The contest between gravity and quantum physics takes a new turn (2015) | Hacker News Reader

The contest between gravity and quantum physics takes a new turn (2015)

nautil.us

67 pointsdnetesn45 comments

> Just as a pixel is the smallest unit of an image on your screen and a photon is the smallest unit of light, he argues, so there might be an unbreakable smallest unit of distance: a quantum of space.

I thought that that role was fuffiled with the Planck Length. I've seen it used in conjunction with the speed of light to calculate a theoretical "refresh rate" of the universe, was that based on a flawed assumption?

Planck units are what you get if you set the gravitational constant, the Coulomb constant, the speed of light, Boltzmann's constant, and the reduced Planck constant all to 1, resulting in the elimination of constants in equations like Newton's law of gravitation. The argument was that using such units eliminated anthropic bias.

Well, the immediate problem with such arguments is that it's not clear that those values are the right ones to set to 1. The original discussion started before Einstein developed general relativity, where 4piG crops up more than G itself. Likewise, it arguably makes more sense to set permittivity of free space, not Coulomb's constant, to 1, which eliminates the constants in Maxwell's equations. On the other hand, using the charge of ⅓ that of an electron would mean every particle (so far as we know) has integer charge. Planck charge (under any of the values mentioned above) is not any comfortable multiple of that charge.

Effectively, arguing that Planck units hold any physical significance amounts to modern-day numerology for the most part, just as much as when some physicists tried to argue numerological principles for the fine structure constant being exactly 1/137.

Was just wondering the same. I suspect the Planck Length is a lower bound on the hypothetically smallest unit of distance, however in practice perhaps it's larger. The difference between this experimental resolution (10^-18m) and the Planck Length (10^-35m) is absolutely enormous though, so a negative result would still be very inconclusive and highly likely.

If there's a smallest unit of distance, shouldn't there be a shortest wavelength? And thus a smallest difference in wavelength and maximum energy? And a smallest energy change? And thus there must be a minimally different relative velocity, to preserve discretization of relativistic energies and momenta? And a thousand other things we also have no evidence for?

> If there's a smallest unit of distance, shouldn't there be a shortest wavelength?

If I remember correctly a photon with a wavelength of the plank length would become a black hole.

> And a smallest energy change?

No, small energy is large wavelength. Not all energy must be representable by wavelength anyway - imagine an energy difference so slight the equivalent photon has a wavelength greater than the size of the universe.

> And thus there must be a minimally different relative velocity

No, as per above.

> momenta

Angular momentum is quantized, so study that and it might help.

> And a thousand other things we also have no evidence for?

We can still theorize.

PS. To whoever downmodded him: Only downmod offtopic and stupid. Questions, even those with an anti-authoritative tone should not be downmodded, but rather answered.

Is there any reason there can't be a photo with a wavelength greater than the width of the (visible) universe?

It seems like a photon with a 96 billion lightyear wavelength has 2 * 10^-52 Joules of energy. (1 * 10^-33 eV); is there any reason a photon can't have that little energy?

Among other things it would take 96 billion years to create the photon.

So that means whatever process created it needs to take that long. I'm not sure what would happen if the process is interrupted before it's complete. But I suspect the photon would "go back in time", and never have been emitted in the first place.

This "time travel" doesn't pose issues because the photon takes that long to be detected, so if it's interrupted it simply wouldn't be detected.

Wouldn't it only take 13 billion years if you'd started at the big bang and a photon that had the wavelength of the then observable universe (since space stretched under it)?

I also have to ask how that jives with it being a quantized change: either the photon emits or it doesn't (since a photon exists with that energy or it doesn't) -- or there's some probability distribution that we'll detect the photon (which might change over time) -- but how can it be half emitted 46 billion years in the process?

In the end, either it provides a kick at its energy level to another property (eg, electron momentum) in one quantum jump.

Hmm. In an expanding universe, what's happening to the Planck length? Is it staying constant? Expanding at the rate of expansion of the universe? Doing something else?

> And a smallest energy change? > And a thousand other things we also have no evidence for?

It's called quantum physics because it was proven that there is a small amount of energy which is basically the indivisible unit of energy. So yes, that exists, and has been proven. It is the whole point of quantum physics, quantum referring to an amount and the smallest packet being a "quanta of light": https://en.wikipedia.org/wiki/Quantum

> that there is a small amount of energy which is basically the indivisible unit of energy

What? No.

Quantum physics means that things happen in "bundles", usually small ones. But it does not define any particular size to those bundles.

They could be big, they could be small.

> "quanta of light": https://en.wikipedia.org/wiki/Quantum

You misunderstand your link. A quantum is the smallest unit of energy in that particular interaction. In other interactions the quantum could have a different magnitude.

