He was born in 1918 and wrote this October 17, 1946, so he was 28 when he wrote this:

I find it hard to understand in my mind what it means to love you after you are dead — but I still want to comfort and take care of you — and I want you to love me and care for me. I want to have problems to discuss with you — I want to do little projects with you. I never thought until just now that we can do that. What should we do. We started to learn to make clothes together — or learn Chinese — or getting a movie projector. Can’t I do something now? No. I am alone without you and you were the “idea-woman” and general instigator of all our wild adventures.

She had died 16 months earlier, so he was only 27 when he lost his soul mate:

Richard and Arline were soul mates. They were not clones of each other, but symbiotic opposites – each completed the other. Arline admired Richard’s obvious scientific brilliance, and Richard clearly adored the fact that she loved and understood things he could barely appreciate at the time. But what they shared, most of all, was a love of life and a spirit of adventure.