On Friday I had a few minutes free, and did an experiment: put my iPhone on a tripod, pointed it at myself, and did a live video on my public Facebook page, taking questions from anyone who happened by. There were some technical glitches, as one might expect from a short-notice happening. The sound wasn’t working when I first started, and in the recording below the video fails (replacing the actual recording with a still image of me sideways, for inexplicable reasons) just when the sound starts working. (I don’t think this happened during the actual event, but maybe it did and everyone was too polite to mention it.) And for some reason the video keeps going long after the 20-some minutes for which I was actually recording.
But overall I think it was fun and potentially worth repeating. If I were to make this an occasional thing, how best to do it? This time around I literally just read off a selection of questions that people were typing into the Facebook comment box. Alternatively, I could just talk on some particular topic, or I could solicit questions ahead of time and pick out some good ones to answer in detail.
What do you folks think? Also — is Facebook Live the right tool for this? I know the kids these days use all sorts of different technologies. No guarantees that I’ll have time to do this regularly, but it’s worth contemplating.
Moe,
Cannot agree more with you. As noted in my original comment, the topic was a suggestion for Sean’s next session, at his leisure. It was not at all meant to be discussed presently.
Anyway, it is a question that has persisted at least since the 5th Solvay Conference. No doubt there will be another 89 years, and more, to tackle it.
It just happened that a week ago Frank Wilczek was asked that question, based on his “Properties need not exist before being measured” statement. I picked position as the property. He wanted an actual experimental setup. And I thought there’s nothing more suitable than the simple double-slit.
Since the “Edit” button is being hit, allow me to correct “Multi-worlds” to “Many-worlds”.
Best,
KC
At first I was skeptical about the Facebook video idea. But after watching the second one on why the hot smooth glowing early universe is really a low entropy state, I now think they’re a good way to go. I thought that Sean found the sweet spot by pre-selecting what he would talk about, but taking some questions at the end of the video, and also by giving some upfront notice on twitter and facebook of when the video will start.
Note that it is not necessary to login to Facebook to watch these videos. I watched this latest one live, and then later re-watched it after it was over, both times without logging in. However, Facebook will bug you with a PITA popup window asking you to login; just ignore it and you’ll be able to watch the video.
So it seems to be a convenient venue for him, apparently doesn’t involve much setup, the video and audio quality was excellent, can be done whenever he has the time and inclination, and if you miss it live you can still see it later. So I’m now thumbs up on the idea of the Facebook videos.
Sean,
Please explain “topological phase transitions” in your next LQaAs.
I not only want to understand it better, but also want to see how you explain it. Here are a few pointers (in reference to the popular explanation put up by the Nobel foundation):
– Referring to the patterns of arrows which together depict the vortices-defining field: The arrows show something flowing (or at least a local tendency to flow). What is it that is flowing here? the multi-electron wavefunction?
– Is there a singularity at the center of a vortex? or is it a hole? What is the value or amplitude of the “flowing thing” at the center point? If the flowing thing doesn’t exist right at the center point, then what does?
– A couple of vortices forming a stable pair may, after the phase transition, get dis-associated from each other. Imagining each vortex to be a hole, the number of holes carried by that “thin slice of cheese” still seems to remain the same even after the phase transition; it’s just that the “boats” seem to get disoriented and displaced a bit. Thus, there does not seem to be a change in the topology of the structure. Why do they then call it a topological transition?
– How flat should be the flatland? 1 mm thick? 1 micron thick? 1 atom thick? In experiments, does it exist as just a thin layer on top of something? If yes, why doesn’t the substrate hinder the motion of the vortices? If there is no substrate, what allows a thin slice of a material to not bend and break down in the middle?
– They already had a theory—the BCS theory—for superconductors, right? It even got a Nobel, way back in 1972. In precisely what way did the 2016 laureates add to it?
– Does this all mean that Google won’t be able to come out with quantum computers in mid-2017?
[… Guess that’s enough, at least for now.]
