Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newton schmewton

  1. Mar 6, 2006 #1
    Lots of scientists have been researching inertial effects in an attempt to develop the "reactionless space drive" (sometimes called propellantless or self-contained space drives). Has anyone made any headway?

    I sometimes find articles pertaining to this that will state things like, "Professor so and so of such and such university has devised a theoretical means of self-contained propulsion." These articles will often state that the results are extremely limited (less than the width of an atom in one case) or the concept is based on the highly theoretical reseach of so and so (meaning it's probably garbage to begin with, I suppose).

    Are these concepts usually developed in an attempt to verify Newtonian Mechanics with the expectation of failure, or do scientists really sense that there must be a way around Newton's laws?

    What's the latest news?
    Last edited: Mar 6, 2006
  2. jcsd
  3. Mar 6, 2006 #2
    There is a way around Newton's Laws... it's called Quantum Mechanics ;)
  4. Mar 6, 2006 #3
    Sure, but can you use it to drive a spaceship?
  5. Mar 6, 2006 #4
    Another way would be Special and General Relativity.

    Yes. I'm not sure if QM could do it but relativity tells us that there is equivalence between mass and energy.

    Mr. Newton taught us that Force = d/dt(mass x velocity) which becomes the familiar F = m x dv/dt when mass is constant but becomes F = (v x dm/dt) + (m x dv/dt) when mass changes. This, as I'm sure you know is the basic principle behind rocket propulsion.

    Since a photon's energy is E = (h x freq.) and since E = mc^2, the effective mass of a photon is m = (h x freq.)/(c^2) .

    So, if one blasts enough photons of extremely high frequency out of the back of a spaceship, the ship will move forward since this action transfers the photons' mass from the spaceship to the outside meaning that dm/dt is not zero and is directed in the backward direction. This creates a net force that would push the ship forward.

    Whether this is a "propellant less" spaceship or not would depend on whether one considers the energy used to produce the photons as a propellant.
  6. Mar 6, 2006 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, that's probably it !
    There isn't much stuff around that has technological applications and that isn't well described by "conventional" physics, you know. And to get away with non-conservation of momentum, you'd be in trouble with classical physics, with relativity, and with quantum mechanics...
  7. Mar 7, 2006 #6

    That's still a rocket. It's called a "photon rocket."


    Tell me more about the conservation of momentum in this regard. It's my understanding that in an isolated system any acceleration of the center of mass, off of its center of gravity, would break this conservation. Is this correct?

    What about in the case where the position changes (like in quantum tunneling) but the relative momentum remains the same? Is this still a conservation violation?
  8. Mar 7, 2006 #7


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes. In Newtonian mechanics (point masses and forces and all that), the conservation of momentum is introduced by postulate, which is the action=reaction postulate. So in elementary Newtonian mechanics, it is put in by hand ; and what you put in by hand, you can take it away, too. You could formulate a force law violating this postulate (for instance, green balls excert a force of 3N on red balls according to the line connecting them, but red balls don't excert a force on green balls). However, the re-formulation of Newtonian physics in Lagrangian terms (the variational approach) includes this action=reaction part in its formalism, from the moment that there is spatial translation invariance (that means, that shifting the entire system over a translation vector T, doesn't change any of the internal physics). You cannot take it out "by hand" anymore once you use this formulation. And this variational principle is the building block of about all of modern physics ; so we understand now the conservation of momentum as a consequence of the translational invariance of physics.
    Now, as with everything, this could, one day, turn out to be wrong. But then about all we know of physics today is wrong. I'd need to see SERIOUS indications for momentum not to be conserved for me to start to take such a drive seriously - and as far as I know, never ever such an indication has been found. Again, one day, this could prove to be wrong. Just as the following assertion could prove to be wrong one day: "when you step out of the window on the 25th floor, you fall down". Maybe one could postulate that this may be true on all days of the year, but not on the 8th of March...a day when you can safely step out of the window of the 25th floor and you'll be floating there, not falling... wouldn't it be great if this were true ? Why don't we look deeper in this possibility ? :biggrin:

    So, apart from this reserve, all of current physics says that momentum has to be conserved, and we think we know the underlying reason for that, which is the symmetry that physics is invariant under space translations.

    In quantum theory, there's no violation of conservation of momentum. The reason is that the overall Hamiltonian is commuting with the momentum operator, which is nothing else but the generator for space translations: if a is a translation vector and P is the momentum operator, then the translation operator in quantum theory is given by T(a) = exp(i a P), where you see the explicit relationship of the symmetry mentioned before.
  9. Mar 7, 2006 #8
    Isn't this the day that the window washer's scaffolding just happens to be positioned outside my 25th floor window for a scheduled washing? :biggrin:

    So you're basically stating that if anyone actually succeeds with the development of a device/experiment that can internally shift it's center of mass off of its center of gravity, then all the physics we think we know falls to pieces?

