Are planetary orbits really ellipses?

Interesting new physics here? I haven't heard about this bleeding of energy before.

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Purely theoretical, never observed.

 

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Very interesting, but not new.
Gravitational radiation was first proposed by Einstein nearly 100 years ago, it seems the theoretical physics has been settled since the 1960s, and there has been experimental support since the 1970's.

PSR B1913+16 (Hulse-Taylor binary)

 

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First paragraph of my reply is for non-moving bodies and second paragraph is for moving bodies. There is no occasion, where the central body is static while orbiting body is moving. Only under such impossible state, the orbiting body's path can be around the central body, in closed geometrical figure.

 

So are we saying the Earth is losing kinetic energy as it orbits the Sun, from the loss of gravitation radiation.
Of does that mean gravity will lessen. So what did that little graph show in your opinion?

 

By observation of relative positions of central and planetary bodies, in a planetary system, planets appear to orbit around the central body. For these apparent paths to be real, the central body has to be stationary without translational motion. Consideration of apparent planetary orbital paths around central body may be used only to predict relative positions of these bodies and for nothing else.

NB: A person can never run around a moving car in circular path, whose radius from the car is constant.

 

From whose view point this this circulation? from the view point of the driver you might be running in circles.

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You never did this, matterdoc.

Ah, but you missed three points. First of all, Jennifer asked about the situation where B was much smaller than A. In the approximation that B is infinitesimally smaller than A then A does not wobble. Second, in a universe with just the two objects A and B, then even if B is not much smaller than A, the only measurable variable is the relative separation between A and B. And relative separation is a vector that also traces out an ellipse for planetary orbits. In such a universe, hypothetical scientists living on B could establish the rotation rate of their planet (by Foucault's pendulum or a ring laser gyroscope), establish an inertial coordinate system and demonstrate that the separation of A and B changes position with time. They would not be able to establish that A is definitely not at the exact focus until they launched circum-A satellites (although if they did so, they would have long had theoretical grounds to suspect it). Three, even with additional test masses or the distant stars, when B is very small with respect to A, both A and B describe elliptical orbits (with the same eccentricity and period, but with different semi-major axes) about the barycenter of the A-B system, and this barycenter can for planetary orbits be located interior to A.

So did I make approximations, yes. I think I was clear about that. I completely neglected the effects of relativity and inertial recoil of A. The mass of the Earth is 3 millionths of the Sun and the typical separation between them is about 150 million of kilometers, so the barycenter of a hypothetically isolated Earth-Sun system is about 450 km from the center of the sun -- that's 0.06% of the radius of the sun. Ignoring the inertial recoil of the sun is a GOOD approximation in many cases.

Making such approximations is pedagogically useful because the OP was confused if the shape of planetary orbits was elliptical or egg-shaped. Ignoring relativity and perturbations from other planets, they are exactly elliptical with the barycenter of the A-B system at one focus, and B can always be made so much smaller than A that the barycenter of the A-B system is within any desired fraction of the radius of A.

Finally, your posting history doesn't lead me to believe you have a doctorate from an accredited university in the field of physics. That you seem uncomfortable with the idea of relative motion tends to reinforce this hypothesis. Are you perhaps a licensed medical doctor?

 

It's losing gravitational potential energy as the orbital distance decreases. Not sure about kinetic energy, but the total energy (potential + kinetic) is decreasing.
For the Earth and Sun, the loss is next to nothing. Wikipedia says the power loss is about 200 watts, or enough to reduce the orbital radius by about the diameter of a proton per day.

The little graph shows that the orbital period of the neutron star binary is reducing. (Note that the reduction is in the slope of the graph, not the overall drop)
It also indicates that the size of the change in the orbital period is exactly what is predicted if they are radiating gravity waves and falling closer together according to the theory.

 

What is the reason that the binary star is showing high recent decay rates? (If you extrapolate the graph back fifty years the slope would have been nearly flat.

 

I think, I will re-phrase my posts.

In the original posting by ‘Jennifer’, planet B is stated to be orbiting around star A.
For this situation to be right, in real terms, star may have no translational motion, while the planet moves in its orbital path around the star at a (constant) linear speed. Both bodies have to be perfect spheres of homogeneous consistency. Orbital path of the planet will be a perfect circle around centre of the star. Since all bodies (including stars) in universe move, said situation is not real. Unreal situations, although useful for certain purposes, give false presumptions and parameters.

Irrationality of this situation is often overcome by the use of relative reference frame. Motions of these bodies are considered with respect to any one body (usually the central/larger body). One of the bodies is considered as static and motion of the other is considered relative to the assumed static body. This is the current, widely accepted method. If the star is considered static, we have the planet orbiting around the star and if the planet is considered static, we have the star orbiting around the planet. Both are equally valid. It is the observer, who determines which body should orbit around the other by his choice of reference body. Although both bodies are capable to (apparently) orbit around the other, these motions cannot take place simultaneously. If such situations can be true, it should be agreed that simple thought process of an observer can instantaneously move heavens. That is why planetary orbital paths around their central body should be considered as apparent or imaginary. Apparent (relative) considerations are handy mathematical operations to determine relative positions of concerned bodies. I consider shape of planetary apparent orbital path as egg-shaped rather than ellipse. (Link to details is available on my website).

