Gravity Waves

Discussion in 'Physics & Math' started by Little Bang, Sep 26, 2015.

  1. paddoboy Valued Senior Member

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  3. The God Valued Senior Member

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    They look good, but most of the time you follow fraiser cain, that poposcience guy....

    and Paddoboy, to tell you honestly, that tutorial of yours is something which can be an achievement for you, able to write so much without basic knowldege of science...But for me its like that ok given by that GR expert of yours.....kind of not to discourage you.
     
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  5. paddoboy Valued Senior Member

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    They are good and factual to boot, the only problem appears it doesn't fit your agenda.

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    The rest of your post, as usual, nil science content and the usual bullshit.
    Do better.
     
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  7. quantum_wave Contemplating the "as yet" unknown Valued Senior Member

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    Here is a very good video about gravitational waves, Ligo, and Einstein's remaining unverified prediction:

     
  8. paddoboy Valued Senior Member

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    Nice video...He actually gives the correct assumption as opposed to your misleading headlines though, at the 4m10s mark, gravity waves not yet directly detected.
    They certainly have been evidenced and most reputable scientists are confident they do exist. Problem is that our present technology is not quite up to the job just yet.
     
  9. quantum_wave Contemplating the "as yet" unknown Valued Senior Member

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    Yes, misleading title, though it is followed with "?!?!". I guess they want us to anticipate the possibility that the "hit" in the early data was a real gravitational wave, and not a passing semi, lol.
     
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  10. paddoboy Valued Senior Member

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    No probs. Here's a nice rundown.
    http://www.astro.cornell.edu/academics/courses/astro201/psr1913.htm
    The Binary Pulsar PSR 1913+16:
    In 1993, the Nobel Prize in Physics was awarded to Russell Hulse and Joseph Taylor of Princeton University for their 1974 discovery of a pulsar, designated PSR1913+16, in a binary system, in orbit with another star around a common center of mass.

    Using the Arecibo 305m antenna, Hulse and Taylor detected pulsed radio emission and thus identified the source as a pulsar, a rapidly rotating, highly magnetized neutron star. The neutron star rotates on its axis 17 times per second; thus the pulse period is 59 milliseconds.

    After timing the radio pulses for some time, Hulse and Taylor noticed that there was a systematic variation in the arrival time of the pulses. Sometimes, the pulses were received a little sooner than expected; sometimes, later than expected. These variations changed in a smooth and repetitive manner, with a period of 7.75 hours. They realized that such behavior is predicted if the pulsar were in a binary orbit with another star.

    The pulsar and its companion both follow elliptical orbits around their common center of mass. Each star moves in its orbit according toKepler's Laws; at all times the two stars are found on opposite sides of a line passing through the center of mass. The period of the orbital motion is 7.75 hours, and the stars are believed to be nearly equal in mass, about 1.4 solar masses. As shown in the figure here, the orbits are quite eccentric. The minimum separation at periastron is about 1.1 solar radii; the maximum separation at apastron is 4.8 solar radii.

    In the case of PSR 1913+16, the orbit is inclined at about 45 degrees with respect to the plane of the sky, and it is oriented such that periastron occurs nearly perpendicular to our line of sight.

    (Figure from Weisberg et al. 1981)

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    Remember that a star in an elliptical orbit will move slower when it is at apastron than when it is a periastron. In an eccentric orbit such as that of PSR 1913+16, the radial velocity varies from a minimum of 75 km/sec to a maximum of 300 km/sec. Hulse and Taylor used their timing measurements of the pulses to infer the details of the orbital motion.

    The pulse repetition frequency, that is, the number of pulses received each second, can be used to infer the radial velocity of the pulsar as it moves through its orbit. When the pulsar is moving towards us and is close to its periastron, the pulses should come closer together; therefore, more will be received per second and the pulse repetition rate will be highest. When it is moving away from us near its apastron, the pulses should be more spread out and fewer should be detected per second.

