back to article Millimetre-sized masses: Physics boffins measure smallest known gravitational field (so far)

It might not have occured to rock lugging early humans in the Stone Age that gravity is a relatively weak force, but it is. Throughout their evolution, humans have become accustomed to living on the surface of a ball with a mass of around 6 × 1024 kg, and our planet being this size makes gravity on Earth a real visceral …

  1. Jan 0

    > Gravity is, for example, about 36 orders of magnitude weaker than the electromagnetic force.

    In what way are the different forces comparable? That statement makes as much sense to me as saying that treacle is hairier than thought, or that light is heavier than doughnuts.

    1. Dr. G. Freeman

      The four fundamental forces (electromagnetism, gravity, the strong nuclear force and the weak nuclear force) hold things together, and all of them have a coupling constant, that it the strength needed to overcome the force.

      like how much effort you have to use to pick up something (gravity), compared to trying to pick up the same thing if it was attached to a big magnet (electromagnetism), which we've measured and it's 36 orders of magnitude bigger.

      1. Cynic_999

        Nonsense. I have an electromagnet which is far weaker than the force of gravity, despite the fact that I am powering it with nearly 1pW of electrical energy.

        1. Eclectic Man Silver badge

          Umm, not sure about this, but I'll have a go:

          One of the intuitive difficulties in comparing the strengths of Gravity with the other three forces is that with the three electro-magnetic, electro-Weak force and strong force, the field lines along which those forces act can get bent, whereas with gravity it is not thought that gets bent. (Physicists help me out here, please.) So whilst I can hold a paperclip off the ground easily if it is attached to a magnet, I have to get the magnet really close to the paperclip to get it to leap off the ground onto the magnet.

          Basically at short distances gravitational effects are much weaker than the other forces, until you get to the exceptionally high density objects like neutron stars. At enormous distances (galactic or observable universe scale) gravity dominates, but electro-magnetism can orient particles. See, for example:

          "Explanation: What role do magnetic fields play in interstellar physics? Analyses of observations by ESA's Planck satellite of emission by small magnetically-aligned dust grains reveal previously unknown magnetic field structures in our Milky Way Galaxy -- as shown by the curvy lines in the featured full-sky image. The dark red shows the plane of the Milky Way, where the concentration of dust is the highest. The huge arches above the plane are likely remnants of past explosive events from our Galaxy's core, conceptually similar to magnetic loop-like structures seen in our Sun's atmosphere. The curvy streamlines align with interstellar filaments of neutral hydrogen gas and provide tantalizing evidence that magnetic fields may supplement gravity in not only in shaping the interstellar medium, but in forming stars. How magnetism affected our Galaxy's evolution will likely remain a topic of research for years to come."

        2. Tessier-Ashpool

          Your electromagnet is vastly smaller than planet Earth. It takes a whole planet’s worth of mass/gravity to defeat your tiny electromagnet. Similarly, the forces in a human arm (that arise through electromagnetism) are enough to raise a pint of beer against the gravitational force of an entire planet.

          One more thing: gravity isn’t a force at all. It is the manifestation of spacetime distortion due to the presence of matter. Einstein’s field equations: matter changes the shape of spacetime; spacetime tells matter how to move.

          1. LionelB Bronze badge

            That seems a bit pedantic. Electromagnetism is also described by field equations - Maxwell's (or more properly the Einstein-Maxwell field equations).

    2. terrythetech

      They are both forces so can be compared.

    3. JDX Gold badge

      I think others have missed your question which is essentially "well surely it depends how strong the magnet is".

      IIRC it is based at the nuclear level based on elementary particles, i.e. the forces between protons and neutrons in an atom. In that context, the gravitation force attracting protons is 36 orders of magnitude smaller than the magnetic force pushing them apart, likewise the strong and weak nuclear forces that hold the protons together are even stronger (otherwise the protons would all push each other apart).

      1. Cynic_999


        ... the gravitation force attracting protons is 36 orders of magnitude smaller than the magnetic force pushing them apart ..."

