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Janus last won the day on November 24 2019

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  1. Nowhere. Assume someone shines a laser off a set frequency of x mhz, as measured by him and he shines it for 1 second by his clock. By his measurement, he is sending at 100 watts, so in the one second he transmits 100 joules of energy. He is moving away from you at some fraction of c so that you see the laser red-shift by a factor of n, so that you measure the frequency of the laser as being 1/n what he measures it to be this means you also measure the beam to be 1/n of 100 watts. However, the red-shift doesn't just effect the frequency, but also how long you measure the laser beam from start to end. From the moment you first detect the beam to when the end reaches you, you will measure n seconds. SO While the source says transmitted at 100 watts for 1 second, you receive the beam for n seconds at 1/n sec. You both say that the beam contained a total of 100 watts of energy.
  2. Janus


    In a year(ship time) You would reach ~0.77c After traveling 0.56 ly. assuming you used another year and another 0.56ly to slow back down,, That leaves 3.18 ly to travel at 0.77c. to reach the nearest star. That takes 4.13 yrs earth time and 2.64 yrs ship time. Total ship time trip to Alpha C is 4.63 yrs . Total Earth time is 6.54 yrs. A far cry from the 1 week suggested by the OP. To achieve the 0.99998999 c needed to cross 4.3 ly in one week (ship time) by accelerating at 1g, you would have to spend almost 6 years ship time.
  3. Janus


