# What happens if everything stops?

Suppose, through some fantastical mechanism, all macroscopic (i.e. reasonably massive) objects in the universe were instantly made at rest. In other words, the relative velocity between every two of these objects is set to 0. Subsequent to that instant, Physics resumes business as usual. What happens next?

Intuitively, it seems like everything would start falling directly toward the nearest largest mass (so earth falls directly toward the sun, sun falls directly toward galactic core, etc.) but I'm wondering if there is enough perturbation due to interaction with bodies of similar masses that things would actually just stabilize into a similar type of arrangement as before.

The intention is that all reasonably massive objects would be instantaneously measured at 0 velocity relative to an observer at any point in the universe (there's a bit of hand-waving involved when you get down to smaller scales, as I don't want all matter to suddenly be at 0K due to thermal kinetic energy being lost.) So, no matter where you are in the universe at that instant, everything will appear to have instantaneously stopped moving (within the limits of the time it takes for the light to arrive notifying you of this event; things would still appear to have been moving when you look into the past prior to the event.) I think there would be a noticeable discontinuity in the observed motion for past events, though. Perhaps there is a more fundamental problem with the idea here?

• Assuming no-one died in the shock of the arrest. Dec 12 '14 at 16:22
• at rest relative to what? Dec 12 '14 at 17:59
• Are we talking rotation of these bodies as well? A sudden stop of the earths rotation should fling most of us off into space, no? Dec 12 '14 at 18:18
• @Twelfth: "A sudden stop of the earths rotation should fling most of us off into space, no?": No, because that would also be relative movement (between us and the earth). Dec 12 '14 at 19:15
• If EVERYTHING stops, then nothing happens ! Of course ! Apr 24 '15 at 2:25

I'm not sure what would happen on a cosmic scale if the expansion of the universe stopped, so I'll talk about what happens if the peculiar velocity of all objects are set to 0. That is, locally all objects appear stationary, but distant objects still have a radial speed exactly proportional to their distance. I'll also ignore the problem of having this happen "instantly" by assuming that all objects are stopped simultaneously with respect to your (the observer's) reference frame.

• All planets fall into their stars. They are destroyed even before they would be incinerated as they pass the Roche limit and are torn apart by the tidal forces of the star. Without the aid of the Earth's velocity, no spaceship would be able to reach heliocentric orbit, meaning humanity is doomed as well.
• Most stars survive (initially). Centrifugal force is negligible compared to the pressures holding up the star, so the fact that rotation is stopped doesn't have a noticeable effect. The impact of one of the gas giants is noticeable: Neptune would strike with a speed of 600 km/s (0.2% the speed of light). However, the huge mass of the Sun absorbs the blow easily and any material blasted off into space will eventually fall back into the Sun. However...
• Binary stars may not survive. It depends on the specific situation: two similar stars will probably just combine into a more massive star. However, star systems with a compact object like a white dwarf or neutron star will probably supernova or collapse into a black hole due to the added mass. Speaking of black holes...
• Many stars are swallowed by a black hole. Specifically, the black holes residing in the centers of galaxies. It takes the Sun about 10 million years to fall to the center of the galaxy, and by then most of the stars in the galactic core have been consumed. With billions of stars, the gravitational interactions are complex enough that some stars are deflected and go into highly elliptical orbits around the black hole. There are probably no stellar collisions, since stars are so small.
• The evolution of the universe continues almost as normal. There are now many fewer stars in each galaxy, but there is no real change to larger-scale structures.
• Would this future universe be fairly barren of planets, then? I'm curious about what the implications would be for some form of life that evolved post-'freeze' and what observations might lead them to theorize that such an event might have happened in the past. Dec 12 '14 at 20:47
• "Stars are [...] small." Well, if I didn't feel tiny before... Dec 12 '14 at 21:17
• @Dan Bryant new planets will be created around new suns from matter ejected by novas and supernovas, exactly like old planets swallowed by old suns. Dec 12 '14 at 22:01
• @DanBryant Post-freeze life would likely notice that at some point in the past the amount of energy in the universe dramatically decreased: that beyond a certain distance galaxies seem to have many more stars than the near ones. This would be a fairly abrupt change, but they would still be able to see many galaxies in the process of collapsing, all at the same distance away. Dec 12 '14 at 23:02
• @ratchetfreak Looking at the wiki article again, the derivation does not make the assumption that the body is orbiting. Basically the Roche limit is where the gravity gradient of the larger body is larger than the gravity of the smaller body, regardless of the relative motion. Dec 15 '14 at 18:09

Relative velocity between any two objects is hard to define. Let's assume you wanted to say that rotation speed of any object in gravitational field of any bigger object is set to 0 (which would account for all solar systems, black hole in the center of the galaxy), and two objects/galaxies with similar mass will stop the rotation around common center of gravity. Close enough?

