I just started my PhD in physics. It has been 6 months. I have had a mentor for sometime and I have wanted to add him as a co-supervisor. My university told me he cannot be added as a co-supervisor because he works in the same field (physics). The co supervisor can be added only if he's from another discipline like biology for example. Is this how these things work? I am confused and also angry. My supervisor says that I can still collaborate with him. But I still don't feel good about it.

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    Presumably your university is correct about their own rules. But there are many other places where a cosupervisor being in the same field or even department would not be a problem.
    – Anyon
    Commented Jul 24, 2022 at 21:08
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    Presumably you want someone from the same field so that in case something happens to your principal supervisor, there is a backup. You sure your university's policy says that? Commented Jul 24, 2022 at 21:12
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    Seems to me a strange policy... No, this is not how things usually work, in my experience. Commented Jul 24, 2022 at 21:28
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    Can you clarify what you don't feel good about?
    – Bryan Krause
    Commented Jul 25, 2022 at 1:08
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    @Newuser7 - It would probably help you to include the funding issue in a "why I don't feel good about it" paragraph in your post itself. The title aside, others may be able to suggest ways to address that and any other consequences of the refusal.
    – user158559
    Commented Jul 25, 2022 at 11:38

2 Answers 2


In the US, at least, every university sets its own policies in such things so you need to ask them for reasons if you really want to know.

But a typical reason might be to optimize (in some sense) faculty usage and efforts. Another might be to assure that you don't get conflicting advice. Some questions here relate to problems with co supervisors. In the case of a dispute between "advisors", should it arise, having a single advisor makes it clear whose advice you need to follow and the dispute can't be used to block you. It happens.

And your assumption about funding travel might be erroneous. Funds don't magically appear and if the place doesn't allow co-supervisors they are unlikely to have funding for such.

External supervisors are another issue as they may get no credit for advising you.

That said, you can certainly seek advice from whomever you wish, local or otherwise. A co-supervisor formal relationship is mostly just an administrative acknowledgement, unless funding is supplied.

The exception for cross-discipline co-advisors is likely there to encourage such things, as is appropriate.

But, as your advisor suggests, you can get advice and collaborate with others. The policy doesn't prohibit that.

There is one caveat, however. You say that the prohibition applies to co-advisors in the same discipline, but your definition of that seems very broad. Physics covers a lot of fields. Make sure you understand the policy. Different physicists in different specialties do quite different things. Maybe different enough in your case.

  • Hello! Firstly thank you. About the funding situation. I think there is a misunderstanding. What I mean is it would be easier for the co supervisor to arrange funding for me to visit him. About external supervisor, he is happy to supervise me. The idea is here it would help funding. And also I would have a backup supervisor. I also need his expertise for my research.
    – Newuser7
    Commented Jul 24, 2022 at 21:47
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    You can get any expertise from him that he is willing to give. That isn't an issue. If you have something to offer, it might be possible for him to fund you for a visit to give a talk. See edit, also.
    – Buffy
    Commented Jul 24, 2022 at 21:57
  • @Newuser7: So you're saying that your "mentor" could obtain travel funding for you from their institution if they were officially designated as your co-supervisor? You might want to lead with that the next time you bring this up with whoever denied the request before. I cannot guarantee that it will help, but IME "I could get external funding for this, if only…" are often the magic words that can convince administration to find some way to make things work. Commented Jul 25, 2022 at 21:23

This is very much institution-dependent. Actually in many examples I’m familiar with, the co-supervisor must be a member of the same (graduate) program so a would-be co-supervisor from biology would first have to become adjunct in physics to be granted co-supervision privileges, effectively becoming a member of the graduate department (here of physics) in addition to their regular duties in their home unit.

As a result, “sharing” students between faculty in the same unit, subject to some formal agreement (which may include sharing the funding burden if required) is not that uncommon.

There is really nothing you can do. In my experience units have (usually good) historical reasons for this or that policy, often driven by the desire to rectify some previous abuse or avoid some tricky situation.

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