Once a long long time ago a guy called Newton came up with a law he called his 3rd law. "The laws of motion"

Everyone knows about "Every action has an equal and opposite reaction"

My question here is, and I may be wrong here; there is nothing in the law that suggest the opposite reaction has to take place in the same space time as the action.

So my suggestion here is that in theory it could be possible to change the space/time in which the reaction takes place. Possible by strange things such as quantum entanglement? Or some other physical law we have yet to encounter.

So just going with this for a second. This would allow us to do things like send rockets into space without any of the dangerous bit happening near the rocket. We would still need all the propellant, but it could be ignited in a safe place and then we "simply" shift the reaction to where the rocket is?

Clearly this is well outside our common experiences, and definitively "syfy," but if the physical universe we live in would allow such things then can you imagine the possibilities.

NOTE I am not talking about moving energy from one place to another. That is a different thing. I am suggesting a way of relocating reactions in spacetime.

The difference can be seen if we were to do the opposite. For example, we could stop an asteroid from striking the earth in a damaging way if we could transfer the excess reaction from the strike to another location, such as the Moon. This could allow the asteroid to simply stop as it hits the earth, and then subsequently create a massive crater on the moon.

So the final question here is: If such things were possible, and the relocation of reactions was "easy" / "and everyday event," then do you think humanity would be better off with this technology?

What advantages could mankind get from this technology?

What disadvantages would we face?

When man invented nukes we were worried it was the beginning of the end, but we are still here.

Would such knowledge be survivable?

  • $\begingroup$ I'm not entirely sure what you're asking. As I have learned it, Every action (force) has an equal and opposite reaction (force). When I backhand your face, you face smacks my backhand. When my hammer smashes a watermelon, the watermelon pushes back on the hammer with the same amount of force. But since the watermelon is less solid, it gets smashed and the hammer doesn't. Every time you jump up and down on the earth, it moves. But the effect is so minuscule it is negligible. $\endgroup$ Commented Mar 14, 2016 at 3:21
  • $\begingroup$ Are you suggestion when I backhand your face, instead the wall behind you is what pushes back, so that I effectively backhanded a brick wall? $\endgroup$ Commented Mar 14, 2016 at 3:23
  • $\begingroup$ You need to remember that so-called natural laws are descriptive, not prescriptive. That is, they are human attempts to describe what actually happens. If we find out that some law doesn't provide an accurate description in all circumstances (as Newton's Laws don't at high speeds or in strong gravitational fields), then they don't, so you have to change the law, not the universe :-) $\endgroup$
    – jamesqf
    Commented Mar 14, 2016 at 5:36
  • $\begingroup$ @XandarTheZenon I am suggesting that the laws still hold true, but the reaction happens somewhere else. So when you hit a watermelon with a hammer the hammer gets no force back on it from the watermelon. The melon is smashed (more then normal as there is no resistance from it) but we are able to redirect the watermelons original opposite force into your face. So when you hit the melon with the hammer the melon hits you in the face (: $\endgroup$
    – Paul Spain
    Commented Mar 14, 2016 at 6:51
  • $\begingroup$ Isn't there a zeroth law of motion: objects response only to forces they feel at the instant they feel them, anyway it is such intuition that give rise to particle zoo(conservation of momentum) $\endgroup$
    – user6760
    Commented Mar 14, 2016 at 10:49

4 Answers 4


there is nothing in the law that suggest the opposite reaction has to take place in the same space time as the action.

Yes, it does. Newton's mechanics did not admit multiple "space times", and if there were such a thing than having this connection would mean they are not separate. That is, you are really talking about separate locations in a single "space time", and perhaps discontinuities in space but really part of the same universe.

Although Newton's later laws concerning gravity would postulate instantaneous force across a separation, now we understand that fundamentally an interaction occurs locally at a mathematical point (although the exact point involved may be "uncertain"). Particles form a vertex of three, and the straightforward case is where one particle enters and leaves (changing momentum) and a boson is emitted or absorbed.

Conservation of momentum takes place locally. That is, in an arbitrarily small region of interaction, and without gaps where the momentum suddenly appears somewhere else.

You can seemingly have something like that, e.g. by slowing down something using magnetism. But you find the field carries momentum, and the transit of momentum from one place to another invisibly is still limited to the speed of light.

But that's a minor difference: in your example the excess energy would appear on the moon a couple seconds later, not instantly.

Having a messy impact magically have effect somewhere else is nothing short of magical. How does the impact on to the rocks become sorted out so that the motion and heat of random atoms simply doesn't happen? That's magic. What is more realistic is to have a way to slow down the asteroid, directing the energy someplace else. A gravitational tractor beam, for example, would have the reaction occur where the tractor projector is mounted, as it carefully lowers the asteroid to the ground.

