# Janus

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1. ## Weight Loss during Solar Eclipse

You can't just use the gravitational pull of the Moon or the Sun on the person to work this out. The Earth (as a whole) is in free fall with respect to each. So, if like in Swansont's example you were to replace the Earth with a man standing on a scale, but located where the center of the Earth is, he would weigh nothing, even though the gravitational pull on him from Both the Sun and Moon are equal to what you calculated. In order to get the effect caused by the Sun and Moon being directly overhead compared to no Sun and Moon, you have to take the difference between the gravitational pull they have on a mass at the distance that the center of the Earth is and that on the mass if located 1 Earth radius closer.
2. ## Why do all cosmological models assume time flow is the same for everyone in the universe?

You have to consider how we measure these star velocities. We do this by measuring the Doppler shifts of stars in in these galaxies. This works best if the we are seeing the galaxy nearly edge on. We measure the Doppler shift from stars on one side of the Galaxy and compare them to those on the other side. By measuring stars on both sides, we can account for any over-all shift that might be a part of the shift we see (by the galaxy itself moving towards or away from us for instance.). From these measurements, we know that any time rate differential is way too small to account for the fast star velocities we see. For instance, for the galaxy M-33, the measured speeds for outer stars are three times what they should be given the visible matter in the galaxy. For that to be a result of any type of time dilation, the galaxy would have to be moving towards us at a good fraction of the speed of light. (gravitational time dilation couldn't be the reason simply because it would cause us to measure slower star velocities, not faster ones, and besides the amount of mass that the galaxy would need to have to exhibit a time dilation of this magnitude would have been more than enough to account for the faster star velocities, so in either case the galaxy would have to have more mass than that we see.) However, when we average out the Doppler shifts for the stars of M-33, we get a relative speed to our Sun of only 179 km/s , which would only result in an apparent increase of 0.06% in the star velocities. In addition, it is not just the higher speed of the stars, that we are dealing with, it is also how the velocities change as you move outward from the center. Near the center, the velocities match pretty much what we would expect and then diverge as you move outward. If the higher velocities were due to some difference in time rate between these galaxies and our own, the velocities would vary from the expected values by the same factor at all distances from the center.
3. ## Inbreeding

The danger of inbreeding comes from the fact that it increases the likelihood of the pairing of recessive genes for harmful traits. I'll give you an example. There are two types of Poly-cystic kidney disease. Both are eventually fatal. One, which is carried by a dominant gene doesn't manifest itself until well into adulthood. If you get the gene from either parent, you will get the disease. But since this doesn't occur until you have likely already had offspring, it doesn't get culled from the gene pool. The other type is carried by a recessive gene, and you need to get it from both parents. This version is fatal within a few months of birth. If it is only passed on by one parent, you are just a carrier and you aren't born with the disease. This genetic disorder, while it does prevent the person born with it from furthering the genetic line, remains in the gene pool because it is recessive and can be passed on from generation to generation with the disease itself ever manifesting. This is true of a good many genetic diseases, they hang around because they are recessive traits. Now let's say that thae chance of any given person in the general populace is a carrier of a particular one of these recessive diseases is 1/100,000. Then is is pretty slim odds that two mated people will both have it and low odds that any of their offspring will suffer from the disease. But what if you have a lot of inbreeding within a given family. If that recessive trait exists and is being passed on from generation to generation, the odds of these any two people in this family having the gene is higher than for the general populace and there are higher odds of offspring from a mating between family members of exhibiting the disease. It is not just the paring of identical chromosomes, its the increased likelihood of the pairing of the wrong identical chromosones.
4. ## Could you slow time using a tuning fork?

