Author Topic: Delayed Choice Quantum Eraser Experiment  (Read 9952 times)

Offline josephpalazzo (OP)

Delayed Choice Quantum Eraser Experiment
« on: May 07, 2014, 11:37:40 AM »
A simple technical explanation of this experiment.




Red → photons from slit A go to D0, {D1, D2}, D4 (but not D3)
Blue → photons from slit B go to D0, {D1, D2}, D3 (but not D4)
D0→ it is a simple quantum eraser experiment, with no delayed choice.
A photon that goes to D1, D2, D3 and D4 is called an “idler”; these photons are entangled with photons going at D0.



D1, D2→ photon comes from either A or B; we get interference like in a double-slit experiment.
D3→ we know it’s a photon coming from the slit A; there’s no interference like in a double-slit experiment with a detector at slit A.
D4→ we know it’s a photon coming from the slit B; there’s no interference like in a double-slit experiment with a detector at slit B.

Interpretation (wrong): some have interpreted that the delayed choice to observe or not the path of the idler changes the outcome of an event in the past. This is not the general consensus. In fact it would violate causality.

The appropriate answer is that in one particular set of circumstances, particle behavior is exhibited, while in a different set of circumstances, wave behavior is exhibited, and this behavior can morph from one circumstance to the next.

Note:  the technical word “eraser” simply signifies that we don’t know which path was taken by the photon. It has nothing to do as in “erasing the memory of the photon” or “erasing our memory of which path the photon took.”

Offline Casparov

Re: Delayed Choice Quantum Eraser Experiment
« Reply #1 on: May 07, 2014, 02:46:31 PM »
http://www.bottomlayer.com/bottom/kim-scully/kim-scully-web.htm

This break down of the experiment was reviewed and approved by Dr. Kim who actually conducted the experiment. The conclusion is:

Quote
Upon accessing the information gathered by the Coincidence Circuit, we the observer are shocked to learn that the pattern shown by the positions registered at D0 at Time 2 depends entirely on the information gathered later at Time 4 and available to us at the conclusion of the experiment.

The position of a photon at detector D0 has been registered and scanned. Yet the actual position of the photon arriving at D0 will be at one place if we later learn more information; and the actual position will be at another place if we do not.

The instance you are referring to when you say: "some have interpreted that the delayed choice to observe or not the path of the idler changes the outcome of an event in the past. This is not the general consensus. In fact it would violate causality,"

Is shown to be a correct interpretation of this more recent version of the Delayed Choice Experiment: "Quantum Erasure With Causally Disconnected Choice," http://arxiv.org/pdf/1206.6578v2.pdf completed in 2013 by Zeilinger's team. You state, "In fact it would violate causality." and Yes indeed it does. This experiment does violate causality as is explained in the paper:

Quote
In all experiments performed to date, this choice took place either in the past or, in some delayed-choice arrangements, in the future of the interference. Thus in principle, physical communications between choice and interference were not excluded. Here we report a quantum eraser experiment, in which by enforcing Einstein locality no such communication is possible.... No naive realistic picture is compatible with our results because whether a quantum could be seen as showing particle- or wave-like behavior would depend on a causally disconnected choice. It is therefore suggestive to abandon such pictures altogether.

Quote
If the observer measures the photons, his choice of the type of measurement decides whether the atoms can be described by a wave or a particle picture. Firstly, when the photons are measured in a way that reveals welcher-weg information of the atoms, the atoms do not show interference, not even conditionally on the photons’ specific mea- surement results. Secondly, if the photons are measured such that this irrevocably erases any welcher-weg information about the atoms, then the atoms will show perfect but distinct interference patterns.

Quote
Since the welcher-weg information of the atoms is carried by the photons, the choice of measurement of the photons— either revealing or erasing the atoms’ welcher-weg information—can be delayed until “long after the atoms have passed” the photon detectors at the double slit. The later measurement of the photons ‘decides’ whether the atoms can show interference or not even after the atoms have been detected.

Quote
Our work demonstrates and confirms that whether the correlations between two entangled photons reveal welcher- weg information or an interference pattern of one (system) photon, depends on the choice of measurement on the other (environment) photon, even when all the events on the two sides that can be space-like separated, are space-like separated. The fact that it is possible to decide whether a wave or particle feature manifests itself long after—and even space-like separated from—the measurement teaches us that we should not have any naive realistic picture for interpreting quantum phenomena.