    (Figure from Weisberg et al.1981)

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    The fact that the negative velocities (blueshifts, approaching the Earth) are larger than the postitive one (redshifts, moving away from Earth) show that the orbit is highly eccentric.

    The pulsar arrival times also vary as the pulsar moves through its orbit. When the pulsar is on the side of its orbit closest to the Earth, the pulses arrive more than 3 seconds earlier that they do when it is on the side furthest from the Earth. The difference is caused by the shorter distance from Earth to the pulsar when it is on the the close side of its orbit. The difference of 3 light seconds implies that the orbit is about 1 million kilometers across.

    (Figure from Weisberg et al. 1981)

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    Since the pulsing of the radio emission from the pulsar can be likened to ticks on a clock, Hulse and Taylor realized that they could look for changes in the ticking caused by relativistic changes in the measurement of time. As seen above, the pulsar's orbital speed changes by a factor of four during its orbit. Likewise, since the orbit of the pulsar around its companion is elliptical, the two are closer together at some times than at others, so that the gravitational field alternately strengthens at periastron and weakens at apastron. Thus the binary pulsar PSR1913+16 provides a powerful test of the predictions of the behavior of time perceived by a distant observer according to Einstein's Theory of Relativity.



    When they are closer together, near apastron, the gravitational field is stronger, so that the pasage of time is slowed down -- the time between pulses (ticks) lengthens just as Einstein predicted. The pulsar clock is slowed down when it is travelling fastest and in the strongest part of the gravitational field; it regains time when it is travelling more slowly and in the weakest part of the field.

    (Figure from Weisberg et al.1981)

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    The relativistic time delay is the difference between what is observed and what one would expect to see if the pulsar were moving in circular orbit, at constant distance and at a constant speed, around its companion.

    Space-time in the vicinity of the pulsar is greatly warped. This curvature causes the pulsar orbit to advance.

    The orbit of the pulsar appears to rotate with time; in the diagram, notice that the orbit is not a closed ellipse, but a continuous elliptical arc whose point of closest approach (periastron) rotates with each orbit. The rotation of the pulsar's periastron is analogous to the advance of the perihelion of Mercury in its orbit. The observed advance for PSR 1913+16 is about 4.2 degrees per year; the pulsar's periastron advances in a single day by the same amount as Mercury's perihelion advances in a century.

    (Figure from Weisberg et al. 1981)

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    Relativity predicts that the binary system will lose energy with time as orbital energy is converted togravitational radiation.

    In 1983, Taylor and collaborators reported that there was a systematic shift in the observed time of periastron relative to that expected if the orbital separation remained constant. In the diagram shown here, data taken in the first decade after the discovery showed a decrease in the orbital period as reported by Taylor and his colleagues of about 76 millionths of a second per year. By 1982, the pulsar was arriving at its periastron more than a second earlier than would have been expected if the orbit had remained constant since 1974.

    (Figure from Weisberg et al. 1981)

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    In the intervening decade, continued timing of the pulsar shows the continued decrease just as predicted by Einstein.

    Because the binary system is losing energy, the orbits are shrinking, and someday the two stars should coalesce. Such a merger might produce strong enough gravitational radiation to be detected by instruments like the Laser Inteferometer Gravitational-Wave Observatory now under contruction.

    The pulsar's orbit is shrinking with time as shown in this diagram; currently, the orbit shrinks by about 3.1 mm per orbit. The two stars should merge in about 300 million years from now.

    (Figure from Weisberg et al. 1981)

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  11. quantum_wave Contemplating the "as yet" unknown Valued Senior Member

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    Yes, that is a very interesting pulsar, and the details in the link you provided certainly are awsome. I am going to have do a little more study on it, to see if any professionals mention if the gradual decrease in timing might have other contributing factors, besides what relativity predicts, i.e. that the binary system will lose energy with time as orbital energy is converted to gravitational radiation. Is there any mention anywhere of looking for any theoretical impacts, like of an influence from a configuration of nearby stars, or lopsided galactic structure, or even the distribution of galactic dark matter.