        Surely the force pushing protons apart is electrostatic rather than magnetic force?

    4. Brewster's Angle Grinder Silver badge

      A cleverer version of this argument would be to say, "In what way is a tensor boson comparable to a vector boson? (And why do we exclude scalar bosons?) "

      1. ThatOne Silver badge

        "Rock o' my soul in the boson of Abraham."

        1. DJ

          Have an upvote for the MK reference. Well played.

    5. Anonymous Coward

      The sense that's usually meant is this: if you take two protons a metre apart, the electrostatic repulsion they feel is about 2.3E-28N. The gravitational attraction they feel is about 1.9E-64N. Dividing those two numbers gives you a ration of about 1.2E36: about 36 orders of magnitude.

  2. wabbit347

    A tiny magnet can hold a paperclip off the ground, so the entire mass of the earth doesn't exert enough gravitational force to overcome the force that the magnet is exerting on the paperclip. That's how weak gravity is compared with the electromagnetic force.

    EDIT: Boffin icon in honour of clever people doing science.

    1. Tom 7 Silver badge

      Surely gravity has some noticeable effect at the nuclear level. While G is quite small the distances involved in the quantum description of, say, a nucleus often involve 0 which tends to make 1/r**2 fucking larger as Newton put it.

      1. Dr. G. Freeman

        Gravity is still tiny compared to the strong and weak nuclear forces inside the nucleus (10^35 and 10^26 times bigger respectively) so they drown out any effect from it.

      2. Richard Boyce

        One of the great unknowns is how gravity behaves at such tiny scales where quantum mechanics is so evident, hence the desire for experiments like this to observe gravity at smaller scales than before.

        Quantum Mechanics and General Relativity (Eintein's description of gravity) are mutually contradictory, but humanity has yet to observe conditions sufficiently extreme to pit them against each other to see which breaks first.

    2. Paul Crawford Silver badge

      It is ironic that while gravity is a tiny force on a particle-particle basis compared to the nuclear and electromagnetic effects, it is ultimately able to overcome all other forces and crush matter out of existence in a black hole.

      Probably, I'm not sure the fate of black hole contents is known (or indeed, knowable).

      1. Pascal Monett Silver badge

        Good point.

        So, while the strong and weak electromagnetic forces are parading about, gravity is siping its drink in the corner, thinking "just you wait, I'll show you who's stronger".

      2. Anonymous Coward
        Anonymous Coward

        @Probably, I'm not sure the fate of black hole contents is known (or indeed, knowable).

        Well you know they (black holes) move with respect to the outer universe, so you know that none of the matter inside is at the limit case of 'speed of light' because they can be moved.

        You can deduce things without necessarily being able to see inside.

      3. Anonymous Coward

        Yes, it is remarkable. And this is really because there are both positive and negative charges and bulk matter is electrostatically neutral (it wouldn't be bulk matter if it wasn't). But that's never true for gravitation: everything is always attracted to everything else. Indeed, if GR is correct particles which were repelled by gravity are not even theoretically possible (and this is something that people are therefore actively looking for as a test of GR I think).

    3. Anonymous Coward
      Anonymous Coward

      Is it tiny though?

      You have a clumping force pulling matter together, that is also pulling atoms together and their internals together..... you then measure one in terms of the other.

      The clumping force *must* exist at all scales, even subatomic.

      So gravity moves planet 1 closer to planet 2 by 1 atom size in 1 unit of time, but in the same time the atoms you measure it with also got smaller too! You measured the net effect of the two clumping effects.

      The difference is small, but the clumping is not.

      If matter at the atomic level was clumping together faster than at the planet scale, then planets would even appear to move apart, simply because they were clumping slower than the local stuff that makes up matter! Again this is clumping, but it is perceived as if it was an outward motion of planets!

      You might then imagine some sort of 'pushing apart' force, and design a model for it, (e.g. inflation and its extra dimension), but it would be better to realize that ALL FORCES MUST EXIST AT ALL LEVELS, and thus a clumping force will be clumping at the subatomic level too. That it's not just planet scale things that are clumping, its the matter itself, and since you're measuring the scale of one in terms of the other, you can realize the cases when you'd see this clumping as if it was an expansion!