Similar logic is used in motions of bodies in a multi-body system about their Barry centers. A central body, with more than one planet, is assumed to move, simultaneously, in many orbital paths around different Barry centers, obtained by different sets of bodies. These considerations are alright for mathematical analysis intended for relative positions of each pair separately. However, they do not give real parameters of bodies’ motions or paths other than their relative positions.

If both bodies are observed from an external point, we can simultaneously observe their actual (real) motions. [If it is easy to routinely assume above mentioned apparent situations required for apparent orbital paths, why should it be difficult or impossible to consider a point outside these bodies as reference]. Under this real situation, it can be seen that no free body can orbit around another (moving) free body, in any type of geometrically closed path (elliptical or circular).

Linear speeds of bodies do affect some of their parameters. In order to determine these parameters or to find how they affect overall performance of a multi-body system, it is necessary to determine linear speeds of each body with respect to an absolute reference.

For example; considering speed of sun in its linear path around centre of galaxy as 250000 m/s, earth’s linear speed in its orbital path about the sun varies between 280290 m/s to 219710 m/s, roughly in the same direction as sun’s motion. At such tremendous linear speeds, earth cannot reverse its direction of motion, in six month period, as is required for a circular (or elliptical) path and yet remain an integral body. Rate of change of angular deflection of orbital path made by central force will be quite different from current estimations. Unlike with apparent orbit, where direction of central force is always perpendicular to orbital path, in case of real orbital path, direction of central force changes through whole circle with respect to direction of motion of the planet. Etc.

Knowing parameters of apparent orbital path and applying principles of reverse engineering, it should be possible to determine parameters of real orbital path (in absolute reference frame).

 

You've misused the word linear. For circular motion, you mean "constant speed" or "constant scalar velocity" and when you use instead "(constant) linear speed" this seems like further evidence that you never mastered freshman physics.

Also, nothing in the presentation of the problem requires circular motion and when you write "For this situation to be right," you again fail to present any reason why your preconceptions about the problem are logically or physically required by what Jennifer wrote or the laws of language or physics.

Finally, there is no need to put scare quotes around Jennifer's online handle. Since I doubt you are any kind of physicist, I would be correct to use scare quotes to write matter"doc" in order to convey such suspicions to the reader.

Not true in Newton's Universal gravitation.

Actually, as discussed above, even in your restricted case, both bodies will orbit the barycenter of the system and this barycenter only corresponds with the exact center of the star in the limit of the mass of B/mass of A ratio going to zero. For the Earth-Sun system, this barycenter would be very close to the center of the sun as a fraction of the Sun's radius. But you still having said why either constant speed or circular motion is required by Jennifer's initial post.

After this point, you being asserting relative coordinate systems are invalid, which is an endorsement of an unphysical absolute space and time. You then point us at your website which has not been endorsed as a reliable source of education.

The best coordinate systems are those coordinate systems where physics is simple and just as accurate. For a universe with just the two bodies, A and B, there is a class of best coordinate systems where the barycenter of A and B is motionless. The second-best class is where the barycenter of A and B moves with non-zero constant velocity. Newton's universal gravitation allows us to move from one coordinate system to another by just subtracting out the constant velocity (as long as we do it consistently). Newton believed in absolute space and time, but his physics and the universe do not require us to make that assumption.

 

Owned!

 

Sorry, missed this post before.
The graph is the accumulated time difference in the pulsar orbit since they started measuring it in 1975. They used the 1975 rate as the baseline, which is why the graph is flat at that time. Extrapolating backward would have the graph curve down again.

- In 1975, the measured orbital period of the pulsar binary at about 7.75 hours.
- The baseline of the graph is a perfect clock tick at that rate, synchronized with the pulsar orbit in 1975.
- The pulsar orbit is getting faster over time, by about 1 s/yr every 10 years.
- in 1975, the rate difference was 0 s/yr (ie the graph is flat), and the clocks were synchronized
- in 1985, the rate difference was about 1 s/yr, and the pulsar orbit was about 5 s ahead of the perfect clock.
- in 1995, the rate difference was about 2 s/yr, and the pulsar orbit was about 20 seconds ahead
- in 2005, the rate difference was about 3 s/yr, and the puslar orbit was about 45 seconds ahead.

Extrapolating backward:
- in 1965, the orbit would have been reading 5 seconds ahead of the perfect clock, but ticking 1 s/yr slower so the perfect clock was gaining

Does that make enough sense?

 

Yes thanks Pete. So they are putting that slowing down to loss of energy due to gravity waves? Yet orbiting bodies seem to slow more to tidal effects that anything else? How would they separate out the various factors?

 

The most convincing thing is the match in the measured quantities.

The theory says that binary pair should lose energy at a particular rate by radiating gravity waves, and that their orbit period should decrease at a particular rate as a result.

The binary pair's orbit is measured to be losing energy at that particular rate.

So there are at least two options:
1) The theory is right, and they are losing energy to radiating gravity waves
2) The change in orbit period is due to factor/s that coincidentally give just the same result as predicted for gravity wave radiation

The second option can't be ruled out.


I don't know what other potential factors are known, and how they are separated.
Tidal effects is a good suggestion - I think they tend to change the orbit over time so as to reduce their effect, for example making the orbit more circular and tidal locking the two bodies, so maybe they figure that for this pair, tidal effects would no longer be significant - but yeah, I don't really know.

 

The math of the binary star system and gravitational waves is beyond me sorry.

 

You and me both.
I think it's beyond most maths professors.