The speed of a shadow is therefore not restricted to be If the wall is very far away, the movement of the shadow will beĭelayed because of the time it takes light to get there, but the shadow's speed is still The speed can even be much faster than this if the wall is at an angle to yourįinger's motion. Shadow's speed will be multiplied by a factor D/d where d is theĭistance from the lamp to your finger, and D is the distance from the lamp to the If your finger moves parallel to the wall, the Using a nearby lamp onto a distant wall and then wag your finger, the shadow will move Relativity?) In this case the relative speed is actually about 0.88c, so this The FAQ entry How do You Add Velocities in Special Speeds must be added using the relativistic formula for addition of velocities. Rocket B observes his distance from A to be increasing. Rocket A relative to rocket B is the speed at which an observer in However, this is not what is normally meant by relative speeds. Greater than c can be observed by a third person in this way. My frame of reference is increasing at 1.2c. If a rocket A is travelling away from me at 0.6c in a westerlyĭirection, and another B is travelling away from me at 0.6c in anĮasterly direction, then the total distance between A and B as seen in The CherenkovĮffect is thus not considered to be a real example of FTL travel. When we discuss moving faster than the speed of light, we are really talking aboutĮxceeding the speed of light in vacuum c (299,792,458 m/s). See the FAQĮntry Is there an equivalent of the sonic boom for The medium, and Cherenkov radiation is produced as a result. Possible for particles to travel through air or water at faster than the speed of light in Refractive index of the medium (1.0003 for air, 1.4 for water). Such as water or glass, light slows down to c/n where n is the Travels at a speed c which is a universal constant (see the FAQ entry Is the speed of light constant?), but in a dense medium One way to go faster than light is to make the light slow down! Light in vacuum The sections are numbered so that they can be referred Many of these things are discussed in more detail elsewhere in the FAQĪnd hyper-links are provided. Finally, we will look at some of the more serious proposalsįor real FTL. Think about what we mean by non-trivial FTL travel/communication and examine some of theĪrguments against it. When FTL is being discussed, and so they are necessary to deal with. Mentioned not because they are interesting, but because they come up time and time again Possible, then causality would probably be violated and some very strange situations wouldįirst we will cover the trivial ways in which things can go FTL. Inevitable that one day technology will enable us to go faster than light. It was inevitable that someone would one day succeed inįlying faster than sound, once technology got around the problems. The truth is that some engineers once said that controlled flightįaster than sound might be impossible, and they were wrong about that. It was known that rifle bullets go faster than sound long before an aircraftĭid. Now they say no-one will ever go faster than light."Īctually it is probably not true that anybody said it was impossible to go faster than It is sometimes objected that "they said no-one would ever go faster than sound and The foreseeable future), but it does cover some of the more common points that are This article is not a fullĪnswer to the question (which no doubt will continue to be discussed in the newsgroups for On the other hand, there are also good reasons to believe that realįTL travel and communication will always be unachievable. In actual fact, there are many trivial ways in which things can be goingįaster than light (FTL) in a sense, and there may be other more genuine It might be thought that special relativity provides a short negative answer to this The moon revolves round my head faster than light! Is Faster-Than-Light Travel or Communication Possible?
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