A Physics Quiz of a different type
 

Last night, the neighbors came over for a visit, and (somehow) we got on the topic of Relativity. One neighbor, a non-science kind of guy (but very intelligent) wanted to know why you can't go faster than the speed of light.

My response was that at one end of E=MC^2 you have pure mass, and at the other end you have pure energy. You can't go past the endpoints. As you go closer to C, M decreases and E increases, since C is decreasing*

*That is, since C is relative to a fixed point, as you hit, say, 0.75 C, there is only 0.25 C "left", so C is actually smaller.

OK, he accepted that as reasonable , but in my mind, that's not right.

So, in one or two sentences, why is there a limit to the speed of light?

Thanks.

Re: A Physics Quiz of a different type
 

From Wikipedia:

Re: A Physics Quiz of a different type
 

This is only a theory. Some scientist believe that you can go faster than the speed of light, we just have no idea how to do it. A particle accelerator in Europe actually detected what they believe to be partials that can travel faster than the speed of light. I personal think it will be possible to go faster than the speed of light in the future.

Re: A Physics Quiz of a different type
 

Are you talking about the neutrinos a month or so back? There was a lot of healthy speculation about that. I heard that was finally shown to be false, they got the timing between the start and endpoints wrong. They had used GPS for the timing and their error was something like they had forgot to completely take general relativity into account for calculating the time (the relative time was different because the satellites were moving fast)
Someone please correct me if I'm wrong or show a link or reference if I'm right. I can't find where I heard that.

I find the Wikipedia explanation pretty good. It should be noted what the gamma factor is (the funny y). The Wikipedia article refers to how it approaches infinity, etc., so it may be helpful to see what it actually is. It often (or almost always) pops up in the math for these types of equations, so they made a variable for it.



Oh, here's a joke. The tachyon is a hypothetical particle that would move faster than light. So...
The barkeeper says, "Hey, we don't serve tachyons here," and a tachyon walks into the bar.

Re: A Physics Quiz of a different type
 

It's fine to believe something, if you have a good reason for it. Here you just need to be a little more careful adding up the facts. Even the experimenters who published those results are skeptical, and are calling for independent replication of the results. (Wikipedia has a fantastic article on those results and their implications.) I wouldn't bet on the original results being upheld.

That's not to say we haven't been wrong about our understanding of the universe before—only that our understanding of the universe today is several orders of magnitude greater than in any other era of history, and because of this, experimental error is the more likely explanation for the results you mention.

Besides, any application of faster-than-light anything is still firmly within the realm of speculation (or daydreaming)—in other words, I don't think there's any good reason to believe "it will be possible to go faster than the speed of light in the future".

Also, "only a theory" doesn't really mean what you imply. Wikipedia has a dense article on theories, but the important concept is that "theories are collections of hypotheses that are logically linked together into a coherent explanation of some aspect of reality and which have individually or jointly received some empirical support". In other words, calling something a theory doesn't call it into question so much as it indicates that there has been systematic scientific inquiry into the subject.

Aside: 4 citations to Wikipedia in 4 replies? That must be a record. (And it's not a bad thing.)

Re: A Physics Quiz of a different type
 

I'm no expert mathematician, but I believe Don said he wanted an answer in "one or two sentences", and nobody has provided anything less than a paragraph related to the subject.

Being a high school student that has not taken physics yet, this question is very interesting, and I would also appreciate a short and concise answer, instead of having to read 4 long paragraphs.

In conclusion, I would very much be interested in a short answer as to WHY it isn't possible to go faster than the speed of light.

Thanks!

"In order for knowledge to be understandable, it must be accessible. We live in a world today where people don't want to read long books to learn things. Bring the knowledge to the people, and they will learn it". - Sal Khan, Khan Academy

Re: A Physics Quiz of a different type
 

Yeah, you're right on the simplification part.
The Wikipedia article mentioned this, I'll try to sum it up.

It takes an increasing amount of energy to approach the speed of light, in a way that you would need an "infinite" amount of energy to travel at the speed of light.
The physics of relativistic motion, which deal with speeds comparable to the speed of light, show that the mass (or relativistic mass) increases as you approach the speed of light and the energy of the object approaches infinity. So, it takes more energy to move something more massive, and infinitely so when approaching the speed of light.

Re: A Physics Quiz of a different type
 

Cool!

Re: A Physics Quiz of a different type
 

Particle "A" is traveling at c/2 in the +X direction in inertial reference frame XY. Particle "B" is traveling at speed c/2 in the -X direction in the same reference frame. See Figure 1.

In a second inertial reference frame X'Y', particle "B" is at rest. How fast is particle "A" traveling in this second reference frame? See Figure 2.

Re: A Physics Quiz of a different type
 

This brings up an interesting situation. If you were traveling at the speed of light with a flashlight in your hand... When you turned it on would no light be emitted? If you are at half the speed of light and turn it on, does the light leave at half the speed of light?

Re: A Physics Quiz of a different type
 

See answers in embedded in red below:

The answer to your question hinges on the same principle as the answer to the question posted here one hour previously.

Re: A Physics Quiz of a different type
 

Wouldn't it be slightly slower than c because of the air and lens of the flash light. :yikes:

Re: A Physics Quiz of a different type
 

I thought Al was in outer space where there is no air.

Re: A Physics Quiz of a different type
 

Still has a flash light with a lens, although I suppose he could remove it.

Re: A Physics Quiz of a different type
 

Of course you still have the glass of the bulb...