What is the theory of relativity?

Factual Questions
1

What is the theory of relativity?

Also, please explain this passage. This passage was found when I was reading about gravity in another article:

In the late nineteenth century astronomers struggled to tease out a tiny anomaly in the orbit of Mercury–an aberration that would never affect a calendar or the prediction of an eclipse. They measured it just because it was there, with no inkling that it would soon emerge as the sole experimental anchor of a revolutionary new conception of space and time–the general theory of relativity.

Please help me.

2

I can’t offer you a simple explanation of the theory of relativity, but I can explain a bit about the passage. Newton’s inverse square law of gravity couldn’t quite explain the “anomaly” in Mercury’s orbit. Einstein’s inverse “square plus epsilon” law could. By “square plus epsilon”, I mean that the exponent of distance in Einstein’s model is slightly greater than two. So that’s that part.

Standard disclaimer: I’m not a physicist, I just wanted to be one once upon a time. If somebody says I’m wrong, they’re probably right.

3

In Newtonian physics you have relativity of position and time. The laws of Newtonian physics do not change when you change position or time. Relativity takes this and extends it to relativity of velocity (Special Relativity) and relativity of reference frame (General Relativity). Newtonian physics agrees with relativity when velocities are much smaller than light and away from strong gravitational forces. Newtonian physics fails to account for the precession of Mercury’s orbit. Relativity accurately predicts this precession.

4

The best thing to do is to find a popular science book about relativity; there are scads of them out there (and I can’t recommend any of them because I haven’t read any). That said, the basic idea of general relativity is pretty simple. To actually USE GR can be quite tedious and complicated, but the ideas are fairly straightforward.

Roughly speaking, GR is what we call a “metric theory of gravity.” Helpful, eh? What this means is that in GR, gravity is only a manifestation of the geometry of spacetime. Basically, matter tells spacetime how to distort, and the distortion of spacetime tells matter how to move. The old rubber blanket analogy gives you some idea of this, but like all analogies with GR, it’s somewhat flawed.

This is about as well as I can do while being brief and avoiding math, but hopefully Chronos (a relativist) or someone who actually works in the area will be by to give a better description.

5

I don’t think that you can put relativity in perspective until you have a good handle on Newtonian models and what intertial frames of reference mean. Still, this might be a good resource. http://www.math.ucr.edu/home/baez/physics/relativity.html

6

There are two theories of relativity. The “special” theory and the “general” theory. Special applies in only one kind of situation, and general applies in all situations.

The very best book you could pick up on the subject is entitled “Relativity” by Albert Einstein, written for the general public. It covers special relativity. General relativity is just too weird for even Big Al to explain it to the general public.

I’m no physicist, but I play one at home. You will learn why the orbit of Mercury “progresses” (I’ve forgotten since college those many, many years ago), you will learn that gravity affects waves of light too, the paradox of the twin astronauts, and why “c” is the universe’s speed limit in works of non-fiction.

Hawking’s a Brief History of Time is good too.

But none of this is anywhere near as weird as quantum mechanics. The quantum stuff is way beyond me. On the other hand, Feynman was not as clear as Einstein.

7

In a nutshell…

SPECIAL RELATIVITY explains that distance and time are not absolute. They depend upon the motion of the observer compared to the observed.

GENERAL RELATIVITY explains how gravity and “spacetime” work. Gravity is a curvature of spacetime. There’s also the Equivalence Principle (gravity pulling in one direction is equivalent to an acceleration in the opposite direction).

There is, of course, much more to it.

8

I suggest renting and watching Carl Sagan’s Cosmos. He explains the theory in quite understandable terms. It is a fascinating documentary.

9

I think it had something to do with women and money being interchangeable.

The equasion is E=mc^2, + a constant, where E=the amount of estrogen in your life, m=money, and c = chocolate.

I think the constant is nagging. But it might be begging. That depends on the position of the observer (special relativity.)

10

FWIW you can experience Special Relativity in everyday life. Or rather, you can get a sense of it.

