Theoretical physicists striving to unify quantum mechanics and general relativity into an all-encompassing theory of quantum gravity face what’s called the “problem of time.”

In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions *x*, *y* and *z* into a four-dimensional “space-time” fabric. The fabric warps under the weight of matter, causing nearby stuff to fall toward it (this is gravity), and slowing the passage of time relative to clocks far away. Or hop in a rocket and use fuel rather than gravity to accelerate through space, and time dilates; you age less than someone who stayed at home.

Unifying quantum mechanics and general relativity requires reconciling their absolute and relative notions of time. Recently, a promising burst of research on quantum gravity has provided an outline of what the reconciliation might look like — as well as insights on the true nature of time.

As I described in an article this week on a new theoretical attempt to explain away dark matter, many leading physicists now consider space-time and gravity to be “emergent” phenomena: Bendy, curvy space-time and the matter within it are a hologram that arises out of a network of entangled qubits (quantum bits of information), much as the three-dimensional environment of a computer game is encoded in the classical bits on a silicon chip. “I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia.

Researchers have worked out the math showing how the hologram arises in toy universes that possess a fisheye space-time geometry known as “anti-de Sitter” (AdS) space. In these warped worlds, spatial increments get shorter and shorter as you move out from the center. Eventually, the spatial dimension extending from the center shrinks to nothing, hitting a boundary. The existence of this boundary — which has one fewer spatial dimension than the interior space-time, or “bulk” — aids calculations by providing a rigid stage on which to model the entangled qubits that project the hologram within. “Inside the bulk, time starts bending and curving with the space in dramatic ways,” said Brian Swingle of Harvard and Brandeis universities. “We have an understanding of how to describe that in terms of the ‘sludge’ on the boundary,” he added, referring to the entangled qubits.

The states of the qubits evolve according to universal time as if executing steps in a computer code, giving rise to warped, relativistic time in the bulk of the AdS space. The only thing is, that’s not quite how it works in our universe.

Here, the space-time fabric has a “de Sitter” geometry, stretching as you look into the distance. The fabric stretches until the universe hits a very different sort of boundary from the one in AdS space: the end of time. At that point, in an event known as “heat death,” space-time will have stretched so much that everything in it will become causally disconnected from everything else, such that no signals can ever again travel between them. The familiar notion of time breaks down. From then on, nothing happens.

On the timeless boundary of our space-time bubble, the entanglements linking together qubits (and encoding the universe’s dynamical interior) would presumably remain intact, since these quantum correlations do not require that signals be sent back and forth. But the state of the qubits must be static and timeless. This line of reasoning suggests that somehow, just as the qubits on the boundary of AdS space give rise to an interior with one extra spatial dimension, qubits on the timeless boundary of de Sitter space must give rise to a universe with time — dynamical time, in particular. Researchers haven’t yet figured out how to do these calculations. “In de Sitter space,” Swingle said, “we don’t have a good idea for how to understand the emergence of time.”

One clue comes from theoretical insights arrived at by Don Page and William Wootters in the 1980s. Page, now at the University of Alberta, and Wootters, now at Williams, discovered that an entangled system that is globally static can contain a subsystem that appears to evolve from the point of view of an observer within it. Called a “history state,” the system consists of a subsystem entangled with what you might call a clock. The state of the subsystem differs depending on whether the clock is in a state where its hour hand points to one, two, three and so on. “But the whole state of system-plus-clock doesn’t change in time,” Swingle explained. “There is no time. It’s just the state — it doesn’t ever change.” In other words, time doesn’t exist globally, but an effective notion of time emerges for the subsystem.

A team of Italian researchers experimentally demonstrated this phenomenon in 2013. In summarizing their work, the group wrote: “We show how a static, entangled state of two photons can be seen as evolving by an observer that uses one of the two photons as a clock to gauge the time-evolution of the other photon. However, an external observer can show that the global entangled state does not evolve.”

Other theoretical work has led to similar conclusions. Geometric patterns, such as the amplituhedron, that describe the outcomes of particle interactions also suggest that reality emerges from something timeless and purely mathematical. It’s still unclear, however, just how the amplituhedron and holography relate to each other.

The bottom line, in Swingle’s words, is that “somehow, you can emerge time from timeless degrees of freedom using entanglement.”

Time will tell.

Theoreticians almost universally reject Einstein's relative time, sometimes explicitly:

Quote: "And by making the clock's tick relative – what happens simultaneously for one observer might seem sequential to another – Einstein's theory of special relativity not only destroyed any notion of absolute time but made time equivalent to a dimension in space: the future is already out there waiting for us; we just can't see it until we get there. This view is a logical and metaphysical dead end, says Smolin."

