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B Could gravity be used to send light speed signals?

  1. Dec 14, 2016 #1
    So theres a spaceship 5 light years away from earth and they want to send a signal to earth but a cloud of interestellar dust dont let them use light or radio signals so they decide to send a gravity signal.

    So they produce a violent huge thermonuclear explosion that will send some particles CLOSE to lightspeed.

    Then the mass of those particles increase hugely due to relativity and hence its gravitational field.

    And this gravitational field would be detected as a perturbation of the force of gravity from earth.

    Would this be correct?
  2. jcsd
  3. Dec 14, 2016 #2
    Why would a cloud of dust not let radio signals through?
  4. Dec 14, 2016 #3
    Well make it a thick lead cloud dust :)
  5. Dec 14, 2016 #4
    If a particle has huge mass (and velocity, as you said), its energy is huge too. So to send a signal detectable on earth 5 light years away, the detonation would most probably vaporize your spaceship.
    Because energy is proportional to 1/r^2
  6. Dec 14, 2016 #5
    yes, such things exist almost everywhere in our galaxy, im sure :D
  7. Dec 14, 2016 #6
    Well imagine the spaceship is comanded by robots and they dont mind being destroyed.

    Could that be used to send a signal to earth theoretically?
  8. Dec 14, 2016 #7
    Well if Houston knows they have to look for this signal and they know what it would mean (if they have some CODE, like Morse ;) ), I guess it could be...
  9. Dec 14, 2016 #8
    However I doubt you would get a patent for your idea.
    -It is extremely expensive
    -it is unreliable (if a star explodes nearby the waves interfere and information is lost)
    -it would destroy anything ni a radius of X (some very big number)
  10. Dec 14, 2016 #9


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    That's a common misconception. The idea of mass increasing with velocity is somewhat misleading (take a look at this Insights article); but even if it weren't, relativistic mass doesn't have the gravitational effect that you're imagining.

    There is such a thing as gravitational waves, and they do propagate at the speed of light. They have been detected in some of the most demanding and technologically sophisticated experiments ever done (google for "LIGO gravitational wave"). However, a thermonuclear explosion isn't the way to generate gravitational waves so your idea won't work. And even if we did have a way of generating gravitational waves, they are so weak and so hard to detect that they it makes no sense to use them for communication; any process that produces detectable gravitational waves is also going to throw off enomously larger amounts of much more detectable electromagnetic radiation.
  11. Dec 14, 2016 #10


    Staff: Mentor

    Ugh, no! There is almost nothing which is correct about this.

    First, the concept of relativistic mass is a bad idea. It is just another name for energy, and we already have a perfectly good name for energy.

    Second, the invariant mass does not increase, it stays the same.

    Third, neither the invariant mass nor the relativistic mass is the source of the gravitational field. The source is the stress energy tensor, which includes contributions due to momentum and pressure. Both of those are significant in this scenario.

    Fourth, if you want a gravitational wave perturbation then you need a changing quadrupole moment. A simple explosion will not do anything if it is spherically symmetric, regardless of how violent it is.

    In principle, you could use gravitational waves for communication, but not the way you describe.
  12. Dec 14, 2016 #11
    Wow that was impressive thanks a lot for the feedback.

    So a mass that acquires a relativistic speed trully will increase its gravitational field?
  13. Dec 14, 2016 #12


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    No. The exact opposite, it will NOT.

    There's an easy way of seeing this: Right now, you are moving at 99.99999% of the speed of light relative to someone somewhere in the universe. Has your gravitational field increased?
  14. Dec 14, 2016 #13


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    Gravitational radiation does exist, but a "huge thermonuclear explosion" would almost surely be spherically symmetrical, expanding in all directions. And this implies that it wouldn't generate significant gravitational radiation - there is no such thing a spherically symmetric gravitational wave (this is a consequence of something called Birkhoff's theorem), so a spherically symmetric explosion wouldn't be a significant source of gravitational radiation.

    See for instance the wiki article on Birhoff's theorem, https://en.wikipedia.org/w/index.php?title=Birkhoff's_theorem_(relativity)&oldid=724240631

    Let's ignore this problem for now, and ignore the fact that we don't have a good mechanism other than a binary inspiral for generating gravitational radiation, and just look at the figures for energy assuming we could arrange such an inspiral - or perhaps come up with some as-yet-unthought of similarly efficeint way of genreating gravitational radiation.

    Looking at known sources of gravitational radiation, we turn to Ligo. The Ligo paper is online, at http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.116.061102. Ligo detected the signal due to a binary inspiral, something that is reasonably efficient at generating gravitational radiation. It involved two black holes, estimated at 36 and 29 solar masses, spiralling together (something that doesn't happen in Newtonian theory, but does happen in GR), to form a 62 (estimated) solar mass black hole. In this process it was estimated that three solar masses of energy were converted into gravitational radiation. So that's maybe 5 percent efficient. Which doesn't sound like much, but it's still better than fusion or fission as I recall - slightly better than fusion if my memory serves (but feel free to check).

    This was about a billion light years (estimated) away (converting the megaparasecs to light years). Scaling this is tricky, because a smaller inspiral would generate a shorter pulse of gravitational radiation, which our current detectors are not designed to detect, and it's unclear to me what the noise floor of detectors that could detect it would be like. But lets take some seat-of-the pants guesses. First we'll assume that we can use the inverse square law to scale this to get a similar signal at 5 light years. The signal will be smaller in amplitude, but happen faster, so it'd have roughly the same power. (This is speaking rather loosely, by the way - power and energy are much trickier in GR than I'm letting on). Three solar masses divided by 200 million squared is on the order of 10^14 kg of energy release. If we assume a first power inverse scaling law, which would give the same amplitude of the signal (but higher power), of we'd need about 10^22 or 10^23 kg. This is the signal power, not the size of the masses you'd need to generate the signal via an inspiral - which would be 20 times larger.

