41

What is the smallest item for which gravity has been recorded or observed? By this, I mean the smallest object whose gravitational effect upon another object has been detected. (Many thanks to Daniel Griscom for that excellent verbiage.)

In other words, we have plenty of evidence that the planet Earth exhibits gravitational force due to its mass. We also have theories that state that all mass, regardless of size, results in gravitational force.

What is the smallest mass for which its gravity has been recorded or observed?

(By the way, I hoped this Physics SE question would contain the answer, but it wound up being about gravity at the center of planet Earth.)

Community
  • 1
End Anti-Semitic Hate
  • 605
  • The comet that ESA just landed a probe on ? – Martin Beckett Nov 01 '15 at 19:27
  • I'm thinking much, much smaller than a comet. – End Anti-Semitic Hate Nov 01 '15 at 19:55
  • 2
    Probably some of the modern-day Cavendish experiments... – Florian Marquardt Nov 01 '15 at 19:56
  • 5
    Perhaps you'll find this question insightful. There's an experiment that measured the gravitational force between millimetre-thick discs. – eyqs Nov 01 '15 at 20:09
  • 5
    Do you mean "the smallest object to be seen as influenced by gravity", or "the smallest object whose gravitational effect upon another object has been detected"? – Daniel Griscom Nov 01 '15 at 20:52
  • @DanielGriscom Great question. I mean the latter: "the smallest object whose gravitational effect upon another object has been detected." I will update the question accordingly. Thanks! – End Anti-Semitic Hate Nov 01 '15 at 21:25
  • 2
    "In other words, we have plenty of evidence that the planet Earth exhibits gravitational force due to its mass." The force is due to the product of two masses and is felt equally but in opposite direction by both objects. Therefore the gravitational influence of gas molecules in the Earth's atmosphere on the Earth is demonstrated by them not flying off into space. – ProfRob Nov 01 '15 at 21:32
  • 1
    @RobJeffries If you can quantify the gravitational effect of an individual molecule in the Earth's atmosphere, then that seems like a good answer to provide. As a sum of molecules, they would not be the smallest by any measurement (mass, volume, etc.). – End Anti-Semitic Hate Nov 01 '15 at 21:33
  • @RockPaperLizard The question asks whether an effect has been detected, not quantified. I guess I really don't understand what the question is asking for. Everything is affected by gravity, including photons. – ProfRob Nov 01 '15 at 21:37
  • 2
    @RobJeffries: I think the question is asking for direct experimental measurement of a small object's gravitational effect on (for example) the Earth, or any other object. As opposed to experimental measurement of the Earth's gravitational effect on the object, together with a deduction that the reverse force also applies. That said, I doubt we have any device for measuring force, whose principles of operation we are as confident in as we are confident in Newton's Third Law / conservation of momentum. So I don't think this is a "real" distinction in the end, but still. – Steve Jessop Nov 02 '15 at 00:51
  • @SteveJessop OK, then I guess the Cavendish experiment it is. Certainly not neutrons (which are more massive than photons). – ProfRob Nov 02 '15 at 07:34
  • I cannot answer because I just joined physics., but Gravitaional Lensing affects photons, which have zero mass. – Christian Nov 03 '15 at 10:41
  • 1
    However, that does not answer the question. The OP is looking for the "smallest object whose gravitational effect upon another object has been detected". – Jake Nov 03 '15 at 10:57

3 Answers3

34

Gravitation effect on neutrons have been demonstrated.

Bouncing neutrons

To obtain neutrons with quantized gravitational energy states, the team used a technique first described in 2011, in which a nuclear reactor produces neutrons travelling at 2,200 metres per second. These are then slowed to less than 7 metres per second and cooled to just a fraction of a degree above absolute zero, before being funnelled between two horizontal plates.

The neutrons bounce off the lower plate, which is a highly polished mirror, while the upper plate is an absorber that creams off those with the highest energies, to leave only neutrons in their lowest quantum state. Neutrons are ideal for these quantum bouncing experiments because they have only weak electrostatic polarization and carry no net electric charge, says study co-author Peter Geltenbort, a physicist at the Laue-Langevin Institute in Grenoble, France, which produced the neutrons for the experiments. “They only really feel gravity,” he says.

I offer the neutron as the smallest particle to display Newtonian gravity.

The team found that the neutrons' energy levels are exactly as if they are being acted on by gravity alone — measured on a scale 100,000 times smaller than ever tested before. This puts limits on additional 'exotic' forces that some have predicted could be seen on these tiny scales.

In this experiment the two objects are "a neutron" and the "earth", obeying Newton's third law of motion:

When one body exerts a force on a second body, the second body simultaneously exerts a force equal in magnitude and opposite in direction on the first body.