Causality is thus violated and the interpretation that the choice to observe or not changes the outcome of the past is and has been a valid interpretation since the year 2013. Further:

The naïve realist theory may be characterized as the acceptance of the following five beliefs:

1) There exists a world of material objects.
2) Some statements about these objects can be known to be true through sense-experience.
3) These objects exist not only when they are being perceived but also when they are not perceived. The objects of perception are largely perception-independent.
4) These objects are also able to retain properties of the types we perceive them as having, even when they are not being perceived. Their properties are perception-independent.
5) By means of our senses, we perceive the world directly, and pretty much as it is. In the main, our claims to have knowledge of it are justified.
« Last Edit: May 07, 2014, 02:56:19 PM by Casparov »
“The Fanatical Atheists are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who—in their grudge against traditional religion as the "opium of the masses"—cannot hear the music of other spheres.” - Albert Einstein

Offline josephpalazzo (OP)

Re: Delayed Choice Quantum Eraser Experiment
« Reply #2 on: May 07, 2014, 02:59:02 PM »
All of these interpretations have proven to be useless as they do not lead to any new predictions. You can find tons of these speculations but none predicts anything new, you can use the paper iit's written on as toilet paper.



Re: Delayed Choice Quantum Eraser Experiment
« Reply #3 on: May 07, 2014, 03:14:13 PM »
Casparov, you again post crap that is not from main stream scientists, but science writers that are also prejudiced by religion, are philosophers, or to promote their agenda that questions science. You are still being disingenuous after being called out on it.

Quote
Ross Rhodes is a science writer and lecturer for both professional and lay audiences, specializing in the philosophical implications of quantum mechanics. He is the founder of and principal contributor to BottomLayer.com, a web resource devoted to the convergence of physics, philosophy, and computer science. Mr. Rhodes has written for publications in science and theology; lectured in the United States and Europe on the foundations of quantum mechanics; and provided editorial assistance to leading researchers in the field. He obtained his B.A. from Tufts University in 1974, and his Juris Doctor from the University of Connecticut in 1981.
Just go play in traffic! Solitary
There is nothing more frightful than ignorance in action.

Re: Delayed Choice Quantum Eraser Experiment
« Reply #4 on: May 07, 2014, 03:23:48 PM »
The Juris Doctor (J.D.) is a professional doctorate and first professional graduate degree in law. The degree is earned by completing law school in the United States, Canada, Australia, and other common law countries. Those who hold the degree of Juris Doctor are professionals committed to the practice of law, and may choose to focus their practice on criminal law, personal injury, family law, corporate law, or a wide range of other areas.

Most individuals holding a Juris Doctor must pass an exam to be licensed to practice law within their jurisdiction. Professionals who pass the required bar examination are known as lawyers or attorneys. Not all J.D. degree holders sit for the bar exam, and thus not all J.D. holders are licensed attorneys, unless the jurisdiction permits otherwise.

The degree was first awarded in the United States in the late 19th century and was created as a modern version of the old European doctor of law degree (such as the Dottore in Giurisprudenza in Italy and the Juris Utriusque Doctor in Germany and Central Europe). Originating from the 19th century Harvard movement for the scientific study of law, it is a law degree that in most common law jurisdictions is the primary professional preparation for lawyers. It is a three-year program in most jurisdictions.

You post good sources Casparov. Still being disingenuous after being called out for it. If it was up to me you would be banned for being dishonest and having an anti science agenda.
There is nothing more frightful than ignorance in action.

Offline Casparov

Re: Delayed Choice Quantum Eraser Experiment
« Reply #5 on: May 07, 2014, 03:34:50 PM »
All of these interpretations have proven to be useless as they do not lead to any new predictions. You can find tons of these speculations but none predicts anything new, you can use the paper iit's written on as toilet paper.

Are you serious? Anton Zeilinger is world renowned as on of the most respected living Experimental Quantum Physicists alive today second only to maybe Alain Aspect! His lab and team in Vienna is one of the most well funded and well staffed operations in the world. Are you kidding me right now?? Your going to wipe your ass with any experimental proof that disagrees with your beliefs? What are you a Fundamentalist Materialist?

