How do I deal with diverse interests when applying for a PhD program? I have a master's degree in Computer Science. I have applied to PhD programs, and decisions are trickling in. I may soon have to choose between two or more PhD programs.

Unfortunately, I have diverse interests. I was advised that a successful PhD application is usually quite specific, so I made each of my applications very specific. I wrote to a professor working in decision making (AI), another working in computational learning theory, another working in computer music, another working in quantum &amp; parallel computing, another working in logical foundations &amp; philosophy of computation. With much enthusiasm, I informed each professor that I was interested in pursuing a PhD in his / her field. This is true. I am interested in each of these fields, and know something about each of them. When advised to apply, I did.

The problem is - being interested in each of these fields also means that I'm interested in all of them. (There are more areas of CS I'm interested in. What's worse - my interests are not even restricted to CS. For the purposes of this post, I'm restricting myself to only the mentioned areas.) So when I choose between PhD programs, I'm choosing between completely different areas - and that is surely going to be excruciating.

This is not a case of not knowing my 'true interest' now, only to discover it later. I have no 'true interest'. I have always pursued numerous unrelated things in parallel. I love all my interests equally. In fact, I love the feeling of 'being interested' more than the interests themselves. Whenever I've tried to restrict myself to one topic of study - even for a month - that feeling is lost, and I am quickly bored. As a result, my skill-set is a classic case of a jack of all trades - master of none. The worst part is - everybody knows that a PhD is about getting deep into one specific topic. It's not about breadth - it's about depth. Even if I were to choose one of the PhD programs (and I must choose one), I have a feeling that I might get bored quickly and be prone to constant topic-changing / transferring. Again, this is not about being fickle - it's about following my heart, and my heart takes me to different things at different times.

Do you think I'm likely to fail / never complete my PhD? Does anyone have any advice about dealing with diverse interests? Has anyone felt the same way and dealt with it? I'd love to hear from you.
(My experience with non-American PhD programs is limited, so some of what I say here may not be relevant or possible.)

First, I think you may have painted yourself into a corner.  More important than being specific, PhD applications should be honest.  Hopefully your applications described the breadth of your interests, perhaps with some extra emphasis and detail in one area, rather than implying falsely that you are only interested in one topic.  It's better to be rejected than to be accepted into a PhD program where you won't thrive.

I think you need to look for overlaps and connections between your various fields of interest.  Decision theory and machine learning are not that far apart.  Quantum computing and philosophy of computing are not that far apart.  Parallel computing and logic are not that far apart.  Decision theory and logic are not that far apart.  Machine learning and computer music are not that far apart.  Finding something at the intersection of five or six different areas is almost certainly impossible, but lots of interesting stuff happens at the interface between field X and field Y.

On the other hand, all the areas you describe are incredibly diverse within their own boundaries.  You may find enough different topics within (say) machine learning to keep your fickle heart satisfied.

One thing you should ask about (quickly!) is the possibility of having multiple (ie, two) advisors in different areas.  Some departments encourage inter-disciplinary co-advising; some don't.  Some advisors encourage collaboration with other faculty; others flatly forbid it.  (In many European PhD programs, students are attached to specific projects of specific faculty from day one, and moving between projects or fields is almost impossible.)

What worries me more than the diversity of your interests is this sentence: "Whenever I've tried to restrict myself to one topic of study - even for a month - that feeling is lost, and I am quickly bored".  You have to get over that.  Research requires long-term focused attention.  Early in your PhD program, you may be able to juggle a couple of different research projects at once, but eventually, you have to focus on a coherent thesis topic.  It can be an interdisciplinary topic, sure, but it has be coherent.  You will have to work on that one topic for years.  If that sounds awful to you, then maybe you don't want to be a PhD student after all.

My department head has changed fields several times in his career, so I think his advice on this topic is worth repeating.  Assuming good health and good luck, you have a 50-year research career ahead of you.  So if you're deeply interested in half a dozen different areas, you can afford to spend seven or eight years on each one.