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RECENT POSTS IN THIS TOPIC

**Georgina Woodward**: *on* 12/6/17 at 6:41am UTC, wrote As discreet sensors of some kind or another have to collect the em signals...

**John Cox**: *on* 12/6/17 at 4:12am UTC, wrote perhaps... lambda/alpha | h/-8pi

**John Cox**: *on* 12/5/17 at 18:49pm UTC, wrote Thanks Robert, You are quite correct. What the ChiCom (for the politically...

**Robert McEachern**: *on* 12/5/17 at 17:34pm UTC, wrote John, Your comment about "making communications undetectable in the first...

**John Cox**: *on* 12/5/17 at 15:28pm UTC, wrote All due respects for Ed Witten, but quantum communications are looking very...

**Pentcho Valev**: *on* 12/5/17 at 12:33pm UTC, wrote Nima Arkani-Hamed (06:09): "Almost all of us believe that space-time...

**Georgina Woodward**: *on* 12/5/17 at 10:45am UTC, wrote Quote: "I tend to assume that space-time and everything in it are in some...

**Georgina Woodward**: *on* 12/5/17 at 10:27am UTC, wrote The space-time continuum has issues. What is seen in a reference frame has...

FQXi FORUM

December 15, 2017

Natalie Wolchover has a fascinating interview with string theorist Ed Witten, in Quanta on dualities and what is real and fundamental. Thank you to Steve Agnew for suggesting that this would be a could discussion topic.

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Edward Witten: "I tend to assume that space-time and everything in it are in some sense emergent. By the way, you'll certainly find that that's what Wheeler expected in his essay. As you'll read, he thought the continuum was wrong in both physics and math. He did not think one's microscopic description of space-time should use a continuum of any kind - neither a continuum of space nor a continuum of time, nor even a continuum of real numbers. On the space and time, I'm sympathetic to that."

One of my comments in Quanta:

"Emergent space-time" is a category mistake. Specetime has already emerged - it is a deductive consequence of Einstein's constant-speed-of-light postulate:

"Special relativity is based on the observation that the speed of light is always the same, independently of who measures it, or how fast the source of the light is moving with respect to the observer. Einstein demonstrated that as an immediate consequence, space and time can no longer be independent, but should rather be considered a new joint entity called "spacetime."

Anything deduced from different premises would fall in a different category and would have nothing to do with Einstein's spacetime - combining the two concepts would be absurd. For instance, bringing granularity to spacetime is equivalent to painting spacetime in yellow. If the original concept of spacetime is unacceptable and should be replaced, then the underlying premise, Einstein's constant-speed-of-light postulate, is false and should be abandoned. The step is unavoidable if logic is obeyed.

Pentcho Valev

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One of my comments in Quanta:

"Emergent space-time" is a category mistake. Specetime has already emerged - it is a deductive consequence of Einstein's constant-speed-of-light postulate:

"Special relativity is based on the observation that the speed of light is always the same, independently of who measures it, or how fast the source of the light is moving with respect to the observer. Einstein demonstrated that as an immediate consequence, space and time can no longer be independent, but should rather be considered a new joint entity called "spacetime."

Anything deduced from different premises would fall in a different category and would have nothing to do with Einstein's spacetime - combining the two concepts would be absurd. For instance, bringing granularity to spacetime is equivalent to painting spacetime in yellow. If the original concept of spacetime is unacceptable and should be replaced, then the underlying premise, Einstein's constant-speed-of-light postulate, is false and should be abandoned. The step is unavoidable if logic is obeyed.

Pentcho Valev

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Nima Arkani-Hamed (06:09): "Almost all of us believe that space-time doesn't really exist, space-time is doomed and has to be replaced by some more primitive building blocks."

Any substitute for spacetime that emerges from "some more primitive building blocks", that is, is deduced from premises different from the principle of relativity and the constancy of the speed of light, has nothing to do with Einstein's spacetime. The two fall in different categories. The new "spacetime" could replace the old one, but then the rejection of Einstein's spacetime would entail that at least one of Einstein's 1905 postulates is false.

Marcelo Gleiser: "The challenge is to somehow bring the notion of granularity to spacetime, bring the discrete to the continuous. This is the problem that has baffled theoretical physicists for at least half a century."

