Why can nothing go faster than the speed of light?

February 14, 2008 4:24 AM

How does Einstein prove nothing can go faster than the speed of light and that Atoms exist?

I've never understood Einstein's theory's on light, How does he prove that the speed of light is Constant and that nothing can go faster than this speed?

And while we are on Einstenion theory's can someone also explain how Brownian motion of pollen seeds in fluid proves that atoms exist? If atoms are moving the pollen whats moving the atoms?

posted by complience to science & nature (20 comments total)

He doesn't. The constancy of the speed of light in all frames of reference is an axiom of the theory of relativity. This means it's one of the underlying assumptions from which he derives the rest of the theory. The axiom can be experimentally verified, but it is not proven in any theoretic sense.
posted by knave at 4:49 AM on February 14, 2008

Similarly, his statistical theory of Brownian motion shows that if atoms exist and have certain plausible properties, then we would expect to see Brownian motion. The common theme is that in each case he works out the math to show that certain axioms give results that are generally consistent with what we see in nature. It "shows that atoms exist" by supplying a good answer to the question, "WTF is the deal with Brownian motion? The atomic theory can't possibly explain it. There must be some kind of magic zig-zag device inside every pollen seed, right?"

As for "what's moving the atoms?", the answer is that the atoms are already moving, and thanks to our friend Isaac Newton, we expect them to (a) keep moving in the same direction when they don't hit something, and (b) bounce when they do. Their motion changes at times, but they don't require constant external forces to keep them dancing around.
posted by grimmelm at 5:04 AM on February 14, 2008 [1 favorite has favorites]

on preview I see some succinct explanations. But here's my WALL OF TEXT!

Here's my total layman's understanding of both things. I'm sure others will clarify or correct me where I might be wrong.

As Knave and grimmelm said, Einstein hasn't PROVED anything, what he did was come up with a fantastic structure that is elegant and consistent. Various predictions as to how light & gravity & such may act have proven to be true and it has withstood numerous attempts to prove him wrong. His theory says that nothing can more faster than light* and so far we haven't found anything that really disproves it. (and he was right on so many other things, we're pretty OK with saying that its true).

Anyhow, Einstein says that as you move faster and faster, your mass increases. This is because you (your atoms) have more energy and physical mass *is* energy in a sort of solid form (that's what the whole E=MC^2 thing is about. Energy = Mass * Speed of Light ^2). Since your mass increases as you go faster, its going to take more energy to make you go even faster. The slope of this is pretty gentle at first...we don't notice it in our regular scale world, but once you start getting to very HUGE objects or very FAST objects, hoo boy will you notice it! When you get right up to the light speed barrier, your mass is approaching infinite. It would take more than all the energy in the universe to get you across the barrier. Which, of course, is impossible.

Now aspects of this have been shown to be true. We have actually accelerated particles and have shown them to increase in mass as they go faster. We've also been able to explain irregularities in the orbit of Mercury through Einstein's theory. (Basically the huge amount of energy the sun puts out is equivalent to a big chunk of mass, which affects gravity, which explained the irregularity. Before Einstein's theory, we thought there might be yet another planet affecting Mercury's orbit).

Okay...Brownian motion. So obviously we can't really see atoms** in our day to day. We can only infer their existence from evidence. When Pollen seeds are floating in water, they move around. They do NOT move around in a nice smooth gradient. Oh no, they kind of look like they're getting jostled, first this way and then that way. Kind if like YOU would get jostled if you were surrounded by a bunch of hyperactive third graders. You're just trying to make it across the room! I mean JEEZ! Without those little bast...ummm....bumps, your trip would be smooth uninterrupted progress. With the atoms, however, you get bumped around. Assuming the existence of smaller particles doing the bumping neatly explained the motion that was being seen.

As to what is moving the atoms...Energy! Atoms are not a solid mass. They're tiny particles in constant motion, whipping about trying break apart but being held together by various forces. The old model was kind of like the solar system. Planets (electrons) orbiting the Sun (the nucleus). In reality, it is more more frenetic than that. A cloud of rapidly moving particles. The more energy (heat for instance) the more these particles want to get the heck away from each other. (turning liquids into a gas for instance..they bump and ricochet each other farther and farther away). The less energy, the slower they move (liquid to solid). At Absolute Zero, motion is absolutely zero! (basically).

