Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: AlexandrKushnirtshuk on 13/04/2021 11:00:37
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Brief description to avoid unnecessary complication.
The distance from the Earth to the Sun is about 8 light minutes, so from the Earth we see the Sun at the point in the sky where it was 8 minutes ago (in 8 minutes the Sun passes through the sky with an angular distance of slightly less than two solar disks) ... It is difficult to both explain and imagine, because most likely it is impossible, that is, cosmic distances are too exaggerated.
(https://i.ibb.co/m9pMJ8b/11eng.jpg)
The distance from the Earth to the Moon is about 1 light second. That is, the apparent and actual position of the moon is almost the same. The shortest distance from Earth to Jupiter is about 32 light minutes. The apparent and actual positions of Jupiter differ 4 times more than in the case of the Sun.
(https://i.ibb.co/2KLTjCv/22eng.jpg)
The question and the most important thing. Why is astronomy not taking into account the actual and visible position of space objects corrected for the speed of light? The motions of the planets are calculated using Kepler's formulas. The calculated positions of the planets (that is, the actual ones) coincide with the visual ones without corrections for the speed of light. I do not question the speed of light, it has been measured and refined for several centuries. The official space distances and the sizes of space objects, respectively, are in great doubt.
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Very good question and the answer is far from obvious.
Changes to a gravitational field (caused by a moving mass) propagate at the speed of light, but in a somewhat surprising way that means the force experienced by a test mass will always point to the current position of the moving mass rather than it’s position x light seconds ago. Below is a popsci article that explains what happens and below that I’ve compared it with a similar effect due to a moving charge.
https://medium.com/starts-with-a-bang/what-is-the-speed-of-gravity-8ada2eb08430
There are similarities between em radiation and gravity and comparisons can be made which help us to understand them better.
Let’s start with the field surrounding an electron. A test +ve charge will be attracted to the electrons position. We know that the electric field propagates at light speed, but if the electron is moving the test charge will move towards the current position of the electron, not where it was. In this way the field behaves in a similar way to gravity.
What is less well known is that if the electron changes velocity the test charge will move towards the position the electron would have been at had it not accelerated, until such time as the change in the field propagates to the test charge’s position. How does the test charge know where the electron will be? I can offer an analogy that shows that this behaviour of fields is not so mysterious.
Imagine a boat crossing a lake, the bow wave propagates away from the boat, but even at some distance from the boat the crests point to the boat’s current position (almost, because waves in water don’t propagate exactly as do em waves). If the boat changes direction it will take time for the change to propagate to the observer, and until it does the bow wave will point to the predicted position of the boat had it not changed course.
In the case of the accelerating electron we know the speed at which the change propagates, because the change causes a discontinuity in the field which we detect as em radiation - light, radio etc (and if we have a quantum detector we can say we detected a photon). We can describe the electron field at any point as being dependent on the history of both the position and derivative of position (velocity) of the electron.
A similar effect occurs with gravity. The force felt by the moon is directed to the current position of the earth rather than it’s historical position. This led early researcher to conclude that the speed of gravity was either instantaneous or extremely fast whereas it does not need to be for similar reasons as I have described for the electron.
It’s worth pointing out that the gravity situation is slightly more complex because there is a higher order effect which allows the masses to ‘point’ towards each other’s current positions despite both being in non-linear motion around a common centre of mass. This higher order effect is also why em waves are dipole and gravitational waves are quadrupole. The gravitational waves that have been detected (which are analogous to the em radiation of the electron) show that the speed of propagation, and hence the speed at which gravity changes propagate, is as close to light speed as can be measured.
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Concerning the accuracy of the diagrams in the OP:
Relative to the nearly-inertial frame of Earth, the sun moves about 15000 km in 8.5 minutes so your picture is orders of magnitude off. It actually moves about 1% of its width in those minutes, not 2x its width as you show.
Likewise, Jupiter moves at about 17 km/sec relative to Earth when it is closest, so in 32 minutes that moves it about 32000 km or less than a quarter its diameter, not ~15 times its diameter as you show it.
In the frame of the Earth-sun barycenter, the sun moves hardly at all (a meter maybe?), so the gravity thing that Colin addresses isn't significantly relevant, and is more of a problem with two similar masses orbiting each other, generating significant gravitional waves. Earth simply effects a negligible change to the gravitational field of the sun as it orbits and radiates a mere 200 watts in the form of gravitational waves.
The apparent motion of the sun in the sky is due to you turning your head, not the sun actually moving around the Earth, which was shown to be false nearly 5 centuries ago, give or take. But your post seems to presume this ancient model.
According to the logic put forth in the OP, if I sat on the surface of a satellite that rotated on its axis every 16 minutes, the Sun that I see would be in the opposite direction from where I see it since it has moved 180° in the 8 minutes it took the light to make the trip. Jupiter (at its closest, and thus in the opposite direction) would have gone around twice in the 32 minutes it takes the light to get to me, and thus would appear to have returned to the same place where it actually is. In other words, despite the sun and Jupiter being in opposite directions, they would appear in the 'sky' to be in the same place.
Let me know how that works for you. That description is actually what you'd see if the sun and Jupiter actually revolved around you instead of the Copernican system used for the last 5 centuries. It makes a nice falsification test.
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The official space distances and the sizes of space objects, respectively, are in great doubt.
No, no they are not (at least not within the Solar System). If this is an attempt to promote your bonkers model, do I need to move this thread?
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Very good question and the answer is far from obvious.