All objects will collapse toward center of gravity. This will restore some rotation momentums. Some old stars will collapse, other will explode. Lots of matter would be "permanently stored" in black holes" but my wild guess is that enough matter will remain out. New stars will form and explode. Collapse and reborn will continue for many billions of years, entropy would keep increasing, and after some time (which I am not capable of calculating, but billions of years) there would be no trace of preceding "momentum freeze".

And so it will continue until the heat death of the universe unless we find any advanced technology (undistinguishable from magic) which can revert laws of thermodynamics.

The fall of the Sun into galactic black hole would take long time, and gravity of any neighboring objects will affect it's trajectory, and stars would keep forming and exploding, even during the fall towards central black hole. Collapse will not be few weeks long affair, and quite of lot of mass would survive the fall.

• Actually it is very easy to define: Velocity in the center-of-mass frame. Well, unless it is supposed to hold for the complete universe, of course, because that has no center of mass. In that case, movement relative to the cosmic microwave background would be the best bet. Dec 12 '14 at 19:13

Larger objects may generate enough rotational energy not to collapse (so for example some of the stars of the galaxy would survive) but essentially yes you are right. Anything in orbit collapses on top of whatever it is orbiting. The results are pretty catastrophic for, well everything.

The only good news is that the collapsing may well trigger supernovas enough to trigger a new wave of star formation.

• What is this “rotational energy” that you speak of? If all relative velocities become 0, nothing is rotating anymore. Dec 12 '14 at 20:47
• @Gilles Objects get pulled towards each other. As a simple example imagine a binary star system with a gas giant planet. The planet is pulled towards both stars so does not fall in a straight line towards either of them as it is pulled towards the center of mass. If that curve is enough to mean it misses the stars then it will end up in an elliptical orbit. Dec 12 '14 at 22:06

I wanted to comment on 2012rcampion's answer, but I don't have the reputation, so I'll try to expand my thoughts into somethat that can pass for an answer. Here's hoping I get enough upvotes to be able to comment in the future.

I agree with 2012rcampion's answer up to the last bullet. I think it's more likely that

• After galaxies are mostly reduced to supermassive black holes with a few eccentrically-orbiting stars, the clusters of these (formerly "galactic clusters") will also fall together. I'm not sure what would happen as they came together, but I would expect most of them to be perturbed enough to miss colliding and form orbital systems of supermassive black holes. Some of these still include stars, and planets, but few (if any) planets remain in orbit around a star.

I'm not sure that the evolution of the universe would continue "as normal":

• Without most of the stars it would be much darker
• With some of the supermassive black holes in small fast orbits, there would be much stronger gravitational waves than there are now
• Cosmic dust and clouds would mostly have fallen in to stars or black holes. With much less mass being emitted by stars they would not reform as before, and where they did accumulate they would be reshaped by the gravitational waves.

But there might be some formations that are more familiar:

• Something would happen to the star material that escaped from the fall into a black hole when the star crossed it's Roche limit and was torn apart. Some of that might form into new stars and planetary systems, some of which might even have stable orbits around or within the black hole systems.

Of course, we have very little idea what would happen with all the dark matter.

• Welcome to Stack Exchange! Dec 13 '14 at 3:17
• I'm hardly new to stack exchange. I've been frustrated by the low-reputation trap on several different stack exchange sites for a few years. Apparently I have a much easier time thinking of small improvements to existing answers than of whole new answers (or questions). Dec 13 '14 at 3:32
• Eventually you get enough rep that they give you a +100 boost when you join a new SE site. Not that that helps you right now :( Dec 13 '14 at 3:43
• ... and by coincidence I hit that threshold today, thanks to some upvotes to an answer on SuperUser. About a year after I started trying. Dec 13 '14 at 7:11

The problem with your Assumption to everything being pulled to the closest heaviest object is that there are a lot of objects in the universe.

given your example of the earth falling toward the sun and the sun falling towards the center of the galaxy, you have one ball attracted to a ball that is already moving.

so what might happen is that the everything starts moving towards these gravitational pulls and then some of them will start spinning and everything starts up again, maybe not the exact same but in a new way, some objects will be destroyed (or turned into something new) by hitting other objects in the Galaxy.

The big thing is that when the Freeze is thawed all things will start moving again, and probably not in the same fashion that they had in the past.

The earth may start spinning the same way that it had, one side falling faster than the other side (thank you Mt. Everest), and because the earth is slightly squished from the many years of spinning that have already occurred the earth will spin from the north pole-south pole axis. The only reason that this would happen though is because of the pull from the rest of the planets.

Let's remember for a second that Mars, Neptune, Jupiter, (not a planet, Pluto), Saturn (not in order) are all going to be falling toward the sun as well, and the earth is also going to be pulled toward them when the earth is closer to them then the sun and the other way around, even though one is heavier than the other they still have an effect on each other, not just a one way street.

• The earth-sun system would both be attracted more or less equally to the center of the galaxy, which would have enough effect that earth still hits the sun. Dec 16 '14 at 23:04
• if the sun is in the Earth-Center of Galaxy vector or close to it, at the time of the stop/restart Dec 16 '14 at 23:06