I am looking for a book that I read when I was a teenager. My dad had owned it but it's gone now.
Some details that I remember are a group of 4 or 5 people travel back in time to various locations/times. For some reason I think a couch was involved.
I sort of remember them traveling as a group but then they are split up. They then gradually travel to each other's time period trying to gather everyone up again. Time has passed at different speeds for the different people.
One or maybe two of them end up in what I think was an alternate universe of the same time period. This is why I think it was set in the late 60s or early 70s because they encounter protestors and the police are trying to break up a riot. I think the police were dressed in pink(?). They were there only a short period.
One guy ended up in the Stone Age. He was there the longest, 20-30 years maybe. During that time he changes the course of human development because he teaches the primative people how to do metalworking, and to live in different communities.
Another guy travels to the future(?) and spends several months there. He is a out of shape guy and while he is there he is trained by someone to endure high levels of pain and push his body to the limits. I remember him doing a high dive from a very high location.
In the end after they have all found each other and they get back to their present time, they discover that it is completely changed, presumably because of what the one guy did in the stone age.
Does any of this sound familiar to anyone? I would love it if I could find this book again. Thanks