    It would be interesting to give a better answer for what would be required with known mechanisms - i.e. a binary inspiral. What would the size of the black holes required be? Would the holes be so small they'd evaporate before they could inspiral, or would it be feasible? But I'm not quite clear on the correct scaling rules for this, so rather than give a bad guess I'll leave it an open question.

    The really short answer is that the energy and power requirements are literally astronomical.
  15. Dec 14, 2016 #14
    I believe gravity also travels at c, like if the sun were to suddenly vanish, we'd still orbit (and see) it for 8 or so minutes. So, no FTL communication using gravity seems possible.
  16. Dec 14, 2016 #15
    If you want to use gravity to send a signal, you'd need a wave. Maybe two massive objects orbiting each other at extremely high speed. That should be detectable by a gravity wave detector. From that, you can alternate the relative rotations to change the frequency and encode some data.

    I feel like there would be way easier ways to do this. Why not use neutrinos? A gas cloud isn't going to get in the way of that, and they travel at nearly the speed of light. They go so fast that when a nearby stars explode, we almost always see the neutrino blast first, because they went straight through the meat of the star while the light had to propagate through it to be released. Even over the light years, the neutrinos travel so fast that the light hasn't caught up by the time it reaches us.
  17. Dec 14, 2016 #16


    Staff: Mentor

    Then the laws of GR would be violated, since this would violate conservation of energy (more precisely, local conservation of the stress-energy tensor). It is actually quite nontrivial to formulate a scenario that lets us test how fast changes in spacetime curvature propagate, because of this limitation; you can't just create or destroy stress-energy and watch how the change propagates.

    However, you are correct that GR predicts that changes in spacetime curvature propagate at c. It's just not easy to test experimentally.
  18. Dec 14, 2016 #17


    Staff: Mentor

    In other words, like two black holes merging, from which LIGO has indeed detected gravitational waves. (If indirect detection counts, by looking for loss of energy in the system, then we have been detecting this for several decades with binary pulsars.)
  19. Dec 14, 2016 #18
    Excuse me for my daring ignorance but ive been checking the forum of this and found this:

    The clearest statement that kinetic energy does indeed contribute to gravitational mass is found in the abstract of this paper.

    According to the general theory of relativity, kinetic energy contributes
    to gravitational mass. Surprisingly, the observational evidence for this
    prediction does not seem to be discussed in the literature. I reanalyze
    existing experimental data to test the equivalence principle for the
    kinetic energy of atomic electrons, and show that fairly strong limits
    on possible violations can be obtained."



    So couldnt you just implode with a fusion explosion two very heavy rotating particles in an inspiral so by conservation of angular momentum they acquire relativistic speed and generate gravity?
  20. Dec 14, 2016 #19


    Staff: Mentor

    No. First, you apparently failed to understand what other posters meant when they said that the source of gravity is not relativistic mass, it's the stress energy tensor. Reasoning that an object's gravity increases just because it acquired relativistic speed assumes that gravity is generated by relativistic mass. It isn't.

    Second, let's re-examine your scenario using correct reasoning. The two rotating particles will acquire kinetic energy as they implode, and you are correct that kinetic energy does contribute to the source of gravity, the stress-energy tensor. But that kinetic energy doesn't come out of nowhere. It comes, ultimately, from the fuel that was used to make the fusion explosion. The fuel has mass, which means it has energy, which means it generates gravity. And by conservation of energy, the energy that appears as kinetic energy of the rotating particles must be the same (assuming no dissipative losses) as the energy that was originally stored in the fuel. Which means the amount of gravity doesn't change at all; the total energy of the system is the same. It's just been redistributed internally.

    (In somewhat more technical terms, kinetic energy of components of a system gets counted in the rest mass of the system as a whole; so does the rest mass of the fuel. So the stress-energy tensor of the system as a whole isn't changed at all by this process.)
  21. Dec 14, 2016 #20
    Maybe a dumb question: In the LIGO detection, is the three solar masses of energy converted into gravitational radiation considered a dissipative loss?
  22. Dec 14, 2016 #21


    Staff: Mentor

    No. Dissipative losses aren't really possible for a black hole merger, since there is no stress-energy present and so there is no viscosity, friction, etc. Dissipative loss would be something like the pulsars in a binary pulsar system heating up as a result of their tidal effects on each other; that would reduce the amount of energy emitted as gravitational waves. (Btw, I'm not saying we have measured this to actually happen; I'm just giving a hypothetical example of what a dissipative loss would be.)
  23. Dec 15, 2016 #22


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    Why yes, of course I can imagine that...
    No!... I refuse to even consider an attempt to imagine [COLOR=#black].[/COLOR]( I did have to... lol) [COLOR=#black].[/COLOR]something so outlandish ... the ethical issues involved from such a proposal are extraordinary... completely beyond, even, the use of words for a description... :oldgrumpy:

    I tell you!... I won't allow it... ever.[COLOR=#black]....[/COLOR] lmao.gif
    Last edited: Dec 15, 2016
  24. Dec 15, 2016 #23

    Mister T

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    The three solar masses worth of energy was radiated away as gravitational waves. That energy is a property of the waves. Before the waves were created that energy was a property of the black holes that merged.

    Many authors make the mistake of saying that energy was converted into something. Energy is a property. It can't be converted into something, it can only be converted into a different form of energy.

    The most famous of these is probably the way an explosion of an atomic bomb is discussed. Matter is not converted to energy. Matter is converted to radiation. The energy that was formerly a property of the bomb is converted into energy that is a property of the radiation.
  25. Dec 15, 2016 #24
    You've never heard of The Borg?
  26. Dec 15, 2016 #25


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