The significance of the Planck length is so far based heavily on conjecture. Some theories of quantum gravity look at it as a minimum distance two events have to be apart for them to be distinguished, others incorporate it in other ways, and some don't imply anything about it at all. As others have noted, we're nowhere near being able to use physical evidence to decide it.

The Planck length has no physical meaning. It's just a combination of physical constants that gives a length. It's considered a scale at which quantum gravitational effects become significant, but it's just that, a scale. It might be a factor of 100 off from something significant, but what that thing is we have no idea.

I have read alternate interpretations of the Planck length/time/volume/etc.

One says that they are a mathematical construct with no meaning, the other says that they provide a bound for the limits of the measurement of reality as we understand it (which many people take to mean the smallest unit).

Do you think these definitions disagree? Does one definitely supersede the other? If so, could you point me at some literature?

Not a physisist, but I found this link [0] to be understandable. The Heisenberg uncertainty principle says that the more accurately we know position, the less accurately we know momentum. When you formalize this, the result is that as the size size of a particle you are trying to measure decreases, the energy necessary to measure it increases. Since energy causes gravity, there is a length, l, where measuring a particle would require so much energy that the resulting gravity creates a black hole.

I think it is fair to say that l is just a mathematical construct, since nothing really happens at this point. But it is a useful mathematical construct that is, theoretically, relevant to our ability to observe the universe.

Having said all of that, all of this is at a much smaller scale than we have ever been able to observe. Seeing as we needed to invent new physics (relativity) to explain scales as small Mercury's orbit [1], and as large as atoms, and that these two theories are still incompatible, it is highly likely that describing physics near the plank scale would require another reinvention of physics.

[0] http://backreaction.blogspot.com/2012/01/planck-length-as-mi...

[1] Arguably smaller, since we have confirmed relativity without leaving Earth orbit.

Yes I too can contrive a thought experiment that combines a bunch of physical constants. It's hardly a construct. It's just a length. It might mean something. It might mean something when multiplied by pi or a tenth or whatever. It's the scale at which quantum gravity might maybe possibly could be important. Nothing more nothing less.

I would not consider the explanation I describe to be a thought experiment, but rather a mathematical derivation; although I might showing my roots as a mathematician instead of a physicist there.

Additionally, all the argument, as I presented it, shows is that there is some length beyond which we cannot observe. It turns out that we can calculate this length, and the result happens to be precisely the plank length. As a mathematician, this seems highly unlikely to be a coincidence; however I am not familiar enough with physics to know if this is a deep result, or a trivial consequence of it's definition.

To clarify the disagreement: do you agree that our current theory predicts that there is some length beyond which we cannot measure?

Yeah, that would make sense, but I don't imagine it's a hard cutoff. It may be that you get diminishing information per unit energy at a length scale related to the Planck length or something.

The first one is true, the second is total speculation.

>The Planck length has no physical meaning. It's just a combination of physical constants that gives a length.

remember a combination of physical constants that gave a speed 100+ years ago? Happened to be pretty meaningful later :)

But it's meaningless today.

> The Planck length has no physical meaning.

Well, if current physics is worth anything, a photon with a wavelength around that scale is probably going to spontaneously collapse into a mini-black hole. So that probably does count as a threshold of some sort.

Yes, some of the attempts at a theory of quantum gravity indeed are hinting at a "quantum of space" around the Planck order of magnitude.

However, theories that assume a "fixed grid" sort of arrangement tend to run into issues. If space is quantized, the pattern is probably not the fixed rectangular sort, but something else instead. Perhaps the notion of space arises as a smooth approximation of some kind of graph of nodes and edges of some kind.

Source: Lee Smolin's books.

(So the search for the "quantum of space" is legit, it's just not likely to be fruitful very soon because of the scale involved. But someone's got to try it.)

My understanding of all this is that "space" and "time" does not exist. There is only quanta of energy (quantum field theory) from which we should be able to derive space and time. Which does not means there nothing smaller than planck length.

But I learn all this by watching discovery channel so I would appreciate if somebody smarter can summarize idea that space and time derive from something else.

> Beneath its layers of conceptual complexity, the holometer is technologically little more than a laser beam, a half-reflective mirror to split the laser into two perpendicular beams, and two other mirrors to bounce those beams back along a pair of 40-meter-long tunnels.

> For the scale of chunkiness that Hogan hopes to find, he needs to measure distances to an accuracy of 10-18 meters, about 100 million times smaller than a hydrogen atom, and collect data at a rate of about 100 million readings per second.

How is this different from LIGO in any way?

It's the same thing but the Holometer is intended to be more sensitive than LIGO.

