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Testing Einstein's Relativity With a Cosmic Neutrino

The arrival of a tiny cosmic particle from a distant, extremely energetic place in the universe enabled researchers to test key principles of general relativity and special relativity. Their results will be especially valuable for scientists attempting to merge quantum physics and general relativity through “quantum gravity” models.

The particle was a neutrino, an elementary particle with a miniscule mass that rarely interacts with anything and travels at practically the speed of light. Neutrinos are prolific – more than 65 billion neutrinos pass undetected through each square centimeter of the Earth in just one second, but this one was special.

Most neutrinos that we see on Earth come from nuclear reactions in the Sun. Others are produced by the collapse of massive stars, cosmic ray interactions in the atmosphere, and radioactive decay. Still others are remnants of the big bang. The origin of a neutrino is determined by its energy and the direction in which it is traveling.

Because neutrinos rarely interact with anything, researchers have built huge, dense detectors to study them. In recent years, a detector that looks for neutrinos with sensors embedded in a cubic kilometer of ice at the South Pole, called IceCube, has seen several extremely high energy neutrinos. After surveying the sky for possible sources based on their trajectories, earlier this year a collaboration led by Matthias Kadler from Universität Würzburg announced that one of these high energy neutrinos was likely emitted during a major gamma ray outburst from a distant blazar, one of the most energetic kinds of objects in the entire universe.

A blazar is an extremely bright, compact area that surrounds a supermassive black hole at the center of a giant elliptical galaxy. Blazars spew out relativistic jets of high energy particles and light pointing toward and away from the Earth. The jet coming our way are what makes blazars appear so bright. They also cause signature changes in intensity, like the outburst that emitted the high energy neutrino IceCube detected.

Artist's conception of a blazar.
Image Credit: NASA's Goddard Spaceflight Center
Over the Chinese New Year holidays, Xiang-Yu Wang, an astronomer at Nanjing University, heard that this high energy neutrino was likely produced in a gamma-ray outburst from a distant blazar. Wang is a long-time high energy neutrino researcher. He realized immediately that this neutrino could be used to test the equivalence principle, a cornerstone of Einstein’s general relativity, and Lorentz invariance, a key principle of special relativity. Scientists had tested Lorentz invariance before using neutrinos from Supernova 1987A, but with a higher energy neutrino from an even more distant source, Wang knew the blazar test would yield more accurate results.

According to the principles, uncharged particles like photons and neutrinos should travel at the same speed in a gravitational field or vacuum, regardless of their compositions or energies. Therefore, it should have taken the neutrino the same amount of time to reach the Earth as the high energy gamma rays produced in the outburst.

Unifying general relativity and quantum mechanics is the holy grail of physics. Some models that attempt to do this predict that in specific cases the equivalence principle and Lorentz invariance could be violated. For example, some models predict that Lorentz invariance violation at very high energies.

By calculating and then comparing the travel times of gamma rays and the neutrino likely emitted from the same blazar outburst, Wang and his colleagues Zi-Yi Wang (Nanjing University) and Ruo-Yu Liu (Max-Planck-Institut für Kernphysik) tested these principles. They did not find evidence that the equivalence principle or Lorentz invariance had been violated.

From this, the team set tighter limits than previous research on the conditions under which the equivalence principle and Lorentz invariance could be violated, if indeed they are. Scientists can now compare models of quantum gravity to these results, which will soon be published in Physical Review Letters, and explore how well the models match reality.

Kendra Redmond


  1. "According to the principles, uncharged particles like photons and neutrinos should travel at the same speed in a gravitational field or vacuum, regardless of their compositions or energies. Therefore, it should have taken the neutrino the same amount of time to reach the Earth as the high energy gamma rays produced in the outburst."

    I'm scratching my head at this paragraph. Don't you mean that uncharged particles accelerate/decelerate the same in a gravitational field, regardless of composition? Obviously, photons (massless particles) travel at a different speed, i.e. c, than neutrinos (as far as we know massive, although very small, and therefore impossible to reach c).

    In the paper they actually put a lower limit on the flight time difference to test EP and LI, so that paragraph cannot be correct.

  2. Useless effort... What's the cost?

  3. Great question - thank you for bringing this up. I just checked in with Xiang-Yu Wang to make sure I got this right. Here is Dr. Wang's response: "The effect of mass on the velocity of the neutrino is very very small and can be negligible, especially for high-energy neutrinos. Thus, PeV neutrinos can be treated as massless particles for our purpose." Also, the researchers actually put an upper limit on the flight time difference, not a lower limit. I hope this helps, thanks for your thoughtful comment.

    1. Thanks for the follow-up and the answer!


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