Basically a scientist makes a stargate type of device. Nonliving things that go through end up fine but living things like mice would either immediately just die or go absolute insane after exiting.
Long story short, the trip is instantaneous. But because they're traveling outside of our normal space time is perceived differently by conciousnes minds. So what seems like a split second to us on the outside things seem almost endless on the inside. Like anywhere from millions of years to the age of the universe. Endless time with nothing but their own thoughts. No body and just white light. Humans they test it out on either also die or end up going all Event Horizon when they exit.
They figured out a work around by knocking people out before putting then through.