Quote: 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…"

Quote: "Nobel Laureate David Gross observed, "Everyone in string theory is convinced…that spacetime is doomed. But we don't know what it's replaced by."

So spacetime is "doomed" but, on the other hand, it is an "immediate consequence" of Einstein's 1905 constant-speed-of-light postulate:

Quote: "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."

Since the "immediate consequence" is doomed, the underlying premise (the postulate of the constancy of the speed of light) is false. Logic forbids the combination "wrong consequence, true premise".

Pentcho Valev

Wow, another Verlinde article. You've really drunk the koolaide.

Thank you, it's drawing a lot of traffic to my own blog.

Keep up the good work!

We think of time as the present moving from past to future and physics codifies this as measures of duration, but the reality seems to be action turning future to past. Tomorrow becomes yesterday because the earth turns.

This would describe time as we experience it, change within the present state. Duration is the present, as events come and go. The reason clocks can run at different rates and remain in the same present is because they are separate actions. Much as an animal with faster metabolism will age quicker than one with slower metabolism.

Alan Watts used the analogy of a boat and its wake to describe this, in that the wake doesn't steer the boat, the boat creates the wake. The past is an effect of the present. Events are first in the present, then in the past. They have to occur, in order to be determined. It is only the occurrence of an event which can fully calculate all possible input into it, so the future remains probabilistic.

This makes time an effect of action and energy, like temperature, color, pressure, etc. It is just that the logical sequencing of our minds, aka narrative, is an effect of this rate of change. Much as temperature and thermodynamics are fundamental to the environment and our bodies.

As conserved energy goes from past to future forms, these forms coalesce and dissolve, going future to past. So time is not so much linear, as it is feedback between the inertia of prior forms, as these forms are consumed by present actions.

I hate to nitpick, but the heat death of the universe and death by terminal expansion are two very distinct concepts that should not be treated as interchangeable (heat death is merely universal thermal equilibrium and doesn't require any spacetime expansion, although perhaps the two may go together). If I hate to nitpick, why am I doing it? While physicists might think conflating distinct things is okay so as not to confuse the unwashed mass readership, members of the mass readership such as myself do not wish to be misinformed. If I didn't already know the concepts in question, this article would have misled me. I hope the author doesn't indulge in that much!

Michael Farries,

Thanks for pointing out the distinction. While it's true that heat death is a more general concept in cosmology, in the emergent space-time framework I describe in the article it seems coincide with space-time's timeless boundary. Thermal equilibrium = maximum entropy = time death. You're right to question this — it is, after all, just a conjecture. But the theoretical work on AdS space makes many physicists think there's something to this whole it-from-qubit notion.

Thanks for reading.

Natalie

Wolchover is up there with the very best, making complicated patterns of ideas accessible and intelligent. Thanks, Natalie Wolchover, thanks very much!

Unfortunately, it from the qubit and globally flat spacetime plus the standard model from the breaking of time reversal symmetry and irreversible thermodynamics, is far more believable for quantum gravity, of which we know very little if nothing about, than it is for dark matter, of which there is a great deal of observational evidence that is available.

I feel like we're entering a period where the word "emergent" is going to see extremely heavy use.

Sometimes complexity obscures, rather than clarifies. While the whole Big Bang theory is gospel, this is science and occasionally primary arguments do need to be revisited.

When Edwin Hubble and company were first measuring the redshift of those distant galaxies, it became clear that whichever direction they looked, the rate of redshift was directly proportional to distance and this created the impression that we are at the center of the universe. Einstein’s explanation for Relativity, the “fabric of spacetime,” was fresh in everyone’s minds and so the argument became that space itself must be expanding and every point would appear as the center.

Which totally overlooks the essential premise of Relativity, that the speed of light remains CONSTANT to the distance! As it is still being assumed the redshift was due to those galaxies moving away and the light taking LONGER to cross. If this is actually relativistic space, than the speed of light would INCREASE as the space expanded. Which, if it did, would totally eliminate any redshift.

So two metrics of space are being assumed from the very same intergalactic light. One that expands, based on the spectrum of this light, compared to one that is stable, based on the speed of this light.

If redshift is an optical effect, it would explain why we appear a the center, because we are at the center of our view of the universe.

The argument for redshift only being due to recession is based on single spectrum photons, but when the light of entire galaxies is distilled to just a few photons of light, these are multi spectrum photons and according to some, multi-spectrum photons would redshift over very long distances.

(http://fqxi.org/data/forum-attachments/2008CChristov_WaveMotion_45_154_EvolutionWavePackets.pdf)

This effect compounding on itself would explain the rate of redshift going parabolic, more effectively than Dark Energy.