You're going to say you can throw out the entire experiment and it's conclusive results on the basis that "it doesn't predict anything new"? First of all, you don't know that that is the case. Zeilinger is applying Quantum Theory to applications within the field of Quantum Cryptology and Quantum Optics, so no these experiments are not worthless. Second of all, even if they didn't predict anything new, that is no basis to entirely disregard conclusive experimental results! I mean wtf is going on here....

Okay well then change your statement to:

Interpretation (correct): some have interpreted that the delayed choice to observe or not the path of the idler changes the outcome of an event in the past. This is has been proven experimentally in 2013. In fact it conclusively violates causality. But you can just throw out all of those experimental proofs because they don't predict anything new and are therefore worthless. So in conclusion, causality is not violated nana nana boo boo!

This is getting sad. You don't debate the evidence, you look for any opportunity to just throw the evidence out instead... Atheist Apologetics at it's best.
“The Fanatical Atheists are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who—in their grudge against traditional religion as the "opium of the masses"—cannot hear the music of other spheres.” - Albert Einstein

Re: Delayed Choice Quantum Eraser Experiment
« Reply #6 on: May 07, 2014, 03:41:07 PM »
 :popcorn:
Winner of WitchSabrinas Best Advice Award 2012


We can easily forgive a child who is afraid of the dark; the real
tragedy of life is when men are afraid of the light. -Plato

Re: Delayed Choice Quantum Eraser Experiment
« Reply #7 on: May 07, 2014, 03:43:58 PM »
Has anybody bothered to point out that JP is an actual Physicist? Casparov is a, uh, what?

Re: Delayed Choice Quantum Eraser Experiment
« Reply #8 on: May 07, 2014, 03:57:50 PM »
Are you serious? Anton Zeilinger is world renowned as on of the most respected living Experimental Quantum Physicists alive today second only to maybe Alain Aspect! His lab and team in Vienna is one of the most well funded and well staffed operations in the world. Are you kidding me right now?? Your going to wipe your ass with any experimental proof that disagrees with your beliefs? What are you a Fundamentalist Materialist?

You're going to say you can throw out the entire experiment and it's conclusive results on the basis that "it doesn't predict anything new"? First of all, you don't know that that is the case. Zeilinger is applying Quantum Theory to applications within the field of Quantum Cryptology and Quantum Optics, so no these experiments are not worthless. Second of all, even if they didn't predict anything new, that is no basis to entirely disregard conclusive experimental results! I mean wtf is going on here....

Okay well then change your statement to:

Interpretation (correct): some have interpreted that the delayed choice to observe or not the path of the idler changes the outcome of an event in the past. This is has been proven experimentally in 2013. In fact it conclusively violates causality. But you can just throw out all of those experimental proofs because they don't predict anything new and are therefore worthless. So in conclusion, causality is not violated nana nana boo boo!

This is getting sad. You don't debate the evidence, you look for any opportunity to just throw the evidence out instead... Atheist Apologetics at it's best.

After his return to Europe, he built up an interferometer for very cold neutrons which preceded later similar experiments with atoms. The fundamental experiments there included a most precise test of the linearity of quantum mechanics and a beautiful double-slit diffraction experiment with only one neutron at a time in the apparatus. Actually, in that experiment, while one neutron was registered, the next neutron still resided in its Uranium nucleus waiting for fission to happen. Solitary
There is nothing more frightful than ignorance in action.

Offline Casparov

Re: Delayed Choice Quantum Eraser Experiment
« Reply #9 on: May 07, 2014, 04:04:52 PM »
Casparov, you again post crap that is not from main stream scientists, but science writers that are also prejudiced by religion, are philosophers, or to promote their agenda that questions science. You are still being disingenuous after being called out on it.
 Just go play in traffic! Solitary

Solitary, let me explain something to you my friend. It is a LOGICAL FALLACY to throw out evidence based on the personal beliefs of a person. You are not debating or even addressing the ACTUAL EVIDENCE you are merely taking the easy way out and saying "THIS GUY IS NOT AN ATHEIST!!! OMG ALL OF HIS EVIDENCE IS AUTOMATICALLY FALSE!!" This does not follow, and thus you are employing a Logical Fallacy rather than an actual argument about the evidence in question. You have done this probably FIVE TIMES now and each time I have pointed out to you that discrediting a person based on your own opinions does nothing to address the actual evidence.