Best,
–Ajit
[E&OE]
KC,
It’s an ill posed question, to ask where a particle is when it is not being measured. You can ask about states when observations aren’t happening, but a “where” question like that is trying to get an observable without observation.
Hi Sean,
Thank you and Henry for the fantastic MinutePhysics videos. Like the Facebook Q&A too.
I am still confused however about your comments about the maximum entropy in the FB Q&A.
Suppose we put a point of observation somewhere in space, anywhere in our observable universe is fine. Let space undergo increasing expansion as in a big rip scenario.
– What would the point’s horizon (Hubble horizon?) do?
– What would happen with the entropy and the maximum entropy within that horizon?
– What would happen with the entropy and maximum entropy outside that horizon if we could even speak of something like that?
Best,
Tigas
That should be cosmic event horizon i just realized.
Doug,
You chose to separate the state and its location. Understood. Whether that is reasonable, given our ignorance about the state, is another matter. In any case, my original suggestion (September 27) to Sean for a future topic, was “Where is, and what is, a particle before measurement?” Perhaps it is simpler to have left it that way.
Since we are back on this topic, at variance with Moe’s admonition (September 30), might as well provide a fuller background, at the risk of boring everyone. The question was raised in part to address Frank Wilczek’s statement “A property that is not measured need not exist.” Recently, that question of “Where is, and what is, a particle before measurement?” was put to him.
Wilczek paused briefly, and asked for an actual situation. I suggested a simple double-slit setup. The long and short of the discussion was that no “non-standard” conclusion could be reached. In short, we’re still in the dark. (Hence the question in the first place, for a second opinion.)
Then I brought forth a little thought experiment. “If we keep adding more slits on either side of the double-slit screen, until the width of that multi-slit screen eventually stretches from one observable horizon to the opposite horizon, we will not affect the validity of the outcome.”
Wilczek nodded in agreement. Then he volunteered, “But the interference bands at the horizons would be very thin.” I agreed. And left it at that, not wanting to take more liberty.
Likewise, I will not take up Sean’s blog space to elaborate too much on what that “multiple-slit” thought experiment could mean. Suffice it to say that it might suggest: “We still do not know ‘what is and where is’ regarding that particle. But perhaps we can be slightly more certain about the opposite: what it cannot be. The physical multi-slit screen may suggest that the particle cannot be physical, before measurement”.
A physical particle (or a physical wave equivalent), as large as the size of the universe, is frowned upon by common empirical experience. [Bohmians excepted. But the validity of their argument remains in question, particularly the faster than light communication, even if their wave is tolerated.]
Surely we could eventually have come to similar conclusions with only two-slits. But this thought experiment, by building on well-established real life observations, takes our vista from focusing on one laboratory bench, to encompass the whole universe, and the space within.
So, at least from the “may be” in the Bohr-Einstein debates, the current “need not be”, to the more definitive “cannot be (physical)”, it constitutes some progress. But more than the physical ontology of a particle, a deeper message could be about space itself.
Whatever a particle is, at the instant when it passes through this multi-slit screen, the space associated with it seems to stretch physically across the width of the slit screen, or in other words, the universe. In fact, some do hold that at the instant upon reaching a black hole, space seems to behave in exactly that manner.
As to how far space stretches, there is no clear agreement. Now, with the help of this multi-slit screen thought experiment, perhaps one could venture that space stretches as far as the observable horizons. What happens to locality then? It’s definitely a topic for another day.
All this talk about a particle and the space it occupies perhaps echoes Sean’s “… how spacetime works in the real world, not just in the minds of theoretical physicists.” (February 12, 2016, The Atlantic Magazine).
Lest we digress too much, here is the original question: “Where is a photon before we detect it in a double-slit setup?” Was it in the photon gun? Really?