    Does it seem then that physics (since Newton) might be founded on a single principle that may or may not be true? Other than by experiment, has anyone tried to verify this by trying to formulate physics in other terms? What happens?
  10. Mar 8, 2006 #9
    Hi ubavontuba,

    From your original post, I was not aware of the requirement for a rocket-less propulsion system (Sorry, I've not read the articles you referenced. Are they available on the web? -- they sound interesting).

    Regarding your recent post:

    The "center of gravity" and center of mass of an object only coincide when the gravitational field is perfectly uniform. So, in the real universe where nothing is really perfect, such coincidence doesn’t actually exist.

    To "shift" the center of gravity (even more) from the center of mass, all one needs to do is increase the non-uniformity of the gravitational field acting on the object of mass in question.

    A dramatic example would be the “spaghettification” an object experiences as it falls into a black hole. As the object gets closer to the black hole, the gravitational field becomes less uniform. Simply put, the gravity at the “bottom” of the falling object is greater than the gravity at the “top” of the object. As such, the object experiences forces pulling it apart from top to bottom and squeezing it together from side to side.

    Even though the object will eventually “fall to pieces”, the physics we know thankfully does not. ;)

  11. Mar 8, 2006 #10
    Well, you can do a Google Scholar. Do one on "inertia" for starters. You can get more specific to propellantless space drive concepts if you use "propellantless propulsion" -tether -"solar sail." Also, substitute out "propellantless" for keywords: antigravity, reactionless-propulsion, reactionless-space-drive, self-contained-propuslsion and inertial-propulsion

    There are a number of interesting papers to peruse.

    Also, do a general Google of the same keywords and you'll find lots of articles and silliness. Adding the keyword "article" cuts out some of the clutter.
  12. Mar 9, 2006 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    yes. (if I rewrite your phrase simpler: that can accelerate its center of mass internally, in an inertial frame)

    Simple as that.
    Last edited: Mar 9, 2006
  13. Mar 9, 2006 #12


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    "other than by experiment" :rofl:

    Of course. I can do it by the force of thought. I can clash together entire galaxies that way (but not by experiment, just in my phantasy...)
  14. Mar 9, 2006 #13

    That's funny... not. I was referring to reformulating the basis for the standard model on different mathematical principles. Do all roads lead to the current paradigm?

    In other words, if these researchers that are trying to develop asymmetry in isolated systems have any hope of success, musn't there be an alternative to our standard model that correctly predicts what is currently observed, plus predicts the effects they desire?

    I know this is obviously too vague a notion (without observable experimental results) to do more than perform some cursory explorations. I'm just wondering if anyone has done this, and if so, what interesting (or not) results might they be finding (interesting as in "too bizarre to be true" would also be interesting). String theory approaches this level of exploration, doesn't it?
  15. Mar 9, 2006 #14


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    No, of course not. That was exactly my point. You can dream up all you want, you can postulate all you want. If you do not need to have agreement with experiment, there's a lot more "wiggle room" to think up new stuff of course. As I pointed out, in Newton's original formulation of mechanics (which is usually taught as introductory course to mechanics), you do not need action = reaction. It's postulated, but you can leave it out. Then you already have your theory which potentially works as you like, and in which there is no conservation of momentum.
    As I said, you could have a toy universe consisting of red and green balls, red balls pulling on green balls, and green balls not pulling on red balls.
    Define the force law:

    F_green = G_greenred xM_red x M_green / r_green-red^2 x 1_green-red

    (force excerted on a green ball, in presence of a red ball).

    F_red = 0

    (force excerted on a red ball)

    We also define that red balls do not exert forces on red balls, and green balls do not excert forces on green balls.

    All Newton's postulates (except for action = reaction) are respected this way.

    It's a funny system to study. For instance, study what happens to a halter made of a red ball and a green ball, helt together by a strong steel bar of 1 meter length.

    Or study what happens if you have a dispersed set of green and red balls, and what happens to the center of gravity.

    Question: it there conservation of angular momentum ? Is there conservation of energy ?