> What if the universe were entirely empty except for two astronauts. One of them is spinning, the other is stationary. The spinning one feels dizzy, doing cartwheels in space. But which one of the two is spinning? From either astronaut’s perspective, the other is the one spinning. Without any external reference, Einstein argued, there is no way to say which one is correct

There is a 50% chance each is spinning, and we'll know which is correct when an external reference "observes" them. The fact that there's no external reference is what makes this situation be "quantum". If the conscious astronauts are conceptually incapable of being 50% "feeling dizzy", then that would mean conscious beings also require the external reference in order to exist.

This example totally confused me. An object spinning in space experiences centripetal acceleration, relative to its own center of mass. For sake of example, let's say that instead of spinning in cartwheels, the one spinning astronaut is spinning like an ice skater. OK, which astronaut feels their arms being pulled outwards due to centripetal force, and which one has their arms hanging limply at their sides? Wouldn't that answer the question?

in the model where inertia is a result of interacting with all the other gravitational fields empty Universe would mean no inertia and thus no centripetal force. (of course there is another model - that of Higgs boson - for which i haven't so far been able to find how it explains equivalence of inertial and gravitational masses and thus it is so far non-starter for me, i mean not the boson itself - that of course exists, i mean the boson's role in inertial mass story until of course the boson happesn to be the main carrier of the "gravitational charge" then all would fall nicely in place)

> would mean no inertia and thus no centripetal force

Wouldn't that allow infinite acceleration?

Without inertia, if two objects collide elastically - what controls the final speeds?

I don't think you can just get rid of inertia in a thought experiment and expect results that correlate with the real world.

Correct me if I'm wrong, but without the Higgs boson and its corresponding scalar field, the concept of mass/inertia does not exist. Without mass, can matter still exist? How would this "matter without mass" behave? My mind is close to exploding.

imagine that there is only gravitational force in the Universe and the matter has only gravitational mass("charge"). Accelerating any given piece of matter with respect to all the other matter would cause propagation of change (ie. wave) in the gravitational field of the whole system (Universe). Wave is energy and to generate wave a work is required, ie. a force has to be applied to the piece of matter being accelerated - the requirement for that force is what we call inertia.

Now of course the question - what is the carrier of that "gravitational charge". It would be great if Higgs boson was the carrier of that charge.

I thought the idea that "the Higgs boson is the field responsible for imparting mass" was the dominant theory, are there reasonable alternative explanations?

I don't think that's right. I think the Higgs creates a large amount of mass (90%-ish), but not 100% of it. (This is from memory of something I read, so I can't point you to a source...)

> An object spinning in space experiences centripetal acceleration, relative to its own center of mass.

In this universe, yes.

But in an otherwise empty universe? We don't know.

I agree - this doesn't make sense. You can detect if you are spinning without looking at the outside world.

In an absolutely empty universe you can still tell if you are rotating.

Wouldn't the spinning one see the other "orbiting" rather than spinning? If gravity is not strong enough to cause that orbit and there's no thrust evident that could explain the delta-V causing the orbit, couldn't you then conclude that you're spinning without an external reference?

I think this thought experiment assumes the astronaut is spinning around an axis orthogonal to their belly. From the cartwheeling astronaut's perspective, the other one is also cartwheeling in their own plane.

But if you tuck in your knees and the other guy spins differently, that's a sign you're the one spinning, no?

Yeah, the choice of a non-inertial reference frame for this example is a little confusing to me too. I also haven't been able to find an independent mention of the spinning astronaut experiment.

No because you technically performed an action which changes the system.

Relativity states that there isn't a measurement you can take if you change the system however it's a different story.

No it will not. Elements from both theories will be used to formulate the structure/theory/paradigm that will then subsume both of them.

Can we stop ignoring the fact that Quantum gravity exists and works for the most part.

The parts where it doesn't work are extremes like black holes where GR also breaks apart.

Where's the evidence for your statement? This is what Wikipedia says:

> Currently, there is still no complete and consistent quantum theory of gravity, and the candidate models still need to overcome major formal and conceptual problems. They also face the common problem that, as yet, there is no way to put quantum gravity predictions to experimental tests, although there is hope for this to change as future data from cosmological observations and particle physics experiments becomes available.

[0] https://en.wikipedia.org/wiki/Quantum_gravity#Candidate_theo...

It very likely exists, somewhere out there in the grand scheme of things, but so far we don't know squat about it.

> The parts where it doesn't work are extremes like black holes where GR also breaks apart.

Wrong. That scenario is exactly the kind that quantum gravity is supposed to illuminate. Of course, we can't be 100% sure about that yet.

Reply on news.ycombinator.com