Thursday, February 21, 2008

Multiverse - Melvyn Bragg - In Our Time podcast - 21st Feb 2008

Download (40mb mp3) / Info on 'In Our Time'.

Melvyn Bragg:
If you look up the word ‘universe’ in the Oxford English Dictionary you will find the following definition:

“The whole of created or existing things regarded collectively; all things (including the earth, the heavens, and all the phenomena of space) considered as constituting a systematic whole.”

That sounds fairly comprehensive as a description of everything, but for an increasing number of physicists and cosmologists the universe is not enough. They talk of a multiverse – literally many universes – to explain aspects of their theory, the character of the universe and the riddle of our existence within it. Indeed, compared to the scope and complexity of the multiverse, the whole of our known reality may be as a speck of sand upon a beach.

But what might a multiverse be like, why are physicists and cosmologists increasingly interested in it and is it really scientific to discuss the existence of universes we may never know anything about?


Lord Martin Rees, President of the Royal Society and Professor of Cosmology and Astrophysics at the University of Cambridge

Fay Dowker, Reader in Theoretical Physics at Imperial College

Bernard Carr, Professor of Mathematics and Astronomy at Queen Mary, University of London

My Notes about the discussion:

Martin Rees: we know about this Universe: since 1920 we know that our galaxy contains 100 billion stars - just one of many billions of other galaxies we can see - started off with Big Bang 13.7 billion years ago - we can see galaxies 10 billion light years away but no further - still a horizon that we can observe - cf in Ocean - what is beyond the horizon - may be parts of space and time that we cannot directly observe; are there galaxies we cannot see but are the aftermath of the Big Bang. Could there be other Big Bangs? We cannot observe other big bangs; can we infer anything beyond what we can observe?; we believe in Einsteins theory of relativity but cannot see inside a black hole. We believe - must be openminded eg big bang was pure speculation 50 years ago - whereas now that is part of serious science.

Fay Dowker: Not part of science to speculate about in principle unobservable objects? No! if theory tested / well founded - then have good grounds - have to sure about theory if cannot see it directly - predicts other non observable universes this is acceptable as good science but the theory has to be strong; regions /other patches. Need greater confidence in (in theory) unobservable stuff.

Bernard Carr: can we explain the big bang - in last 20 years - half a dozen theories of big bang - theories are tested - is tricky - expanding universe in inflationary universe - due to nature of vaccuum - according to inflationary theory it expanded exponentially with time - accelerating - reason is the nature of the vacuum- the universe that we can observe is tiny; other bubbles / domains - say the laws are differant - other patches - these domains / patches may NOT have the same law of physics - these are the multiverse - when inflation occured - microscopic size grew to size of grapefruit. Inflationary theory predicts:-

  • what we can see of our universe is only a minscule amount of what there is
  • there are other domains or patches or bubbles with entirely differant laws of physics - the multiverse
MR: surprising that laws of physics of observable universe are the same; but are their other domains - from the aftermath of different big bangs - have differant laws

FD: black holes , quarks, LHC - working with unobservable is still science. Sort of unobservability - quarks; branch universes. In a multiverse nothing that occured in there could cause something to happen in our universe. This is unlike quarks which although unobservable could have an effect on our universe.

MR: idea of other universes is conjectural. metagalaxy - part of the universe we can observe.

FD: big bang: big bounce theory or proposal - for theory of a multiverse - many big bangs /followed by expansion then big crunches/ laws of physics could be different in each bounce - spread in time (rather than space); each cycle lasts longer than the previous cycle; universe must be flat in beginning

BC: inflation - huge number of universes spread out in space - bouncing universes are spread out in time

MR: extra spatial dimensions cf bugs walking on a 2D universe- another universe a millimetre away with a 4th dimension - speculative ideas - idea will be narrowed down - ideas based on very early hot universe - one day may be able to test in a lab.

FD: anthropic principle - we should take account of fact we are special - we Carbon based life late on in evolution of the universe - we see universe now because of the beings we are; stars must form and die for carbon to be made - carbon is produced late in life cycle of stars; carbon is formed only when universe is OLD; this can give some explanatory power.

BC: anthropic principle based on fine tuning Various forces in nature - the coupling constantants - relationships between constantonts - there is fine tuning between constants of nature - masses of elementary particles and cosmological constant (describes acceleration of the universe). Defines constants of fine tuning required to within 10%. Physics does not explain it. No fine tuner or God - most physicists do not want to bring in a creator; small fraction if multiverse exists. Multiverse theory the is legitimisation of anthropic principle; if you have many universes then it a natural selection effect - we have to been a universe where the correct constants are required for life to exist.