    So here we have a theory, which is perfectly self-consistent, and in which there's no conservation of momentum. So clearly things do not HAVE to be that way. You can think up such theories, it is not difficult. The point is that they do not correspond to what's observed. I wouldn't know how to make it into a quantum theory, for instance. What's the point ?
  16. Mar 9, 2006 #15


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I think one of us is mis-reading ubavontuba's post. I don't think he's talking about the difference between how gravity works upon an extended object, versus the centre of mass. He's more concerned with the possibility of a "propulsion-less" space drive or something, which would require non-conservation of momentum.
    I'm trying to explain that conservation of momentum resides somewhere deeply in our understanding of physics, and that if momentum turned out not to be conserved, it would effectively shatter about all we know about physics to pieces. Now, I know that in GR, on large scales, things like "conservation of momentum and energy" become delicate issues, mainly because global quantities in GR are difficult to define (see for instance http://math.ucr.edu/home/baez/physics/Relativity/GR/energy_gr.html). So I restrict myself here to conservation of momentum locally, in flat enough space (which is in any case the situation of a potential space drive).
    Newton's theory, without action=reaction, is the last time we could accept violation of conservation of momentum. After that, we got Lagrangian and Hamiltonian formulations (which have it build into them if there is translation symmetry) of classical mechanics. And if you give up on that, you have no leg to stand on to do quantum theory. So most of 20th century physics would indeed fall to pieces if momentum were observed not to be conserved.
  17. Mar 9, 2006 #16

    I think I now understand his original post (thanks in large part to some of your explanations). It was this statement to which I was responding.

    As an engineer, I take that as a challenge. :rolleyes:
  18. Mar 9, 2006 #17

    Thanks. Your letters cleared it up for me quite well. I now only have one question:

    Why are these researchers so eagerly seeking asymmetry then? What's the point?
  19. Mar 9, 2006 #18
    I understand where you're coming from. I'm glad that you now understand what I was asking.
    Last edited: Mar 10, 2006
  20. Mar 10, 2006 #19


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Making fuzz ? Craving for attention ? Too much time on their hands ? I don't know.
  21. Mar 10, 2006 #20
    How 'bout this?

    I was thinking about this subject today while taking a shower and came up with this wild invention.

    It seems to allow for space travel without relying on Newton's law of conservation of momentum. Instead it relies on special and general relativity and ironically is basically a "photon rocket" turned inside out. This ship also emits light but as “exhaust” and not as a “propellant”. As such, I'm unsure as to its overall efficiency compared to Newtonian propulsion systems.

    I call it the Photon Sphere Drive. Though the technology does not now (nor may ever) exist, the principles are fairly elementary. Thus it’s not a matter of physics but a matter of engineering.

    (I wish I could include a picture – is that possible on this forum?)

    “Reaction-less Space Drive using a Photon Sphere”

    The Engine: The Photon Sphere

    The photon sphere is a device used to create primordial black holes for the purpose of space travel. Its radius (which relates to its power) might be on the order of thousands of meters if not more.

    Its inner surface is technically quite amazing. It consists of an array of extremely high frequency lasers directed toward the sphere’s center. Each laser is capable of creating extremely high frequency gamma ray photons. Once firing the photons, the lasers shrink drastically in area so that the photon sphere becomes virtually transparent. After some period of time the lasers return to their normal size and fire again. The laser array almost completely covers the entire inner surface of the sphere and is extremely dense. In fact, each laser’s maximum diameter is only the wavelength of the gamma ray photons.

    Each laser fires at a precise time (based on the speed of light, the sphere's speed and the laser's position) so that all photons reach the center of the sphere (in phase and at the same moment in time) which makes the energy density great enough at that point to curve space-time into a primordial black hole (which “absorbs” all consequent photons from the lasers). After some short period of time the primordial black hole evaporates and its Hawking radiation escapes into space through the holes in the photon sphere created when the photon sphere becomes transparent.

    The Spaceship:

    Attached to the part of the inner surface of the photon sphere not covered by the laser array is a long flexible and stretchable arm that at its end is a pod containing the control deck, crew’s quarters and power supply for the lasers. The vast majority of the mass of the ship is here and its location is slightly removed from the center of the photon sphere. Its’ position relative to the center of the photon sphere can be changed by the flexible arm connecting it to the sphere.

    The principle of operation:

    The mass of the primordial black hole (at the center of the sphere) and the center of mass of the ship (essentially the control pod) fall toward each other. But before the pod enters the primordial black hole’s event horizon and disaster ensues, the primordial black hole evaporates into space through the openings in the photon sphere when in it’s transparent mode. The cycle continues and the spaceship continues to accelerate for as long as the lasers can be powered.

    The ship is steered, stopped and speeded up by moving the pod relative to the center of the photon sphere with the flexible arm.

    Maybe to increase efficiency, the leading surface area (“front”) of the photon sphere would not turn transparent thereby utilizing the energy of the Hawking radiation to add to the ship’s kinetic energy. But this gets Newtonian so ....

Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Newton schmewton
  1. Newton to Joule ? (Replies: 3)

  2. A Newton Question (Replies: 13)

  3. Newton's bucket (Replies: 2)

  4. Newton's rings (Replies: 3)