DanMP

Yes, but the energy you obtain may/should be greater with nuclear power, for the same mass M. The ratio/equation you derived is correct, but it doesn't mean that you cannot have a greater exhaust speed, and benefit from it (m is reduced for the same exhaust KE / energy budget). I described how aliens may visit us (from a colony in our solar system, centuries after the probe arrival - that may or may not include a passing in Earth's atmosphere -, when they would have new ships, suited to visit Earth), as a reply to your question in the title of this thread. UAP sighting reports are not my concern, because I'm sure that most of them (if not all of them), are not related to alien activity. You may be right. On the other hand, the problem with Bussard Ramjet concept is that you cannot achieve fusion, not that you cannot scoop hydrogen ... Don't be so drastic. Part of what happened was due to my poor English ... 🙂

Limit?! You started well, but ended nowhere 🙃 There are so many issues with your "analysis": 1 - the mass diminishes a lot in the process, if you use chemical fuel 2 - KErocket is a fixed value, the kinetic energy you need to shed, but KEexhaust is not, so you should put in the right side of the equation, and see that the exhaust speed does matter. 3 - if the rocket/thruster you use to decelerate the probe is nuclear, the energy output, for the same mass M, may be much higher, so definitely, KEexhaust is not the same in both cases. You can even reduce M ... 4 - you can use the hydrogen you scoop as the exhaust material, so you both start lighter and brake while collecting it ... We already have some projects for nuclear thrusters, as you can see here and here. Imagine what we may have in a billion years or more (when the Sun become a problem). In such a long time, the Himalayas would be completely eroded, the continents no longer the same, probably not even us ... In the interstellar space, where you start the deceleration, you cannot use solar panels. And the Xenon must be shipped. I doubt that you can scoop Xenon. Anyway, there are magsails, etc. Did you, or anyone, read the arxiv article I offered? I never said that the scooped hydrogen would be for fusion. Read again. Also, I proposed planet atmosphere skimming just for the final deceleration, after you use all the other methods I mentioned. Please read what I wrote before answering. Yes, the durability of everything may be an issue. We must provide the robots with what they need for repairs, and also maintain such conditions that would prevent damage. As I mentioned, the payload would be small (there are no people, no crops, no habitat) and it may be even smaller if we send LUCA-s. Also, nobody from the planet of origin would die in such a mission. So we may send a lot, in different directions. In a billion years we would be able to do it just for fun (there are people who seriously consider that we live in a virtual reality made by an advanced civilization just for fun). That means that, especially if there are more advanced civilizations, more than one alien colony may be present in our solar system.

Yes, if we use nuclear power, apart from the fuel itself, we need a material to eject + a nuclear reactor and the ejector. That would add to the rocket/thruster mass ... But we don't necessarily have to carry everything from Earth. Since we are already deploying a massive magnetic field (the magsail), we could theoretically use it in reverse during the cruise/early deceleration phase as a magnetic scoop, similar to a Bussard Ramjet concept. We collect interstellar hydrogen and than eject it. Also, a nuclear/ion system has a much higher exhaust velocity, so you need fewer particles to get the same kick. Anyway, we send the most efficient rocket/thrusters we get at the time, nuclear or/and chemical. You don't need to maneuver in the atmosphere. You adjust your trajectory far before the entry, with minimum use of thrusters. And then you go "straight" (free fall). The planet may be a gas giant. And you can do it multiple times, until you can land on an appropriate moon or planet. Yes, you lose time on elongated orbits, but you have no living creatures in the ship, so it doesn't matter too much.

Ok, but you may use nuclear energy ... The fuel mass should decrease substantially if you switch from chemical to nuclear. Did you read the arxiv article? The acceleration phase is pretty much what I suggested and they proposed magnetic and electric sails for deceleration. If you add that to what I already proposed, including atmosphere skimming, you may have a viable solution.

In fact, such rockets would be obsolete. I'm pretty sure that some lighter and more efficient thrusters would be developed/available. I learned that there are more methods for deceleration: Combining Magnetic and Electric Sails for Interstellar Deceleration

With low speed travelling, they couldn't know at the departure that the Earth is inhabited by an intelligent species. I don't know about them, but I wouldn't colonize a planet already occupied. So, I'd either abort the mission completely or choose some moon to dig and build habitable places and so on. Therefore it is possible to have aliens on some moon in our solar system. We may solve this problem by improving and extending the surface of the sails, by increasing de time for acceleration (using not only the lasers near the Sun, but also lasers situated much further, and powered from the belt of power stations near the Sun) and by improving the resilience of the shipment (robots, devices, etc.), in order to withstand longer travel time, with lower speed. I also thought about a way to flip the sails when overheated (if sails are divided in smaller sections). Due to the fact that the acceleration is provided by external sources, we can send a big multistage rocket for the deceleration. After that, we redeploy the solar sails. If we still need deceleration, we may consider skimming through some planet(s) atmosphere. I want to add another use for the solar power stations belt: we can use it to direct heat to some frozen moon, in order to make it habitable. So, the investment would not be solely for colonizing other solar systems.

OK, we don't discuss imagined physics, but we should acknowledge that there are many things we don't really understand ... and this should change/improve with time, especially in thousands or even billions of years. Also the technology would advance a lot. What I mean is to use the Sun in order to propel the starship. We can do that by orienting & focusing large mirrors, in orbit near the Sun, towards the ship light sails. I also mentioned a mirror that would both protect and propel the starship. A variation would be to use laser beams from solar power stations orbiting near the Sun. Men are donating sperm. Why women cannot donate/sell eggs? For a good price they would be happy to do it. Especially if there are signs/concerns that the Sun is dying or a supernova or some other catastrophic event would wipe out the civilization. To have your DNA selected for a colony meant to preserve the species and all its knowledge (in a database) would be, I think, desirable. Also, in millions of years, we may be able to 3D print the embryos, plus animal embryos, seeds, etc.. So we only send the database and few sophisticated devices, plus robots, etc.. And the embryos would get to be transferred into an artificial womb only after the robots would build a safe environment, with air, water, food, etc.. In my proposal, with the use of lasers from solar power stations orbiting close to the Sun, those power stations should be already deployed as a better way to produce/harvest energy. The lasers would be used to transport the energy to/near the Earth. This would be a miniature Dyson sphere. To use, briefly, part of this power stations in order to propel a ship meant to preserve the species and all its knowledge from annihilation would be, I think, acceptable. I mentioned such power stations before, when we discussed how to deflect an incoming asteroid. With such a power station near the asteroid, receiving energy, through laser beams, from the ones near the Sun, we may deflect the asteroid. I proposed this scenario as a way to explain alien presence on/near Earth. It is not very probable, but it is possible. If the payload is still too big, an inteligent species might send a LUCA (Last Universal Common Ancestor) in order to seed a planet with the kind of life they originated. So, we may be the aliens ...

Hello, I have few thoughts on this subject: I wonder if anyone offered Alcubierre drive or wormholes as possible explanations for an alien presence on/near Earth. They are not rejected entirely by mainstream. If nobody did, it's probably because all of us consider them BS 😄 A new/better understanding of gravity (I may have one) could offer a way to reverse or block it ... It may be possible but I really doubt it. (I also doubt that there are aliens on/near Earth) Someone proposed hidrogen harvesting as a refuel method. Interesting. I thought about dark matter "harvesting" as a possible fuel source for a fast travelling spaceship. Now, back to our reality, I imagined a simpler way to colonize another solar system. We send a small ship with few advanced /AI robots, plus spare parts, 3D printers, spare materials, etc. and a freezer with embryos. We collimate light (from the Sun?) in order for the ship to achieve a high speed. Of course, the ship must have a big mirror and solar sails. For deceleration, the solar sails and some thrusters may be used. Also, we don't need to decelerate all the ship, at least in the last part of the trip. We decelerate only a probe with the things really needed for colonization. If the technology is advanced enough, with the materials on that probe, the robots/AI and the solar power, plus, of course, the materials available on the destination (asteroid/planet?), in a century, a full grown colony, with the embryos transformed in newborns and then raised/educated by the robots, plus habitats and ships, can be achieved. What do you think?

Ok, please explain why the speed of light is that speed. Also, please explain time dilation for a GPS satellite (kinematic and gravitational time dilation).

For DM particles, as I wrote above (see especially what I underlined), the photons "inside" time is very very short and the same for all frequencies, unlike the "inside" time for atoms and molecules. So, the index of refraction in DM gas is the same for all frequencies. No different speeds and no "rainbow" at the refraction responsible for gravitational lensing. Also, the change in direction (dispersion) is very limited. I'll be busy next days, so I won't be able to reply for a while.

Exactly (thanks), I just tried to describe what is happening. I know it sounds silly, but I consider the DM particle as a very stable "system". When a photon gets in, the particle/ system becomes unstable and promptly "spits" the photon out. Don't ask me how, because it's like asking Koch when he just discovered bacteria about its DNA. Take it as a definition of my model. Let experimental tests I proposed to decide if the model is good. You need a clock to measure the mirror speed. So it's not much different than the Fizeau apparatus in the opening post. You obviously didn't read what I wrote 6 years ago. I explained/proposed that, in the Hafele-Keating type experiments, kinematic time dilation is caused by the movement of the clock through Earth’s DM gas atmosphere, the reason being easy to explain with a light clock (the original use of the light clock). So, in very high orbit, where the instrument/clock can be surrounded by its own, comoving, DM atmosphere, there is no movement of the clock trough DM gas, hence no kinematic time dilation. Simply to detect but/and never attempted.

Quasi (or failed) absorption immediately followed by re-emission is not scattering. The definition of scattering is: "the process whereby a beam of waves or particles is dispersed by collisions or similar interactions". There are no collisions ... and the change in direction at re-emission is very limited, because the time between quasi-absorptions and re-emissions is too short. Normal matter, as opposed to DM, can form atoms and molecules, structures able to use (actually absorb) the photons if the photon energy is the one needed by the electron to "jump" to a higher energy level. So, the time between quasi-absorptions and re-emissions is longer, giving the time for the electron to move around (Bohr model) and emit in other directions than the original. The time needed to "decide" if the photon can be used depend on its energy, therefore light/photons travel at different speeds in transparent materials and the refraction is energy/frequency dependent. DM gas atmosphere, like any atmosphere, is denser near the massive object it surrounds (like stars, planets, black holes) and, going upward, gets less dense. The gravitational time dilation is therefore greater near the massive object and smaller as we go up. No contradiction with reality. They are all equally affected. Please read my explanations for the invariance of the speed of light in vacuum posted 6 years ago. I really have not enough time to write/explain everything again. When I'll have time, I'll try to cover/explain what I didn't.

If you read what I wrote 6 years ago, you understand how. In short, I proposed that light, actually the photons, travel through both ordinary and dark matter, by quasi-absorptions quickly followed by re-emissions. DM particles never really absorb photons, so the tiny delay of each such interaction is the only effect (in fact this causes one more, a refraction, responsible for gravitational lensing). I initially used this idea for ordinary matter, successfuly explaining (with math and all) the Fizeau experiment and the Sagnac effect. You should read what I wrote 6 years ago ... Maybe, but the Earth centered non-rotating frame is the one we usually choose (although the clocks at rest in the dragged frame would be the fastest ... while rotating - with respect to distant stars - and having centripetal acceleration ...). Ok, I admit I didn't quite understand, so please elaborate. Also, my questions, even seemingly unrelated, still stand. See the beginning of this post.

I really appreciate you and your effort to find and offer links to interesting papers regarding relativity, but after following at least 3 of them, I realized that you didn't notice the requirment to prove/disprove my DM gas theory. We need to measure the time in a very high orbit, in a place where the Hill sphere of the instrument/satellite is large enough, because otherwise the instrument/satellite cannot have it's own DM atmosphere. If/when the instrument/satellite has it's own DM atmosphere, the kinematic time dilation dissappears, because the instrument becomes static in relation with the DM atmosphere surrounding it. That's it. It is a simple, powerful experiment, and it was never done. When there is no own DM atmosphere, there is absolutely no reason to disagree with GR. As for the Moon and other distant celestial bodies, like stars, we only have indirect measurements, redshift observations, and the redshift is caused by many different things, besides kinematic time dilation. It is impossible to exactly tell how much of it is from kinematic time dilation. Also, the observations are flawed by the fact that we obtained our data considering the kinematic time dilation as existent. And, when the data disagree with our theory, we usually add a peculiar velocity ...

You obviously didn't read/understand what I wrote. I presented a thought experiment (there is no actual Fizeau apparatus) aiming to show why the speed of light in vacuum is invariable, not to deny it. The title of this thread is wrong/misleading. It should be: "A thought experiment meant to show that c invariance and gravitational time dilation are caused by a dark matter atmosphere". Being now in the speculation forum, I can be less subtle. Let's try again. In a box we have an imaginary device that is sending a pulse of light towards a mirror and receives the pulse reflected by the mirror. In vacuum, the total time from emission to detection is exactly 3 nanoseconds (as explained in the first post). The light clock is "ticking" exactly 3 times. The atomic clock situated on the same table agrees. Now, we introduce a gas in the box, a gas that is not detectable (like DM), but makes the light travel slower, exactly as any known gas would do. The light clock would still tick exactly 3 times (3 ns) for a round trip of the light pulse in the measuring device, so, for the people in the box, the speed of light in what they think is vacuum, is invariant. The atomic clock would measure more than 3 ns, and everybody is wondering why the speed of light in what they thought is vacuum, is decreasing. Not knowing about the DM gas, they declare that the space is warped, and this is what GR pretends to happen in the regions where in fact light is traveling slower due to the increase density of the DM gas. In real life, the atomic clock is also affected by the DM gas, because, being made of extremely tiny particles, the DM gas cannot be contained. So, if you measure the speed of light in "vacuum" at the sea level, you get c, but only if the clock is also at the sea level, at the same DM gas density. If the clock you use is on a mountain top, at lower DM gas density, you obtain a value lower than c for the sea level speed of light in "vacuum". You know any thing or device that can be used to measure the time and is not made of parts held together by a force that is transmitted similar with the light in vacuum? Even if you know one (the muon?), how can you be sure that the proposed DM gas has no influence? You obviously didn't read what I actually wrote: Read what I underlined. I wrote that it's not true and I presented one reason (there are more). So, your comment, although good and interesting (thanks), was uncalled for. So, the electromagnetic force acts/propagates slower than light in vacuum? Ok, but you should notice that in real life there are preferred frames. See the Hafele-Keating experiment. The relevant frame there is exactly the DM atmosphere I'm proposing. It is a kind of aether, but one that not only agrees with Lorentz transformation, time dilation, and so on, but is also explaining them. Follow the link to my theory I provided. I'm pretty sure that no one searched the experimental test I offered/mentioned, so here they are: Thread 'Suggestion for 2 more GR tests' https://www.physicsforums.com/threads/suggestion-for-2-more-gr-tests.1056055/ Thread 'Time dilation in a planet-moon system' https://www.physicsforums.com/threads/time-dilation-in-a-planet-moon-system.1053613/ https://www.physicsforums.com/threads/is-there-an-atomic-clock-experiment-on-the-moon-testing-relativity.1053545/