It is implicit in the gravitational time dilation formula. For example, use it to work out the time dilation for the surface of the Earth vs. the surface of Uranus, and then compare the surface gravity of the two. You will note that, for a far off observer, clocks will tick slower on Uranus than on Earth, but the surface gravity of Uranus is less than that of Earth. This is because even though you are starting from a point where the gravity is locally weaker while on Uranus, the amount of energy needed to lift a mass far away from Uranus is greater than that needed to lift an equal mass as far away from Earth. This is the problem, you are trying to assign the difference in tick rates as being due to some local physical effect acting on the clocks. It is due to the fact that the clocks simply measure time differently. This is a much more abstract concept, than a "physical cause". Let's describe it this way. You have your two clocks, and they are sending signals to each other at 100,000hz. This means that for every sec ticked off by a clock, its sends out 100,000 complete waves of electromagnetic radiation. The light going up from the lower clock to the upper clock is fighting the gravity the whole way and loses energy in doing so. Light's energy is tied to its frequency, so it exhibits this as a decrease in the signal frequency as it climbs to the upper clock. So let's say that by the time it reaches the upper clock it has a frequency of 99,999 hz. This means that the upper clock receives 99,999 waves every second, and thus will take a tad bit more than 1 sec to receive the 100,000 waves, the lower clock sent in its measured sec. Since the number of waves sent off by the lower clock between each second it ticks off is fixed, The upper clock must also "see" the lower emit 100,000 waves between each second it sees the lower clock tick off one sec. Since we have established that it takes longer than one sec by its own clock for the upper clock to receive 100,000 waves, it must also take longer for it to see the lower clock tick off 1 sec. The longer we let these clock run, the greater their difference in reading grows. If after some time, we stop both clock and bring them together again, they will show different times. (Imagine the top clock has seen the lower clock stop and then stops itself. Then it approaches the lower clock, while watching it. at no point during the approach will the top clock see either the lower or itself change time readings. so when they meet they will show different times. Now its is true that some time will pass between the lower clock stopping and the upper clock seeing this, so we must account for this offset. But this is a fixed amount and doesn't change depending on how long we let the clocks run before stopping them. It could be 1 microsecond, and we could let the clocks run until the upper clock saw the lower one 1 sec behind. When brought together, that 1 microsecond only accounts for a small amount of the difference. If we turn things around and watch the upper clock from the lower, the signal gains frequency as it travels down and increases its frequency. The lower clock sees the upper clock as running faster than itself. And when the clocks are brought together again this difference in clocks will remain. Nothing is physically acting differently on the two clocks, they are just measuring each other across a gravity potential.
5. ## Could you slow time using a tuning fork?

Consider the following example: You have a rocket which is accelerating. There is a clock in the nose and tail. As far as anyone in the rocket is concerned, they are accelerating at the same rate. Relativity predicts that the clock in the nose will run faster than the clock in the tail, even though there is no difference in the acceleration felt. In the same way, two clocks at different heights in a uniform gravity field (one that does not change strength with height.) will run at different rates even though they feel no difference in gravity. While you need acceleration or gravity for these differences to show up, it is not the difference in acceleration/ gravity that causes the difference. To go back the the centrifuge. If you are in the lab frame, you have a sample traveling at a high speed and you expect it to show a time dilation effect from it. If you are in the centrifuge frame, it is like there is a gravitational force acting outward from the axis and which gets weaker as you move towards the axis. You would expect a time dilation effect that would be related to the potential difference between the end of the arm and the axis. (the aforementioned work needed to move in to the axis). This will work out to be the same factor as that expected in the lab frame. The two time dilation effects, as seen in the lab and as seen by the centrifuge are just two different views of the same thing. What you cannot do is "double up" by applying both.
6. ## Could you slow time using a tuning fork?

g- forces have no effect on aging, or put another way, there is no time dilation factor due to acceleration forces. The only time dilation effect that would be seen would be from the speed of the vibrating ends of the tuning fork. That acceleration forces have no time dilation effect has proven by spinning radioisotopes up to extremely high speeds in centrifuges so that they experience 1000's of gs. By varying the size of the centrifuge, you can vary the ratio of speed and g force experienced. Such experiments show that the only factor that determines the time dilation is the speed at which the sample travels. This is no different than Gravitational time dilation. Gravitational time dilation is related to the amount of work it takes to move a test mass from one height to another, not to the difference in g force felt by this test mass at the two heights. In the same way, with the centrifuge, it is related to the work needed to move the test mass in from the end of the spinning arm to the axis of rotation, and this is directly related to the speed at which the end of the arm is moving, and not the g force difference between the end of the arm and the axis.
7. ## Why do we think there is dark matter and dark energy?