    4. Nick Ryan Silver badge

      Gravity is not a particularly weak force, it's the density and scale that matters. What tends to let it down is that in most instances atoms by and large are mostly not there and therefore there's not a lot of mass in the space of your average atom to exert much gravity. If one was to find a way to stick lots of them next to each other somehow, I suspect gravity would be considered slightly more useful.

      1. Anonymous Coward
        Anonymous Coward

        Yet if you took that matter and made it into magnets it would be a long stronger!

        But, a side point, Gravity cannot be a fundamental force.

        If I arrange my charged particles +-+-+- then they clump, if I arrange then ---+++ then the sames repel and the opposites attract till they rearrange themselves till they all clump. The same charges, but the way you arrange them determines the strength of the clumping effect.

        This is distinct from electric force, its a function of how I organize those charges, as to how strong they clump.

        This is "electric as a clumping force". Magnetic has a similar clumping effect, throw magnets into a bag and shake and they'll clump, regardless of their original arrangement.

        These are common yet ignored clumping effects, they are non-zero, so they must be components of gravity, (or all of it).

        i.e. gravity is not a fundamental force.

        But why so small?

        You could hypothesize that sum of these clumping forces as zero at planetary distances and thus gravity was a different force, but here in this experiment, they're measuring gravity at small distances that we know these clumping effects exist at.

        So why so small?

        All the time in physics, you measure the properties of matter by comparing it with other matter, then you get a constant local RATIO and declare it to be a constant VALUE. Magic numbers, everywhere, are actually magic local ratios.

        So you don't know that its small, only that the effect of the sum of these clumping forces at planetary level is slightly more than it is at subatomic level, so relatively its a small force.

        1. Anonymous Coward
          Anonymous Coward

          Minor digression...

          Above I point out that gravity cannot be a fundamental force since it must include these clumping effects.

          Ergo any particle that experiences gravity, must also experience electric and magnetic in order to experience the sum of these electric/magnetic clumping effects. Because it experiences gravity it must experience the components of gravity.

          So all neutral particles that experience gravity must actually be made of charged particles.

          Since the photon bends under gravity, so the photon isn't a fundamental particle. It must be made up of an equal amount of +ve and -ves.

          Kinda of obvious given you can split photons between two slits in a simple slit experiment, so of course its not a fundamental, indivisible particle! You might wonder why it sticks together then, acting like a particle.... if only there was some clumping effect it experiences.....

    5. Cynic_999

      Is electromagnetic force proportional to the mass of the electromagnet? If not, the comparison you make is meaningless.

      1. Anonymous Coward

        Other things being equal, yes. It's proportional to the number of turns of wire, and the mass of the wire is proportional to the number of turns.

  3. Peter Prof Fox

    From which we can conclude

    If the Earth was made of bees it would be about 20,000 Km in diameter.

    I think that rules out the possibility that the Earth is made of bees.

    1. Tom 7 Silver badge

      Re: From which we can conclude

      I dunno my hifi has taken to buzzing at 50hz which is the frequency the bee that evolved with tomatoes uses to shake the pollen from the flowers. And I had a salad last night.

    2. Nick Ryan Silver badge

      Re: From which we can conclude

      Shhhh... you give the flat earthers or even hollow eathers ideas... Ideas borne out of absolutely no logic and impossible to hold up to the most basic of trigonometric scrutiny, but nothing like that has stopped these kind intellectually challenged believers. These people would lose an intellectual argument with a fruit fly if they ever stopped frothing at the mouth long enough to listen.

      My sincerest apologies to fruit flies and this unfair intellectual comparison.

      1. Neil Barnes Silver badge

        Re: From which we can conclude

        Things change... I am reliably informed that a sign alongside the A1 north of London which has authoritatively stated 'EARTH IS FLAT' for many years, recently decided instead to state 'EARTH IS PEAT'.

        Now I don't know what to believe.

