The Doctor is a Time Lord who travels through space and time aboard a sentient vessel known as the TARDIS (short for Time and Relative Dimension in Space). Thanks to a broken chameleon circuit, the TARDIS has the appearance of a London police box from the 1960s, and thanks to its advanced technology, it's famously bigger on the inside.

In our universe, astronomers can't travel into the past, but they can look into it by studying light from distant objects. The farther away we look, the longer light takes to reach us, so studying objects at different distances lets astronomers piece together the history and development of the cosmos.

Scientists are also looking for mechanisms that, like the TARDIS, defy the known laws of physics. By searching for cracks in current theories, scientists hope to expand and improve their understanding of the universe. One type of crack scientists are searching for is a violation of a principle called Lorentz invariance, and gamma rays detected by Fermi are providing one means to do so.

Lorentz invariance states that the result of a measurement doesn't depend on how fast an object is moving or where it is in space. The concept is a linchpin of major scientific theories, like Einstein's theory of relativity. Modern searches for a Lorentz invariance violation (LIV) take many forms, one of which is measuring the speed of light from distant parts of the universe.

Intense cosmic explosions called gamma-ray bursts (GRBs) are associated with the births of new black holes and occur on average once a day somewhere in the universe. Light from GRBs typically takes a billion or more years to reach us. Einstein predicted — and experiments show — that light travels at a constant speed, so all of the light from the initial blast of a GRB should arrive at the same time even after traveling billions of light-years.

Some physical theories predict that light with the highest energy moves a bit more slowly than light at lower energies, which would violate Lorentz invariance. GRBs are prime tools to test these theories because their initial pulse of gamma rays occurs at the same time, so all the photons reaching us have traveled the same distance. And because GRBs are so far away, they provide a sensitive probe of any differences in arrival times across different energies.

In one case, scientists studied two gamma rays detected by Fermi from an event named GRB 090510. The two photons, one carrying a million times more energy than the other, traveled over 7.3 billion light-years before arriving at the telescope. Despite traveling this enormous distance, both photons arrived at nearly the same time, meaning that any violation of Lorentz invariance must be extremely small. Scientists continue to test new LIV theories using Fermi data.

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