The James Webb goes up in two years and I would predict we find the CMBR to be the light of even more distant galaxies, shifted completely off the visible spectrum. Essentially the solution to Olber's paradox.

@ Natalie Wolchover

Infinite slow passing of time, does it not orrespond with minimal entropy?

Hi, I am a bit confused by the very beginning of the article, namely "In quantum mechanics, time is universal and absolute; its steady ticks dictate the evolving entanglements between particles. But in general relativity (Albert Einstein’s theory of gravity), time is relative and dynamical, a dimension that’s inextricably interwoven with directions x, y and z into a four-dimensional “space-time” fabric."

I thought that QM and Special Relativity – which is the "time is relative" part, compare to classical perspective – have been merged in Quantum Field Theories and that the real challenge was that space-time (but not time itself) is a rigid background in QFT while it is dynamic in General Relativity.

It is indeed nice to see these long standing fundamental puzzles are finally being attacked by the tools in quantum information that we have developed over the last 10 years. I would like to bring to your attention recent work in arxiv:1611.08581 which is very relevant to the problem addressed in the article. Using the machinery of tensor networks (MERA, to be specific) time has been obtained as the emergent holographic direction for de Sitter spacetimes — so yes, we are one step closer to understanding what's up in de Sitter spacetimes. Additionally, the tensor network perspective gives us very interesting hints on the nature of the entropy of the cosmological horizon as well as cosmological perturbation theory in de Sitter.

Time is the potential of space, in Erick'van der Linde's view, gravity emerges out of information. information as a geometric construct. And as waveguide QM shows us how 2 systems can interact (droplet bouncing on its own waves), that partly explains our world, its not the droplet bouncing on its own wave, there is no single droplet, no single particle, it comes down two sides of the same information entity, in computer terms the zero and the one. The curvature as caused by mass in space-time is therefore a description of information within this construct. And it works on any scale. because information (computer analog) is only recent in what we saved last.

it doesnt matter if you save multiple copies (as quantum mechanics does all the time).

eventually it solves itself to result in the most balanced energetic solution. (ea best information description continuously. In simple terms a quantum computer can never finish on a solution, if the problem keeps evolving as its working on its own results.

Its something programmers do often, just look at fractals for example some programs never end. Despite that their math notation is finite.

note to the writer what Erick wrote explains gravity as emerging from information theory /string theory. from the smallest possible information containment to the largest size of star clusters.

"But the state of the qubits must be static and timeless."

Not at all. That is a boundary assumption.

Must say I tend to agree wth John about the speed of light increasing in empty relativistic space, and based on that, it seems that the idea of time stopping at the edge of our universe (ala de-Sitter) is backwards. Isn't the relativistic idea that speed of light is only constant relative to an observer in their particular space-time frame of reference? It seems we tend to extend our observational frame of reference to other gravitational/energy states, in a sense anthropomorphizing the universe. In high-energy gravity relativistic system, space-time is compressed relative to lower energy states and while light travels across the same relative distance in the same relative time in the high-gravity system compared to a low-gravity system, to an observer in a lower gravity system, light crossing the high-gravity system would appear to travel more slowly. As John mentions, in intergalactic relativistic space-time (very low-energy/gravity) time and the speed of light would increase relative to an observer in our gravitational/energy state.

And as John mentioned, wouldn't time as we experience it simply fall out as rate of interaction within a particular system. Within a particular space-time system the rate of potential interactions will be constant, limited by the speed of light, but observations of that system from a different space-time system would tend to show time running faster or slower depending on the energy/gravity/speed state of the observing system to the observed system.

Seems to me this would argue against our universe being a de-Sitter space as I understand its description from the article, since, if the universe is expanding relativistic space, at the extreme low-energy -gravity state at ultimate expansion of space-time at the boundary of de-Sitter space, time would run relatively faster–NOT slower or stopped–and light/information would travel across the relatively expanded space-time in the same relativistic manner as it does for high-energy/gravity and mid-energy/gravity systems in the "bulk". Relative to us then, time at the edge of de-Sitter space would seem to run extremely fast or infinite, not stopped. And, if you have infinitely fast and distributed system wouldn't all states exist at once relative to our space/time reference? So maybe holography is right, they are just thinking about the boundary conditions wrong? Fun stuff!

In the article I can see this effort to grasp what is in front of you. It is a reversed way of looking and giving a lot of credibility to the grasped material. What is the hand that is grasping? Isn't the shape of material the shape of the grasping hand? To me history of science is evolution of the watching eye. Exept that it is not becoming better at explaining or in any way more precise. You twist it and see differently. Why something is still missing? Will observation ever be complete?