Before committing ourselves, it is good to be reminded of this textbook statement, “… if you insist on absolute certainty in the outcome of a second measurement of momentum, you must hit the particle so hard with your first measurement that next time you measure it, it’s equally likely to be found anywhere in the universe.” (Binney and Skinner, “The Physics of Quantum Mechanics”, 2014)
Or, encouraged by Sean’s remarks above, in order to answer the question, we could entertain the reverse of space stretching at the slit screen: space pulling together (emergent) at the moment of detection.
After all, we already have theoretical bases (Van Raamsdonk, Swingle, KAVLI etc.) for spacetime emergence out of entanglement. And entanglement is what takes place when the back screen in the double-slit setup measures the photon. So this could be one practical mechanism of spacetime emergence. One that could be generalized to all measurements including, by the way, environmental decoherence.
What then does the multi-slit screen in the little thought experiment suggest? It seems to say that before measurement (or between measurements) a particle carries all the spatial position (of the slits) information with it. There is no reason it could not carry similar information on other observables.
Sorry about this long-winded response.
Best,
KC
KC Lee,
I wasn’t admonishing at all. I was saying that a discussion here would be great, and if Sean wants to say something, then he can.
I like your question about the location of the photon between measurements. I think this is one of those things that leads the students and general public astray in understanding physics.
Most people assume that a photon released from a star 1 million light years away has been actually traveling through space, at the exact speed of light, for 1 million years along a defined trajectory. Like a bullet.
Yet, if you conduct a double (or million) slit experiment with light from that star 1 million light years away, it still works the same. The number of slits in the screen at the end of the journey determines the available path for each single photon independently.
This is mostly about understanding what is really meant by the “wavelength” of single particles, and what is meant by its “phase”. And what in meant by the “interference” of a single particle with itself. How can a particle “cancel itself out” by simply being “out of phase” with an alternative version of itself in superposition?
These are the questions that will lead to better answers about “where the photon was” between measurements.
It’s just that “where” is a bad question when you are not making an observation. Observations are local operators, they have locality. You can’t have locality without a local operator.
Or maybe the answer to “where” is “just in Hilbert space.” Is that more satisfying? Perhaps not, because that certainly isn’t a place, but it does have the word space in the name. But it’s important to think this way. If you have “stretching out” or the like for your photons, you get a-causal behaviors and other problems.
@KC Lee
People just confuse the heck out of themselves and tie themsleves in knots over quantum mechanics because they are trying to force-fit everything into a physical picture of the world. If we add some non-physical properties to our ontology of reality, then QM actually becomes very clear and simple.
Look, wavefunctions simply can’t exist inside ordinary physical space. This is because physical wavefunctions would violate the principle of ‘locality’ (which says that no physical influences can travel faster than light). ALL realist interpretations of QM that try to put the wavefunctions inside physical space suffer from this problem.
To repeat: Wavefunctions don’t exist in ordinary physical space. They exist in Hilbert Space. But Hilbert space is completely mathematical (or computational).
Therefore, the correct conclusion to draw is that there exists another ‘layer’ of reality beneath the physical level. This ‘computational’ (or mathematical reality) is more fundamental than physical reality, and what we call ‘physics’ is then just another emergent property of what is really pure computation or mathematics. And ‘Hilbert Space’ is the description of this sub-physical layer of reality.
Q Where do particles exist prior to measurement?
A They exist as ‘mathematical’ (computational) entities in Hilbert Space (wavefunction). When measurements are made, they ’emerge’ into the physical level and appear as particles.
Thanks to all who have left comments. Doug’s Hilbert space idea has been addressed by Zarzuelazen. The word “admonition” is from me, feeling guilty and apologetic about taking up Sean’s blog space for this topic.
When in doubt, always a good idea, as Moe suggested, to get back to basics. I offer this from the Binney and Skinner 2014 textbook (p.161) “Given the unphysical nature of ideal measurements, we should be worried that quantum mechanics attaches such basic physical concepts as position and momentum to operators that are defined in terms of ideal measurements.”
