I'm afraid you ask a difficult question. [He takes a sip of his drink, thinking how he can properly explain this. When he starts to speak, his expression is completely serious, and just because Luke is a kid doesn't mean he's going to sugar coat any of this.]

There are certain materials that need to be found. They need to be destroyed entirely in order for you to be sent home, but you cannot destroy them yourselves. I must do it. If you find and destroy the materials yourself, you will kill yourself--and you won't come back to life. You will be destroyed outright and you will cease to exist in your own universe.

I thought I found them last year, but they were merely decoys. In order to stop you from coming back here ever again, everything needs to be destroyed.