Go sit in a train in a train station that has other trains lined up and waiting (such as you would find on the Metra trains at Union Station in Chicago). Pick a seat where all you can see out the window is the train next to you. Eventually either you or the other train will start moving out of the station. The thing is (assuming your train didn’t lurch when it started) you can’t tell if it is you or the other train that is moving. This isn’t a perfect experiment (since acceleration is involved) but it gives you an idea of what Einstein was talking about.

If you were in space, with no external reference points (such as a nearby planet), and another spaceship was speeding towards you there is NO experiment you could do to determine who is actually moving. It is just as reasonable to say you are standing still and the other guy is moving as it is to say the other guy is standing still and you’re the one moving (or you are both moving).

You can again see something like this by standing at the edge of a highway. If you did you’d see cars whizzing by at 60 MPH or so. If you’re in a car and going 60 the guy passing you at 65 seems to YOU to only be moving at 5 MPH. Granted, while in a car, we happen to know everyone is moving along at a good clip but again, if you removed all external reference points (such as the road and signs and everything) all you could say for certain is that the car passing you is going 5 MPH faster than you in the direction you are travelling OR that he is standing still and you are going ‘backwards’ at 5 MPH OR that you are both moving at some speed that when combined gives a 5 MPH closure rate.

Notice, all of these measurements are relative to the observer and each observer’s measurements are just as valid as any other observer. In short, there is no absolute motion (is the car going 60 MPH or 5 MPH…depends who you ask) and no preferred frame of reference (you are a frame of reference…are you in the car or on the side of the road watching the cars?).

11

When I attempt to explain relativity to non-technical people, and there is time for the long version, I begin with the wave nature of light, and lead up to the point where it becomes obvious that there was something wrong with our old picture of the universe.

A few hundred years ago (like around the time of Newton), scientists didn’t know the nature of light, but they visualized it as a particle, since it goes in straight lines. But then someone did the two-slit experiment, and discovered that light behaves as a very small wave. A guy named Michelson invented something called an interferometer, which was just a way of measuring the interference of light, and it would let you measure distances very precisely by counting the number of light wavelengths between two points.

No problem so far. But then people started pondering, if light is a wave, what is the medium that’s “waving”? For example, a water wave is just water that’s moving in such a way that we see a wave pattern. The wave itself doesn’t exist as a separate entity, it’s just a the collective behavior of water. Also, when football fans do a wave in a stadium, the wave is just a pattern of individual people moving in a collective way. You can’t have a wave without something there that’s doing the waving.

So with light, what is it that’s waving? It must be something really hard to detect, since we had never observed it. They gave this unknown thing a name - the “aether”, which is from the same Greek work that gives us “ethereal,” so it was a word that meant something is there, but there’s not much to it.

Still no problem, other than that no one had separately observed the aether. But then someone had the idea to detect the action of the aether using Michelson’s interferometer. If you have a fluid that’s moving, such as a river, the speed of a wave will be faster (relative to you) in the downstream direction than upstream, because the speed of the wave in the medium is constant, and the medium is moving with respect to you.

So a guy named Morley did an experiment where he measured the upstream speed of light versus the downstream speed. He measured it once in his lab, then waited six months until the Earth was going the opposite direction in its path around the Sun, and measured it again. The interferometer was accurate enough that he would be able to detect the difference. But he found none! He presented his results, and no one believed him. So he got help from Michelson himself, and they made improvements to the interferometer design so that it was way more accurate than what would be needed, and they got precisely the same speed!

The whole scientific world was in a quandary. Since the time of Newton, they had been nailing down everything they needed to know about how physics works. They were just polishing off a few last details. Then this Morley guy comes in and made precise measurements that showed that their Newtonian understanding of physics was fundamentally wrong.

So if Newtonian physics was wrong, what was right? What kind of a universe could explain the Michelson-Morley experiment? The speed of light seemed to be constant with respect to each observer! It just didn’t make sense. Along comes a physicist with very good math skills named Albert Einstein, who took as a first principle that the speed of light is constant for each observer, and worked through the heavy math to describe such a universe. It was pretty bizarre, suggesting things like time going at different rates in different locations, and describing space and time as interrelated, and curving.

But every experiment to test this bizarre theory has confirmed it. It also explained the bit about Mercury’s orbit. It’s explained everything since, so it’s bizarre, but apparently correct.

