Against Spacetime by Giovanni Amelino-Camelia

Dear Giovanni,

I read your essay with great interest. A few remarks/questions.

1. I agree wholeheartedly that modeling classical spacetime as a manifold over the continuum is at the very best redundant. Some would argue that it "does no harm" and that it "makes no difference" what the structure is at arbitrarily small scales, but something that bothers me about this view, in addition to the issues you raise (tunneling, etc.), is the fundamental role the representation theory of the Poincare symmetry group of Minkowski space plays in constraining the properties of particles in quantum field theory. I do not think it is at all obvious that altering this symmetry at small scales would have no practical effects.

2. What is your favorite interpretation of quantum theory? I ask this because it seems to me that the conceptual role of spacetime depends on this to a large degree. For instance, if you prefer a sum-over-histories view, you can separate the properties of classical spacetime from quantum effects arising through superposition of universes. If you prefer to think about a single universe and view classical physics as a limit via the correspondence principle, then it seems clear that such a universe is very unmanifold-like.

3. On a related note, one possible way of thinking about nonlocality is to provisionally banish the spacetime metric and simply regard direct influence between events as defining locality. The idea is that spacetime is a way of talking about relations among events, that it closely resembles a Lorentzian 4-manifold at large scales, but that "nonlocal interactions" with respect to the metric of this "nonphysical manifold idealization" are simply indications that the manifold is really fictional. Personally, I tend to think that the most prominent nonlocal phenomena such as entanglement have more to do with superposition than nonmanifold structure, but I do think that the large-scale properties of spacetime arise from more primitive microstructure, or, more conservatively, that models based on primitive microstructure will eventually outperform manifold models experimentally. See my essay here for more on this: On the Foundational Assumptions of Modern Physics.

3. I will have to have a look at your papers about observer-dependent association of processes to points in noncommutative spacetime. That is an idea I have not heard of before.

Thanks for the enjoyable read. Take care,

Ben Dribus

I will think about this more as well. There may be a more general isomorphism of some type.

I find myself wondering if twistores might be some type of connection to between loopy and stringy approaches to quantum gravity.

Cheers LC

Giovanni,

If the absurd "energy-dependent but observer-independent" speed of light is a failure, why don't you try, for a change, "energy-independent but observer-dependent" speed of light? I am not sure about the energy independence but "observer-dependent speed of light" sounds quite reasonable. I have tried to show this in my essay:

http://fqxi.org/community/forum/topic/1398

Pentcho Valev

Dear Giovanni,

I appreciate the response! I think we agree on how quantum theory should be viewed in general. As you suggest, however, different interpretations might generalize in different ways. For example, in Feynman's original sum over histories formulation (the 1948 paper), he sums over "particle trajectories" on a fixed background. If you believe GR, though, you know that the "spacetime" itself should respond in different ways to different trajectories, so each trajectory has its own spacetime, and suddenly you are summing over universes, but the underlying concept of a sum over histories comes through unscathed. The alternative formulation of a probability density function on spacetime, which works fine for a fixed background, doesn't generalize as easily to the background-independent scenario. Take care,

Ben

Giovanni

Can you advise if light passes through the visor material of your helmet at the constant speed c/n irrespective of the speed at which you are driving?

If so, have you considered that, as your visor is equivalent to a lens medium, CSL may then be simply explained by light changing speed on arrival at your visor or lens. Wavelength and thus frequency also change (inversely) because c (as c/n) is conserved in both the rest and co-moving (bike) frame.

The boundary condition between frames is the equivalent to dynamic fluid coupling at the surface fine structure of all dielectric media, including the diffuse ISM.

You did indicate you'd try to read my essay, I hope you may soon get there and comment. It considers this 'discrete field' model and boundary mechanism in detail, also with a little theatre. I have been unable to falsify it, found the implications broader than they may at first appear.

Many thanks in advance. PS, I confirm I still think yours is worth a far higher placing and intend to help it achieve that.