Thanks, Britton.

It's surprising how difficult is to get anyone to consider even the most basic logic when it's outside convention. Sometimes the keys are not under the streetlight.

The Amplituhedron and Holography are related by the horizonal structure of Gravitons.

RE: John and Britton. I could well be wrong, but I believe the piece that John is overlooking in his questioning of the existence of dark energy is the manner in which the galaxies are theorised to be moving apart.

Dark energy is not producing any acceleration of the galaxies motion, per se. Dark energy is moving them apart by 'creating new space' between the galaxies. Literally.

One analogy I've heard is to a sidewalk (footpath) where new squares of concrete are constantly mysteriously appearing and pushing apart the ones to either side. Perhaps neither you nor your friend, who is just a block a way, are walking at all. Yet you will still see the other person receding from you, and at an ever increasing rate, as new sections of footpath appear and push you apart.

If you think about it, this resolves the paradox you believe exists between the constant speed of light and the red shift attributed to dark energy's effects. Your friend down the footpath shines a torch at you. Because it is light, it still travels to you at a constant speed. But the appearance of new sections of footpath between the two of you as it travels causes the light to be red shifted.

Evidence may some day emerge that calls into question the current thinking on Dark Energy, but there is no inherent contradiction between the thinking about Dark Energy and the nature of light ala Einstein.

Scientists try to unify gravity (GR) and quantum physics (QP) within the same methods. No one can see that Nature shows that such unification is impossible? The real problem is very different – we just need to seek answer to the following question: why can not we unify GR and QP?

And the answer is very simple. Gravity is classical, not quantum. Gravitational fields are the gradients in the non-gravitating classical Higgs field (HF) whereas quantum particles are the entangled states of the components of the gravitating spacetime (of the Einstein spacetime (ES)). Properties of HF and ES are very different and were fixed irreversibly during the inflation.

In the two-component spacetime there are the closed strings but their properties differ significantly from those postulated in the string/M theory. Instead the higher spatial dimensions, there are the HF, entanglons responsible for quantum entanglement and the ES characterized by respectively 6, 10 and 26 degrees of freedom.

Christopher,

What seems to be overlooked is that the speed of light constitutes a ruler. Such as a lightyear as a measure of distance. So if the transmitter and the receiver of that light are moving apart, such that it takes light longer to cross and thus is redshifted, then there are more lightyears between them, not stretched lightyears. If these lightyears were stretched, that would mean the speed of light would increase, to match the increasing distance. Then that would be actual relativistic space, where the speed of light remains constant to the distance.

So the speed of light is being used as the denominator and the redshifted spectrum is the numerator. If they were to use the spectrum as the denominator and the speed of this light as the numerator, then the question would be 'why is the light taking longer to cross' and it would be a "tired light" issue.

As for dark energy, what Perlmutter and company expected to find was that the rate of redshift, which is quite extreme at the very edge of visibility, would decrease evenly the closer the sources are to us, under the assumption the initial expansion is slowing down at a constant rate. What they actually found is that the rate of redshift drops off rapidly, but then continues at a more measured pace. To use an analogy, it would be as if the universe were shot out of a cannon, but after it started to slow, a rocket kicks in and keeps it going at a more measured pace. The Big Bang being the cannon and dark energy being the rocket.

Now if you look at it from the other direction, rather then from the edge of the universe in to us, but from our point of view, out to the edge of the universe, what you see is a redshift that starts off slowly, but then increases exponentially. Basically a parabolic curve upward.

Now keep in mind the reason why this is not considered to be just an expansion in space, with normal doppler effect, but an expansion of space, is because this effect of those galaxies moving away at rates directly proportional to distance makes it seem as if we are at the center of the universe and that seems farfetched.

Now we are at the center of our view of the universe and if redshift is an optical effect, then it would be quite logical for us to appear at the center and if this optical effect were to compound on itself, then that would explain why the rate goes parabolic.

Time in relativity symply the metric of space. The distances are mesured by time of light speed. So the light speed must be constant between all objects however they move. But it is possible for linear moving objects (so called inertial frames) only. This is the postulate of relativity theory.

This means that in relativity principlly impossible to mesure distances between objects moving not linear. Even for rotating objects.

According to the Big Bang all objects moves not linear. Applying relativity theory there scientists get big calculation mistake.

And this is the real reason of appeaeing "dark matter/energy".

