Image © by Adam Walanus

*Claus Kiefer is one of the leading experts in quantum gravity and a professor at the University of Cologne.*

*He authored a highly popular textbook called Quantum Gravity, which is a great starting point for any research in quantum gravity. We are very glad he took the time to answer some questions. He tells us about his biggest discovery, shares many tips for students who are interested in quantum gravity and gives some insights on his current research. Enjoy!
*

* *

**Physics Insider:** Mr. Kiefer, what are you currently working on?

**Kiefer**: I’m trying to understand, within simple models, how the arrow of time could emerge from quantum cosmology. The general ideas are outlined e.g. in my essay on http://arxiv.org/abs/0910.5836 ; Further I’m trying to understand, within simple models, how the singularities predicted by Einstein’s theory of general relativity can be avoided in quantum gravity.

**Physics Insider:** Why is this important?

**Kiefer**: While the microscopic laws, as we know them, are time symmetric, most of the observed phenomena are not, i.e. they exhibit an arrow of time. The origin of this irreversibility may be found in a theory of quantum gravity. Such a theory must be free of singularities.

**Physics Insider:** What was the biggest advance/discovery in your field in the last 20 years?

**Kiefer**: On the observational side, the discovery that the expansion of the universe is accelerated and the determination of cosmological parameters from the CMB spectrum and other data. On the theoretical side, there were many small advances, but no big discovery.

**Physics Insider:** What was your biggest discovery?

**Kiefer**: Perhaps the understanding of the quantum-to-classical transitioning quantum gravity/cosmology and the insight on how quantum gravitational effects may be calculated and eventually observed within concrete approaches.

**Physics Insider:** What is your advice to a student who wants to make a career in your field?

**Kiefer**: Be aware that this is a dangerous field, where progress is slow, but if you are really interested and talented, go ahead.

**Physics Insider:** If some fairy would offer to answer you one question about nature; what would it be?

**Kiefer**: Is there a consistent quantum theory of gravity?

**Physics Insider:** How far do you think we are away from answering this question?

**Kiefer**: Between 10 and 100 years.

**Physics Insider:** If you could give your 20 year old self one piece of advice, what would it be?

**Kiefer**: Follow your own interests and try your best (this is what I have actually done).

**Physics Insider:** What math is necessary to be able to work in your field?

**Kiefer**: Besides the standard math that every physicist needs, differential geometry and some functional analysis.

**Physics Insider:** Which books do you recommend to someone who wants to do research in your field?

**Kiefer**: I hope it is not too immodest to recommend my own book *Quantum Gravity*.

**Physics Insider:** Which books did influence you the most?

**Kiefer**: Many books, among them Misner, Thorne and Wheeler, Gravitation; Weinberg, Gravitation and Cosmology; Zeh, The physical basis of the direction of time.

**Physics Insider:** What was the best physics or math book you’ve ever read?

**Kiefer**: There is no “best” one, but there are ten or so excellent books that I could recommend (including the three of the last question).

**Physics Insider:** What do you wish you would’ve known earlier in your career/when you started studying physics?

**Kiefer**: I was happy with my career, but many things I had to find out myself; so a good advice about the relevance of the various fields and about good courses/literature would have helped me a bit.