Final answer:
Albert Einstein developed scientific theories that were essential to the development of the first atomic bomb in the United States.
Step-by-step explanation:
The historical figure who developed scientific theories essential to the development of the first atomic bomb in the United States was Albert Einstein. While he did not directly work on the atomic bomb project, his theories of relativity provided the foundation for understanding nuclear reactions. Einstein's famous equation, E=mc², demonstrated the equivalence of energy and mass, which was crucial to the understanding and harnessing of the immense energy released in nuclear reactions.
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