Very interesting article. Plain to see though, that although they're making progress, they don't yet "get it", time that is. Isn't it about time we put time in its proper place? It cannot be some esoteric "thing" that has some infinite quality displayed by its purely relative nature. At the speed of light, or at the event horizon of a black hole, time stops relative to a stationary or outside observer. But, the insistance of physicists in declaring time to be purely relative (and this can only be opinion), means they do not view the stopped clock as really stopped at all. They still consider entities or events moving through the event horizon and further in towards the centre but this is only sensible with a purely relative nature of time. If time were some absolute, finite "thing", then it will actually run out at the speed of light and at the event horizon. The clock really will have stopped, still relative to the outside, but also in absolute terms. Movement towards the center will become impossible. Correction:- it will still be possible mathematically but it will take until the end of the universe and longer to move much at all. The ultimate conclusion from this view is that for all the black holes in the universe, not one of them has a singularity inside it (yet).

Love the isomorphic end with entanglement. Borderline spiritual and still Platonic

I am going to have to argue with the toy universe giving us time as an illusion. This theory eliminates the very complexity that makes QM nondeterministic.

The problem is with complexity's introduction. Complexity itself is conditional on the structure it resides and yet an assumption to have its own ontology. The only way out that may ever be computationable feasible is symmetry groups. Either finite and probable in recurrence for transaction or infinite and quite trivial.

Perhaps the whole of space is undeciable betwixt Sitter and Anti de Sitter. Of course then you have to explain away panpsychism.

Good stuff. Natalie's been killin' it lately!

Great reporting there, Quanta! Thanks.

I wonder whether we are silly or supersmart to consider the articles such as "Evolution without evolution: Dynamics described by stationary observables" from 1983.

The cosmos provides some timekeeping devices, but the atoms and other quantum mechanical systems represent the best clocks available.

I don't believe that the question is why time flows from the left to the right like the system of musical notes.

The sources of oscillation, natural frequency keepers are all around us. They function in the gravitational field and respond to its gradient. The wavelengths are elongated or compressed accordingly.

You can put the quantum systems in orbit around the massive star, and consider how the time parameter t is different depending from the height.

The quantum state is governed by e^(-iHt). The probability amplitude of that state is analogous with the partition function from the statistical mechanics. The time factor has some analogy with the temperature.

In one hand, we have the "perfect atomic clocks", and on the other hand we have the statistical systems that are perhaps chaotic.

It seems a bit counterintuitive to find the sources of precise frequencies as the constants of nature.

After speicial relativity appearance, the instant action structure of QM looked underdeveloped, and it was the miracle that the quantum entanglement acts through space and time. Till we`ll understand this miracle,all the attempts to explain anything with the aid of the miraculous QM is senseless,like quantum gravity.More possible is the explanation of QM with the help of some other theory, may be the General Relativity.

Ditto Bob.

The statement at the beginning of the article that time plays a fundamentally different role in quantum mechanics than it does in general relativity is only true if one restricts oneself to non-relativistic quantum theory, where the state of a system evolves according to Schrödinger's equation. Such a restrictive approach is already overcome in relativistic quantum field theory, where all of the pertinent equations of motion are Poincaré covariant. The transition to a quantum theory of general relativity can be accomplished by adopting the sum-over-histories approach, where the path integral would taken over all geometries connecting two spacetime hypersurfaces. Such an integral has yet to be defined rigorously, but progress has been made in calculating approximations to such integrals via the use of causal dynamical triangulations.

Tthe words "emerge" and "emergent" are becoming mere placeholders for explanations we'll never get. Every attempt to show time emerging from something more fundamental looks, to me, like this: "something happens, and time results". Is it really necessary to point out the obvious circularity?

Jim,

Would it make sense to say temperature emerges from non-linear activity?

Color emerges from the spectrum of light?

Pressure from the non-linear activity in a closed volume?

Are measures of time also measures of frequency? Do we have any experience of time that is not a rate of change? So would it be reasonable to consider that time simply emerges from this activity? Duration is only the present, as events come and go.

I'm no expert, but I thought Nobel Laureate Schwinger's use of proper time in the path integrals solved the reconciliation of time, as far as a quantum field theory of gravity goes (e.g. up just above the inflation scale). In the 50's, no?

On the other hand we know that Verlinde's geometry/entropy basis doesn't work for fundamental reasons (relativity and the usual absence of a lowest energy scale of such ideas).

Time might be all of these (absolute, relative, reversible) and much much more concurrently in the multiple domains of understanding (AdS) and research (QM, Classical,QFT, ES,HF,MERA etc.,). Some day sooner than latter we might be able to integrate both of those using quantum computing to go from the outer (spacial) to the inner (psyche) and know all there is to organize and apply. Applied Knowledge (AK) for everyone without even the need to write/read/speak. Is that too optimistic?