Changes to a gravitational field (caused by a moving mass) propagate at the speed of light, but in a somewhat surprising way that means the force experienced by a test mass will always point to the current position of the moving mass rather than it’s position x light seconds ago. Below is a popsci article that explains what happens and below that I’ve compared it with a similar effect due to a moving charge.
https://medium.com/starts-with-a-bang/what-is-the-speed-of-gravity-8ada2eb08430
There are similarities between em radiation and gravity and comparisons can be made which help us to understand them better.
Let’s start with the field surrounding an electron. A test +ve charge will be attracted to the electrons position. We know that the electric field propagates at light speed, but if the electron is moving the test charge will move towards the current position of the electron, not where it was. In this way the field behaves in a similar way to gravity.
What is less well known is that if the electron changes velocity the test charge will move towards the position the electron would have been at had it not accelerated, until such time as the change in the field propagates to the test charge’s position. How does the test charge know where the electron will be? I can offer an analogy that shows that this behaviour of fields is not so mysterious.
Imagine a boat crossing a lake, the bow wave propagates away from the boat, but even at some distance from the boat the crests point to the boat’s current position (almost, because waves in water don’t propagate exactly as do em waves). If the boat changes direction it will take time for the change to propagate to the observer, and until it does the bow wave will point to the predicted position of the boat had it not changed course.
In the case of the accelerating electron we know the speed at which the change propagates, because the change causes a discontinuity in the field which we detect as em radiation - light, radio etc (and if we have a quantum detector we can say we detected a photon). We can describe the electron field at any point as being dependent on the history of both the position and derivative of position (velocity) of the electron.
A similar effect occurs with gravity. The force felt by the moon is directed to the current position of the earth rather than it’s historical position. This led early researcher to conclude that the speed of gravity was either instantaneous or extremely fast whereas it does not need to be for similar reasons as I have described for the electron.
It’s worth pointing out that the gravity situation is slightly more complex because there is a higher order effect which allows the masses to ‘point’ towards each other’s current positions despite both being in non-linear motion around a common centre of mass. This higher order effect is also why em waves are dipole and gravitational waves are quadrupole. The gravitational waves that have been detected (which are analogous to the em radiation of the electron) show that the speed of propagation, and hence the speed at which gravity changes propagate, is as close to light speed as can be measured.
Because of the inverse square nature of electromagnetism and gravitation the strength of the fields die away fairly rapidly. The reason that the centre of mass is always the centre of attraction could therefore be thought of as a local phenomenon. At vaster distances the effect would not be apparent. Although it would be interesting to know if it persists over greater distances. It brings to mind spooky action at a distance.
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Because of the inverse square nature of electromagnetism and gravitation the strength of the fields die away fairly rapidly. The reason that the centre of mass is always the centre of attraction could therefore be thought of as a local phenomenon. At vaster distances the effect would not be apparent. Although it would be interesting to know if it persists over greater distances.
For two point masses, relative to a given mass, the centre of mass (CoM) of the pair is in the exact same direction as the other mass, so the attraction is in the same direction over any distance.
For more than two bodies, the attraction is rarely towards the common CoM and can even be away from it. Hence the unpredictability of 3+ body motion in the long run. So in reality, with masses here, there, and everywhere, if you widen the scope, the attraction (the acceleration vector of a given mass) is not necessarily towards the CoM of anything. For instance, the average acceleration of the sun (over say the next million years) is not especially in the direction of the the CoM of the galaxy, but rather a vector that points significantly out of the galactic plane, much of which is due to matter being attracted to a disk shape in a different manner than it would to a spherical distribution of a similar mass.
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Is there a difference between visual and actual location in space?
There is an effect in astronomy called "aberration of light", where the position of objects in the sky varies as the Earth orbits the Sun.
- The size of this deviation is affected by Earth's motion, and not by the number of light years to the star/galaxy being viewed. (Unlike parallax, where the size of the deviation is affected by the distance to the star.)
- The maximum deviation is only 20 arc-seconds.
- But it's large enough that astronomers noticed it
- Trying to explain this phenomena drove a number of theories of light over the centuries
See: https://en.wikipedia.org/wiki/Aberration_(astronomy) (https://en.wikipedia.org/wiki/Aberration_(astronomy))
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Below is a popsci article that explains what happens and below that I’ve compared it with a similar effect due to a moving charge.
https://medium.com/starts-with-a-bang/what-is-the-speed-of-gravity-8ada2eb08430
Thanks for recommending this thread from elsewhere ("what would happen if gravitational mass were different than inertial mass"). I've read through it.
That Popsci article looks frightening and is exactly the sort of thing that caused me some trouble when I started learning about GR.
Could I take a moment to highlight some concerns and get some discussion? Also, where would that best be done (in this thread, in the thread from whence you pointed to this one, or in a new thread)? If it was left to me, I would be thinking of starting a new thread with a title like "Some problems with the presentation of General Relativity in Popular science articles" and placing it in the Physics, Astro & Cosmo section of the forum.
Late editing: It would not be my intention to create a monologue, no one wants that and I don't have the time to write it either. A better title would probably be a question and much more limited in scope. I'll think about it a bit more but the basic question remains - can I discuss some issues I have with that Popsci article, please?
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...can I discuss some issues I have with that Popsci article, please?
I have issues with every Popsci article so yes you can discuss them, but I would suggest this is not the thread.
If you look at the OP you will see that he is unlikely to comprehend anything more complex as he is already lost on how we measure the size of planets and thinks they are much smaller than they are.
Sometimes, for people like the OP, the Popsci explanation can be useful, but as Stephen Hawking said about virtual particles, don’t take them literally.