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Doom: The Dark Ages system requirements demand ray tracing gaming GPU
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Game engines don't have to simulate sound pressure waves bouncing off surfaces to get good audio. They don't have to simulate all the atoms in objects to get good physics. There's no reason to have to simulate photons to get good lighting. This is a way to lower dev costs and increase spending on the consumer side, I would not be surprised if Nvidia was incentivizing publishers to use ray tracing.
The digital foundry video explains better than I can why the dev cost is lower with ray tracing only.