First off, dark matter and dark energy are two different things and the reasons for expecting their existence are completely unrelated. The only thing they have in common is that they have "dark" in their names. The initial evidence for dark matter came from observations of how stars move in galaxies. Galaxies are formed from stars that are mutually orbiting each other. If we look at a galaxy, and estimate its mass by the matter we can see, we find that there does not appear to be enough to hold the galaxy together. At the speed the stars are orbiting, they should fly apart. We also know how these stars should orbit if the mass is contained to the shape we see it as having. Not only does the galaxy have more mass than that we can see, but the unseen mass must be distributed a lot differently than the part we do not see. For example, in a typical spiral galaxy, a good deal of the mass must be located above and below the disk-like shape we see. If it was made of normal matter, we should see it, if not in the visible spectrum, it should be visible at some other spectrum. This leads us to believe that whatever is causing that extra mass is not made of normal matter, but a type of matter that does not emit or interact with light or any part of the electromagnetic spectrum. Thus the term "dark matter. There have been attempts to explain the discrepancy through developing different models for how gravity behaves, but to date, none have been consistent with all the observations we have made. Dark energy concerns itself with the expansion of the universe. We have known for a long time that the universe is expanding and that distant galaxies are moving away from us. But until couple of decades ago, we assumed that the mutual gravity between the different parts of the universe was slowing this expansion down over time. What we did not know was whether this was enough to eventual stop the expansion all together. In the 1990's a study was made to try to determine if this was the case or not. Basically it worked because as we look at distant galaxies, we are seeing them as they were when the light left them. Thus as we look further away we are looking further into the past. Thus, to explain it simply, by comparing various galaxies' distances to how fast they appear to be receding from us, you can work out how the expansion of the universe has change over time. The surprise came when it was discovered that the universe's expansion was not slowing down, but was speeding up. Not only was it mutual gravity not enough to stop its expansion, but something was overcoming the gravity and pushing the universe apart. They decided to call this unknown influence "dark energy" (mainly because they had already coined the term "dark matter") . We really know very little about dark energy, and the term really just is a place holder for whatever it turns out to be. (Much in the way the terms "X-rays" was coined before we learned that they were just a certain part of the electromagnetic spectrum.)
8. ## Missing Matter Found:

That makes sense, I as understood that the universe couldn't have very much more baryonic matter than presently predicted without it having had a large effect on the early universe, and thus an effect on what the universe should look like today.
9. ## Why don't light bulbs implode/ break in it contains a vacuum?

You don't need them to contain an inert gas to keep them from caving in.(after all, vacuum tubes don't collapse in on themselves. The trick is that the pressure is equally distributed over its surface. In this case the force is a compressive force and the compressive strength of glass is actually in the 1000's of psi. A glass bulb will shatter when hit because the force is not evenly distributed over its surface.
10. ## New definition for the second

Neither of these are unchanging. The Earth's rotation is constantly slowing and can even be effected by major earthquakes. The Earth's orbit also slowly evolves. While at one time both of these were used as the basis for the definition of the second, they were abandoned in favor for a non-changing standard.
11. ## The Physics of Star Trek

It's just orbital velocity. The only way to transfer rotational energy is through tidal interaction, but this is slow and inefficient. Consider our own Moon. It gains only 4cm per year in orbital height. That's a gain of 1.33e-6 joules per kg per year gained by the Moon. This amount of energy gain in terms of equivalent velocity gain is 0.163 cm/sec per year. And a probe using a planet as a slingshot spends only a relatively short period of time close enough to the planet for this interaction to take place. It is inefficient in that the majority of the rotational energy lost by the planet is given up through waste heat and not transferred to the orbiting body. To explain a gravity slingshot, first imagine the probe some distance to the left of a planet. It is approaching it at 2 km/sec on a trajectory that will follow a parabolic path around the planet, so that some time in the future it will again be the same distance to the left of the planet, but now moving away from the planet to the left at 2 km/sec. Now imagine that this whole arrangement was moving right to left like the planet is orbiting the Sun. the planet's orbital velocity is 4 km/s. So from the sun fixed view, the probe is moving at 2km/ per sec to the left (the relative velocity between probe and planet is still 2km/sec.) the probe whips around the planet as before, and ends up once again moving at 2 km/sec to the right relative to the planet. But now it is moving at 4km/sec + 2km/sec = 6km/sec relative to the Sun. It started out moving at 2 km/sec to the left and ends up moving 6 km/sec to the left. Of course, it doesn't quite gain a total of 4km/sec. Because in the above, we ignored how the planet behaves during all this. It reacts to the gravitational pull between it and probe also, In the first scenario, it would pick up some speed moving to the left as the probe came in from the left, and then start moving to the right as the probe moved away. But since its mass is so large compared to that of the probe, its resultant velocity to the right will be infinitesimal, and in the second scenario, we will see it as an equally small decrease in its right to left orbital speed relative to the Sun.
12. ## The Physics of Star Trek