        1. a pressbutton

          Re: From which we can conclude

          Definitely not pete.

          Unless the earth has grown a beard and has taken to homebrew over the last year or so.

    3. John Robson Silver badge

      Re: From which we can conclude

      Certainly not solid bees.

      1. Nick Ryan Silver badge

        Re: From which we can conclude

        Are you referring to Eric? Who could be termed as half-a-bee?

  4. Persona Silver badge

    General relativity

    According to a fairly popular theory, gravity is not as a force but a consequence of masses moving in a curved spacetime.

    1. Paul Kinsler Silver badge

      Re: General relativity

      It would be slightly better to say something like: gravity is not really a force, but a result of the spacetime curvature caused by masses.

    2. ibmalone Silver badge

      Re: General relativity

      "fairly popular theory"

      Would you say it was generally relatively popular?

      (Aside, if it turns out there are gravitons, will that make gravity in some sense a force regardless?)

      1. Eclectic Man Silver badge

        Re: General relativity

        It used to think it was *special*.

    3. Nick Ryan Silver badge

      Re: General relativity

      ...and what is the medium of force transfer? How fast is the effect of gravity felt? It is possible to convert energy to matter and vice-versa gravity and therefore local gravity should be directly alterable. And thus the real headaches begin. While it's possible to visualise a 3D deformation of a 2D plane, doing the same for 3D space is much harder.

      1. Eclectic Man Silver badge

        Re: General relativity

        I believe that the current theory at LIGO is that gravitational waves travel at the speed of light.

      2. Anonymous Coward

        Re: General relativity

        Gravitational waves propagate at c, the speed of light, if GR is correct. We've now seen gravitational-wave events with electromagnetic counterparts which confirm this is the case to to within a few parts in 10^15 (and probably better than that: this was my simple-minded calculation for GW170817).

        Matter and energy are indeed interconvertible, but what GR 'hears' is not either: it's the energy-momentum tensor, which encapsulates both. So, for instance, if you built a giant hollow sphere, mirrored on the inside, and then exploded a bunch of nuclear weapons inside it (thus converting some matter to energy), its gravitational field, as observed from the outside, would not change. (The mirrors are so the photons bounce around inside the sphere, rather than heating it up and thus effectively leaking out, which would cause it to lose mass.

        1. Nick Ryan Silver badge

          Re: General relativity

          I should have made it clear that I understand that gravitional waves propogate at C, it's just one of those things that when added to the melting pot of concepts produces quite a headache.

          Good description of the mass-energy equivalence in a sphere and that external measuring would produce the same value. In the end it's no different to removing a quantity of matter and then measuring the new gravity.

    4. Brewster's Angle Grinder Silver badge

      Re: General relativity

      I'd hold on for a quantum theory of gravity before making that assertion.

      For now, we view a force as a process which causes a change in momentum of an object. There are four such fundamental processes,* exchange of photons (electromagnetism), exchange of gluons (Strong nuclear force), exchange of W and Z boson (Weak nuclear force), and gravity. Yes, we model gravity as geometry. But there are well known problems with that model (dark energy, dark matter, closed time-like curves, black holes). If spacetime really does exist, it is probably stepped, not curved. And since the end outcome is probably the exchange of gravitons, we may not even need spacetime at all; we could exist in a sea of gravitons and distance be the measure of the mean number of gravitons between us.

      We can also model electromagnetism and geometry and merge it with general relativity, but we don't trend to talk about electromagnetism as consequence of curved spacetime.

      * Debate, should we include the Higgs as a fifth force?

    5. Anonymous Coward

      Re: General relativity

      That's Field Marshall Relativity to you.

  5. Aladdin Sane

    Golden balls and no mention of Beckham? Shameful.

    1. Ochib

      Golden balls and no mention of Jasper Carrot? Shameful.

  6. Anonymous Coward
    Anonymous Coward

    Gravity as ratio

    Let me take you on a short walk.

