Three light pulses travel upwards from a transmitter on the floor.
We watch from the outside and see the pulses travel at light speed.

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Three light pulses travel upwards from a transmitter on the floor.
We watch from the outside and see the pulses travel at light speed.

The laboratory accelerates upward while the light pulses continue upward.

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The receiver at the top of the laboratory is starting to move relative to the pulses.

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The receiver at the top of the laboratory is moving upward when the pulses hit it.

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The receiver at the top of the laboratory is moving upward when the pulses hit it.

It records a greater time interval between the pulses than the transmitter sent.

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The receiver at the top sees a lower frequency than the source at the bottom.
The equivalence principle says that the same thing would happen to light pulses moving upward in a gravitational field --- their frequency drops as they climb.

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