Tuesday, September 15, 2009

Physics Quote Of The Day.

From David Tong's Lectures on String Theory:
It is often said that general relativity contains the seeds of its own destruction. The theory is unable to predict physics at the Planck scale and freely admits to it. Problems such as non-renormalizability and singularities are, in a Rumsfeldian sense, known unknowns...On the other hand, some aspects of quantum gravity suggest that general relativity isn’t as honest about its own failings as is usually advertised. The theory hosts a number of unknown unknowns, things that we didn’t even know that we didn’t know.
For those who missed it, this quote alludes to Rumsfeld's poetic words:

As we know,
There are known knowns.
There are things we know we know.
We also know
There are known unknowns.
That is to say
We know there are some things
We do not know.
But there are also unknown unknowns,
The ones we don't know
We don't know.

Indeed general relativity does contain the seeds of it's own destruction, and herein lies perhaps the greatest challenge to theoretical physics.


  1. The system I did built does permit to think everything from the same basis, not like with general relativity and quantum mechanics.

  2. Postscript : there is quite a lot to come about it and its results on my blog.

  3. In sciences different theories can exist together, so I am not targeting exclusivity ; but with the string theory, what I do not like is the carryall dimension (it is a bit easy to put all problems in a dimension).

  4. Cartesian, thank you for your thoughts. The reason to add an extra dimension is in the past it has worked.

    Einstein added time to being on equal footing as space and the new four dimensional theory describes gravity wonderfully.

    Then Kaluza and Klein added a fifth dimension to general relativity and showed that once you "compactify" this theory, which means make it so creatures would only see 4 dimensions, then you have a theory that describes both gravity and electricity and magnetism. (plus a dilaton which isn't observed yet.)

    The saga continues with string theory with the extra caveat that it forces you to go to 10 dimensions. (String Theory is only consistent in these higher dimensions.)

  5. I think the best thing is to try in a research field (not directly in a general teaching) to use these theories in order to know what they bring really to science, and if it is easy to use them.

  6. I think Kaluza - Klein really did introduce this way of thinking. And let's be honest, first time you worked through their equations you just were blown over by how cool it is. And even now folks not embracing strings still often turn back to Kaluza - Klein to think about how to get going. While I'm dubious about string theory the criticisms of the extra dimensions always struck me as an odd criticism. There is something just so elegant and beautiful by translating talk of odd force relationships into movement in extra dimensions.


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