Secondly, if the evidence at hand actually does rule out Realism/Materialism then it is quite reasonable for a person who understands and faces this fact head on to not be an Atheist. You cannot therefore dismiss the evidence solely on the basis that the person presenting it is not an Atheist. The evidence in question disputes that Atheism as a logical conclusion based on the fact that Realism/Materialism is a false assumption about reality. "Look! That guy is not an atheist!" is not an argument silly.

And third, the source I provided is one of the only sources I have provided that is not a peer reviewed scientific paper. The only reason I provided this source is because it breaks down the actual experiment by Dr. Kim and Dr. Scully in layman's terms and can be trusted to do so accurately because it was reviewed by the actual experimenter himself before it was published:

Quote
Your commentator wishes to thank Dr. Kim for reviewing this commentary before posting.

A presents evidence for X
A believes Y
Solitary disagrees with Y
Solitary rejects evidence X because A believes Y

Solitary never addresses evidence X, he rejects it outright on the basis that A believes Y, and Solitary has pre-concluded that Y is definitely false. The subtext is, "Anyone that believes Y cannot be trusted about anything ever therefore any evidence presented by someone who believes Y can be dismissed entirely without a second thought."

The problem here Solitary my friend, is that you are only looking for "someone to trust" and arguing that "this guy can't be trusted" but my dear friend, I am telling you "don't trust anybody, look at the god damn evidence!!". Do you see? It matters not who is presenting it or what they believe, you have still not addressed the evidence itself.

The fact that you continually argue that people can't be trusted based on their personal beliefs demonstrates that you only are looking for trusted people that can give you beliefs. Perhaps you have found such trusted people in Richard Dawkins or Daniel Dennett, but clinging to someone whom you trust and believing everything they tell you is no better than clinging to the Bible and believing everything it says. You are no different unless you pull yourself up by your own bootstraps, take responsibility for your own beliefs, and do the work of examining the evidence for yourself to come to your own conclusion.

Going from Fundamentalist Christian to Fundamentalist Materialist is no real move at all. You are running in place. You have moved sideways but not forward. Your mindset is still one that cherishes belief over evidence. You are no more open minded than a Creationist. Your mind is closed to your belief and you automatically dismiss everything that disagrees with it unless it comes from someone you trust based on their beliefs agreeing with yours. You are out in the world looking for beliefs and choosing who you accept them from.

Address the evidence or stfu.
« Last Edit: May 07, 2014, 04:10:42 PM by Casparov »
“The Fanatical Atheists are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who—in their grudge against traditional religion as the "opium of the masses"—cannot hear the music of other spheres.” - Albert Einstein

Re: Delayed Choice Quantum Eraser Experiment
« Reply #10 on: May 07, 2014, 04:05:05 PM »
Are you serious? Anton Zeilinger is world renowned as on of the most respected living Experimental Quantum Physicists alive today second only to maybe Alain Aspect! His lab and team in Vienna is one of the most well funded and well staffed operations in the world. Are you kidding me right now?? Your going to wipe your ass with any experimental proof that disagrees with your beliefs? What are you a Fundamentalist Materialist?

You're going to say you can throw out the entire experiment and it's conclusive results on the basis that "it doesn't predict anything new"? First of all, you don't know that that is the case. Zeilinger is applying Quantum Theory to applications within the field of Quantum Cryptology and Quantum Optics, so no these experiments are not worthless. Second of all, even if they didn't predict anything new, that is no basis to entirely disregard conclusive experimental results! I mean wtf is going on here....

Okay well then change your statement to:

Interpretation (correct): some have interpreted that the delayed choice to observe or not the path of the idler changes the outcome of an event in the past. This is has been proven experimentally in 2013. In fact it conclusively violates causality. But you can just throw out all of those experimental proofs because they don't predict anything new and are therefore worthless. So in conclusion, causality is not violated nana nana boo boo!

This is getting sad. You don't debate the evidence, you look for any opportunity to just throw the evidence out instead... Atheist Apologetics at it's best.

Aspect is a graduate of the École Normale Supérieure de Cachan (ENS Cachan). He passed the 'agrégation' in physics in 1969 and received his master's degree from Université d'Orsay. He then did his national service, teaching for three years in Cameroon.