This is insane. Spacetime is not an ab initio model that one can modify, e.g. by introducing granularity. It is a DEDUCTIVE CONSEQUENCE of Einstein's constant-speed-of-light postulate, and if the consequence is unacceptable, the postulate is false (logic forbids the combination "true postulate, unacceptable consequence"). In other words, you cannot "retire" Einstein's spacetime without declaring the postulate false:

What scientific idea is ready for retirement? Steve Giddings: "Spacetime. Physics has always been regarded as playing out on an underlying stage of space and time. Special relativity joined these into spacetime... [...] The apparent need to retire classical spacetime as a fundamental concept is profound..."

Pentcho Valev

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Any substitute for spacetime that emerges from "some more primitive building blocks", that is, is deduced from premises different from the principle of relativity and the constancy of the speed of light, has nothing to do with Einstein's spacetime. The two fall in different categories. The new "spacetime" could replace the old one, but then the rejection of Einstein's spacetime would entail that at least one of Einstein's 1905 postulates is false.

Marcelo Gleiser: "The challenge is to somehow bring the notion of granularity to spacetime, bring the discrete to the continuous. This is the problem that has baffled theoretical physicists for at least half a century."

This is insane. Spacetime is not an ab initio model that one can modify, e.g. by introducing granularity. It is a DEDUCTIVE CONSEQUENCE of Einstein's constant-speed-of-light postulate, and if the consequence is unacceptable, the postulate is false (logic forbids the combination "true postulate, unacceptable consequence"). In other words, you cannot "retire" Einstein's spacetime without declaring the postulate false:

What scientific idea is ready for retirement? Steve Giddings: "Spacetime. Physics has always been regarded as playing out on an underlying stage of space and time. Special relativity joined these into spacetime... [...] The apparent need to retire classical spacetime as a fundamental concept is profound..."

Pentcho Valev

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All due respects for Ed Witten, but quantum communications are looking very realistic today. And it's the Chineese whom are taking the lead, at least publicly, with the first of a planned 20 satellites. It takes no great leap of understanding to recognize that an instantaneous correlation of paired events making communications undetectable in the first place, let alone decipherable, would render...

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John,

Your comment about "making communications undetectable in the first place, let alone decipherable" is not quite correct. The communications are potentially both detectable and decipherable. But they are non-compromisable, since, for example, it is possible to determine that someone has attempted to intercept a distributed key. In effect, an adversary can jam the system by constantly attempting to intercept such a system. Thus, while one might know when the system is in a secure state, that state can be denied by a suitable denial-of-service attack. Denying an adversary their secure form of communications, in order to force them to use an insecure one, whenever it is essential for them to communicate, is a common strategy in communications warfare.

Rob McEachern

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Your comment about "making communications undetectable in the first place, let alone decipherable" is not quite correct. The communications are potentially both detectable and decipherable. But they are non-compromisable, since, for example, it is possible to determine that someone has attempted to intercept a distributed key. In effect, an adversary can jam the system by constantly attempting to intercept such a system. Thus, while one might know when the system is in a secure state, that state can be denied by a suitable denial-of-service attack. Denying an adversary their secure form of communications, in order to force them to use an insecure one, whenever it is essential for them to communicate, is a common strategy in communications warfare.

Rob McEachern

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It is all a question of numbers after all...if a continuum is composed of 1e39 bits per angstrom, why is that different from a continuum?

Einstein's spacetime is a continuum approximation of a very large number of discrete quanta. So spacetime works very well and quantum works very well and all we need is a connection that also works very well...

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Einstein's spacetime is a continuum approximation of a very large number of discrete quanta. So spacetime works very well and quantum works very well and all we need is a connection that also works very well...

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The space-time continuum has issues. What is seen in a reference frame has to be a sensory product or product of a device, post receipt and processing of EM signals. Vision requires that causal order of EM signal receipt preceding product seen. The process from detection of signal to product generation gives a resolution to the product so that it is not a continuum. Whether because of the limited number of cells of the visual cortex or pixels display-able on a screen.

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Quote: "I tend to assume that space-time and everything in it are in some sense emergent." Edward Witten. Yes, it has to be because Einstein was referring to what is *seen*. Being a physicist and not a biologist the process of seeing was not built into the work.

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As discreet sensors of some kind or another have to collect the em signals prior to their processing, discreetness is input to the production of seen space-time products. Even though some aggregation and averaging of the information generated from the signals occurs it isn't amalgamated into a fully continuous whole. The information is carried by discreet neurons or circuits. The information that ends up as the product is confined to the discreet components of the host surface.

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