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Danijel Gorupec

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Danijel Gorupec last won the day on July 19 2020

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  1. Sorry, if I am a bit off topic, but just a side question out of curiosity - can bouncing light in a fixed box have any wavelength, or only some discrete values?
  2. "He is Novak, Serbian, he won't cry, so I cry for him." Was this his debacle, or a debacle for a man is to give up his principles?
  3. What an uneasy thought, considering that it seems to me the homo sapiens was moslty on the receiving end of this 'interbreeding' process.
  4. It is something. Although it does not show that the distribution of mass/energy is as expected from the energy density formula. I have personal interest in confirming the density formula, so I searched quite a lot. All measurement proposals seems out of our reach (for many years to come).
  5. Interesting... I was under impression that we generally believe that static field indeed gains mass due to stored energy - and this mass is distributed exactly as you would expect from the energy distribution. Are there any theoretical reasons to doubt this? I do know that we don't have any experimental confirmation of this (or I couldn't find any).
  6. @Peterkin, I basically agree with everything you say. But why would you think that it is a problem if man creates an environment where a special kind of animals (farm animals) would thrive? Is it wrong to create a new type of ecological niche and populate it with adapted animals? I agree that in practice animal framing often turns into a horror story, but here I am asking purely theoretical 'in-principle' question... In my opinion, animal farming is not inherently morally problematic.
  7. One (minor) problem with veganism - as I understand it, the vegan diet is not instinctive - it requires education (especially for vegan parents). Implemented wrongly, vegan diet might cause some individual suffering and increased medical cost for society. One (minor) note about ecological footprint - while in rich societies using freezers is probably more eco-friendly way to deal with crop surpluses, in other parts of world there are still families that cannot afford freezers. Storing surpluses into animals (pigs) would then be more eco-friendly than letting the crop rot on the field. One (minor) note about animal welfare - while unnecessary animal suffering is a very unnecessary thing, it is not clear to me if responsible animal farming is that much immoral. I wouldn't be surprised if average life span of some farm animals is greater than closely related wild species. Some of most successful animal species are domestic animals. Some of them are so aggressively selected that they cannot live outside of the farm-machine - one could argue that for these beings, being eaten is the life destiny and they can only succeed in farming environment.
  8. So, you think that when GR is accounted for, then it will turn out that only the mass within Earth's orbit will affect Earth's orbit - by making the orbit tighter - even if the whole Universe is uniformly filled with certain mass density? And, consequently, by examining revolution speed of stars in a galaxy, scientists were able (and actually did) calculate that there is no significant uniformly-distributed component of dark matter density?
  9. Sadly, I might never arrive to learn GR... But if we limit ourselves to the old Newton space, we can luckily analyze certain questions from this thread by simple geometry. First, can mass evenly distributed outside Earth's orbit have any gravitational effect on the Earth? Down there is a picture of 2D newtonian-like universe (linear, infinite): In the center there is Sun. The Earth is orbiting it - Earth's position at some instant is shown as point 'E''... I added some mass density (hatched) everywhere except for one circular Sun-centered 'bubble. The 'bubble' is slightly larger than the Earth's orbit. So, can this distributed mass (hatched) have any gravitational influence on Earth? Easy; due to symmetry all the gravitational effect of distributed mass on Earth that is outside of the Earth-centerd green circle will cancel out. Only the mass within green circle (the crescent-moon-shaped region 'A') will provide net gravitational influence to the Earth. So, It looks quite obvious to me that the distributed mass will indeed affect the Earth (it pulls the Earth in the positive direction of the vertical axis). The same approach... Down there I depicted the same case, but now the 'bubble' is exactly the size of Earth's orbit. Again due to symmetry, I can actually ignore all the distributed mass except for the region 'a' inside the green circle... Because the region 'a' is circular, it is very easy to calculate its mass and therefore its influence on the Earth. In fact, the region 'a' is the same size as the region of the 'bubble'.... [This means that if we fill the 'bubble' also with the same mass density as the rest of space, there will be no net gravitational effect on the Earth... Something that was just obvious to me even without this complex discussion.] If you ask for more realistic space (GR, limited observable space, limited universe age, finite gravity propagation speed etc.) I cannot analyze this.
  10. I try to put all observers down to Earth No, I don't see how the Earth's orbit will get tighter if I add uniform mass density to the whole Universe. It would get tighter if I spread the added mass in a large sphere centered on the Sun - and only the part of the mass inside Earth's orbit would affect the Earth, exactly as you said. But I didn't do that - Instead I distributed the mass uniformly over the whole universe (I didn't center it at the Sun). In my opinion, adding uniform mass density (positive or negative) to the whole Universe, does not change local trajectories of gravitationally bound objects. It might affect, as you said, the expansion of the Universe as a whole, but this is an even darker topic. It seems to me, there is still enough wiggle space in our theories and observations that we cannot say for sure there is no additional (positive or negative) offset to the observed distribution of dark matter density.
  11. [I always enjoyed Susskind's lectures - did watch quite a few of them, but didn't know he has the cosmology series too.] Are you only considering the case of uniformly distributed mass density? Because if I put a black hole just outside Earth's orbit, it will affect the Earth trajectory. But even with uniformly distributed mass I find your claim confusing... Suppose you only have Earth orbiting the Sun. Now you add uniform mass density to the whole universe - you say that the Earth will only 'feel' the part that is inside its orbit? Does it mean the Earth will go into a tighter orbit with greater velocity? Ok... but remove the Sun now - will Earth go into some other orbit (because it feels the gravitational attraction of the mass inside its orbit) or will the Earth continue straight?
  12. This is exactly what I am talking about -> is this claim a jumping in conclusion? Because of effects like dark energy I cannot dare to claim that dark matter attracts. It might as well repulse... But I know very little, and maybe I am missing an important bit of information that indeed shows it is attractive.
  13. Sure, but there can be negative mass within the galaxy that will produce an effect on the galaxy. I suppose it would be possible to came out with some negative-mass-density profile (as a function of the distance from the galaxy center) that would give the observed gravitational effect.
  14. Why? It seems the same to me - you can have positive-mass around galaxies, or you can have negative-mass everywhere else. How could you tell the difference?
  15. Thanks for the video, I liked it. So, without the dark energy nuisance, we should be able to estimate the portion of uniformly distributed dark mater. But because of the poorly understood dark energy, I guess we cannot do much of an estimation? [Looking at equations derived by Susskind, purely mathematically speaking and putting things upside down, if the dark mater has a negative mass it might cause accelerating universe... supposing large majority of it being actually distributed uniformly, and being only slightly less concentrated around galaxies; as we can detect from its gravitational influence.]
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