In our last online poll to find out what Plus readers would most like to know about the Universe you told us that you'd like to find out how long a day is. We took the question to the physicist Nicholas Mee and here is his answer — and it's not 24 hours!
In our online poll to find out what Plus readers would most like to know about the Universe you told us that you'd like to find out if time travel is allowed. We took the question to Kip Thorne, Feynmann Professor of Theoretical Physics, Emeritus, at the California Institute of Technology, and here is his answer.
I know a little GR; but no Quantum Gravity and very little QM.
I interpret the article as saying that the consensus is: Quantum Gravity can not be fit into Godel's time-closed solution to GR. Is that correct? In that case the global topology would constrain the deeper theories. Put another way Quantum Mechanics can not be embedded/formulated on an arbitrary manifold.
These are questions; not statements.
I read references 2,3
3 is quite readable.
2 is tougher and I am not through.
The idea that some local properties can't be extended globally in some topologies is not as strange as it might seem at first glance; there are other examples.
I do have doubts about some of the reasoning; but that doesn't fault the reasoning just the presumptions.
I think its possible that the "energy conditions" are not the right analysis tool. Something along the lines of Entropy (Maxwell's daemon ) might be sharper in the mathematical sense.
Th relation to Casimir vacuums was fascinating. So is the solidarity of Hawking's argument.
Although the possibilities of time travel would lead to the age old grandfather paradox, and I am not sure as to what would be a right explanation. Maybe the Copenhagen interpretation of splitting states to maintain Quantum decoherence.
Cannot say anything conclusively.
Are you a committed non-smoker who loves rock climbing? Then, like most of us, your have an ambivalent attitude to risk. Some risks are so much fun they're worth taking, and others you definitely stay clear of. But what's the attitude of a risk professional like David Spiegelhalter, Cambridge Professor of Public Understanding of Risk (and regular contributor to
Plus)? Find out in this short video posted on the Cambridge Ideas website.
Vaccination is an emotive business. The furore around the MMR vaccine and autism has shown that vaccination health scares can cause considerable damage: stop vaccinating, and epidemics are sure to follow. But how do scientists decide whether a vaccine and a vaccination strategy are effective and safe? We talk to Paddy Farrington, Professor of Statistics at the Open University. You can also
read the accompanying article.
Infectious diseases hardly ever disappear from the headlines. If it's not the disease itself that hits the news, then it's the vaccines with their potential side effects. It can be hard to tell the difference between scare mongering and responsible reporting, because media coverage rarely provides a look behind the scenes. How do scientists reach the conclusions they do? How do they predict
how a particular disease will spread, and whether it is likely to mutate into a more dangerous strand? And how do they assess the impact of an intervention like vaccination, and make sure that a vaccine is safe?
Two answer these questions, we have put together a package of five articles, a podcast, and a classroom activity.