    To me as a layman, the theoretical loss of energy in the binary system as it is converted to gravitational radiation means that the curvature of spacetime, which represents potential energy to move the two binary members on their geodesics, can be converted to gravitational waves without any actual interruption in the stars as they follow their geodesics.

    Of course, when the two binary stars collide, there is a clear physical interruption in their geodesics, and then I can certainly see how the conservation of momentum might result in GR gravitational waves that are described as ripples in spacetime.
     
    Last edited: Oct 31, 2015
  12. Farsight Valued Senior Member

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    Sorry to be tardy all. I was on holiday for a week, and didn't want to advertise it, and I promised the wife I wouldn't spend time on the internet.

    Nowhere. Energy is conserved. It's like a wave moving through glass then space.
     
  13. Farsight Valued Senior Member

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  14. Farsight Valued Senior Member

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    The mass of a body is a measure of its energy content. Saying it isn't that, and instead is a measure of its interaction with some cosmic-treacle field contradicts E=mc²

    Electrons and protons are stable as far as we can tell.

    You're missing the point. When you trap a massless photon in a mirror-box, it increases the mass of that system. You can think of photon momentum as resistance to change-in-motion for a wave moving linearly at c. But when it's in the box going round and round at c we don't call the resistance to change-in-motion momentum any more. We call it mass.

    The wave nature of matter is not in doubt, nor is spin angular momentum or the Einstein-de Haas effect or the fact that in atomic orbitals electrons exist as standing waves. The electron is like a 511keV photon in a box of its own making.
     
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  15. Farsight Valued Senior Member

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    Because at that location the "coordinate" speed of light is zero.

    In a planetary gravitational field the ascending photon speeds up, and it doesn't lose any E=hf energy. It's emitted at a lower frequency because everything's going slower at a lower elevation. In similar vein the descending photon doesn't gain any energy. Gravity is not a force in the Newtonian sense. When you send a 511keV photon into a black hole the black hole mass increases by 511keV/c², not by more.
     
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  16. OnlyMe Valued Senior Member

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    Obviously you call it mass, but what you mean by we in that statement is suspect!

    Going back to the paper where Einstein introduced the idea that E=mc^2, he never claimed that the photon had any independent mass. What he argued was that the mass of an atom increases and decreases with the absorption and emission of a photon. Which could be interpreted to mean that the mass of the box increases and decreases as the photon bounces around in the box.., assuming that bouncing included the process of photon absorption and emission.

    But even this assumes to some extent that mass is an intrinsic characteristic of a particle. It becomes a little more complicated if what we think of as mass is emergent, from the interaction between the particle field and say zero-point energy of the vacuum.
     
  17. Farsight Valued Senior Member

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    It shouldn't be. The E=hf photon moving linearly exhibits a resistance to change in motion, see for example Compton scattering on Rod Nave's hyperphysics:

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    Think of yourself as the electron. You're at rest, and you get a "kick" from the photon. Now I trap the photon in a gedanken mirror-box such that it's effectively at rest even though it's going round and round at c, then I throw you at it. Instead of the photon kicking you, you kick it. It still exhibits a resistance to change in motion, but you don't call it momentum any more.

    You shouldn't think in terms of absorption and emission, focus on the photon being effectively at rest and remember that the mass of a body is a measure of its energy-content. Have a read of http://arxiv.org/abs/1508.06478 where the 't Hooft is not the Nobel 't Hooft.

    Einstein never said that. He said the mass of a body is a measure of its energy-content. If you think it's a measure of something else you're effectively saying E=mc² is wrong. It isn't. It's E=hf and p=hf/c when the photon isn't in the box and E=mc² when it is. You divide energy by c for momentum, then divide by c again for mass, and they're all just different ways of looking at energy-momentum. It's like inertia is just the flip side of momentum.
     

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