Would Bobby Fischer at his prime be able to beat a 2500 Grandmaster in today's world? (in a match)

In a 12 game match vs. a 2500 GM in 2021 ....

easy. 

How about Fischer vs Carlsen?

Yes...

Yes, 
Would Capablanca be able to beat a 2500 in 2021, would Morphy ?

He was 2700, ofcourse he will beat him. 

true is, this question lack of circumstances. give these guys (Capabalanca, Fisher, Morphy etc.) some time to prepare, and they crush all 2500, 2600, etc.  because they true chess born players

Fischer's peak rating would still put him in the top 10.  You could have figured this out yourself without any trouble by digging into it online.

Of course.

Look at it this way. Kasparov played a tournament a few years ago (it was blitz, but it is enough to roughly show playing strengths) and won a few games against some super GMs, so people over 2 700. That is not prime Kasparov for sure. Fischer's top playing strength is not that much weaker compared to prime Kasparov. He might be a bit weaker, but that is it, so his prime is surely better than Kasparov a few years ago, and that weaker version of Kasparov would still beat 2 500 players most of the time. So there is my logic.

Morphy would lose--Fischer would of course win.

If Kasparov could do it, while not in his prime, Fischer can do it in his prime for sure.

There's no assurance that the MEANING of a 2700 rating (or even, the meaning of the word "rating") has remained stable over the past 50 years.

This question is one of the great unanswerable questions, along with "would Bruce Lee in his prime beat Mike Tyson in his prime?", or any other attempt to compare people from different eras. For Chess, I think the most compelling answers use statistical analysis with engines. One interesting attempt (full article here) used High-performance Computing and Stockfish to build a statistical model of all world champions from Steinitz to Carlsen. From the model they predict the win % for each pairing. 

According to that model, Fischer comes in at the all-time number three, behind Carlsen and Kramnik, but ahead of Kasparov, Anand and the rest. Based on that, if the answer to "Would Kasparov in his prime beat a 2500 GM over 12 games" is a pretty clear "yes", then it's also yes for Fischer.

Of course, you probably came here for an argument.

The two main schools of thought are that ratings have inflated, or that chess players have just gotten better...but either way, Fischer at his peak would be near the top.  He would either be 2785, or >2785.

In fact, Fischer could have been rated higher at his peak if the other players of his time were not 2600s.  Elo ratings are like an inflating balloon.  You can't leave the balloon, you can only poke at the edges and try to make it slowly expand further.  The balloon inflates over time...or probably more accurately, the ratings pool attenuates over time on both ends...but the low end players will always drop out/disappear faster than the high end, because it's not actually a closed system.

There are several studies that attempt to reconcile ratings with actual playing strength, and Fischer does extremely well in these studies.  The fact that super GMs still routinely talk about their admiration for the strength and accuracy of Fischer's games is also noteworthy.

This question is one of the great unanswerable questions, along with "would Bruce Lee in his prime beat Mike Tyson in his prime?"
I'm confident Mike would knock Bruce Lee's head in orbit around earth, Buddeh.  

I agree regarding the outcome of Mike vs Bruce. I meant it's another form of the same question, one that's seen a lot in other forums (or maybe this one if we had a chess boxing thread).

If fischer had access to todays technology and resources, he would destroy most 2500's. 

If he was only limited to the technology, knowledge and resources of his time, he would struggle a lot. 

Yes.  It wouldn’t be easy.  But, yes.

2500?! Just speculation but my guess is he would be quite competitive against low ranked GM's. And I'm definitely in the camp that thinks today's players (in anything) are better than yesterday's and worse than tomorrow's.

duh