Monday, 28 January 2013

Black Hole Event Horizons & Quantum Entanglement

Question from Peter (A-level student) -

An aspect of physics which has always interested as well as confused me is
quantum entanglement. I believe that this is a part of the new theory,
perhaps incorrectly dubbed "burn up", for one of the theories of what
happens past the event horizon of a black hole. I would be greatly
appreciate it if you could pass this on to someone with the expertise
to explain what quantum entanglement is and how it affects this "burn
up" theory of black holes.

Quantum Entanglement - credit

Answer from Dr Marika Taylor - see her personal website below.

Your questions about burning up at the surface of a black hole and quantum entanglement were passed along to me to answer, since I work on black holes.

I think your questions must be in the context of some recent works on black holes, involving so-called firewall problem, see this popular article

which discusses these ideas.

The question of what happens at the horizon of a black hole has been hotly debated for close to forty years now, since Stephen Hawking discovered that black holes aren't really black, because quantum effects cause them to emit radiation. There's a nice heuristic picture of how this radiation gets created, which relates to the firewall and entanglement discussion - in quantum theory particles are continually created in pairs, live for a short time and then annihilate. When such pair creation happens near a black hole horizon, however, one member of the particle pair can fall behind the horizon where it gets trapped, and so can no longer annihilate with the other member of the pair. The particle outside the black hole horizon can escape off t infinity, far from the black hole horizon, where we could measure it with a detector. In this picture the particle behind the horizon and the particle which escape to infinity are said to be entangled, which really just means that they have complete information and knowledge of each other.

Soon after Hawking discovered black hole radiation, he realized an important consequence of his calculations - regardless of how the black hole was formed astrophysically the particles measured coming out of the black hole seemed to be the same! That means that what we measure doesn't seem to depend on what happened earlier, in the formation process of the black hole. However, physical measurements at a given time not depending on what happened at earlier times (technically, non-unitary evolution in time) is not what we expect for a physical theory. For about 25 years people debated whether black hole evolution was indeed non-unitary or not until various breakthroughs in building  a quantum theory of gravity (string theory) convinced people that actually black hole evolution indeed had to be unitary - Somehow we must actually be able to do measurements which tell us how the black hole was formed.

Right now the growing consensus is that the only way we will be able to recover enough information from the black hole to deduce how it was formed is if the description in terms of Enstein's theory of gravity isn't quite right, it must miss some important physics near the horizon. There is not however consensus about what physics is missing near the horizon. The authors mentioned in the article above think that the basic problem is in the entanglement of particles produced near the horizon. In other words they don't think it is true that the particle which fell behind the horizon and the particle which escapes to infinity actually have complete information about each other, they think that the one which falls behind the horizon gets strongly entangled with a gas of particles living there. They also think that this gas of particles is so hot that anything falling behind the horizon would be burnt up by it, hence the name firewall.

As I said the consensus is indeed that something happens behind the horizon, there is some gas of particles which record the history of the black hole's formation. But there is very little evidence for the firewall - the authors mentioned in the article above were the only people in a recent black hole conference to think that the firewall exists! A more popular viewpoint, which I have worked on, is the fuzzball picture. In this picture there would be a gas of particles behind the horizon telling us about how the black hole was formed but something passing behind the horizon wouldn't get burnt up, it would just interact weakly with this gas and then eventually escape back across the horizon, out of the black hole.

I hope this explanation helps in understanding what entanglement means in this context. Please feel free to ask if there is something I can explain further.

Thursday, 17 January 2013

Stargazing Live 2013

We participated in 2 stargazing live events this January.

All the Team at the Newbury Stargazing event on the 9th Jan.

For our event on 8th at the Univerisity we had loads of activities going on- see the event brite website here

Eventbrite website for Event (scroll down to below the tickets bit) 

 We sold out all of our free tickets and were expecting over 350 people. As with all stargazing events access to the telescopes and roof observatories is the most popular activity but unfortunately for us it was cloudy and rainy and the roof observatory part of the event got cancelled. 

However, over 150 people still came along and the planetarium Soton Astrodome was full and there was a good turn out for all the astronomy talks and the Astrophotography talk. All the people I spoke to seemed extremely enthused and asked us to run more evening stargazing events like this. 

 On the 9th at Newbury our 4 planetarium shows in SotonAstrodome sold out in first 10 mins of the racecourse opening its doors to the public. At this event over 1500 people attended and the sky was perfect. As well as the planetarium my team and myself took along the inflatable planets and talked about the solar system, and showcased the world wide telescope software on a flat screen next to the astrodome- where we talked about the life cycle of stars. I estimate our Team of 7 engaged with over 160 people. 

 Therefore, cross the 2 events we engaged with over 300 people.

Media from events

There are 2 videos from our stargazing event on campus here:

The press release about the event at Uni which just went up on the website is here (please feel free to knick parts of this etc)

We did an event on the 8th at Uni and were at the BBC South event at Newbury Racecourse on the 9th. All the photos taken over those 2 days are here and you can again steal any of these for your article.

Additional Comments

I think the stargazing events are important and amazing because they get everyone involved and inspired. It introduces astronomy and space to a lot of people who don't realise they can simply go outside and look up. For me personally I am always shocked by my friends from school etc who come up to me when I am back home after the shows and they tell me they have watched stargazing live and they want me to send them information about getting telescopes and stargazing because they are really inspired! I am used to being approached and interested people on stargazing live events at University but the TV shows make astronomy much wider reaching. 
For the children and young adults who visit stargazing events I think the scientists/PhD students and academics they meet are great role models. For them to see that they are just normal people at these events is very important. I think the great thing about stargazing events is that people will come along and speak to researchers etc about things which they consider crazy and 'out there' like 'Black Holes' etc and when they realise all these students at the University actually study these subjects everyday. I think knowing that this career/future is ‘within reach’ can inspire them to take up Physics degrees and see a future for themselves in Physics and Astronomy