Imagine also that the valve separates this container from a second container that is completely empty. Imagine that, rather than turning on a light, our switch connects to a valve for a container of air (Figure 9.19). If you keep going even faster, you seem to be able to make the light turn on before the switch is flipped!Īnother example might be even more instructive. But, in the case we are studying, the two events actually occur at exactly the same time. A real light switch seems simultaneous to us because the delay is so short. In the last activity, you should have noticed that if you increase the speed of the observer enough, you can make the flip of the switch and the lighting up of the bulb simultaneous. We can explore when the other events occur. Then Event A will also occur at the origin for the moving observer, and so the switch is flipped at t' = 0. But, what about for a moving observer? Imagine that an observer comes flying past, moving in the x-direction, and passes you just as you flip the switch. The other two events occur only 9 seconds after. We will call this Event D.įor an observer at rest, Event B occurs 15 seconds after the switch is flipped. ![]() This is farther than light could travel in the same amount of time. The final event to plot should occur at a point with x = 15 and t = 9 at the speed v = 5/3c, the signal would travel 15 light-seconds of distance in 9 seconds of time.The signal will travel 3 units of space in 3 units of time, so an event at x = 9, t = 9 can represent it. Next, plot an event to represent the case where the signal travels at the speed of light.This signal can travel 3 light-seconds of distance in 5 seconds, so an event at x = 9, t = 15 can represent this case. Now plot an event, assuming that the signal from the switch travels to the bulb at 3/5 the speed of light.It will be easier to understand this example if the first event (call it A), flipping on the switch, occurs at the origin.To begin, use the Spacetime Diagram Tool to plot several events. We will compare this situation with signals moving at the speed of light and slower than the speed of light.ġ. Imagine the following thought-experiment: We flip a switch, and a signal is conveyed to a bulb at a speed 5/3 the speed of light-nearly 70% faster than light could transmit the signal. With that in mind, we will examine the causal relationships between two events as observed in two different reference frames in uniform motion. To reverse the order of causally connected events is easily seen to be completely absurd. There is nothing in the laws of physics, not the ones we have discussed so far, that would prevent these "backwards" events from happening. These sorts of events never happen in the reverse order. ![]() Conversely, we never see milk collect itself from a puddle on the floor and arrange itself into a glass, as the glass itself spontaneously assembles from scattered shards. For instance, you flip on a light switch and then the light begins to stream from the bulb you push a button on your iPod, and the music begins playing a glass of milk slips from your hand, and then it falls to the floor and shatters. ![]() This is just the notion that some events are the causes of other events. We are familiar with the idea of causality in our daily world. We will perform another thought-experiment to understand why this is so. However, other even more fundamental reasons explain why particles cannot exceed the speed of light. The discussion in Section 9.6 has, we hope, helped you to understand why no particles can be accelerated from our frame of reference to a speed equal to (or in excess of) the speed of light: Sufficient energy is not available to move any material particles to such a speed. What Do You Think: Traveling Faster Than Light?
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