Lois & Clark Forums Forums General Discussion Off Topic Nerd overload or How Did einstein Figure Out That Nothing Moves Faster Than Light?

Well, this is a favorite story of mine, and since I'm feeling nerdier than usual, here goes.

How did Einstein figure out that nothing can move faster than light? Well, he started by imagining that he was riding a beam of light. Here is a beam of light:



This is a beam of light from a newborn star:


This is a jet of matter from a newborn star. As you can see, it ends very abruptly:



The reason why the jet ends so abruptly is twofold: first, the jet has run into some rather dense interstellar nebulosity which caused it to slow down precipitously. But second, this jet of matter also turned on quite abruptly. Therefore is has a distinct, sudden beginning. A front end, if you will. A prow! A front bumper! And so it is with beams of light, too. They, too, have turned on at a very specific moment, and they, too, have a front end. All right, all right, of course this front end of a light beam will be diluted and spread out, but but the front of it will be there, nevertheless.

Now, Einstein imagined that he was not only riding the beam of light, but that he was sitting at the front end of it, dangling his feet.




Now Einstein imagined that he was going to shave, as he was riding the beam of light. Imagine that he needed a mirror to see what he was doing:



Ah, but - here's the trick question! Would he be able to see his own face in the mirror? Rmember that he is riding the beam of light, so he is moving just as fast as the light itself. And he is sitting at the very front end of the beam of light. He is holding the mirror a few centimeters ahead of the front of the beam of light. Can he see his own face in this mirror?



For us to be able to see ourselves in the mirror, light must bounce off ourselves, go to the mirror, and reflect back to our eyes, so that we can see ourselves. But if we ourselves are moving at the speed of light (as we would have to be doing if we were riding a beam of light) then the light that reflects back from the mirror must overtake itself and move faster than the rest of the beam of light in order to catch up with the mirror, which is held a few centimeters ahead of the front end of the beam of light!

Einstein concluded that if you could really ride a beam of light and sit at the front end of it, you would not be able to see your face in the mirror which you held in your hand. Watch, an empty mirror!



It took Einstein several more leaps of logic to conclude that the speed of light is constant, and a person can't move as fast as light (and therefore can't ride a beam of light), and nothing at all made of matter can move as fast as light. Me, I just love the idea of imagining myself riding the beam of light, trying to see myself in the mirror, realizing I would not be able to see myself and concluding that - wow, e = mc2 and all the rest, you know. Ah, nerd overload!

Ann

That's one of the best explanations I've ever run across, Ann. Thanks!

The e=mc2 equation is also the reason we can't travel at light speed in the natural universe. In order to do so, we'd have to expend infinite energy, and in order to use any kind of energy, we have to release it from some kind of matter. Because the universe (and therefore the supply of matter within it) is not infinite, we don't have an infinite amount of energy available to us.

On top of that, our subjective time frame would be absolutely unmoving and still. Even if we could travel at light speed, we wouldn't be aware of it.

Love these discussions. I make my daughter crazy talking about this kind of stuff.

Why couldn't my teacher in high school make it sound so good? Go you!

The jury is still out on whether or not the universe is infinite, Terry. At least my astronomy magazines tell me so!

Back when I became interested in astronomy, in 1969 (the same year I became a diehard Lois and Superman fan! ), astronomer's model of the universe was quite simple. They thought that there were basically only two forces acting on the universe: one, the universe's own, demonstrable expansion, and two, the universe's own intrinsic mass, which exerted a lot of gravity on the universe and slowed its expansion down. (As for the cause of the expansion itself, astronomers assumed that all of this outward motion originated in the "explosive" birth of the universe, known as the Big Bang.)

When this old simplistic universe model reigned, there were really only two possible fates for the universe. If it was not massive enough to fully halt its own expansion, then the universe would keep expanding forever. In this case, the universe was considered infinite. However, if the universe really was sufficiently massive to stop its own expansion and reverse it into contraction, then this contraction would inexorably make the universe fall in on itself and collapse under its own prodigious weight. In this case, the universe was considered finite.

Of course, since then astronomers have found that the expansion of the universe is not slowing down at all, but rather picking up speed, which is impossible if nothing but good old gravity is acting on it. So astronomers have concluded that there is indeed another force at work, which they have dubbed dark energy. This dark energy works as a sort of antigravity, making the universe spread out rather than concentrate, but otherwise astronomers haven't got a clue what it could be. Anyway, the current model of the universe looks something like this:



As you can see, astronomers foresee an exponential and eternal expansion of the universe, in which case the cosmos we live in can certainly be considered to be infinite. On the other hand, quite a few astronomers argue that the unexplained acceleration of the universe proves that we really don't know what is going on. Existing mathematical models can't explain the nature of the driving force behind the acceleration. Therefore, some astronomers say, we can't foresee the future behaviour of this mysterious force, and for all we know it might just as well reverse its own "charge" some time in the future and start acting as a strong contracting force instead. Well, who knows?

I have a little more to say about why nothing made of matter can move at the speed of light, but it will have to wait for a future post....

Ann

I talk too much, I know , but I just have to say a little something about what happens when you try to accelerate anything made of matter to speeds approaching the speed of light.

In order to make something accelerate, you have to give it a "push". This push contains energy. When an object isn't moving at all, or moving very slowly, all of the energy of the push will cause the object receiving the push to move faster.

But now imagine that you've kept pushing and pushing this object so that its speed has increased prodigiously. As the object's speed increases, each new push becomes less effective at imparting more speed to the object. For simplicity's sake, let's forget about such things as friction, and assume that for a non-moving object, 100% of the energy of a push will add more speed to the object. But as the object moves faster and faster, less and less of the energy of a new push will actually increase the object's speed. More and more of the energy of each new push will be converted into something other than speed. What is that?

The well-known formula, e=mc2, gives us the answer. Energy, mass and speed are interconnected. The faster something is moving, the less an added push of energy will increase the object's speed. What happens, instead, is that more and more of the added energy will increase the fast-moving object's mass!

This effect has already been demonstrated in particle accelerators, where particles of matter have been made to move at speeds close to the speed of light and then collide. The "debris" this collision gives rise to proves that the particle was clearly more massive than this kind of particle "ought" to be. Purely and simply, the fast-moving particle has gained weight (or at least mass)!



Particle tracks that resulted from a collision inside a particle accelerator. The shape and number of these tracks tell particle physiscists a lot about the properties of the particle that collided as well as of the elementary particles that were produced in the crash.

But the speeds we humans have managed to impart to large objects such as cars, airplanes or spacecraft, are just a minute fraction of the speed of light.



The spaceships we have built don't move fast. Sorry.

Therefore, the mass increase even for our fastest spaceships is so small that the increased mass almost certainly isn't even measurable with our man-made scales. Yet the mass increase is there. And if we were to build very much faster spaceships the mass increase of these ships as they move at very high speeds would become a real problem. This is also why it is impossible to build a spaceship that moves just as fast as light. As you add more and more and more energy to make the ship move faster and faster and faster, you will eventually just make the ship become more and more and more massive.



So if you think these ships here could move as fast as light, or faster... well, it just can't be done, according to Einstein.

Then again, who knows what will happen in the future? Many astronomers today suspect that Einstein didn't get everything completely right. Maybe scientists will really find a loophole some time in the future that will allow us to move as fast as light. (But boy, will that require a lot of energy! Wheew!)

Anyway, I found a cartoon which I suspect I will not be allowed to post here, so I'll describe it. A woman is trying on a new dress, and she is looking at herself in a mirror. And she is asking the store assistant the following question: How come they are making size 16 smaller these days?



Ann