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Do all the objects in the universe exert force on all other objects? Like a type of gravity; also, how much does it decrease as it gets farther away?

Timtech
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  • Curious about your thinking behind the wording "like a type of gravity". Are you seeking a force with infinite reach other than gravity? – Bob Stein Sep 24 '13 at 23:51
  • The responses given so far have a distinctly "Newtonian" flavour. The appropriate gravitational theory for this question is of course Einstein's and from that we learn that everything within our causal horizon effects us gravitationally. Whether material beyond our causal horizon can have influence is technically far more complex and depends on a number of assumptions about the initial conditions for the cosmic expansion. Anyone care to address that? – JonesTheAstronomer Jan 13 '15 at 22:48
  • Anybody that thinks they can answer this question needs to think back to the days before we knew of the existence of dark energy and dark matter. We detect things (including those mentioned above) by the effect (force) they have on other things. If there was an object in space not exerting any force on another object, we wouldn't know that it is there. – Jack R. Woods Sep 14 '15 at 15:39
  • Also, the only known "free" mass-less particle (now that the neutrino is thought to have mass) is the photon which does exert a "force" in the sense that it has momentum p = h/(wavelength). This is what "pushes" solar sails (radiation pressure). Particles with mass, of course, "exert" gravity. – Jack R. Woods Sep 14 '15 at 16:04
  • One more thing. We only know of 4 forces. The strong and weak forces fall off with distance so quickly (short lifetime of particles that carry the force) that they are confined to distances around the size of a nucleus. The other two we know well (electromagnetism and gravity). There is no "kind of like gravity" force as far as we know. – Jack R. Woods Sep 14 '15 at 16:21

2 Answers2

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Yes - this is the formula:

$$F = G\frac{m_1m_2}{d^2}$$

Using this equation, we can say that all atoms in the universe exert force upon eachother. One carbon-12 atom has a mass of $1.660538921(73)\times10^{-27} kg$. That's a crazy small mass.

Now let's say that these two atoms are 100,000,000 light years apart. That's $9.461\times10^{23} m$, which is a very long distance.

Now, if we plug these values into our equation, we get that the force is: $1.709191430132 \times 10^{-59} N$

That's a very, very small amount of force. But it's still force.

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No. It's impossible for every object to interact with every other object, due to the assertion by general relativity, that the universe can, and is, expanding faster than the speed of light.

I then assume that the universe initially was expanding at, or close to the speed of light, and that it immediately after the big bang was expanding faster than the speed of light.

Some of the particles/forms of energy that would have reached us are also bound to have been "held back or deflected", even in the young stages of the big bang, and are now at a distance at which they can never reach us. They could have been held back by for example a black hole.

Potentially, if the expansion of the universe at one point was so slow that gravity from every particle had time to propagate to every other particle, then yes - every particle and energy in the universe affects every other particle.

