particles yet detected. Each has more than the energy of a bullet, 10**20 electron volts."

While there is evidence that these most powerful cosmic rays must be coming from outside of our galaxy, if they come from elsewhere in the universe they really shouldn't reach us at all. The problem is that at the speeds these particles are traveling, they should be bumping into the cosmic background radiation. Thought to be remnants of the Big Bang that created the universe, the cosmic background radiation is made up of microwave photons that have less energy than visible light.

These cosmic rays are thought to move so fast that when we picture ourselves riding along with such a particle we see it colliding not with an oncoming microwave photon, but with a gamma ray (the opposite end of the electromagnetic spectrum, with more energy than X-rays).

Remember E=mc2? With the energy of a gamma ray involved each collision would cause a nuclear reaction, releasing so much "E" that it materializes into sub-atomic particles called pions; so much energy that the collisions should cause, according to us on Earth, the cosmic ray to lose both energy and speed. After a few of these collisions the highest energy cosmic rays should be slowed down far too much before they reach the outskirts of the Milky Way to reach Earth carrying 10**20 electron volts of energy.

"But the fact that we detect them means that they survive the journey," said Lieu.


The laboratory experiments that showed pions being produced from high energy collisions used stationary ions and gamma rays, said Lieu. "We assumed that all uniformly moving platforms must experience the same physics."

If that were true and the calculations of the energy carried by the highest energy cosmic rays are accurate, then the collision of the cosmic ray with a microwave photon should have the same result as the collision of a gamma ray and a stationary ion. And if that's the case, those highest energy cosmic rays shouldn't survive their intergalactic voyage.

"This is where the loophole comes in," said Lieu. "No one has ever observed under lab conditions how a 10**20 ev nucleus interacts with a photon. We try to picture what's happening as if we were moving together with the particle. We can then infer a collision that's familiar to us.

"But, when a time uncertainty as small as the Planck time according to us is in fact a lot larger from the perspective of the cosmic ray, this grossly increases the uncertainty of our estimates concerning what happens to the cosmic ray. Our 'error bar' gets much bigger.

"Because space-time fluctuations prevent us from identifying the correct frame of reference with respect to which the nucleus is at rest, we don't know what energy the microwave photons assume in that 'correct' frame of reference."

If those highest energy cosmic rays aren't really doing what we think they are doing, their collisions with microwave photons won't release enough energy to create pions. Instead, photons from the cosmic background radiation would bounce off of the cosmic ray nuclei like ping pong balls bouncing off a battleship -- and the nuclei keep coasting along.

"At least half of those nuclei should be able to cross the universe," said Lieu.

Since the highest energy cosmic rays do reach Earth and the energy they carry appears to be 10**20 ev, in cases such as this, says Lieu, "it appears that we can no longer use ordinary, everyday physics that you can verify in your lab to understand them."