FD: existence of multiverse - if established multiverse - many differant possibilities; multiverse are many differant possibilities. We see these constants or laws of phyics- only type of universe that we could have evolved in.

BC (22 mins): this observable universe is special - that is what the multiverse concept is going against - that is the history of science - we always want to think we are unique - the earth is at centre of the universe is wrong - the Milky Way galaxy is at the centre of the Universe - that is wrong - Some of us think that the observable Universe is all there is. Now I think we have the glimmering of evidence that 'this is the only Universe' is wrong! Its the story in our continual humiliation that we are not special or unique.

MB: but is it humiliation when ? Because nobody has so far said anything about something producing what is hear!

BC : universe is designed for intelligence - mankind is physically insignificant - continual humilliation - humilliation in sense that we are physical insignifiance - could be wiped out by an asteroid - intelligence has occurred - man in 200 years now understands universe - some bubble are stillborn; other bubbles may be far more advanced than our

MR: (25 mins) first atoms, first stars, understand of laws of nature have particular values - special place - rather than a stillborn universe, around a special star - entire cosmic volume of our universe is special. Most of other bubbles of Multiverse (in BC terms) would be stillborn - other universes could be more advanced than ours. We are not necessarily the most complex organisms.

BC: Physicists have independantly come up with multiverse idea - not just to explain the fine tuning of our universe. Since the Multiverse is a sensible thing to discuss - that is the explanation for the anthropic fine tuning. That is why the anthropic principle has become more acceptable. not just as an explanation of anthropic (greek word for man) - nothing to do with man - too conceited - really means life can arise somewhere.

MR (26mins): laws of physics in observable universe is the same; surprising they are the same throughout; but that could be just a tiny fraction of what is out there. eg other domains further away or from the aftermath of differant big bangs were the laws could be different (he repeats himself).

MB (27' 20"): Can you explain how the Multiverse and the universality of the law of physics relates to search for fundamental theory or theory of everything

MR: we are searching for a formula that combines the very small with the very large. Will theory give
unique formula - for basic numbers in nature eg strenght of gravity or mass of electron etc. Are numbers just brute facts - if so there is no room for the anthropic selection arguement. The theory of everything will no predict the exact values of the laws of nature. Some say that strength of gravity or mass of an electron is just an environemental accident. So in a differant big bang the strength of gravity could be completelty differant.
What aspects of physics are truly universal.

Will theory of everything give unique results or just environmental accidensts of electron; what laws of nature are truly universal and which laws of nature are environmental accidents cf. snowflake - all hexagonal - that is feature of water molecule - really fundamental . Which laws of nature are really fundamental - which are environmental accidents (eg differant shapes of snowflakes - even though they ALL hexagonal). Until we know which are the fundamental properties of nature and which are created by the environment - we will not be able to give a complete theory of everything. String theory may not pan out

FD (30mins) : string theory predicts not one unique vacuum state or set of laws for particle physics - but a large number of 10 to 500 power laws for vacuua; each of differant vacua; theory of quantum gravity; granular theory of spacetime - may be able to detect granularity. Multiverse could be formed out of vacuaa.

MB: what made you to a convert of Multiverse possiblitity?

FD: bouncing cyclic model of universe - quantum gravity proposal - explains fine tuning - shape of early universe has to be flat. if wait long enougth.

MR: long haul to unified theory - which are truly universal? Understand nature of space.
link quantum effects - space is trillions and trillions smaller than protons or neutrons. It is part of science - speculative science - not just metaphysics.

MB: 5% of physicists 20 years thought multiverse could exist - now many more think multiverse exist - how do you TEST it!!

FD (35mins) : string theory for quantum gravity.
only a dozen physicists work on granular theory of spacetime - is not smooth - test depends on the theory. How could theory of granularity change the property of light from distant sources?

BC (36 mins): test for inflationary theory - tiny fluctuations give rise to galaxies - can see fluctuations in Cosmic Background Microwave radiation; 5 years ago seen; fluctuations in temperature are 1 in 1000 - inflationary theory predicts formation of galaxies - testable.

MR: relates tiny fluctuations - link between small and large. Essential

FD: link gravity and quantum theory - interplay with theories and cosmology - observations made constrain theories. Look out to cosmology - interplay between quantum gravity - will inform cosmology - constrain the theories.

BC: is multiverse part of legitimate science - multiverse is predicted by string theory - if can test string theory - then can show indirectly that multiverse is possible; string theory / M theory or quantum gravity - may never be testible - energy is too high; or is M theory just maths not physics / science; legitimate science - may have to wait 200 years before we can solve mystery of universe

FD: quantum gravity - very confident that it can be tested; we will know that multiverse is possible; deep confidence in unity of physics - we will know whether multiverse is real
MR: must be optimistic and try very hard to find any answers at all!!

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