Best wishes

Peter

For better clarification my approach

I sending to you Frank Wilczek's 3 keen articles

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits388.pdf

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits393.pdf

http://ctpweb.lns.mit.edu/physics_today/phystoday/Abs_limits400.pdf

All the best

Yuri Danoyan

Giovanni,

{link:http://fqxi.org/community/forum/topic/1304]The problem as I see it[/link], is that we perceive time as a sequence from past to future and physics re-enforces this by treating it as a measurement issue, ie. clocks, detectors, days, etc, but the physical reality is the changing configurations turn future into past. It is not the earth traveling a narrative dimension from yesterday to tomorrow, but tomorrow becoming yesterday because the earth rotates. In this way, time is an effect of action. Like temperature it emerges from that basic thermodynamic activity. Clock rates vary, as levels of activity vary. More activity, faster clock rate. If time were a dimension from past to future, one would think a faster clock rate would travel into the future more quickly, but the opposite is true. As it ages/burns quicker, it moves into the past faster. The twin in the faster frame is dead when her twin in the slower frame returns.

Duration is not some dimension that transcends the present, but is the state of the present between detections.

Since the lightcone of any event is incomplete prior to the event, the future is probabilistic, even if the laws determining its outcome are deterministic.

It is the collapse of probabilities which yields actualities, so the cat is not both dead and alive, because there is no external timeline moving the present from past to future, but the actual occurrence of events turning future into past.

Cause and effect is not sequence, but energy exchange. Yesterday didn't cause today, any more than one rung on a ladder causes the next. It is the sun radiating on this rotating planet that creates this sequencing called days. Time is an effect, not a cause.

Knowledge is created inductively, as future becomes past, but is used deductively, as the past is used to predict the future.

Giovanni,

You will get maximum rating from me. Something tells me you share Magueijo's and Smolin's conviction that "the root of all the evil is special relativity":

Joao Magueijo, Faster Than the Speed of Light, p. 250: "Lee [Smolin] and I discussed these paradoxes at great length for many months, starting in January 2001. We would meet in cafés in South Kensington or Holland Park to mull over the problem. THE ROOT OF ALL THE EVIL WAS CLEARLY SPECIAL RELATIVITY. All these paradoxes resulted from well known effects such as length contraction, time dilation, or E=mc^2, all basic predictions of special relativity. And all denied the possibility of establishing a well-defined border, common to all observers, capable of containing new quantum gravitational effects."

Pentcho Valev

Hello Giovanni,

I found your essay one of the best I've read, and you argue very well for weaknesses in the spacetime interpretation at a small scale in some areas. But you talk as if all is well with spacetime elsewhere and generally, as intuition says it should be. Of course SR is right, but a close look and you'll see that all is not well generally with the spacetime interpretation. You say:

"The redundant abstraction of a macroscopic spacetime organizing all our particle detections is unproblematic and extremely useful in the classical-mechanics regime."

In fact it is not 'unproblematic', as spacetime leads unavoidably via a rigourous proof to block time, which requires motion through time not to exist, which in turn removes cause and effect. This loss of cause and effect is somewhat problematic in the classical mechanics regime.

You also say:

"Of course, the spacetime abstraction is unrenounceably convenient for organizing and streamlining our description of observations done in the classical-mechanics regime."

That is all true, except for the word 'unrenounceably'. It is convenient, and it seems harmless. But we don't understand time, and Minkowski spacetime contains a set of assumptions about time. Because we don't understand time, we have to look at the clues about time, as I've argued in my essay. If we don't look at the clues in front of us, however unexpected, I'm not sure how we're going to reach a point where we understand time. There are also missing pieces of the puzzle, and we need to allow for their existence.

Because it leads to block time, spacetime is in contradiction with the standard view of quantum theory about whether the future currently exists. That is a deep contradiction. And there are some places where the implications of spacetime jar with our picture that deeply, and conflict directly with what we observe. So although it is convenient to use, it looks like it might be flawed.

This means it is not like your analogy with the aether, which was portrayed in your quote as a useful concept, whether or not it actually exists. You sound almost like it makes little difference whether spacetime exists - and yet if it does exist, motion through time doesn't, and if it doesn't, motion through time can. That's a big difference.

So to me, you've not gone far enough in criticising spacetime, and could have strengthened your argument by pointing out other weaknesses. Of course SR is right, that has been extremely well confirmed by experiment. But the spacetime interpretation has not been confirmed, and if one gets conceptual problems with an interpretation, then one probably needs a new interpretation.

I'd appreciate any thoughts you might have on my essay. Yours and mine together show the problems with spacetime very well, though they deal with different areas.