    Mass is motion:

    1. Detect a stable particle that is stationary, it is in place P1. Detect it again, it is in place P2. Ergo it has moved. It really did move, you did not "collapse a wave function by measuring it", the particle doesn't know which interactions constitute 'measuring' anyway and it is constantly interacting with the universe, so it would be constantly being detected. It really is moving, yet overall it is not in motion, it was a stationary particle remember? So those movements must cancel out. Ergo it is doing some sort of oscillatory motion.

    2. The same must be true of the electrons in our detection circuit, so they are also doing an oscillatory motion, so electric is an oscillating force. Magnetic has a fixed relationship to electric, so magnetic also must be an oscillating force.

    3. You never detect a particle, you only detect the net effect of the oscillating force to the particle. All those weird effects, like particles travelling backwards in time? No, they're no different than the spokes of a bike turning backwards when recorded on video. The net effect of a slight differences between the shutter speed and the wheel speed. Here the slight difference between the oscillating electric field you're measuring with and the oscillating particle.

    4. If you had a slightly different oscillating field from your detector, then your particle would be in motion over that field. It's only stationary with respect to a particular observer with a particular oscillating field. In the bike analogy, the wheel is stationary, but if I change the shutter speed on the camera, then it is moving. The particles motion is not a function solely of itself, it is a function of the net effect of the oscillating electric field from the observer and the oscillations of itself.

    The motion is with respect to a given observer. Hence ALL MOTION IS THIS MOTION. Every form of motion you've ever detected is this motion. Velocity, spin, the weird oscillatory motion of light, the orbit of planets and movement of stars, all is the net effect of the oscillation modes of the matter in your detector vs the oscillation of the particle.

    The velocity of those stars is the same motion that is there at the subatomic level, because it is the ONLY mechanism of motion.

    5. Point 4 is true all the way up to the [zero mass/speed of light] limit case. Light is moving the same way. It moves the same way through the vacuum between atoms of glass as it does through the vacuum between atoms of gas, as it does moving between atoms of really low density gas, all the way to a vacuum, at no time does it move any other way. Always near 1 resonant oscillation of the local field.

    6. And since the size of a particle is from its the extent of this motion, hence the *scale* of space is also from this motion. Light moves N electrons in M oscillations of this resonance field, this is true regardless of N. N could be 1 in one direction and 100 in another, and it would still be true. We measure lights speed by comparing it against matter, and since light and matter both get their scale/speed from the field, so the speed of light in a vacuum always appears to be a constant when compared to matter. The apparent constant, "speed of light in a vacuum" isn't a constant, its just a fixed ration we chose when we chose our unit of time.

    OK, so we almost got mass here, you have a) a motion that goes nowhere that can be converted into movement, but you also need a clumping force, gravity. Mass has gravity, not just an energy that can be converted into motion.

    7. Take a bunch of magnets, shake them up, they end up clumped. Why? Because the repel ends push apart, the attract ends pull together and there is a net clumping effect. They organize themselves to clump.

    8. The same is true of oscillating forces. Notice how things will try to settle to resonance? And the ability to clump is a function of the component of overlap of the two oscillations, stationary mass clumps with stationary mass at the same velocity because it is oscillating the same way and hence maximum overlapping oscillation component.

    The overlapping oscillation clumps the most, light clumps with light.... ~same oscillation mode. Matter doesn't clump with light... ~zero overlapping component, stationary matter clumps with stationary matter ~same oscillation mode.

    9. If you have a clumping universe, and clumping planets, then both the planets are clumping together AND SO IS SPACE, and you observe it as the ratio of the two. That tiny clumping effect only *appears* to be tiny with respect to the matter clumping. See wabbit347 comment above....

    Can I point out:

    You might want to realize that to be stationary and in resonance, it must return to the same place after an *Integer* number of resonant oscillations and given this is mass, you might then realize the ~ near integer atomic masses of stationary/stable atoms is a consequence, not accidental. You might also then notice that non-stable (i.e. non-integer) particles are not stationary when in an even field, and you might then explore pushing an oscillating particle with an oscillating force and realize that you're not making mass, it's just harder to push....

