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PhD Inquiry, Modified Gravity or String Theory

  1. Feb 10, 2019 at 5:07 AM #1
    Hello everyone,

    I am currently finishing my Masters in Theoretical Physics and looking forward to continue in a PhD.

    I always thought I would love String Theory, and now that i am actually doing it I see that my feeling was correct. I do not know why but trying to understand its concepts amazes me and I really love its mathematical formalism as well. Therefore when applying I always stated that my primary interest is the field of Superstrings, Supergravity and Ads/CFT correspondence.

    However, I have realized that it is quite difficult to get a PhD in string theory, not to mention that it is already difficult to find a PhD position in the first place. Therefore, plenty of my advisors have consulted me to be open minded.

    Another important thing that I would like to mention is that while I study at the UK I come from Greece and would love to return back for my PhD. However, while I know that there are some Professors and research staff in Greece that do a splendid job in research, my first concern is if me doing my PhD there would affect my CV even if it turns out to work well and I do a very good job.

    Now, in my effort to be open-minded, I have come across the aspect of Modified Gravity. My experience in that field is limited but I understand that it tries to find an alternative description of Gravity rather than GR. The second question I want to make is if you can provide extra details and if you believe someone that really likes String theory would also like Modified Gravity as well.

    Any help is appreciated,
     
  2. jcsd
  3. Feb 11, 2019 at 8:29 PM #2
    Modified gravity is a rather exotic topic for string theory. String theory definitely "modifies gravity" at very short distances, because many other string states come into play alongside the graviton. But when people talk about modified gravity they usually mean macroscopic, astronomical scales. It's a lot harder to find string research on modifying gravity at those distances. You have Thibault Damour on dilaton phenomenology, and you have a few papers looking e.g. for corrections to the Kerr metric of Sgr A*. But in general, macroscopic gravity in string theory will look like Newton and Einstein.

    In my opinion MOND is the best reason to be interested in modified gravity. My favorite explanation of the velocity aberrations is the superfluid of Khoury and Berezhiani, which is not technically modified gravity, but they sometimes describe their theory through a kind of modified gravitational potential. It would be nice to see string theorists thinking about whether you could obtain MOND from the theory somehow. Lubos Motl had a blog post about what he called "HOND", holographic Newtonian dynamics, in which the modification was supposed to be due to holography. What Erik Verlinde does is not really string theory, but he does motivate his ideas with some reflections on the subject. There may be others I don't remember.

    But in general, modified gravity has nothing to do with string theory, instead it's mostly about the phenomenology of all kinds of modifications of general relativity. Such research is going to be heavily about data analysis, using cosmological time series to fit parameters in a generalized modification of the gravitational potential, comparing it to the null hypothesis of Lambda CDM, and so on. Of course there's a theory side as well.

    I'm making these remarks either just as an objective characterization (from what I see of these fields), or from the perspective of what I think has scientific merit. I don't have any pragmatic thoughts with respect to your PhD choices, hopefully someone else will.
     
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