    From world to world, do you mean star system to star system or between planets in the Solar system? To travel from world to world in our solar system in a week doesn't require speeds anywhere near light speed (though with present technology, it would require impractical amounts of fuel. To get to Mars when it is at its closest would 2.24e12 kg of mass for every kg you want to deliver to Mars. If we assume a craft with an mass equivalent to what we sent to the Moon with the Apollo missions, it would take an amount of fuel equal to the mass of one Jupiter's small moons. If you mean star to star, then you have to ask: One week as measured by who. There is just no way to get to even the nearest star in a week as measured by the Earth without going faster than light (it takes light 4.3 years to travel the distance) But due to Relativistic effects, if you could travel close enough to the speed of light, you could make it in 1 week of ship time. But to do this would require reaching 99.9989999% of the speed of light. The amount of fuel needed to do that with today's technology exceeds the mass of the observable universe. Another problem is that even if you could find a type of engine that could reach that speed with a reasonable amount of fuel, the acceleration needed to reach that speed in so short a time would flatten any passengers into jelly.
  4. So was I. Essentially, if you can go 50 ly in 1 million years under constant acceleration, then it would take ~450,000 yrs to go 10 ly starting from "rest". Waiting until you get warning of an imminent supernova is likely not an option. But that doesn't mean we couldn't try and keep our distance from supernova candidates. We know that only stars above a certain mass supernova. So maybe the strategy would be "Why wait for trouble? Let's just keep our distance from any stars that could even possibly supernova." Pluto shouldn't be left behind. If we go by the 50 ly in 1,000,000 year at constant acceleration scenario, then the acceleration needed is 4.27e-10m/s2, While the centripetal acceleration for Pluto is 3.8e-6 m/s2, several magnitudes larger.
  5. I get about 450,000 yrs to move 10 ly. That's assuming a constant thrust-acceleration scenario. You'd start slowly at first and then pick up speed, and after 450,000 yrs you would be moving at 4.5e--5 ly/yr ( ~13.5 km.sec) having traveled a bit over 10 ly. In the next 550,000 yrs you would accelerate up to .0001 ly/y (~30 km/sec) while covering a bit under 40 ly.
  6. There have already been a number of supernovae that occurred close enough to be easily visible with the naked eye. The last one occurring at the start of the 17th century. All of these were stars thousands of light years away. But even Betelgeuse, at 640 ly is at a safe distance. It would make quite a show though.
  7. Strange has already answered your question. Why do you think there is something wrong with the way we calculate mass?
  8. I'm 100% sure that the type of clock doesn't matter. And atomic clocks are not "as rubbish as wrist watches", they are much, much more accurate. A typical wristwatch might gain or lose a second per day, while an atomic clock could run for a 100 million years before losing or gaining a sec. The tick rate difference between a ground and orbiting clock is not due to the clocks not accurately measuring time, but due to the fact that time itself is different for the orbiting clock frame than it is for the ground frame.
  9. The type of clock used makes no difference. Your statement makes it seem like you think the Relativistic effects are caused by something acting on the mechanism of the clock. This not the case. The method by which the clock operates plays no role. Relativity is about the fundamental nature of "time" and "space" and how they relate to each other and it doesn't care about what kind of clock you use, only that they are accurate enough. ( For instance, if you are trying to measure an accumulated difference of 1 nanosecond over a 24 hr period, you need to use clocks that you know that, if placed side by side, would drift apart in their readings much less than 1 nanosecond per 24 hrs.)
  10. If the Earth were perfectly spherical, and you put a clock in a geosynchronous orbit around it, that clock would still tick at a different rate than one on the ground. This is not in accordance to what Newton would predict, so yes, you would still need Relativity to explain this. I'm curious as to why you would think that perfectly spherical bodies and synchronous orbits would have any effect on whether or not Relativity holds.
  11. As swansont has already stated, dark matter is matter that is hypothesized to exist due to the apparent gravitational effects we see, but which doesn't interact electromagnetically like the regular matter we are used to dealing with. This means it does not emit, reflect, or absorb light either, thus the description "dark". And as he said, we don't really know just what it is made up of yet. There are a couple of possibilities. Massive Compact Halo Objects (MACHOs) would be objects like neutron stars and black holes. These are object formed from "normal" matter (or at least a far as black holes go initially from it), but compact so much mass into a small area that they are just to small and dim to see individually. However, we need quite a bit of dark matter to explain observations, and there are reasons why we don't think the universe could have this much mass in the form of MACHOs, as it would have effected how the universe evolved, resulting in one that looks a bit different from what we see. Another possibility is Weakly Interacting Massive Particles (WIMPs) These are sub-atomic particles that have a rest mass, but just don't interact via the electromagnetic interaction. (And pretty much everything having to do with how we interact with regular matter, from touching it or seeing it, to chemical reactions involves electromagnetic interaction) These "ghost-like" particles would pass right through you like you weren't even there. While this this seems bizarre, we actually already know of a particle that behaves like this, the neutrino; Billions of them pass through you every day with your ever noticing it. Neutrinos ( or at least the type we know of) don't work for dark matter for various reasons, so the WIMPs of dark matter would be something like, but not identical to neutrinos. It is also possible that both of the above play a role in making up the total effect we see. swansont also mentioned attempts to explain things by a modified theory of gravity. The problem with this is that a number of observations are not compatible with such an explanation. An example would be galaxies that appear to be identical but exhibit different gravity profiles. Even if the rule of gravity were different than what we presently think they are, they would still need to be consistent from galaxy to galaxy. So while its perfectly possible for different galaxies to contain different amounts of dark matter and thus as a whole act differently in terms of gravity, it is hard to explain why the actual rules governing gravity would change between galaxies. Having said this, it is still possible for a modified gravity theory to play some role, if combined with dark matter. A new theory of gravity which also incorporates a mix of MACHOs and WIMPs could end up being the final answer. Right now we are at the stage of continuing to make observations in order to narrow the playing field. We have eliminated some possibilities, but there are more to explore.
  12. But you need one of the observers to be close to something like a black hole in order for this to be significant. So for example, lets say we have a star orbiting a galaxy at the same distance from the center of the galaxy as our Sun is from ours( _26,000 ly). We we also assume that the mass of that galaxy is 150 billion solar masses. We can plug these numbers in and at least get a ball park figure of how much time dilation you could expect. if you do that, you get an answer of 0.999999548 for the time dilation factor. This is totally insignificant and many, many many factors too small to account for the rotation curves we see. The only way time dilation could be a significant factor is if the galaxy contained a great deal more mass than what we see, much more "extra mass" than what we presently calculate for Dark matter. You would have made the problem worse in terms of extra mass needed rather than better. And, as I already mentioned, if time dilation were the culprit, it would betray itself as a shift in the light spectrum. In a typical galaxy rotation curve, the star' orbital speeds remain fairly constant as you move out from the center. According to the amount of visible mass in the galaxy and how it is distributed, We would expect the star's orbital speeds to decrease with distance. If star B was twice as far out as star A, you would expect it to have around 70% the orbital speed of star A. If the reason we saw Star A with the same speed as star B was time dilation, this is a large time dilation factor, one that would also effect the light we see coming from these stars. The spectral lines produced by star A would be greatly shifted compared to those coming from Star B. This is not something we see in the spectrum from these stars. And while we use a shift in the spectrum due to Doppler shift to measure these differences in star orbital speed, the shift we measure is very small compared to what would be needed if time dilation were the cause. A star orbiting at 200 km/sec has a Doppler shift factor of 0.999335, compared to a factor of 0.999533 for one traveling at 70% of that speed. This only something in the order of a 0.01% difference, compared to the 70% difference needed for the measured speed to be due to a time dilation effect.
  13. This is really dependent on where the probe is relative to the Earth and Moon. Assuming, you could see a "muzzle flash" caused by scattering of light from the laser, and then a reflection off the Moon when the beam hit: If you are positioned so that the Earth- Moon line is at a right angle to your line of sight and are an equal distance from Earth and Moon, then 1 sec after the muzzle flash you would detect the reflection off the Moon. However, if your line of sight is along the Earth- moon line (and neither of these bodies block your view of either flash or refection), Then: If the Moon is on the far side of the Earth, you will see the reflection 2 sec after the muzzle flash, as the refection has to travel to the Moon and back to the Earth before it gets to you,while the muzzle flash only has to travel from the Earth. If the Moon is on the side towards you, you will see both at the same time, as light of the muzzle flash will be passing the Moon at the same moment as the reflection is produced at the Moon. Other viewing angles to the laser path line will produce results between these extremes.
  14. Wave action, even on the same stretch of beach can have varying effects. For example, not too far from here is a beach with a large basalt rock formation on the tide line. At high tide it is completely surrounded by water and during low you can walk up to its base. Now the point is that on its landward side during low tide, there is a rock debris field that on some days is exposed and on others can be completely buried under sand. You never really know what you'll find any time you visit. The same is true for a small cove near a place we stay at further South. On one day you go down to it and it is pretty much just sand, and then the next it will have bands of pebbles and rocks. You can have two bands of with a band of just sand between. There is sorting going on, but just not in a steady large to small fashion. The interaction between the next incoming wave and the water going back out from the previous one is not a simple one.
  15. Right. In fact, the reason we need to add leap seconds from time to time is that the second was defined based on the mean solar day at the time it was first defined and since then the Earth's rotation has changed, changing the length of the solar day. Thus, once in a while, we need to make an adjustment to keep civil time from drifting too far from solar time. (while it is a small difference in solar day length, the effect is accumulative.)
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