In the early 1980s, while working on his PhD thesis[1] from the lesser academic rank of lecturer, he performed the elusive "Bell test experiments" that showed that Albert Einstein, Boris Podolsky and Nathan Rosen's reductio ad absurdum of quantum mechanics, namely that it implied 'ghostly action at a distance', did in fact appear to be realised when two particles were separated by an arbitrarily large distance (see EPR paradox). A correlation between their wave functions remained, as they were once part of the same wave-function that was not disturbed before one of the child particles was measured.

If quantum theory is correct, the determination of an axis direction for the polarization measurement of one photon, forcing the wave function to 'collapse' onto that axis, will influence the measurement of its twin. This influence occurs despite any experimenters not knowing which axes have been chosen by their distant colleagues, and at distances that disallow any communication between the two photons, even at the speed of light.

Aspect's experiments were considered to provide overwhelming support to the thesis that Bell's inequalities are violated in its CHSH version. However, his results were not completely conclusive, since there were so-called loopholes that allowed for alternative explanations that comply with local realism. See local hidden variable theory.

Stated more simply, the experiment provides strong evidence that a quantum event at one location can affect an event at another location without any obvious mechanism for communication between the two locations. This has been called "spooky action at a distance" by Einstein (who doubted the physical reality of this effect). However, these experiments do not allow faster-than-light communication, as the events themselves appear to be inherently random.

After his works on Bell's inequalites, he turned toward studies of laser cooling of neutral atoms and is now mostly involved in Bose–Einstein condensates related experiments.Why do you lie so much Casparov? Solitary
There is nothing more frightful than ignorance in action.

Offline josephpalazzo (OP)

Re: Delayed Choice Quantum Eraser Experiment
« Reply #11 on: May 07, 2014, 04:14:17 PM »
:popcorn:

Are you kidding me right now?? Your going to wipe your ass with any experimental proof that disagrees with your beliefs? What are you a Fundamentalist Materialist?


This might bore you to death, but the DCQE experiment gives exactly the same results as the double-slit experiment:

1) If the path of the photon is unknown, there is an interference - you get that with the double-slit.
2) If the path of the photon is known, there is no interference - you also get that with the double-slit.

The only safe conclusion is that it doesn't matter if you place an eraser and/or a delayed choice in the experiment. Now if you had had a different result than the double-slit experiment, then you would have something to chew on. But that is not the case, and when you think about it, that makes sense that you would get exactly the same result: why would the photon behave differently just because you made it go through a longer path in one part of the experiment than its entangled partner? Answer: no reason whatsoever, which is what nature is telling us.

Any other interpretation is hogwash.


Note: appealing to authority won't save you as I have a PhD in physics, and I have been teaching this stuff for over 25 years. FYI, Zeilinger is small potato. Had you quoted Witten, Maldacena or Susskind, I might have paid a little more attention. 


Offline josephpalazzo (OP)

Re: Delayed Choice Quantum Eraser Experiment
« Reply #12 on: May 07, 2014, 04:24:19 PM »


After his works on Bell's inequalities, he (Aspect) turned toward studies of laser cooling of neutral atoms and is now mostly involved in Bose–Einstein condensates related experiments.


That is an important point. Most people who have done some work on Bell's theorem have moved on as there is nothing to be accomplished, except to come up with a better theory than QM.  Fat chance this will happen. But needless to say, there are still those die-hard's who continue to try to prove QM is wrong. It's either bravery or stupidity... whatever.

Offline Casparov

Re: Delayed Choice Quantum Eraser Experiment
« Reply #13 on: May 07, 2014, 04:29:42 PM »

This might bore you to death, but the DCQE experiment gives exactly the same results as the double-slit experiment:

1) If the path of the photon is unknown, there is an interference - you get that with the double-slit.
2) If the path of the photon is known, there is no interference - you also get that with the double-slit.

The only safe conclusion is that it doesn't matter if you place an eraser and/or a delayed choice in the experiment. Now if you had had a different result than the double-slit experiment, then you would have something to chew on. But that is not the case, and when you think about it, that makes sense that you would get exactly the same result: why would the photon behave differently just because you made it go through a longer path in one part of the experiment than its entangled partner? Answer: no reason whatsoever, which is what nature is telling us.

Any other interpretation is hogwash.