One last point - Einstein also participated heavily in the development of quantum mechanics, which is so bizarre that it makes relativity look unremarkable. The problem is that no one has been able to come up with a model of the universe that accounts for relativity and quantum mechanics at the same time - they aren’t really compatible. So there will be at least one more change to propose a system that puts them together. This is the Holy Grail of physics right now. You hear theories of super-strings, or branes, and that’s what they’re trying to do.

12

I forgot to mention but it should be noted that things start to get strange when you start approaching the Speed of Light (the ‘C’ in E=MC[sup]2[/sup]).

A few things happen as you approach a significant percentage of the speed of light (I htink 85% is where effects might become a bit noticeable but they really kick in above 95 or 98% or so…Chronos could tell you for certain). Note: All of the things I am about to mention happen when you are moving at any speed but the effects are so miniscule as to be practically zero.

For starters your mass increases (while slightly misleading you basically get heavier). If you actually managed to hit the speed of light your mass would be infinite. Since it takes an infinite amount of energy to move an infinite mass (maybe) this is one of the reasons actually going the speed of light is impossible.

Next off time slows down for the person moving. This leads to some really weird issues. For instance, if I sent you on a ride to the center of our galaxy at something like v=0.9999999999999999999999951 c (very nearly the speed of light) it would take, as perceived by the traveller, about 6 seconds to go there and back. Not too bad huh? Unfortunately everyone you knew on earth would be looonnnggg dead by the time you returned (on the order of over 52,000 years dead). Again, if you could actually go the speed of light time would stop which is another problem with going that fast. If you hit the button on your super-duper light speed travelling spaceship to go that fast you’d never hit the button to stop again before the Universe ceased to exist.

Yet another issue when going that fast is your rulers shrink in the direction of travel (they get shorter). Space also seems to contract in the direction of travel. If you actually went the speed of light the Universe would contract (to you) to a singularity (a point…[or would it be an infinitely thin line that you are travelling perpindicular too?]). Essentially this would allow you to be everywhere at once.

I hope that doesn’t confuse the issue too much but I think it is some of the fun stuff when contemplating this sort of thing. If some of these things seem too outlandish you’ll need to study the math behind it to see that everything really seems to work this way (you can also read up on the numerous experiments that have verified these aspects of relativity over and over again).

13

Didn’t “they” fly a nuclear clock around for a while, then brought it down to earth and find ever-so-slightly ahead of a nuclear clock that never left the ground?

This was one way to show how gravity impacted time/percieved time…

Speed and gravity “affect” time. Just one of the many things one has to get used to thinking about.

14

Whack-a-Mole or others who are physicist types,

Isn’t the reason that time slows, mass increases and lengths shrink towards lightspeed is the need to maintain the constance of “C”? In other words, because “c” is constant something else has gotta give.

My question is - why is c constant? One could argue mathematically it is because by holding it constant, the rest of the model makes sense and reflects observations. But I wonder (without wanting to go into GD’s over religion AT ALL - not my intent) what lead to a universe model whose “natural equilibrium” requires the speed of light to be constant? Is this understood and can it be explained to lay people like me? Something along the lines of why the earth has life having to to with it’s relative position from the sun and how that enables a tempurature that supports the chemical reactions required to sustain life. Is there a similar understanding as to how the speed of light has lead to the universe we know??

Sorry if this is bone-headed question from a professional’s POV.

15

I think so. I also heard it mentioned that the cosmonauts who stayed aboard the Mir Spacestation for 6 months were about 3 seconds behind those they left on earth.

You have two effects to consider how time moves while in orbit. The first, of course, is movement. Is the satellite in a geosynchronous orbit or is it like the space shuttle which orbits the earth a few times an hour (time will be ‘slower’ for the Space Shuttle astronauts than it would be for those in a geosynchronous orbit).

Second, as you leave earth’s garivty well, time moves at different rates. Time moves slower for those on the earth at sea level than it does for those on top of a mountain (the speed of light changes as it climbs out of earth’s gravity well). This effect was measured by placing extremely accurate clocks at the top and at the base of a water tower (a difference of only a few hundred feet). After some while (a few months) the two clocks started disagreeing about what time it was. [NOTE: I may have that reversed (about which clock moves ‘faster’) but I once wrote about this the opposite way and I think Chronos corrected me so I think this is the correct pairing of clocks and their timekeeping status.]

