Spyman

I think it might help to understand that even objects with mass doesn't physically bump their atoms together when momentum is transferred, the electromagnetic repulsion between electrons keeps them apart during the interaction and electromagnetic forces are meditiated by photons, so a transfer of momentum between objects with mass could be said to happen with a cushion of photons meditiating the "push". If you only stick to Earth's frame of view then nothing special happens, from Earth the ship travels with half c relative the buoys, light travels with c relative the buoys and light travels with half c relative the ship. From Earth's view there is nothing wrong with the fact that the lightbeam has only traveled 5,580,000 miles further than the ship after one minute. This is wrong, relativity says that light has traveled 11,160,000 ship-framed miles from the ship, in a ship-framed minute. Yes, relativity claims that constant 'c' only holds true for the observers length contracted miles and time dilated minutes. Neither the crew on the ship or the observers on Earth notices anything strange or contradictory when they measure distance, time or speed of objects. It is when they compare their measurements that they find inconsistencies and start to disagree on measurements, this is the core of the 'mystery'.

Start view from Earth: > Spaceship - Lightray o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o bouys Finish view from Earth: ________________________ 30 buoys __________________________> Spaceship ______________________________________________________________________________________ 60 buoys ________________________- Lightray o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o_o bouys From an Earth view nothing happens to the ships velocity, it is moving in the trail behind the photons it emitts, getting further and further behind the front peak of the lightray. The mystery is how the crew on the ship can measure the lightray to move away from the ship with c when a measurement from Earth shows a speed difference of 1/2 c between the ship and the lightray. The answer from relativity is that the crew is in *distorted* space and time compared to observers on Earth, rulers and clocks on the spaceship are no longer calibrated to an equal scale as if they where on Earth. (Of course from the crews point of view it is the observers on Earth that are in *distorted* space and time.)

I am only repeating what others already have said, but sometimes it helps understanding when reading/viewing it from different sources/angles. It's not like you mean it. Such a recipe for cake baking : "Bake for 20 UTUs" is useless wherever you baked this cake, even on Earth. If in the recipe you specify that cake should bake for 20 UTUs, at 200 ° C, at a pressure of air of 1 atm , etc. , then you bake this cake in 20UTUs in any place in universe. I do not understand this.Could you explain it more detail? Let's say that we put this measuring robot on a large and powerful spaceship that is currently burning fuel for a one g acceleration in a safe trajectory through the mighty gravitational field of a nearby supermassive black hole. The robot is inside a pressurised cabin that keeps the enviroment inside identical as on Earth and has a camera with a videofeed transferred to Earth. The camera view shows the measurements of the UTU by the robot, the display of a very accurate clock and the inside of an oven through the glass lid, with a cake being baked inside. An observer on Earth watching this live videofeed on a screen notices that 20 UTUs on Earth equals 20 UTUs for the measuring robot on the ship, but due to gravitational time dilation 20 minutes as measured by a clock on Earth is only measured to 10 minutes by the spaceship clock. Question: If the period of 20 UTU units is measured to last 20 minutes on Earth and a duplicate cake needs 20 minutes in an identical oven on Earth, for how many UTU units should the robot in the spaceship bake the cake? Answer: According to the theory of relativity the cake needs to be baked 20 minutes according to the local clock, which in this case is the clock in the ship. Since time passes at half the rate in the region the spaceship is travelling through compared to time on Earth, the double time needs to pass on Earth for the cake in the ship to be done, which in this case is 40 minutes. 40 minutes on Earth equals 40 UTU units on both Earth and ship. Conclusion: We can NOT bake the cake for 20 UTUs in any place in the universe with equal results.

Why don't you use the English version of Wikipedia? In general the English pages contains more information than what other languages provide. * Elliptical galaxies. Some elliptical galaxies show evidence for dark matter via strong gravitational lensing, X-ray evidence reveals the presence of extended atmospheres of hot gas that fill the dark haloes of isolated ellipticals and whose hydrostatic support provides evidence for dark matter. Other ellipticals have low velocities in their outskirts (tracked for example by planetary nebulae) and were interpreted as not having dark matter haloes. However, simulations of disk-galaxy mergers indicate that stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories, explaining the low velocities even with a DM halo. More research is needed to clarify this situation. http://en.wikipedia.org/wiki/Dark_matter#Galactic_rotation_curves

A steady beam of light is not like a solid steel beam, it's more akin to a rapid stream of bullets from a machine gun. If a spaceship with a machine gun mounted in the front would continuously fire a stream of bullets ahead of the ship, then the ship would not catch up with the bullets and as such not travel into the stream it is fireing either. Photons in the beam are also increasing their distance from the ship.