Gravity is actually a really weak force. You need a huge mass to produce any significant amount. As for the "gravity" slingshot. Gravity is not being tapped to speed the probes up. Instead, gravity is just used as a means to transfer momentum from the planet to the probe. We are robbing the planet of some of its orbital velocity and giving it to the probe (this can also be used to transfer momentum the other way and decrease the speed of the probe.) Artificial gravity is likely a pipe dream. The only theoretical known way to do this is with mass or energy, and you need a lot of it to produce much. That being said, in a way, this is how the warp drive in Star Trek is supposed to work. Gravity is the curvature of space-time. The warp drive allows faster than light speeds by compressing space in front of the ship while expanding it behind the ship. It just takes a huge amount of energy to do this, which is why it needs matter-antimatter conversion to generate it. Now there are some theories that allows for such warping of space. However, they rely on the existence of "exotic" types of matter(matter which has characteristics unlike normal matter) and we have no real reason to expect that these types of exotic matter can even exist in our universe.
13. ## Photon

E=mc2 has a specific meaning where "m" stands for the rest(or invariant) mass. Its only meant for those cases when there is a rest mass and that mass has no motion with respect to the measurer. It deals with a limited case of the more general formula: E2= (pc)2+(mc2)2 Here p stands for the relativistic momentum. For light, m=0, but that still leaves (pc)2. Light does have a momentum (While in Newtonian physics you can't have momentum without mass, this is not the case in relativity.), so it has energy even though it has no mass.
14. ## The Physics of Star Trek

As already noted, it is a bit difficult to answer questions about a fictitious TV show with real physics . (particularly when what these systems are and are not capable off can change from episode to episode, depending on the plot. (Did you ever wonder why, in the episode where the transporter splits Kirk in two, they didn't just send a shuttle down to get the crew stranded on the surface? . The answer is that, at this point of the series, they didn't have a shuttle. (the network wouldn't budget them the money needed to build a model or mock up) It wasn't until later that they got their shuttle. But that isn't to say that they at least didn't make an effort to answer some questions. How did the Enterprise avoid objects? First off, that big disk on the front was the part of the main deflector, which had the job of deflecting the small stuff. Really large objects like planets are pretty well spaced out and in known positions. They also had long range sensors( based Sub-space radio technology?) that would give them enough warning to steer around anything too big for the deflector to handle. Photon torpedos, were antimatter contained in a magnetic bottle and propelled by a small warp engine. The original concept was that they were meant to be used while in warp flight where the Phasers, being energy weapons, would have been useless. This rule ended up falling to the wayside. I have a copy of "The Physics of Star Trek". It isn't about trying to explain things in Star Trek, but more in the lines of talking about what things in Star Trek do or do not make sense using real physics.
15. ## Value of an Asteroid

Assuming an ISP in the order of Our best present day rockets, and a typical relative velocity for a near-Earth asteroid, Your asteroid would have to have ~3/4 of its mass to be made up of volatiles capable of being used as fuel. An asteroid with that high a volatile percentage make-up is unlikely to contain a great deal of precious metals. Rail guns need power to run, which would most likely be Solar, which would require building huge solar farms on the asteroid. And the amount of asteroid you would need to "chuck" away would depend on much power you can generate for your rail gun. For any of this to be anyway feasible or economically viable, we would need to have a much greater space presence than we have now or are expected to have in the foreseeable future. I don't know where you got that 50% figure from, as the typical iridium content is in the order of 0.5 ppm (parts per million). This is relatively rich compared to the Earth's crust, but still fairly rare.