Good luck. Best wishes,

Jonathan

Dear Giovanni Amelino-Camelia,

your introduction immediately caught my attention and compelled me to read to the end of your very fine essay. Though I disagree with the answers to those initial questions we are both saying that space-time is not required 'throughout'.I am not saying these following things to be critical of your writing but to highlight another option that is available. It is good to find someone who might be "on the same wavelength".

You wrote ".... Bob's reaction surely would have been going something like "what a stupid question! of course we all share the same time!" We now know that this question is meaningful and actually the answer is no: we established experimentally that observers in relative motion do not share the same time."

It is a fascinating conundrum. What we have IMO is two observers occupying a simultaneously existing (actualised) arrangement of the universe but observing different fabrications formed from different data. That data has been processed to give those outputs, creating the illusion of different times within the fully simultaneous external reality. So the answer isn't a simple yes or no we do or don't occupy the same time but- are we considering what exists without observation , the actualised material reality? Or the observed fabricated manifestation which is an emergent output of data processing?

You then wrote "sometimes the question we are not asking is about the meaningfulness of a notion we are taking for granted: particularly in the second half of the 19th century we were very busy attempting to establish the properties of the ether, but we then figured out that there is no place in physics for any property of the ether."

However if the observed reality is an emergent fabricated output and "beneath that" there is an actualised reality that is unobserved and the source of the data, then there -is- a place for the ether, even though it can not be directly detected.

You wrote: "To me it is irresistibly intriguing to speculate that our insistence on the availability of the spacetime abstraction might at this point be limiting our opportunities for discovery. And I am contemplating a meaningful question: it is for experiments to decide whether or not the reliability of our spacetime inferences is truly universal." I agree. If space-time is an emergent fabrication it overcomes many long standing problems. There is a diagram of an explanatory framework set out in my essay and also as a high resolution diagram in my essay thread.

You wrote:" And I argue that, as we try to get ready for going even beyond quantum mechanics, in the context of quantum-gravity research, we must contemplate even more virulent departures from the "spacetime paradigm."" Virulent may not be the best term. I regard it as the cure, the best medicine rather than a disease within theoretical physics.

Very well written, relevant, accessible thought provoking. Good luck in the competition.

Dear Giovanni,

First I must agree with you about reconsidering space-time. i think of Space-time as a very convenient lie. We build this lie with the relationships between individual particles and then fall in love with the mathematical beauty of the field and call the field real.

And space-time is very, very convenient field, it is almost as if it were genetic. Breaking away is going to be difficult, many worthwhile things are.

You may be interested in my essay in that it joins quantum mechanics and special relativity via boundary conditions. Some earlier posts indicated you had an interest in this. http://www.fqxi.org/community/forum/topic/1403

Thanks for your though provoking essay.

Don L

Jonathan,

You wrote: "Of course SR is right... (...) If we don't look at the clues in front of us, however unexpected, I'm not sure how we're going to reach a point where we understand time. There are also missing pieces of the puzzle, and we need to allow for their existence. Because it leads to block time, spacetime is in contradiction with the standard view of quantum theory about whether the future currently exists. That is a deep contradiction. And there are some places where the implications of spacetime jar with our picture that deeply, and conflict directly with what we observe. So although it is convenient to use, it looks like it might be flawed. (...) Of course SR is right..."

If spacetime is flawed, SR cannot be right. Logic forbids a situation in which the premises (Einstein's 1905 postulates) are true and the consequence (Minkowski spacetime) flawed. So let us "look at the clues in front of us, however unexpected", Jonathan.

Pentcho Valev

See my discussion with George Ellis

http://fqxi.org/community/forum/topic/1337#addPost

Pentcho, briefly,

you and I have already discussed this question at length on my page, and I have shown you to be wrong, in a way that even you eventually didn't argue back about. The reason we call it "the spacetime interpretation" is because it's an interpretation. It's untested - unlike SR, which is extremely well confirmed by experiment. People often imply that spacetime is an unavoidable consequence of SR, but no-one will actually say that it is, because it isn't.

Jonathan

(That was me, Jonathan Kerr. It sometimes logs you out just as you post. Any further discussion of this to my page, but it's already been said. Best wishes, JK)

Giovanni wrote:

"In quantum-gravity research there is a long-standing effort

of understanding how spacetime should be described when both

Planck's constant ~ and Newton's constant GN are nonnegligible.

We cannot claim much success addressing this issue."

Giovanni,

what mean "nonnegligible"?

Give me please more detail..