From this thread: http://www.scienceforums.net/topic/50773-posting-pictures/

You can choose a reference point from where you measure a differential charge to another point in the circuit whereever you like, but usually it is the negative pole that is chosen as ground. In electrical engineering, ground or earth is the reference point in an electrical circuit from which other voltages are measured, or is a common return path for electric current, or a direct physical connection to the Earth. http://en.wikipedia.org/wiki/Ground_(electricity) Don't know much about lightning but here are two seeminly revelant quotes with links to different chapters on the Wikipedia page of lightning: Unlike the far more common "negative" lightning, positive lightning occurs when a positive charge is carried by the top of the clouds (generally anvil clouds) rather than the ground. (...) Positive lightning makes up less than 5% of all lightning strikes. http://en.wikipedia.org/wiki/Lightning#Positive_lightning As a thundercloud moves over the surface of the Earth, an electric charge equal to but opposite the charge of the base of the thundercloud is induced in the Earth below the cloud. The induced ground charge follows the movement of the cloud, remaining underneath it. http://en.wikipedia.org/wiki/Lightning#Leader_formation_and_the_return_stroke

I think meows was worried that the transmission could be inefficient...

Conventions A flow of positive charges gives the same electric current, and has the same effect in a circuit, as an equal flow of negative charges in the opposite direction. Since current can be the flow of either positive or negative charges, or both, a convention for the direction of current which is independent of the type of charge carriers is needed. The direction of conventional current is defined arbitrarily to be the direction of the flow of positive charges. In metals, which make up the wires and other conductors in most electrical circuits, the positive charges are immobile, and the charge carriers are electrons. Because the electron carries negative charge, the electron motion in a metal conductor is in the direction opposite to that of conventional (or electric) current. Reference direction When analyzing electrical circuits, the actual direction of current through a specific circuit element is usually unknown. Consequently, each circuit element is assigned a current variable with an arbitrarily chosen reference direction. When the circuit is solved, the circuit element currents may have positive or negative values. A negative value means that the actual direction of current through that circuit element is opposite that of the chosen reference direction. In electronic circuits the reference current directions are usually chosen so that all currents flow toward ground. This often matches conventional current direction, because in many circuits the power supply voltage is positive with respect to ground. http://en.wikipedia.org/wiki/Electrical_current#Conventions

Agreed, at least an resistor in series should have been allowed.

I am sorry Joatmon, I did not try to reprimand you, I apologize for being unclear with my first comment. I meant for everyone that reads this thread now, to be warned that this is an old thread, YT is not likely around anymore and the game has expired. Of course you are allowed to comment! Yes, you are fully correct, connecting a battery to a standard battery without something limiting the current will destroy it. I am NOT accusing you of anything, but AFAIK, the game was to build the battery, not the electronics needed to tune a LED to a power source. The goal was to make the battery weak enough to not blow the LED up, but yet be able to power it for a long duration.

First off, I better start with an resurrection varning, this is a very very old thread, consider yourself warned. Secondly, Joatmon, I think the challenge was about building a long standing battery without blowing up the LED.

There are two very different concepts that happen to be named very similar and cause a lot of confusion, they are called dark matter and dark energy: Dark matter represents the problem that stars inside galaxies orbit the center at higher speeds than they should be able to, unless there is a large amount of unseen mass increasing the force from normal gravity and helping them hold on to their stars at higher orbital velocities. Dark energy represents the observation that the universe on large scales, between clusters of galaxies, seems to be expanding at an accelerated rate, gravity should counter and slow down this rate, but measurements of redshifts from very distant supernovas reveals that the expansion is speeding up. As such the Milky Way and Andromeda galaxy are not "speeding up", although they are on collision course and will likely merge in the future. (The interaction of their mutual gravity are of course causing them to accelerate towards each other, but that is no "dark" mystery.) Photons don't speed up, they always travel at the speed of light through space. But photons carry energy which according to relativity also interacts with gravity, they both bend spacetime and follow the curved path of spacetime, just like ordinary matter. There is no true vacuum anywhere in the universe, there are tiny amounts of matter spread out sparsely everywhere and a large amounts of photons crossing all volumes of space, even if we would shield off a part of space it would still contain the zero point field with its fleeting particles popping in and out of existence. (In the context of lightspeed, you are correct that photons are not noticeably affected and travel freely.) Asking questions here or in other parts of this forum, is a very good way to increase your knowledge on whatever interests you at the moment.

Science Forums > Sciences > Physics > A forgotten theory of mass

I am not an expert on cameras or photographing but if you couldn't see it with your eyes then I would guess on an artifact made by the camera.