frodeborli
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  • This is incorrect. The prediction from General Relativity is that the graviton is massless, and therefore travels at the speed of light. Additionally, the universe is not expanding faster than the speed of light within our cosmological horizon. – astromax Jan 14 '14 at 19:19
  • @astromax But there are objects outside of our cosmological horizon? The diameter of the universe is estimated to be some 93 billion light years. – frodeborli Jan 14 '14 at 23:48
  • The graviton is also a hypothetical particle which has yet to be observed, and it will most likely never be observed unless it has mass. – frodeborli Jan 15 '14 at 00:04
  • The graviton has not yet been discovered directly, but I don't know why you say it will most likely never be observed unless it has mass. It is believed to be a massless particle, and this is why it travels at the speed of light. However, there is indirect evidence that gravitational radiation (gravitons) do in fact exist, and a nobel prize(http://www.nobelprize.org/nobel_prizes/physics/laureates/1993/illpres/discovery.html) was awarded for this work. We do not know whether or not there are things outside of our horizon. We can neither see them nor feel their effects since all particles are.. – astromax Jan 15 '14 at 00:27
  • restricted to traveling at the speed of light. Your original statement remains incorrect. – astromax Jan 15 '14 at 00:28
  • According to wikipedia; "a detector with the mass of Jupiter and 100% efficiency placed in close orbit around a neutron star would only be expected to observe one graviton every 10 years, even under the most favorable conditions". Also according to wikipedia: "gravitational waves must propagate slower than "c" in a region with non-zero mass density if they are to be detectable". Wikipedia: http://en.wikipedia.org/wiki/Graviton, indirect sources: http://arxiv.org/abs/gr-qc/9709011 and http://arxiv.org/abs/gr-qc/0601043. – frodeborli Jan 15 '14 at 07:36
  • If gravitational waves can propagate slower than "c", then my argument above stands; it is unlikely that every particle is "gravitating" on every other particle in the universe. In my original statement I say that I assume that at or soon after the big bang, the total expansion was faster than the speed of light. Are you saying that at one time "edge-to-edge" expansion was slower than the speed of light? – frodeborli Jan 15 '14 at 07:42
  • No your argument is incorrect. You are confusing gravitational waves with gravitons. Somebody correct me if I'm mistaken on this, but the only point at which the universe has expanded faster than the speed of light so far was during the epoch of inflation. I'm not convinced you have any knowledge in the field of cosmology. I'm not trying to discourage you from answering questions, but pretending to know things when you absolutely do not contributes to the spread of misinformation. You also do not present sources within your answer. – astromax Jan 15 '14 at 15:20
  • @astromax, as usual in GR, for any observers the spacetime is locally flat, even during inflation, so I am not sure, what do you exactly mean by superluminal expansion. Cosmological expansion is a non-local phenomenon and even today gives the superluminal velocities at sufficient distances. So, well, the objects behind the horizon do not gravitate in weak-field approximation, but do affect us anyway as they affect the global metric. – Alexey Bobrick Jan 19 '14 at 22:01
  • @AlexeyBobrick Nonesense, the speed of expansion (or contraction) between any point within the observational horizon with respect to a local observer can be defined with no problems whatsoever. If you are confused about how I'm using the term 'superluminal velocity', why do you use it in your next sentence in exactly the same way I do? Secondly, no - any superluminal expansion today is necessarily outside of our observational horizon. Also, things which were inside our horizon in the very early universe may not be now. They affected us in the past, but they do not .. – astromax Jan 20 '14 at 04:04
  • currently have any way to affect us in any way whatsoever - metric and all. I'm sorry, what you say is wrong. – astromax Jan 20 '14 at 04:05
  • @astromax, 1) I would ask you to be polite, I am not interested in these kinds of discussions here. 2) As I said, relative velocity is a non-local quantity and depends on the distance between two selected points. Therefore, there is no good definition of such a quantity as "velocity of expansion of the Universe", which you were using before. 3) If anything is unclear, please be welcome to point out. – Alexey Bobrick Jan 20 '14 at 12:11
  • @AlexeyBobrick I want to point out that I wasn't being impolite, and I apologize if you got that impression. Secondly, I would also agree - these discussions in basic cosmology are meaningless here. If you'd prefer I could make an appearance on chat at some point. There absolutely is an accepted rate of expansion of the universe between two non-local points. In fact, this is the very definition of the Hubble parameter. The Hubble parameter is the normalized rate of expansion at any redshift (or scale factor) in the universe, and if one would like to find the "speed" at which galaxies at said – astromax Jan 20 '14 at 17:10
  • distance appear to be moving away from us due to the expansion of the universe, one can do so very easily. I'm really confused about why you think otherwise. If you were to find the speed at which the horizon appears to be moving away from us, you would find that it necessarily has to be less than the speed of light. – astromax Jan 20 '14 at 17:12
  • @astromax Isn't our horizon is irrelevant? Every particle within our horizon has its own horizon, which is different from ours. And the question on this page is about every particle in the universe. – frodeborli Jan 20 '14 at 21:33
  • @astromax: Hubble constant has the dimensions of velocity over distance. So, to use your phrase "the universe expanded faster than the speed of light", which I have been commenting on so far, you have to have a distance measure between some two points. The choice of distance measure is arbitrary. So the concept of "superluminal expansion of the Universe" is also arbitrary. But yes, Hubble constant is surely well defined. – Alexey Bobrick Jan 21 '14 at 14:49
  • To both of your comments: I think the comments here are getting a little much (though not necessarily off topic). I spend a couple of minutes yesterday discussing this topic with my colleagues and we came up with a calculation which I think is relevant to the discussion (and may prove that yes, in fact @frodeborli may be correct in that within our comoving horizon it is possible for galaxies to truly be moving faster than the speed of light with respect to us). Once I have the details worked out a bit more I will try to get a chat going with you two. Would this be okay? – astromax Jan 21 '14 at 15:18