Thursday 31 January 2008

Gravitational Lensing - Wheeler's "Participatory Universe"

In Karl's posting on Gravity the responses have been fascinating . In her comment Susan Marie asked about how Karl's question related to CTF. I thought that I would take the opportunity to explain how I feel that one particular implication of gravity being a warping of space-time can be shown as a potential proof of CTF. Indeed what I am about to discuss now is a section that was in the original, 'harder-science' version of ITLAD.

Particle physicist John Wheeler says that according to The Copenhagen Interpretation we can regard some restricted properties of distant galaxies, which we now see as they were billions of years ago, as brought into existence now. Perhaps all properties – and hence the entire universe is brought into existence by observations made at some point in time by conscious beings. The order of the universe is brought about in some way by the manner in which these observations are made self consistent. Wheeler calls this anthropic manner a “Participatory Universe”.

Wheeler uses the famous Two-Slit Experiment as the basis of his amazing, and very "itladian" suggestion that the act of observation creates not only everything that is, but also everything that was. For those of you who have the opportunity to attend my latest series of talks the description below will be presented in a graphical format that, I hope, will clarify exactly what Wheeler suggests. If you cannot get along I hope that this somewhat long posting may help clarify exactly why I am so fascinated by the implications of this experiment.
In the two-slit experiment, particles of light — photons — stream through a single vertical slit cut into a screen and fall onto a piece of photographic film placed some distance behind the screen. The image that develops on the film isn't surprising— simply a bright, uniform band. But if a second slit is cut into the screen, parallel to the first, the image on the film changes in an unexpected way: In place of a uniformly bright patch, the photons now form a pattern of alternating bright and dark parallel lines on the film. Dark lines appear in areas that were bright when just one slit was open. Somehow, cutting a second slit for the light to shine through prevents the photons from hitting areas on the screen they easily reached when only one slit was open.

Physicists usually explain the pattern by saying that light has a dual nature; it behaves like a wave, although it consists of individual photons. When light waves emerge from the two slits, overlapping wave crests meet at the film to create the bright lines; crests and troughs cancel out to produce the dark lines.

But there's a problem with this explanation: The same pattern of light and dark lines gradually builds up even when photons pass one at a time through the slits, as if each photon had somehow spread out like a wave and gone through both slits simultaneously. That clearly isn't the case, because the distance between the two slits can be hundreds, thousands, or in principle, any number of times greater than the size of a single photon. And if that isn't confusing enough, consider this: If detectors are placed at each slit, they register a photon traveling through only one of the slits, never through both at the same time.

Yet the photons behave as if they had traveled through both slits at once. The same baffling result holds not just for photons but also for particles of matter, such as electrons. Each seems able to exist in many different places at once — but only when no one is looking. As soon as a physicist tries to observe a particle — by placing a detector at each of the two slits, for example — the particle somehow settles down into a single position, as if it knew it was being detected.

This illustrates a key principle of quantum mechanics: Light has a dual nature. Sometimes light behaves like a compact particle, a photon; sometimes it seems to behave like a wave spread out in space, just like the ripples in a pond. In the experiment, light — a stream of photons — shines through two parallel slits and hits a strip of photographic film behind the slits. The experiment can be run two ways: with photon detectors right beside each slit that allow physicists to observe the photons as they pass, or with detectors removed, which allows the photons to travel unobserved. When physicists use the photon detectors, the result is unsurprising: Every photon is observed to pass through one slit or the other. The photons, in other words, act like particles.

But when the photon detectors are removed, something weird occurs. One would expect to see two distinct clusters of dots on the film, corresponding to where individual photons hit after randomly passing through one slit or the other. Instead, a pattern of alternating light and dark stripes appears. Such a pattern could be produced only if the photons are behaving like waves, with each individual photon spreading out and surging against both slits at once, like a breaker hitting a jetty. Alternating bright stripes in the pattern on the film show where crests from those waves overlap; dark stripes indicate that a crest and a trough have canceled each other.

The outcome of the experiment depends on what the physicists try to measure: If they set up detectors beside the slits, the photons act like ordinary particles, always traversing one route or the other, not both at the same time. In that case the striped pattern doesn't appear on the film. But if the physicists remove the detectors, each photon seems to travel both routes simultaneously like a tiny wave, producing the striped pattern.

Wheeler has come up with a cosmic-scale version of this experiment that has even weirder implications. Where the classic experiment demonstrates that physicists' observations determine the behavior of a photon in the present, Wheeler's version shows that our observations in the present can affect how a photon behaved in the past.

Imagine a quasar— a very luminous and very remote young galaxy. Now imagine that there are two other large galaxies between Earth and the quasar. As we now know from the programme Karl mentioned in his posting, the gravity from massive objects like galaxies can bend light, just as conventional glass lenses do. In Wheeler's experiment the two huge galaxies substitute for the pair of slits; the quasar is the light source. Just as in the two-slit experiment, light— photons— from the quasar can follow two different paths, past one galaxy or the other.

Suppose that on Earth, some astronomers decide to observe the quasars. In this case a telescope plays the role of the photon detector in the two-slit experiment. If the astronomers point a telescope in the direction of one of the two intervening galaxies, they will see photons from the quasar that were deflected by that galaxy; they would get the same result by looking at the other galaxy. But the astronomers could also mimic the second part of the two-slit experiment. By carefully arranging mirrors, they could make photons arriving from the routes around both galaxies strike a piece of photographic film simultaneously (see the diagram at the top of this posting). Alternating light and dark bands would appear on the film, identical to the pattern found when photons passed through the two slits.

Here's the odd part. The quasar could be very distant from Earth, with light so faint that its photons hit the piece of film only one at a time. But the results of the experiment wouldn't change. The striped pattern would still show up, meaning that a lone photon not observed by the telescope traveled both paths toward Earth, even if those paths were separated by many light-years. And that's not all.

By the time the astronomers decide which measurement to make— whether to pin down the photon to one definite route or to have it follow both paths simultaneously— the photon could have already journeyed for billions of years, long before life appeared on Earth. The measurements made now, says Wheeler, determine the photon's past. In one case the astronomers create a past in which a photon took both possible routes from the quasar to Earth. Alternatively, they retroactively force the photon onto one straight trail toward their detector, even though the photon began its jaunt long before any detectors existed.

Clearly this is exactly what I suggest in ITLAD. This universe is your universe - the being reading these words. It is one of trillions of personal Everett Universes rooted in David Bohm's 'Implicate Order'.
For a graphical example of the Two Slit Experiment please check out:


Hurlyburly said...

I'm worried.

I understood all of that! Shall post my reaction when i get time, great read though.

Karl Le Marcs said...

As Tony knows, I am in full agreement with Wheeler on this. I too shall post more, when not hurried over time by some grumpy yet strangely attractive librarian stood behind me.

Anthony Peake said...

Thanks HB. When I do my lectures I go through this in a graphic format that really goes back to the basics. Indeed this can also be used to explain why Hugh Everett came up with his Many Worlds Interpretation.