I am in what should be the final year of my PhD. Essentially, the problem is as follows:
The goal of my thesis is to reduce the uncertainties in a study performed by a previous PhD student. I have probably thought about the problem more than anyone else on the planet over the past four years and have come to the conclusion that this goal is impossible to achieve for technical reasons. In fact, last Fall I discovered that it had been known in the literature for nearly 30 years that there is a physical mechanism that can explain our results and why it might be very difficult or impossible to obtain the precise measurements we are looking for.
Unfortunately my professor does not agree. He thinks I am making some sort of mistake in the analysis. This is of course possible but he refuses to acknowledge the possibility that the project has a negative result. This has been the case every time I questioned the project's viability over the past 2 years. When I discovered the papers explaining the physical mechanism that describes our results I was very excited but he just brushed it off as something he knew already and that was of little significance (which led me to nearly having a nervous breakdown).
He recently stopped responding entirely to my e-mails. He also made it very clear that if I am unable to finish the project I will fail my PhD. Unfortunately, e-mails I have sent to academic advisors and my own committee members have largely been ignored. A co-author tried to help for a bit and then also stopped replying. All of this has led me to feel like I am the problem here somehow and the situation is very unpleasant for me. I am wondering what to do.