  • $\begingroup$ I never said a couple of seconds later, I said subsequently. It could be that it is instantaneous. The storage of anything is not my question here, "simply" the displacement of effect. Even limiting it to within a radius of the speed of light would suffice for the concept. And since the speed of light is not infinite you could term this as a sort of storage if the displaced the effect to the other side of the galaxy. As in that case it would take many years (from the observers point of view) $\endgroup$
    – Paul Spain
    Commented Mar 15, 2016 at 2:12
  • $\begingroup$ You seem to be speculating on the "reaction" being somewhere else, as opposed to what is directly touching it. Cort got the same interpretation of what you described. That's (non-localoty) specifically what we're saying is not possible. Maybe you have better ideas after reading the Answers. $\endgroup$
    – JDługosz
    Commented Mar 15, 2016 at 2:22

The options are limitless, because to make this happen you would have to have a universe with physics completely unrelated to our own. Quantum physics does allow what Einstein called "spooky action at a distance," but it comes with some really odd caveats that prohibit the transfer of classical information faster than the speed of light. Redirecting energy like that transfers information, so QM says you can't do it. Relativity handles everything at spacetime pointlike events, and doesn't permit any interaction outside of your own light cone.

This means the rules have to be completely constructed from scratch. Perhaps you can construct wormholes so that the action goes where you want, and the reaction goes through the wormhole to "elsewhere." Regardless, one would have to consider the exact rules of the approach to determine how well we could survive. If there are no limits to it, we'd certainly slaughter ourselves. The ability to fire a bullet and project the energy elsewhere in the universe would make assassins unstoppable and undetectable unless you craft a rule of physics that make them trackable and/or permits the deflection of said energy.

  • $\begingroup$ Your "assassin" idea would not be quite that simple. Your assuming only the assassin has the technology. In a world where this is possible it is safe to assume that the target would be able to redirect any fatal strikes using the same such technology. $\endgroup$
    – Paul Spain
    Commented Mar 14, 2016 at 7:09
  • $\begingroup$ Note that you cannot transport matter, only force. And for the bullet to work you would still need the force in the gun. You could somehow fire the bullet from a second gun else where but the first gun would still need to be at the correct location for an assassination. $\endgroup$
    – Paul Spain
    Commented Mar 14, 2016 at 7:11
  • $\begingroup$ @PaulSpain Is there any reason to believe force is not the thing that a bullet uses to kill you? If the technology can take care of the forces and momentums involved with launching a rocket into space, why can it not put force onto someone's skull? And as for the ability to redirect such strikes, you should specify how that would be done in your question. If one looks at the history of warfare, the ability to prevent an opponent from applying force is traditionally orthogonal to the ability to apply such forces yourself. You would need something exotic like Dune's force shields... $\endgroup$
    – Cort Ammon
    Commented Mar 14, 2016 at 14:11
  • $\begingroup$ ... which Herbert explains were explicitly constructed to remove long range weapons like guns from combat because he wanted a world where swords and knives still had power. $\endgroup$
    – Cort Ammon
    Commented Mar 14, 2016 at 14:11
  • $\begingroup$ A shield should still be possible, If you assume a shield is a protective volume and not a shell. This would allow you to protect against internal injury. However assuming some sort of energy limitations it may not offer protection against a bomb blast or other larger scale reaction. $\endgroup$
    – Paul Spain
    Commented Mar 15, 2016 at 23:23

You're asking for a two body system that is in a momentum eigenstate, in which each component body is NOT. So, when you clamp down on the momentum of one body, putting it in an eigenstate, the other assumes the appropriate momentum eigenstate, thereby effecting an effective delivery of action-at-a-distance impulse by quantum entanglement.

But each measurement destroys an entangled state, so you can only do it one-shot. To make this effective, you need a whole lot entangled state pairs divvied up with one of each pair in the nearby body, the other in the remote body.

To get the remote body to go one way, you have to bias the (random) outcome of momentum measurements on the nearby body so that it tends to register a momentum eigenstate that prefers one direction more than the other so that the other body tends to push the other way. But if the measurements are biased, then the nearby body will also already have a tendency toward moving in the direction of the momentum (and the remote body, a tendency to move the other way, already).

The closest equivalent to that general idea is propellant-free rocket thrusting. There is now (well-funded) R&D currently underway to try out an idea for propellant-free thrusting.



  • 1
    $\begingroup$ Welcome to the site Rock Brentwood, when you have a few minutes, please take the tour and read up in our help center about how we work: How to Ask. As you come forward to more recent questions, you may reflect that these days, this one would be closed as "primarily opinion based", or "unclear what you're asking". Pretty good first post on your part though. :-) From review. $\endgroup$ Commented May 22, 2019 at 9:55

Basically you are talking about having a rocket on the ground pushing at the back-side of your ship through a wormhole. The problem with wormholes is that moving that 1cm through a wormhole to cover that 500km distance in regular space follows the same rules as moving 1cm in regular space, meaning if your ground rocket explodes, your space capsule is still 1cm from the explosion for all practical purposes.

If you could make and move wormholes for free, then it would save you some serious launch mass, but my guess is that punching that hole through space-time will take a lot more energy than you'd save. Also, if you have the technology to make wormholes anyway, a better application may be to simply drive your ship into space by going through one, or better yet, forget space and just make interplanetary highways.

That said, there is an actual technology that embodies this description without breaking spacetime or Newtonian physics: solar sails. Using a solar sail, you can use a laser to propel an object at a distance from your power source, and if your laser explodes, it does not harm the distant ship.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .