Jump to content


Resident Experts
  • Content Count

  • Joined

  • Last visited

  • Days Won


Janus last won the day on May 11

Janus had the most liked content!

Community Reputation

1029 Glorious Leader


About Janus

  • Rank
  • Birthday August 28

Contact Methods

  • Website URL

Profile Information

  • Favorite Area of Science

Recent Profile Visitors

The recent visitors block is disabled and is not being shown to other users.

  1. The source clock runs slower (not backwards), if you are accelerating away from it( by your measure). The rate does depend on the distance. The further away, the slower the clock runs.* Now, for any acceleration value there is a distance in that direction where the coordinate speed of light goes to zero. This results in a Rindler horizon beyond which the accelerating observer cannot make any measurement. Light originating from beyond the Rindler horizon will not reach you. This is true even if the source of that light shares the observers acceleration. * this in addition to any time dilation due to a difference in velocity.
  2. The bicycle rider has to measure the speed of light as being c, relative to himself, regardless of what direction he is traveling. So let's say, that when he is ( by his measure) 1 light hr from a source that he has been moving away from at 0.5 c. That means that the light reaching him from that source at that moment had to leave the source when they were closer together, when they were only 2/3 of a light hour apart. Thus it took the light just 2/3 of a hour to reach him, during which time the source traveled 1/3 of a light hour. to a distance of 1 light hour. Further assume that that light carried an image of a clock at the source reading 12:00. Since he knows that 2/3 of any hr passed since that image left, and that due to time dilation, The source clock runs 0.886 times as fast as his own, the source clock should read between 12:34 and 12:35, at the moment he sees this light. Now let's say that he is moving towards the source at 0.5c, when he sees the light while still 1 light from the source. Now he has to conclude that the source was further than 1 light hr away when the light left ( 2 light hrs away). He still has to measure the light as traveling at c relative to himself, So, by his clock it took 2 hrs for the light to reach him, during which time he and the source closed to a distance of 1 light hr. He sees the same image of 12:00 for the source clock*, But he now it has been two hrs since that image left the source, during which time the source clock advanced to read 13:44, which is the time on the source clock when he sees the image This is quite a bit later than what he concluded when he was moving away from the clock. So before the bicycle reverses direction, he would say that is 12:34 on the source clock, and afterwards he would say that it reads 13:44 ( even though the image he sees for the source clock remains 12:00) For this we assume an instantaneous change in velocity, which can't actually happen. The bicycle has to spent some non-zero time accelerating, during which period it is non-inertial. It is a bit more complicated to work out what someone concludes when working from a non-inertial frame. For one thing, it is only the Local proper speed of light that is measured as being c. The "coordinate" speed of light changes with distance from the observer. So said observer cannot conclude that the light traveling from the source traveled at c relative to him for the entire trip. The upshot is that for this accelerating observer, clocks in the direction of the acceleration, run fast by a rate that depends on the magnitude of the acceleration and the distance to the clock. This leads our bike rider to conclude that while he decelerates to a stop and then accelerates back up to speed back towards the source, the source clock runs very fast and ticks off the time between 12:34-12:35 and 13:44, over a very short period by the bike rider's clock. * If it helps, assume two bicycles, one moving towards and one moving away from the source and passing each other when they are 1 light year away from the source. They both need to see the same image of the source at that moment.
  3. Have we jumped back to dark matter again? If so, then what you said is again completely based on misconception. No one claims there is more mass than exists. Dark matter has mass. It just a type of matter with mass that doesn't participate in the electromagnetic spectrum, and thus doesn't emit, absorb, radiate, or block electromagnetic waves of any frequency. Despite what some people seem to think, this is not some bizarre thing. The neutrino, an already known subatomic particle, has these properties. It is even possible that a type of neutrino, the sterile neutrino, is what makes up dark matter. What is it that makes people that know very little about a subject feel like they are qualified to lecture others on it?
  4. There is no way to be still in an absolute, universal sense, But you can always find a reference frame that you, or whatever else you want to consider, is at rest relative to, and that all that needed. Einstein did have an different way of approaching a problem. Instead of starting with an observation or experimental result and attempting to explain it, he would start from basics principles and would see where they naturally took took him. Often it led him to the same results as the observation/experiment. But the fact that Einstein wasn't an experimenter himself means nothing, as he knew that other could and would perform the experiments/observations needed to confirm or disprove his theories. All he needed to do was to show them where they needed to look and what they should look for. Nobody is now or has ever elevated Einstein to "Godhood". People tested his theory from the get go, and continue to do so to this day. They do this in order to see if it fails at some point, and if so, where and how. Einstein is revered because he "with just his pencil" and little experimental evidence to go on, came up with an entirely new way to view time and space, and one that has survived every experimental challenge thrown at it to date. I is one thing to take existing experimental data and come up with an explanation for it, it is an entirely other thing to be able to start from basic principles, and not only explain existing experiments, but to accurately predict the outcome for experiments yet to be made for another 100 yrs. ( one of his predictions was the existence of gravitational waves, something that has just recently proven to exist by experiment.) As far as the expansion of the universe is concerned, when Einstein published GR in 1915, it wasn't even clear if the "universe" extended beyond our own galaxy. It wasn't until 1923, that observations of Cepheid variables finally confirmed that the Andromeda nebula was in fact a galaxy in it own right, distant and separate from our galaxy. A static universe was the accepted view of the time, and Einstein was just trying to make his theory consistent with this. Later, when observations showed otherwise, he admitted himself that it was his "biggest blunder". Ironically, his "biggest blunder" happened due to trying to force his theory to fit with accepted "wisdom", rather than letting it stand on its own.
  5. What is it going to take to get this into your head? There is no such thing as "true" speed. That term has no meaning in our universe.
  6. Which thought experiment are you referring to, as there is more than one. You keep harping on about "motionlessness" as if it is an absolute, and keep ignoring the fact that everyone is telling you that it isn't. When any of Einstein's Thought experiments, if the term "at rest" is used, it just means "at rest" with respect to the reference frame from which the experiment is being considered, not "at rest" in any absolute sense. Put another way. If you were in a spacecraft off in space far away from any source of gravity and perform one of Einstein's experiments*, You will get a certain result. If you then fire the rocket engine, change your velocity by however much you want, cut off the engines, and repeat the experiment, You will get exactly the same results. The fact that you have changed velocities between the two experiments will have no effect. And for the same reason, you don't have to know the Earth's exact motion around the galaxy's center, or the motion of our galaxy with respect to other galaxies, as it will have no effect on the results. As far as Dark matter goes. The observations that led to its hypothesized existence were made on other galaxies, and had nothing to do with knowing how fast our solar system traveled around our own galaxy. You can measure how fast an object like a star is moving relative to us by looking at the Doppler shift of its light. If we look at a distant rotating spiral galaxy nearly edge on, stars on one side will be coming towards us relative to that galaxy's center, and the one on the other side will be moving away. Now the galaxy as a whole my be moving towards or away from us, and some of that may be due to out own solar system's orbit in our galaxy, but that doesn't matter. Because that factor is going to added or subtracted equally from all the star velocities we are measuring. Thus all we need to note in the difference we measure in the velocities to work out how fast they are orbiting their galaxy. And by looking at stars at varying distances from the galaxy's center we can plot a rotation curve. And when we look at these plotted curves, we find they don't look like they should for the amount and distribution of matter we see for the galaxy in question. Something else is supplying a gravitational effect besides what we can see. Not only that, but there has to be a lot more of that something than what we see, and it can't be distributed the same way as what we see. If it were made of normal matter, even if it were cold gas or dust, it still would give us some visual clue. That part of it located between us and the galaxy would absorb some of the light of the galaxy,( and not evenly for all frequencies of light), and no matter how cold it was, it would radiate, even if only at radio frequencies. The conclusion is, that whatever is is, it doesn't react with light or any electromagnetic frequency at all, but still interacts via gravity, thus Dark matter. Another observation involve how gravity bends light. By looking at light from further objects as it passes near to the galaxy we can see how the gravity of the galaxy acts like a lens, bending the light around it. And like we can determine the curve of a glass lens from the way it bends light, we can determine how the mass of the galaxy is distributed by looking at how light passing near it bend. This gravitational lensing gives the same result for mass distribution as the rotation curves give. So two independent observations confirm the same thing. It is clear that you have not done any real study of of these subjects, and are basing your conclusions on a number of misconceptions. *And despite you claim, we can do versions of these experiments. While this wasn't possible at the time Einstein formed them to do many of them, since then we have gained the ability to measure time much more accurately, and accelerate at least subatomic particles up to near light speed. There was one experiment/observation that was within the capabilities of the time, and that was measuring the bending of light by gravity. Though this did have to wait for a solar eclipse for the right conditions. It was the verification of this observation during an eclipse that was the first step towards the acceptance of Relativity.
  7. There is no such thing as absolute "motionlessness" to achieve. The Solar system orbits relative to the center of galaxy. we can give its speed relative to this center. The Earth orbits relative to the Sun. Our galaxy has a velocity relative to the center of gravity of the local group. The local group has a relative velocity larger galactic cluster it is a part of... But none of these motions are with respect to some absolute state of "rest", as no such state exists, and it is meaningless to talk about one. You can only consider velocities as being measured relative to some reference. And any reference you chose is just as valid as any other. Thus you are free to choose whichever one is the most convenient to work from. If the one where you consider yourself or anything else as being at rest is best for your purposes, you are free to use it. If you have a velocity of 100 mph relative to me, it is the same as I having a 100 mph velocity relative to you, and that is as far as it can go. There is no absolute rest frame that either or both of us are "really" moving with respect to.
  8. But the question is, is this difference significant compared to what you are measuring. If the difference is between traveling at 600,000,000 miles per hr and 600,000,001 mph, the the difference in distance traveled in a billion years is insignificant compared to the total distance traveled (a difference of ~ 1.5 ly over 900,000,000 ly) But more importantly it's insignificant if you are trying to test between two models that give answers that differ by over 100,000 ly, for example. But the main point that swansont was making is that there is no object in the universe has an "absolute" velocity, All velocities are relative. There would be no difference in the answers you get by assuming the galaxy has zero velocity and the galaxy having a velocity of 180,000 miles per sec. The effects Swansont refer to are the non-inertial ones due to things like Earth's gravity, etc. And those can be directly measured and accounted for. With the experiments he is talking about, these things have very small influence compared to the size of the effect we are measuring. It's like gravity accounting for the 1.5 ly difference above, when you are looking for the 100,000 ly difference.
  9. Martoonsky pretty much addressed it. The weight of the Air at the Lip of the hole is ~14 psi. Even if there were no gravity all the way from the surface to the center, the Air filling that hole would have to be at a pressure of 14 psi just to support the weight of the air above the Hole. But since air at every part of the whole except for exactly at the center is going to add its weight to the mix which has to be supported by the air underneath it the air pressure has to go up as you near the center.
  10. I'll reiterate what I said elsewhere. "dark matter" can be either MACHOS (objects like black holes) or WIMPs ( non-electromagnetically interacting particles). While the majority is expected to be the later, this does not rule out some of it even if only a small part, consisting of the former. And it is only the latter that would be truly "transparent".
  11. Gravitons would be quanta of gravitational radiation( gravitational waves). You would not "find them" anywhere in the nucleus. They would be produced under the type of conditions that generate gravitational waves, in the same way that photons are produced by the generation of electromagnetic radiation ( radio waves, light, etc)
  12. The Big bang was not an explosion in space and time, it was an explosion of space and time. It happened everywhere in the universe, as it involved the entirety of all space.
  13. "Worse" and "better" are subjective and depends on the circumstances. The other factor to keep in mind is that evolution acts on populations, not individuals or single family lines. So for example, If you have a population of animals, there is going to be a variation in genetic characteristics within that population. These variations will tend to cluster around an "average" or "norm". If we examine a trait like length or thickness of fur, you will find that it varies around a median. Individuals can have longer than normal or shorter than normal fur. As long as this variation isn't too great everything is hunky dory. Now occasionally an offspring will be born that has fur that is a bit too short or too long for the present climate. They won't do as well and will be less likely to survive to produce offspring. This will end up being a pretty stable scenario as long as environmental conditions remain constant. But, if the climate begins to change, the "ideal" fur length also changes. If the temp goes up on average, offspring which, up until then, would have been born with too short of fur, will fare much better, and those that had been born with long fur but still within an acceptable range will find themselves faring much worse. The change in conditions will lead to slightly The climate change will drive the population towards shorter fur. "Survival of the fittest" means a higher chance of survival( and thus produce offspring) for those that have characteristics that are the best "fit" for the present circumstances. If circumstances change, then the characteristics that "fit" best also change. With your robot example, for it to be more in line with evolution you have to start with a large number of robots at the beginning. Thus while the odds of any individual robot at the start having surviving descendants after 100,000 generations might be low, if you start with a large enough population of robots to begin with, enough of those lines will still be around after 100,000 generations to maintain a robust population.
  14. Janus


    Rotation has nothing to do with the space-time curvature that results in gravity*. The curvature is caused by the presence of mass. "Mass tells space-time how to curve, and space-time tells mass how to move." -John Wheeler *Except for causing frame-dragging, which is a very weak effect, hard to detect, and for most practical purposes, can be ignored.
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.