Community
  • 1
anna v
  • 233,453
  • 11
    Photons have been shown to be affected by gravity; are they smaller than neutrons? (Does a single photon have size?) – Daniel Griscom Nov 01 '15 at 20:53
  • 14
    Very cool experiment - but the question asked about the smallest object whose force on another object was recorded. I suppose you can invoke Newton's third Law to conclude that if Earth pulled on the neutrons, then the neutrons pulled on the Earth. – Floris Nov 01 '15 at 21:49
  • 2
    @Floris yes, the earth is the other object and Newtons law. – anna v Nov 02 '15 at 05:05
  • @DanielGriscom No, a single photon doe not have size, in our current standard model it is an elementary particle. One might call it an "object" with difficulty – anna v Nov 02 '15 at 05:08
  • 1
    @DanielGriscom well, the answer is within Newtonian gravity, and the third law. It is general relativity with photons http://physics.stackexchange.com/q/6197/ – anna v Nov 02 '15 at 05:15
  • 1
    Wait, if photons are deflected by gravity, and photons have (non-rest) mass, does that mean that they also attract other objects gravitationally? The gravitational lensing effect has been observed around black holes, so by Newton's third law, the photons also attract those black holes (very weakly and almost isotropically, but still should result in a theoretical net force towards the strongest light source near the black hole). – John Dvorak Nov 02 '15 at 08:34
  • @annav elementary particles still have mass, don't they? And mass implies a Swarzschild radius. So, by which measure do neutrons count as objects but photons don't? – John Dvorak Nov 02 '15 at 08:40
  • 3
    @JanDvorak The photon has 0 mass, it just has energy and according to newtonian physics there should be no gravitational interaction. It is general relativity that generates interactions for the photons through their energy . Electrons are also elementary particles but they have mass. I checked on the internet whether gravity affected electron positron beams. There is the effect of the tides, but I think it is just the deformations happening on the bases of the magnets that affects the electron positron beams. – anna v Nov 02 '15 at 12:01
  • The neutron is a composite particle so definitely it is an object. – anna v Nov 02 '15 at 12:04
  • here is the state of the art for electrons. https://indico.cern.ch/event/361413/session/1/contribution/4/attachments/1137816/1628821/WAG2015.pdf . – anna v Nov 02 '15 at 12:12
  • 1
    @JanDvorak It helps not to mix newtonian gravity with einsteinian. When you're discussing gravity with photons, forget Newton's laws - they're no longer a good approximation. Still, yes, photons contribute to the curvature of space, and as such, create gravitational attraction. I think this has been (theoretically?) explored with something like "gravitational laser" - due to relativity, lasers in a vacuum should auto-collimate due to their own gravity. I'm not sure if there was any practical experiment, but if there was one that disproves this, it would be major evidence against GR, so... – Luaan Nov 02 '15 at 12:59
  • @Floris: One can use Newton's Laws to predict that each of these neutrons exerted a gravitational force on the Earth, but the question ask for an observation. – Beta Nov 03 '15 at 01:02
  • @Beta If you read the link, you would see that the gravitational interaction was oberved acting on neutrons, which directly answers the title. When one reaches "smallest" dimensions quantum mechanical formulations are necessary, and I think the link makes a good sum up of the experiment. – anna v Nov 03 '15 at 09:50
31

The classic gravitational measurement is the Cavendish Experiment, and the masses involved were a pair of 0.73 kg lead weights. So that forms an accessible reference. Other versions of the experiment may have used smaller weights, though.

WhatRoughBeast
  • 6,402
  • 1
    I think this is what the question actualy wanted as both of the bodies involved where 'small', thus demonstrating that gravity is not a propertiy confined to 'massive' objects – Jekowl Nov 02 '15 at 17:36
  • The speculation in your last sentence was well founded. I Found a reference that has yours beat by many orders of magnitude... – Floris Nov 03 '15 at 02:40
  • https://arxiv.org/pdf/1602.07539.pdf a 2016 proposal for milligram sources of gravity – anna v May 05 '16 at 16:17
17

I found a 1988 paper by Mitrofanov et al which describes a Cavendish style experiment where the "big" mass was 706 mg - where Cavendish used balls of over 150 kg. The "small" mass ( the one on the torsion pendulum) was only 59 mg.

This experiment was done to examine possible deviations of Newton's law at extremely short distances, and established a lower limit on the size of such deviations at a distance below 1 mm. The write-up is quite interesting.

This may not be the lowest value - so I encourage others to find credible publications that show the effect of smaller masses (note that Anna's answer sets a pretty high bar for "lowest mass that was subject to gravity" - but this is an attempt to find the "lowest mass that is seen to attract another mass gravitationally")

Floris
  • 118,905