MadSci Network: Physics

Re: Does light really move?

Date: Fri Feb 4 14:48:25 2005
Posted By: Jeff Yap, Physics Teacher
Area of science: Physics
ID: 1102456893.Ph

Hi Chesley,

Wow. That is one brain shaker of a question. I really applaud your question, because the kind of thinking you're doing right now (trying to make sense of the new stuff you learn by combining it with the stuff you already know) is how we’re going to eventually get a unified theory of physics. The biggest problem with all of this is that we (as a species) may not be intelligent enough to comprehend this all. I know I’m not. So we’re going to be ice-skating uphill on this one.

To rephrase (and get the rest of the class caught up), Chesley is basically asking us to define "time" and "movement" from the perspective of light. Relativity maintains that as a mass (hint!) approaches the speed of light, time dilates (slows down) and space contracts (the mass will get shorter in the direction of movement) from the perspective of an exterior observer. However, from the perspective of the mass, time is going normally, and the size is constant. This leads to predicaments such as the twin paradox. (Here's a tip: Anytime people come up with something that leads to a "paradox", they haven’t completely figured it out yet...)

Anyway, since light is traveling at the speed of light (300,000,000 m/s) it should have absolute time dilation, and length contraction. So a beam of light should be traveling at zero speed, and either it has no length, or worse yet, the distance it is traveling is zero and it’s in both places at once. So IF light is affected by relativity, it doesn’t move at all, and it's everywhere at once. That doesn't make ANY sense, because I know there's no light under the basement stairs, and I'm scared of the dark in there. So the conclusion must be that light does not follow relativism. There's two oft used reasons for this: 1) Light has no mass, and only objects with mass are affected by relativism, and 2) Because it has to. Seriously, at a certain point, relativistic concepts don’t work any more. We go back to standard Newtonian model, where things are where they appear to be and your watch says exactly what mine does, no matter where or how fast you’re moving. As far as we’re concerned, light behaves according to standard non-relative motion. It doesn’t travel "instantaneously", but rather it travels so quickly (again, 300,000,000 m/s) that we cannot perceive or see the time pass between events. Once we accept that, we’re off the hook from entanglement and space-time disturbances.

That last part of your question is basically a different question entirely. String Theory basically states that if we consider sub-sub-sub-atomic particles as strings instead of particles, then we can account for both quantum and relativistic theories. Most people, myself included, have problems with quantum theory and relativistic theory, so a theory that handles both is quite a mental challenge. The authority on the matter, Brian Greene, explains it very well in the PBS series, which is free to view. The Event Horizon is the border past which things are unable to escape the gravitational pull from a black hole. Weird physics takes place at and beyond this border, including relativistic effects on light (Hey! We just said that wouldn’t happen!) and light that can be considered "standing still". Dark Matter is an idea that some astronomers use to explain the gravitational forces that we see in fast moving galaxy clusters. The mass that we can detect cannot possibly account for the forces that are present that hold those galaxies together. Therefore, there must be mass that is completely un-detectable by us. This mass is NOT black holes. We can detect black holes because they produce radiation, so dark matter and black holes sound similar, but they are unrelated. The concept of dark matter is still under debate. Lastly, the universe is either expanding or contracting, and while we’re not sure which, practically everyone agrees that it's doing one or the other.
Whew. That’s a lot of geek speak. As far as we know, strings and black holes are not related to each other, so strings are not a mathematical description of event horizons. Neither strings not black holes help to explain dark matter.

It is worth mentioning that a lot of these ideas are just working models. They’ll work in some instances, not in others, but for all intents and purposes, they will explain what happens in the real world. Within your lifetime, we will probably come up with ideas that will make the “truths” that you learn in science class seem like flat earth stuff. That doesn’t get mentioned nearly enough to students.

Lastly, Heisenberg is driving along and gets pulled over by a police officer. The officer asks, "Do you know how fast you were going?" and Heisenberg replies, "No, but I can tell you where I am."

I hope this helps!
Jeff Yap
Mad Scientist

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