I think Tired Light is the concept you are asking about: Tired light is a class of hypothetical redshift mechanisms that was proposed as an alternative explanation for the redshift-distance relationship. These models have been proposed as alternatives to the metric expansion of space of which the Big Bang and the Steady State cosmologies are the most famous examples. The concept was first proposed in 1929 by Fritz Zwicky, who suggested that if photons lost energy over time through collisions with other particles in a regular way, the more distant objects would appear redder than more nearby ones. Zwicky himself acknowledged that any sort of scattering of light would blur the images of distant objects more than what is seen. Additionally, the surface brightness of galaxies evolves with time, time dilation of cosmological sources, and a thermal spectrum of the cosmic microwave background have been observed - these effects that should not be present if the cosmological redshift was due to any tired light scattering mechanism. Despite periodic re-examination of the concept, tired light has not been supported by observational tests and has lately been consigned to consideration only in the fringes of astrophysics. http://en.wikipedia.org/wiki/Tired_light

There is no need to download or install any software, LaTeX is a built in feature on this site, all you have to do is learn how to use it. You activate the LaTeX by using the [math] [/math] tags, here is a link: Quick LaTeX Tutorial

Yes, of course.

Gilded was last active in April 2011 and your quote is from September 2008...

Wikipedia is a very good resource to find answers to such questions and similar information: Measurement There are different ways to determine the value of c. One way is to measure the actual speed at which light waves propagate, which can be done in various astronomical and earth-based setups. However, it is also possible to determine c from other physical laws where it appears, for example, by determining the values of the electromagnetic constants ε0 and μ0 and using their relation to c. Historically, the most accurate results have been obtained by separately determining the frequency and wavelength of a light beam, with their product equalling c. In 1983 the metre was defined as "the length of the path travelled by light in vacuum during a time interval of 1⁄299,792,458 of a second", fixing the value of the speed of light at 299,792,458 m/s by definition, as described below. Consequently, accurate measurements of the speed of light yield an accurate realization of the metre rather than an accurate value of c. http://en.wikipedia.org/wiki/Speed_of_light#Measurement But if you want you can measure it yourself at home with a chocolate bar, microwave oven and a ruler. Link to mooeypoo's experiment thread: http://www.scienceforums.net/topic/31094-experiment-measuring-the-speed-of-light/

Yes, agreed.

Umm, imatfaal, maybe you should review the conversation from post #50...

No, AFAIK, Albert Einstein developed the theory of general relativity but he himself first thought that black holes could not form. Considering the exotic nature of black holes, it may be natural to question if such bizarre objects could exist in nature or to suggest that they are merely pathological solutions to Einstein's equations. Einstein himself wrongly thought that black holes would not form, because he held that the angular momentum of collapsing particles would stabilize their motion at some radius. This led the general relativity community to dismiss all results to the contrary for many years. However, a minority of relativists continued to contend that black holes were physical objects, and by the end of the 1960s, they had persuaded the majority of researchers in the field that there is no obstacle to forming an event horizon. http://en.wikipedia.org/wiki/Black_hole#Formation_and_evolution (Bolding by me.)

If you are writing a science fiction and not a scientific paper or trying to draw a blueprint with the intention to actually build such a travel device, then I don't think you have to worry to much about current scientific knowledge. There are plenty of successful science fictions where they are able to travel faster than the speed of light despite it is currently thought to be impossible to do so. You already have an alien visiting us from a far away world, of course that would be impossible unless they have access to more advanced technology than what we currently have. The only true limit for an fictional advanced alien technology is your own imagination. The aliens could have knowledge that we do not yet possess which supersedes ours and possibly even goes against what we currently knows. Depending on how you want your story they could have invented an expensive proccess to manufacture unobtainium, exotic matter or whatever strange material you like to use in their stargate to open a wormhole or some other kind of portal through space, alternatively if you want them to travel in spaceships the material could be used to craft the core in their engines or simply used as fuel which allows the jumpships to enter hyperspace or to power their warpdrive to superluminal speeds. My advice would be to make your own 'theory' similar to how people are used to view it in movies because then they will have it easier to accept it, so watch some Sci-Fi movies and choose how you want the travel to be performed and read up on the ideas behind in the movies and other similar ways. This Wikipedia article has a list of several well known sci-fi stories and their made up technologies to travel faster than light: Hyperspace is a method of traveling sometimes used in science fiction. It is typically described as an alternative region of space co-existing with our own universe which may be entered using an energy field or other device. Travel in hyperspace is frequently depicted as faster-than-light travel in normal space. Astronomical distances and the impossibility of faster-than-light travel pose a challenge to most science-fiction authors. They can be dealt with in several ways: accept them as such (hibernation, slow boats, generation ships), find a way to move faster than light (warp drive), "fold" space to achieve instantaneous translation (e.g. the Dune universe's Holtzman effect), access some sort of shortcut (wormholes), or sidestep the problem in an alternate space: hyperspace. Hyperspace is sometimes used to enable and explain faster than light (FTL) travel in science fiction stories where FTL is necessary for interstellar travel or intergalactic travel. Spacecraft able to use hyperspace for FTL travel are sometimes said to have a hyperdrive. Detailed descriptions of the mechanisms of hyperspace travel are often provided in stories using the plot device, sometimes incorporating some actual physics such as relativity or string theory in order to create the illusion of a seemingly plausible explanation. Hyperspace travel is nevertheless a fictional technology. http://en.wikipedia.org/wiki/Hyperspace_(science_fiction)