Well I am in total agreement with you here. There are however some extra things that both Eraser and Delayed Choice Experiments teach us that the double-slit experiment by itself cannot.

A Quantum Eraser for instance shows us that the measuring device alone does not cause collapse unless a conscious observer is able to obtain the which-path information from it. If a measuring device makes a measurement and then the which-path information is made unretrievable (no conscious observer will ever see it), the interference pattern remains as if no measurement had ever taken place. The double-slit alone could not have told us that.

Also the Delayed Choice experiment shows us that the wave or particle behavior is not dependent on space-time and violates causality. In Wheeler’s words: “We, now, by moving the mirror in or out have an unavoidable effect on what we have a right to say about the already past history of that photon.” A choice in the future determines the past of the particle, whether or not it passed through the experiment as a wave the entire time or a particle the entire time.

Further the violation of both Bell's and Leggett's inequalities show us that The Principle of Locality is violated as well. All of these details the double-slit alone could not tell us. But agree that yes, in the end it is still the case that:

1) If the path of the photon is unknown, there is an interference - you get that with the double-slit.
2) If the path of the photon is known, there is no interference - you also get that with the double-slit.

This is an undisputed fact at this point. And doesn't have to just be a photon, it could be a 430 atom large molecule.
“The Fanatical Atheists are like slaves who are still feeling the weight of their chains which they have thrown off after hard struggle. They are creatures who—in their grudge against traditional religion as the "opium of the masses"—cannot hear the music of other spheres.” - Albert Einstein

Re: Delayed Choice Quantum Eraser Experiment
« Reply #14 on: May 07, 2014, 04:44:55 PM »
The Existence of the Physical World


Most people idly wonder, at some point, whether the physical world exists, or whether it is all one great illusion. We could be under the sway of a deceptive demon, or (in a more modern alternative) brains in the vat of some mad scientist, or (in an even more contemporary version) stuck in The Matrix. When it comes time to have a cup of coffee, talk to a friend, or even do some work, we tend to put these possibilities out of our heads. But we might wonder whether doing these things has any point if there is no physical world. If we can't prove that there is, are we justified in thinking and acting as if there were? And what does it mean to wonder whether there is a physical world, anyway?

The last question is, of course, the first that we have to consider. It is actually tougher than we might suppose. The common-sense answer is: a three-dimensional world filled with objects, each of which has a certain colour, shape, smell and so on. This view seems simple, but (as we shall see) it is actually quite mysterious and ultimately untenable. A less ambitious answer is that the physical world is something external to us, at least in part causing our sensations, ideas and so on. This must be a part of the notion, but it cannot be the whole of it. For many 'idealists', who have denied a physical world, nevertheless think that there are some things external to us - for instance, other minds, or Berkeley's ideas in the mind of God, or Leibniz's rudimentary minds.

 Material things must then be both external to us and not mental. It is harder to get a more precise grip on the notion of 'material' than that, which may bode poorly for it. Often, when we cannot articulate a notion, and only say what it is not, this is because it is not really reflective of anything.

Before we go on to examine this, someone might want to ask whether we really can assume there is an external world. Perhaps all the places we pass through and people we talk to are just figments of our imagination, as in a dream. This position is known as solipsism, and it really would alter the way you thought about the world, placing all the objects and people (other minds) that we interact with solely within your own mind. It is important to realize that it cannot ever be completely disproved: any evidence could be dismissed as the invention of a bloody-minded mind, intent (for some obscure reason) on deceiving itself.

While solipsism cannot be disproved, most people find certain considerations against it persuasive. The (subtly different) questions of whether these considerations would convince the strongest sceptic, and whether they should convince us if not, are dealt with in another essay. Here, I shall assume that our judgements are reasonable, seeing only where they lead us.

What convinces us (rightly or wrongly) of the falsehood of solipsism is that we are not conscious of creating the world around us, but often seem to passively experience it, encountering new things (like elephants) we had never come across before. The idea of an elephant could admittedly be fabricated from pre-existing concepts, but some of these (like the colour grey) also come from experience, and could not. It could always be argued that these are creations of the unconscious mind, but this is something external to our immediate consciousness, 'discovered' by science and psychology rather than any direct awareness of it, so would strictly count as part of an external world. We can thus convincingly argue that the theory that something external affects our mind fits much better with our everyday experience.