    1. Nick Ryan Silver badge

      Re: Gravity as ratio

      Please define "stationary".

      Stationary is relative to the observer. The Earth spins on its axis, the Earth wobbles on its axis, the Earth and Moon orbit around a point that's not quite the center of the Earth, the Earth (and moon) orbit our sun in a mostly elliptical orbit, the sun is moving relative to other local stars, these are moving relative to the centre of our galaxy, our galaxy is whizzing through space. If you manged to find a particle and somehow, somehow, make it "stationary", relatively speaking it would piss off into the distance at such speed that you'd never ever be able to take a second observation.

      As for the other stuff, yes some things are observable oscillations. These are usually called "waves" and the duality of waves and point particles is a whole field of study in itself - particularly as it is not possible to have a partial wave because for something to be a wave it must traverse the whole wave path, as in up and down, which imposes a certain limit to division. One can measure a fraction of the wave, however the wave itself is always complete - although overlaid waves do produce some useful patterns. Magnets clump with each because they are trying to move to a zero energy state (usually failing spectacularly), therefore this has nothing to do with gravitational attraction, without which we could not exist.

    2. Boothy Silver badge

      Re: Gravity as ratio

      Quote: "Mass is motion"

      Nope, mass is essentially resistance to acceleration.

      The relationship between mass and motion, is that the more mass something has, the more energy it takes to impart motion on that mass. As such, the two, mass and motion, cannot be the same thing.

      1. Steve K Silver badge

        Re: Gravity as ratio

        IIRC the resistance to acceleration (mass) is imparted to by the "drag" of the Higgs field?

  7. Long John Silver

    Dark matter exemplifies Kuhn's thesis

    Thomas Kuhn in his work "The Structure of Scientific Revolutions" highlighted the role of aberrant observations in promoting ferment leading to what he termed "paradigm shifts" e.g. from a geocentric universe to heliocentric.

    The geocentric stance enabled prediction of planetary position but this required introducing the notions of 'epicycles' fine-tuning underlying major cyclical movement. Heliocentricity simplified matters considerably and, on the principle of parsimonious explanations (and tools of prediction) being preferable to elaborate constructions leading to the same outcome, the paradigm shifted.

    Kuhn gave examples of how seemingly secure physical theories came under challenge from anomalous observations. When such observations become accepted as reproducible the natural (indeed sensible) course was to suggest adjustment of the core theory. Eventually these adjustments become so ad hoc that serious attention begins to shift towards as yet tentative alternative theories. This is well illustrated by move from classical description (and prediction of behaviour) of atoms to Quantum Theory.

    'Dark matter' is propping up General Relativity. It remains to be seen whether direct observation of dark matter will remove need for the prop. Should that not be the case then expect a roller-coaster ride to the new paradigm.

    1. Anonymous Coward

      Re: Dark matter exemplifies Kuhn's thesis

      Exactly: everyone knows GR is wrong in some limit cases. Unfortunately those limit cases are extremely hard to explore experimentally. If GR turns out to be wrong on large scales with weak fields as well then, well, whoever shows that to be the case is going to be at least as famous as Einstein. So people really are mad keen for it to be wrong. Sadly for everyone we keep finding things which say that, if dark matter really is just GR being wrong, then whatever theory which replaces is it must just be so hideous that some unexplained dark matter seems like a better option, almost.

  8. The Oncoming Scorn Silver badge

    Well, it’s your habitat, not mine.

    It’s not a question of whose habitat it is, it’s a question of how fast you hit it!

  9. billat29

    I'm sure you lot have all been spending too much time in the bar with AMFM

  10. HildyJ Silver badge


    Could someone convert the odd measures of distance and mass into standard Reg units so I can understand them?

    1. Robert Forsyth

      Re: Please

      The mass of a bee, 1/15 length of a bee (shame bees aren't 16mm long)

      1/9 the force of a blood cell due to gravity, oops.

      The force to accelerate a blood cell at 1.09m/s²

  11. IanRS

    There is no such thing as gravity.

    Everything just sucks!

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