The word “unphysical” above bolsters the same conclusion from the universe-wide multi-slit screen idea Wilczek and I discussed back on September 19. As for whether that means mathematics as Zarzuelazen (and others particularly Max Tegmark) suggests is open to question. Meanwhile, I will just stay with the pure idea of “unphysical”.
Unphysical certainly rules out most items in the classical domain, likely including space and time (will address). We’ve already discussed particle and wave not being candidates, leaving fields unaccounted for. I submit that even energy cannot be an unphysical candidate of this substrate from which physical classical domain items emerge. That quantum fields face ontologic angst is nothing new (e.g. Meinard Kuhlmann’s many publications), specifying the cause of that being unphysical is.
Next I will introduce the idea that what we observe is a function of two items: our experimental setup, and the location in the universe where the observation is made. Will limit first to just the setup, because location then brings in the element of space (to which we will return). That would be way too much for one post.
Energy is there in space whether observed or not. Two parallel plates placed appropriately can demonstrate the Casimir effect. Intriguingly, we could possibly apply the same idea to explain the observation (if ever) of Hawking radiation due to the popping into existence of a new black hole. The plates and the black hole are just “experimental setups”.
Same with our particle before measurement. It is there before measurement. The only new proposal, based on the universe-size multi-slit screen thought experiment, is that it cannot be physical, until measured (emergence). Not as particle, not as wave, not as energy. So back to “what is it then?” I submit unphysical here simply means potential.
The idea of potential was expressed by Heisenberg in 1958 (Source: “Physics and Philosophy: The Revolution in Modern Science” Harper Perennial Modern Thought, p. 27, May 8, 2007). The only difference is that here we mean unphysical potential. Surely, experimental confirmation, indirectly of necessity, will be needed.
Before picking back up on location of observation, will go to another quote by Sean from his February Atlantic Magazine article, “… imagine a future in which we do understand the underlying quantum theory of the universe, in which general relativity appears simply as a useful approximation.”
Why consider this quote? To point out classical accuracy in measurement, paradoxically, is just an approximation when it comes to something inherently uncertain, if we are serious about classical being emergent from quantum. The more accurate (classical), the less quantum.
My favorite example, tying things back to “unphysical”, is the use of a physical ruler to measure something unphysical. What do we get? Uncertainty! Exactly what nearly a century of QM observations such as the double-slit experiment have been showing us.
Running out of space to get to space, will leave with this thought: the more successful we are in applying QM (physical accuracy), the more it “becomes” just classical, and the farther from the essence of QM. Worse, the less physicists are willing to look, more than lukewarmly, at questions such as “Where is, and what is, a particle before measurement?”
Not stated as explicitly as the above, but Binney and Skinner made the point repeatedly that the only justification of QM is that our calculations work (for cell phones, MRI etc. and soon quantum computers and encrypted quantum communications). We have zero idea why the math works at all, let alone working so well. Importantly, this addresses Zarzuelazen’s (and Doug’s) point.
Not pushing British products, but much as one may not agree with Roger Penrose’s treatment of string theory and cosmic inflation in his new book “Fashion, Faith, and Fantasy in the New Physics of the Universe”, he also laments, like Feynman and many others before him, about how little we understand QM, despite using it so successfully. I submit that the lament includes our topic of where a particle is.
“Unphysical” is just one attempt at elucidating the substrate mentioned by Zarzuelazen (and Sean and many many others).
Best,
KC
I wonder if we should actually *define* the forward cosmological arrow of time as increasing complexity? That is to say, suppose no observers can ever see the complexity of the universe as decreasing? This would lead to some curious physical effects I think. For instance, all observers in the universe could only ever see the cosmological complexity as increasing towards a fixed value but never quite getting there. Could this explain some cosmological puzzles such as dark energy?
Zarzuelazen,
Please correct if misunderstood. I recall you seem to, like myself, also favor an unphysical substrate from which physical entities emerge? Further, you deem that substrate to be mathematics. Could be wrong, but psi-epistemic type of thinking generally offers no overt explanation for the arrow of time? But there is no reason not to work on one in your math-based reality.
