How has chess changed?

I keep hearing how past player like Fischer would lose against today's GM because the "game has changed so much." How? What?

I also read a comment on reddit that: "Assuming you mean Fischer in his prime, teleported to 2016 to play the WC match against Carlsen, then zero chances for Fischer. Chess has changed tremendously over the past 40 years."

Also, Nakamura on one of the thread said: "Fischer would almost certainly lose to all of us, but this is due to the fact that the game has so fundamentally changed."

What has changed? And how would past Grandmasters would lose in today's world?

Opening theory has changed and the technique has gotten better, both mostly due to computers. (Technique applies both to the precision of moves and the understanding of tactics/attacking principles.)
Given time to update his theory, I'm sure Fischer could be a match. If Kasparov is only off the best after a 12 year retirement, and he was only slightly better than Karpov 10 years out of his prime, we could assume that Fischer would still be in the top 50.

Oddly enough, I draw a line around 1948. I believe that Botivinnik could hold his own, but Alekhine and before would be struggling. is a partial explanation, but the other is the study habits.

Chess has become faster. Fischer-Spassky played at 16 moves per hour. Now 20 moves per hour.
Fischer Spassky could adjourn a game after 5 hours so as to resume the next day after analysis by seconds. Now you have to ply it out in one session.
Chess players got younger. Now we have 12 year old grandlmasters.
Meanwhile Fischer was in 1992, 20 years older and 20 years without play, still very strong.

Reading the "60 memorable games" you will notice that many things are outdated. He simply had no clue about some future opening developments.

Admittedly, it wasnt't written in his prime. But that's just cosmetics IMHO. He was inferior compared to modern stars.

Alekhine probably did more for opening theory than any other player, but he still did a lot of mistakes in the openings.

Alekhine, like his contemporaries and previous masters, suffers from sustained motivation. He did study well for one event at a time, but he didn't really accumulate the knowledge. I believe that the lack of material, the lack of good transportation, and the lack of time (due mainly the holding down a job) prevented anyone from trying to exceed the normal greatness. Given 15 years to study, Alekhine would still be a GM, but I doubt that he would crack the top 100.

Bent Larsen was asked once if he were taken back to the 1920's could he be World Champion. Even a GM as humble as Larsen said that people knew so much new theory, even by the 80's, that any master would easily be the 20's Champion.

#7 I doubt that Larsen, who got smashed 6:0 by Fischer, would be able to defeat either the 1921 Lasker or Capablanca.

I doubt that Larsen, who thought that he was better than Fischer, was humble.

Fischer was more humble: he ceded the first board to Larsen in the match USSR - rest of the world...