* Tachyons are theoretical particles that do travel faster than light, but they can never slow down. I don't think they've been proven by experiment yet, but if I understand it correctly, Einstein's math allows for another set of particles that ALL travel faster than light, but can never really interact with our slower than light universe except under the most unique of circumstances. Its kind of like when some mathematician said, "Hey look, even if I assume more than 3 dimensions, Euclid's geometry stays consistent in this new framework." thus inventing non-euclidean geometry. (which helped Einstein with some tricky bits in his theory, iirc)

** we've actually sort of taken pictures of atoms now, so we don't need a stinkin' theory.
posted by Wink Ricketts at 5:13 AM on February 14, 2008 [3 favorites has favorites]

It was Michelson and Morley who proved that the speed of light is constant among other things.
posted by GuyZero at 6:28 AM on February 14, 2008 [1 favorite has favorites]

What's already been said. If you have the axiom that light always goes at light speed, you can't ever accelerate to that speed because any ray of light going in the same direction as you still has to pass you by at light speed. The same sort of thing applies if you start off faster than light and try to slow down, as with the tachyons Wink mentions.

So it stems from the principle knave says.
posted by edd at 6:29 AM on February 14, 2008

I read an interesting book that explained all this by Bill Bryson called A Short History of Nearly Everything.

Ok, I'm lying, I listened to it on audiobook, but still.

Ok, actually, I listened to half of it on audiobook, but still.

It does a good job of explaining all this business without requiring a doctorate in experimental physics.
posted by kbanas at 6:46 AM on February 14, 2008

Einstein hasn't PROVED anything, what he did was come up with a fantastic structure that is elegant and consistent.

Well, thats now how complex theories and frameworks work. If you want a simple "taking photos" answer you wont get it, especially in physics.

Most everything in science works on preditions. So if I come up with a theory about how bees act and it turns out I can predict bee behavoir then I can propose a theory on why and how this works. If my predictions are very good then that theory is very close to the reality of nature.

I think you need to accept that in fields that are more theoretical than then they are 'lab-based' you're going to have to starting accepting scientific theory as a way to predict the natural world, and the 'lab based' ideas of 'taking photos of atoms' dont apply as much. The 'lab' here is predicting the movement of the planets with x amount of precision.

Also its worth mentioning that thinking purely in prove/disprove is a problematic thinking. Work people like Einstein does is not exactly "Aha youre wrong, no youre wrong!" Its developing a big picture and seeing how well it can predict things.
posted by damn dirty ape at 6:49 AM on February 14, 2008

Hey look, even if I assume more than 3 dimensions, Euclid's geometry stays consistent in this new framework." thus inventing non-Euclidean geometry. (which helped Einstein with some tricky bits in his theory, iirc)

Uh, what? If Euclid's geometry were consistent with higher dimensions, it would still be Euclidean, not non-Euclidean. Non Euclidean geometry involves getting rid of the axiom about parallel lines, that they won't ever cross. In non-Euclidean geometry, parallel lines can cross.

(here is the wikipedia article)
posted by delmoi at 7:05 AM on February 14, 2008 [2 favorites has favorites]

It's also worth mentioning that he didn't pull the speed of light being constant thing completely out of his ass. The equations that describe how electric and magnetic fields interact, the Maxwell equations, very naturally give you light waves. The light waves move with, as you might expect, a speed based on a couple of measurable properties of free space (permittivity and permeability) that winds up being the speed of light. In fact, they can't travel at any other speed. If you want this equation to be valid no matter how fast you are moving, and what Einstein realized is that "how fast you're moving" is a question with no objective answer, it is clear that you have to give the speed of light a special place in your theory.
posted by Schismatic at 7:21 AM on February 14, 2008 [1 favorite has favorites]

Firstly I think this wikipedia article may help answer the bulk of your question, especially on the differences between general and special relativity. (General relativity allows FTL travel, but with theoretical anomalies)

When you take into account that everything is moving, there is no basis really on what "perfectly still" means. When an object moves near the speed of light within our reference frame, it usually causes odd anomalies when observed. If objects are moving FTL, they would consequently be difficult to observe because of the reference frame we are in....or in other words, they may observable if the point of observation is also moving in the same direction at less than light speed differential.

It's all different theories however with lots of math in between...I'm certain that we'll continue to surprise ourselves with new discoveries, the debunking of old theories, and introduction of new ones. Einstein was a brilliant man, but he only the information that was available at his time. So many new and important discoveries have been made since.
posted by samsara at 7:26 AM on February 14, 2008

It's also worth noting that Einstein wasn't comfortable with the theory of quantum mechanics, which is now largely accepted as a good way of explaining how sub-atomic particles work. Here's a Wikipedia article, which might be a bit dense, but is still a good overview.
posted by Guy_Inamonkeysuit at 7:55 AM on February 14, 2008

May I suggest Walter Isaacson's Einstein: His Life and Universe. I just finished reading it, and as someone who only passed high school physics because of a slightly off-kilter, ex-nuclear physicist teacher, I found the explanations of Einstein's theories to be tremendously easy to understand. Einstein came up with some incredibly simple thought experiments to illustrate his theories, and most are explained in the book. Not only a great source for understanding exactly the questions you're asking, but also a great read about an incredibly complex guy.
posted by undercoverhuwaaah at 7:55 AM on February 14, 2008

Damn Dirty Ape: I know that's how predictions and theories work. I was trying to explain just what you said (perhaps not clearly). Its one of those things that seems to come up often:

"Darwin/Einstein/Mendel PROVED this with the theory of X."