So if we say that our experiences likely come at least in part from something external to us, where do they come from? What about the possibility that there is an external world, which causes our perceptions and ideas, but it is completely unlike the world we assume exists? This is the case in Descartes' famous supposition that an evil demon might be feeding him false perceptions. It is interesting to note that in the common thought experiments of the brain-in-the-vat, The Matrix, and the evil demon (if you think demons are physical, that is - Descartes spoke of a 'spirit'), there is a physical world which causes our experiences; we are just deceived about the contents of that world. But when we talk about the physical world existing or not, we tend to mean a physical world somewhat like the one we seem to experience.

Again, these possibilities cannot be completely disproved. This is because we can only know about the physical world through our senses and what we learn from them. Descartes thought that he could banish the evil demon altogether by reasoning a priori, without reference to experience. His attempt to do so, by first proving God's existence and then arguing that since God is no deceiver, the physical world must exist, is largely discredited. But his general a priori approach was also misconceived, as the physical world is simply what we seem to perceive through our senses, so cannot be known a priori.

But several things convince us that we are not being deceived about the external world. For a start, it seems an entirely superfluous suggestion. The world seems quite unconcerned with us, or any conscious life - so far as we can tell, the laws it obeys are entirely impersonal. Sometimes they favour us, sometimes they don't; we seem to be accidental byproducts of nature. Granted, this appearance could all be part of an extraordinarily clever, apparently purposeless, illusion, but it is reason enough for supposing that the world around us is probably in some way real.

As mentioned earlier, several idealists have come up with worldviews that accept an external reality that we perceive, more or less correctly, but account for it purely in terms of mind, not matter. One of the most famous systems of this kind was that of Bishop Berkeley, an 18th century Irish philosopher. Like the solipsists, Berkeley noted that all we are ever directly aware of are the ideas in our mind (some philosophers would disagree with him, but this exploratory essay is not the place to dismiss their arguments). From this observation, he tried to show that it makes more sense to suppose that there are only ideas in the world than to suppose that there is matter, too. If we believed in matter, we would only be positing the existence of some mysterious, totally hidden substance, divorced from anything that we experience.

And this would violate Occam's Razor. Berkeley also exploited the confusion over how physical events could cause states of mind, arguing that they required mental causes. J.L. Mackie illustrates this argument with the analogy of a billiard ball moving because it is hit by another billiard ball, not because of some unrelated event halfway round the world. The implicit principle is that an event requires a similar cause, so physical causes could only give physical events. Modern neuroscience to some extent undermines this by revealing a close correlation between the physical and the mental, which are clearly not so separate as they were once thought to be.

Berkeley's simpler account of the world was that things exist only in being perceived: the Latin slogan was esse est percipi. However, he avoided the claim that the world ceases to exist when we are not aware of it (the weakness of solipsism.) Objects continue to exist, independent of our perception of them, by virtue of being ideas in the mind of God, which are what God makes us aware of when we experience the real world. So there is an external world, found in God's mind, just not a physical one. Matter is dispensed with altogether.

There are, however, great difficulties with explaining the world in purely mental terms. As Bertrand Russell pointed out in The Problems of Philosophy, though ideas and perceptions are by definition in our minds, that does not mean that their objects are. For instance, idealists generally accept the reality of other people (or at least other minds), yet when we 'bear them in mind', that does not mean that they only exist in our minds any more than the fact that the idea of an orange can only exist in your mind implies that there is no orange outside it. There seems to be an inconsistency here.

The metaphysics behind Berkeley's system has since been undermined. If all the traditional arguments for God's existence fail, and we see no reason to believe in any sort of 'divine mind', it is difficult to see a mental basis for facts about the world. This either takes us back to the problematic view that cats and the rest cease to exist when we are not around, or requires that they have a non-mental existence. And by our definition, a non-mental existence is a material existence.

At this point, we must return to the question of just what material existence is. We know what mental existence is, because our experience of our own thoughts and perceptions is as direct as one could want. But we can only experience the physical objects we take to exist through the medium of such perceptions, so strictly speaking we only know about their power to cause these. This raises the interesting question of whether we directly perceive the external world, or are aware only of our minds' representations of it. And, if the answer is the latter, can we have any idea what the physical world is like?