16

Yup…‘C’ is a constant so if you write down the formulas and do the math you need to adjust the other variables in the formula to make the math work. The other variables that get adjusted are mass, time and so on which give some of the funky results mentioned above

It’s actually not a boneheaded question. Indeed, it is one that philosophers, physicists and others have been arguing for a long time. Unfortunately there is no one ‘right’ answer. I’m really not qualified to get into the finer points of this but what you seem to be looking for is a discussion on the Anthropic Principle or, otherwise stated, why is the Universe the way it is? Does the Universe have to be this way or are we just lucky that it formed in such a way as to support life that can ask these questions? If we are just lucky then the chances against life are astronomically high (there are a LOT more initial conditions to the Universe that would be hostile to life evolving than to conditions that allow life). Since scientists don’t like to think something happened on an insane long-shot of a cosmic dice roll some try to answer this question…hence the Anthropic Principles.

The Anthropic Principles come in two main flavors, the Weak and the Strong. The Weak variant basically says the we are here because the Universe evolved in such a way as to allow us to be here and observe it this way but it doesn’t answer the question of why the universe is the way it is. The Strong variant basically says that for the Universe to exist at all it must possess properties that are beneficial to life so that we can evolve and observe the universe.

There are also a variety of other forms of the Anthropic Principle such as the Participatory or the Final Anthropic Principles. Studying all of these principles and their consequences can be a lifetime’s study in and of themselves (or at the very least subjects for a doctoral thesis).

In final answer to the question of ‘C’ being a constant is it is that way because it is a fundamental property of this universe. If you think of the universe as a framework of rules and values that allow everything to be the way we see it then the Speed of Light would be an integral part of this structure. Did it have to be that way? You need to see the Anthropic Principles for that discussion.

17

Time is slower here than in the sky (there’s some over-simplification). The idea is that if I were to look at someone in high gravity, say under the effects of a black hole, he would be moving in mega slow mo, while I would be a blink to him.

Gravity slows time, as does speed. Take a clock (nuclear for the tiny measurement potential) and fly it around and it will be ahead of ground bases nuclear clocks. The decay of the unstable material was faster in lower gravity.

18

That was just my point when I responded to what you posted. Get the clock out of earth’s gravity well and the clock will speed up (in relation to ground based clocks). However, the act of moving slows clocks down.

So, which is the greater effect? The faster clock because it is further away from earth’s center of gravity or the slower clock because you are flying it around? I’m sure either could be greater depending on total speed and distance from earth.

(As an aside how far do you have to get away from earth before you are considered free from its effects as a gravity well or…stated differently…how far does a photon have to get from the earth before the time dilation effects disappear?)

19

WordMan wrote:

I think you’re asking how we came up with that model, not why the universe chose that one. I wrote a longish post earlier in this thread about this - probably too long to expect people to read. It boils down to the fact that if c were to behave according to our intuitive ideas about waves, then there would have to be some favored reference frame which is at rest in the universe. But precise measurements made in the famous Michelson-Morley experiment showed clearly that c is independent of the observer’s speed. This contradicted hundreds of years of Newtonian physics. Einstein took the priciple that c is constant for all observers, and worked from there through all the math to come up with relativity. It described a bizarre universe which apparently is the way it really is.

20

Philster wrote:

Just an aside, atomic clocks don’t get their time reference from nuclear decay. They use Cesium atoms, and stimulate electrons to higher energy states around the atom. When the electrons fall back, they emit a photon that has an absolutely precise frequency of something over 9 MHz. The clocks have tuned circuits oscillating at this frequency, using Cesium to tune the frequency of the oscillator. Since it’s just tuning an oscillator, an atomic’s clock output is not exactly at the frequency of the Cesium transition, so that’s why the clocks have a specified error (commonly seconds in many thousands of years).

I heard an NPR report recently of a new clock design that’s a thousand or so times more accurate. Aha - I found it - you can listen to it here.