Doug,
Your <> has been addressed by the notion that before an observation, the state cannot exist based on that universe-size multi-slit screen (with which Wilczek* concurred). This is a step further from Wilczek’s own “need not exist” and Bohr-Einstein’s “may not exist”. It is in that light that we can wonder “where” it exists, right? I happen to think it exists in an unphysical substrate.
Moe,
Your <> is worth revisiting. A quick reminder: every time we switch on a light, trillions of trillions photons come gushing out. But first, will address the second part of “experimental setup and location of experiment” both affecting measurement results. The experiment setup part has been dealt with in my last post (October 12).
*Disclaimer: Even though the “universe-size multi-slit screen” idea was discussed with Wilczek, what that screen could mean, starting with a particle being unphysical before measurement, is my own doing. As are all subsequent speculations based on that screen, including “location of experiment”, the subject of this post.
Will pick up from where Wilczek and I agreed: That the interference fringes or bands at the ends of the universe-size multi-slit screen, located at the observable horizons, would be very thin. What follows next would appear, initially, to contradict that agreement.
I submit that the location of measurement affects those interference bands. If the same experiment is now performed near one edge of the universe, the interference bands there will suddenly become much thicker, even though the absolute location, in space, has not changed. It is still the old edge of the universe. Why the difference? Our location of experiment has changed.
Picture a newspaper page lying open on a desk surface. Place the largest circular magnifying glass you can find on that newspaper. The edges of the magnifying glass correspond to the edges of the observable universe. As we slide the magnifying glass across the newspaper (changing the location of our experiment and, more importantly, the boundaries of the observable universe), the spot which used to be at one edge is now near the new center, where the magnification is highest (and the interference bands thickest).
Viewed this way, there is no inconsistency regarding the changes in interference band thickness at a specific location in space (corresponding to changes in magnifications on the newspaper). The result of measurement depends on the location where a measurement is made.
Next, will tie “unphysical” noted in the last post with “space” (location) discussed in this post. Staying with the magnifying glass analogy, space outside the magnifying glass, like pre-measurement particles, can be regarded as unphysical potential (the “mathematical substrate” of Zarzuelazen). Both particle and space acquire physicality at emergence due to measurement (entanglement), as the magnifying glass is moved to the appropriate spots.
Actually, the magnifying glass serves the same purpose of parallel plates in the Casimir effect. Both lead to physical manifestation of unphysical potential. We can view this manifestation as emergence via measurement or entanglement.
Being unphysical does not lend itself to being expressed using mathematical formulas. But the habit of expecting an equation with each new “theory” is unshakable. So if one insists, consider: “Unphysical substrate + Entanglement = Classical domain”.
When applied to spacetime, it becomes a practical means for spacetime emergence via entanglement. One theoretical basis for that has been provided by Van Raamsdonk and others (cited in my post on October 10).
If quantum gravity includes describing classical spacetime using quantum thinking, this emergence of classical spacetime from a “quantum” unphysical substrate might answer the call.
Moreover, it does so with relative simplicity compared to string theory’s 10^500 equations (not to mention extra-dimensions etc.), and to Loop Quantum Gravity’s spacetime atoms which, being physical, cannot shrink past the Planck length.
Please note that the “Casimir plates” can be placed anywhere inside, and replace the function of, our magnifying glass. This suggests the unphysical substrate exists both outside and inside the magnifying glass purview. In other words, “functionally”, the unphysical substrate overlaps the physical classical universe, whether the latter is deemed limited or unlimited.
If and when we substitute black holes for the Casimir plates (the possibility was pointed out in my October 12 post), the plot quickly thickens. But it is definitely beyond this post.
Meanwhile, we return to emergence via entanglement. To conform to nearly a century’s worth of experimental observations, the result of measurement should be random. For simplicity, this is the only stipulation placed on this emergence (measurement) under discussion.