That sort of phrasing is being a bit loosey/goosey with the word proof and prove (imo). And I think that it is important to differentiate between explaining something / predicting something (sort of a logic/geometry/mystery novel Proof) and then finding the evidence to support the theory (prove it).
posted by Wink Ricketts at 8:07 AM on February 14, 2008

That made me think of this thought experiment: If you are traveling in direction X just a fraction shy of lightspeed (99.99999_%), how would a another vehicle be perceived if it is travelling at the same speed in the opposite direction? What would happen when the differential between these referential points is faster than light?

Now imagine instead of vehicles we are talking about the entire known universe and the relation of galaxies to one another. Could it be possible that there are entire galaxies traveling faster than the speed of light (or even a hundred, million, trillion times faster) in relation to our own? Granted they would be far outside our field of view....but it's an interesting thought (even with my limited knowledge on current science fads).

I think the root of the issue here is not whether its possible, but rather if it's observably (scientifically) possible. The rules that we have today only really only apply to how things can be observed from our point of reference....which makes sense as we are the ones trying to do all the predicting.
posted by samsara at 8:36 AM on February 14, 2008

The upshot of all of this stuff is that:

1) The math shows that it's mathematically possible
2) The portions of the theories that we can observe and prove scientifically fit.

We can only guess and assume that everything else fits too.
posted by gjc at 8:54 AM on February 14, 2008

'If you are traveling in direction X just a fraction shy of lightspeed (99.99999_%), how would a another vehicle be perceived if it is travelling at the same speed in the opposite direction?'
It be seen as travelling just a fraction shy of lightspeed towards you, albeit a very much smaller fraction than from the 'rest' frame.

'Now imagine instead of vehicles we are talking about the entire known universe and the relation of galaxies to one another. Could it be possible that there are entire galaxies traveling faster than the speed of light (or even a hundred, million, trillion times faster) in relation to our own?'

Two different kinds of movement involved here. There's 'peculiar velocities', which is the sort of thing your vehicle has, and there's the Hubble flow, which is something you want General, not Special Relativity for. Peculiar velocities act as in your question. The Hubble flow is somewhat different - it's the expansion of the universe. You can have the distance between two galaxies change in a given amount of time by more than the distance light can cover in that time, but that's not the same kind of movement as is important for the lightspeed limit. A good way to see this is that things never move past each other due to the Hubble flow, and you'll never see a galaxy go past a ray of light.

If you have your galaxy or vehicle in a race against a ray of light starting at the same point, the light will always win. If you have enough of a head start, and the space between the vehicle and the ray is expanding fast enough, you can keep your head start and never have the ray catch you. You never get to overtake a ray of light that way - that's the important thing.
posted by edd at 9:28 AM on February 14, 2008 [1 favorite has favorites]

s/the ray is expanding/the racetrack is expanding/ - although the ray of light does get stretched out at the same time, giving you your cosmological redshift.
posted by edd at 9:32 AM on February 14, 2008

If you have enough of a head start, and the space between the vehicle and the ray is expanding fast enough, you can keep your head start and never have the ray catch you.

Achilles is in a whole new world of hurt trying to catch that tortoise these days!
posted by jamjam at 9:54 AM on February 14, 2008 [1 favorite has favorites]

Don't forget The Mechanical Universe on Demand... 52 episodes of physics that pretty much covers all this stuff with nice pretty graphics and easy explanations.
posted by zengargoyle at 11:49 AM on February 14, 2008

The key insight of Relativity is that there is no standard frame of reference. Think of measuring the velocity of a spaceship moving away from you faster than the speed of light. Since we cannot instantly perceive a distant object, but only perceive the light traveling from it, our measurements would be strange indeed.

Another observer watching the ship approach faster than the speed of light would have a completely different idea of how the thing was traveling. Not just the speed of the ship, but its size would be different. If you could zoom in with a telescope on the people on board, you would also perceive that the flow of time is different for them.

All this is because light is essentially the medium in which our measurements are bound, and since science is highly concerned with things that are observable and testable, it starts making sense to make the speed of light your reference point, rather than the size of space, or the speed of time (1 second per second last i checked).

Another useful way to think of it - if you started slowing down the speed of light, then the light rays from a receding object would take longer to reach your eye. How could you tell whether the object was accelerating or the light was slowing?
posted by 31d1 at 6:48 PM on February 14, 2008