Kenneth Greisen, also known as Ken Greisen, is a prominent theoretical physicist known for his work in high-energy physics and particle astrophysics. He is particularly noted for his contributions to the understanding of cosmic rays and the implications for fundamental physics. One of his significant contributions is related to the GZK cutoff, which discusses the limitations on the energy of cosmic rays that can be observed from distant sources due to interactions with the cosmic microwave background radiation.
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