The universe-size multi-slit screen explains why a previously unphysical particle, at measurement, could show up, randomly, as a physical particle, anywhere in the physical universe. By the way, the Schrodinger equation still gives us the probabilities where the particle could show up, subject to the “location of measurement” constraint added.
So”Where is a photon before measurement?” Answer: it cannot have physical existence (in the classical domain) until measured. And when measured, the universe-size multi-slit screen allows the photon to show up anywhere, per the Schrodinger equation, constrained by the location of measurement.
With this frame of mind, we re-visit the photon mentioned by Moe. Unlike Einstein who imagined seeing the world while riding on a photon, armed with what the universe-size multi-slit screen suggests, we can imagine, directly, what the photon itself sees. At emergence (flipping of a light switch for example), each photon should be able to see the entire universe, as suggested by the universe-size slit screen.
But does that make sense? In principle, any photon from a star one million light years away can tell us something about that star. But now our vista has been expanded by the universe-size slit screen, to include: any photon anywhere can tell us everything we want to know, plus also things we don’t even know we want to know.
Another analogy might help. Before emergence (birth), during gestation, an unphysical photon is “schooled” about the physical universe. There is no reason such prenatal education should exclude any one part of the universe. That’s why at birth, a photon sees everything everywhere.
If one photon could serve the entire universe, one only needs one photon to act as messenger. If that reminds us of Wheeler’s “There’s only one electron in the universe”, it is not too far fetched. The trick is to look at the universe at one single instant. Then both what Wheeler, and what the universe-size multi-slit screen, imply makes more sense. The redundancy, in this “light”, of the light bulb switching on, is astounding. Then again, so is the number of instants in time!
To be pointed is that whereas the speed of light underscores spacetime in general relativity (GR), it is also the foundation of the “magnifying glass” causality universe. Just that the unphysical substrate has extended that universe, both under and beyond the magnifying glass, without resorting to additional physics, or otherwise affecting existing physics.
Recapping once more, there could be an unphysical substrate, where clocks do not tick. Time starts running only at the moment a measurement is made. Once clocks start ticking, GR takes over. It rules everything that has emerged from that timeless unphysical substrate. So another form of the above “equation” is: “Unphysical substrate + Time = Classical domain”.
And since that substrate is required, by observation, to produce a random result at emergence, in our GR tinted vision, it could be “existing” in a completely random “state”. In other words, a state at maximum entropy. And GR being derived from that substrate, we might think of GR as carrying that maximum entropy DNA, thereby accounting for the arrow of time in the classical domain.
Sean’s prediction of “GR appears simply as a useful approximation” was mentioned in my October 12 post. We could speculate with another prediction, this time based on Wilczek’s “Physics in 100 Years” (arXiv:1502.07735v1), “‘What is’ and ‘What happens’ will be understood as inseparable aspects of a single, transtemporal reality”.
For the unphysical substrate, devoid of clocks, things are always ‘What is’. In the classical domain, with clocks ticking, things are always ‘What happens’. Bridged by entanglement (measurement), they are inseparable aspects of a single transtemporal reality.
Best,
KC
These cut-and-paste quotes could be missing for unknown reasons.
From Doug, it is “where” is a bad question when you are not making an observation.
From Moe, it is … a photon released from a star one million light years away …
KC
Just moments into the first Minute Physics video with Sean, I found myself wondering how long it would be until someone took some of Sean’s lines and AutoTuned them into a rap video.
Still wondering….
This is a good format. It works well. I think Prof Carroll should select interesting questions ahead of time and simply repeat and answer them live. Prof Carroll is so good at communicating that I think single topics in physics traditionally awkward to grasp for interested non-specialists should be interspersed with Q&A sessions.
I suspect that many people would like to hear more of Prof Carroll’s arguments for naturalism and hear his opinions on the extent to which philosophy, traditionally, at least of late, the butt of casual contempt on the part of ‘hard science’ physicists, has anything to offer science in the modern age.