Locke and the British empiricists in the early centuries of the scientific revolution made a famous distinction between 'primary' and 'secondary' qualities of objects. Primary qualities are the basic elements of modern physics: extension (shape), solidity and motion. Secondary qualities are things like colour, taste and smell, which are a major part of our perception of the world but can be understood in terms of the primary qualities that cause them. For instance, modern science tells us that colour can be understood by the propensity of a surface to reflect certain frequencies of light, dependent on its structure. So an orange doesn't really 'have' a colour, only the ability to cause the experience of orange in a mind.

By itself, this is a radical alteration of the way we look at the world: every object we see appears coloured, so to that extent we are not seeing the world as it 'really' is. Unreflective common sense tends to assume that oranges really are orange. Yes, they may look different given different lighting, perspective or eyes, but there is one right colour: the one you would see in bright sunlight, with normal human eyes. But, as Russell pithily observed, this is "favouritism".

This implies that the physical world is made up of objects with shape and solidity, in motion or at rest, and that colour and other 'secondary' qualities are just representations of these basic, 'primary' properties. However, even this view is extremely problematic. We can understand the primary qualities only by considering how they affect us, the observers. The shape of an object, for example, looks different from different perspectives, none more privileged than any other. So the three-dimensional shape we attribute to it cannot be directly perceived; is is constructed out of these two-dimensional perceptions.

And when we consider what it is that fills this shape, what it is that moves around, we are left able only to say that it is something 'solid'. Locke was very taken with this quality, but even his description reveals that it can only be understood in terms of the resistance solid objects offer us. Berkeley argued that this meant we were back to understanding the object solely in terms of our perceptions of it, which can differ from creature to creature: an orange will offer more resistance to an ant than to a woodpecker.

So we experience objects purely through their capacity to cause certain sensations in us, or alter other objects, which in turn cause sensations in us. This conclusion sounds tautological once we have laid it out: how else could we experience them? We have seen that there is good reason to suppose that objects have an external, non-mental existence. But we seem now to have concluded that we can have no understanding of what this existence is except by reference to our internal mental states. Imagine an orange losing its potency to cause various experiences one by one. First, its taste goes, then its colour, then its smell, then the sound it makes when bounced. Finally, with the sense of touch, we lose the last manifestations of what we call its shape, as well as its texture and resistance. We now seem to be left with no concept of what the orange is, in itself.

We may be reminded of Kant's claim that we can know nothing of the real, 'noumenal' nature of objects, and our limited to considering the phenomenal world of our experience, which is dependent on the mind for its structure and appearance. One of Berkeley's main arguments against those who believed in matter was: "you know neither what it is in itself, nor what relation it bears to accidents."[1] If this is really the case, we may be worried about what it means to say the physical world exists.

The picture of the world that modern physics offers is that of a vast, interrelated network of forces, operating on a mysterious thing called matter. This can be seen in the way matter is merely defined as "the object of inertial mass", which says nothing about what it is in itself. Scientists would doubtless dismiss a request to do more as meaningless.

However, what physics does offer is a powerful, coherent and elegant way of understanding why we have the perceptions we do, or - if your inclinations are anti-realist - of systematising them. For instance, the different resistances oranges offer to woodpeckers and ants can be understood in terms of woodpeckers' greater force and sharper beakes, letting us assign a constant number to the amount of resistance offered by our orange. Likewise, all observers in scientific enquiry can agree that oranges reflect those frequencies of light labelled 'orange', no matter how such frequencies appear to their different eyes. Similar stories can be offered for all the other secondary qualities. Finally, physics treats matter as susceptible to certain apparently constant physical laws: oranges interact with other objects we perceive in predictable ways. So positing its existence goes a long way to explaining our experience.

This view of matter as a theoretical construct brought in to explain our experience is admittedly a long way away from our ordinary picture of the world. It treats our perceptions of the world not as neutral representations of the 'real' structure of things, but as to a large extent reflecting the structure imposed on that world by our minds. The similarity this picture bears to Kantianism is readily apparent: our knowledge of the world can only come from its effects on the five senses of sight, hearing, smell, touch and taste, while any 'noumenal' nature it might have remains forever hidden. Our senses have evolved to give us what we need to survive, not to satisfy our desire for knowledge of how the world really is (a desire which even if it is intelligible.  We should be thankful that they seem to do at least that.
There is nothing more frightful than ignorance in action.