## Enter the Relativity Challenge 78

An anonymous reader writes

*"Any slashdotters wanna pick up a lazy 25,000 Euros? All you have to do is explain Einstein's theory of relativity in a five minute multimedia presentation. The Pirelli Group have laid down this 'Relativity Challenge' to anyone as part of the International Year of Physics. Entries close on 31 March 2005."*
## Like Dirty Harry said: (Score:3, Interesting)

I found mine in Physics 21 when we hit Relativity. I just flat out don't get it. I can do the math, and get the right answers, but I couldn't truly explain it.

## Re:Like Dirty Harry said: (Score:3, Interesting)

After several years of thinking about it, I've decided that the reason is because I kept asking "why".

I've never seen an adequate explaination for *why* a Lorentz Transformation is necessary and, since everything about relativity hinges on it, the entire theory breaks down for me at that point.

Of course, as soon as I finally understand why, I'll probably go "duh! of course!!!!" or something like that.

## Re:Like Dirty Harry said: (Score:2)

## Re:Like Dirty Harry said: (Score:5, Interesting)

Maxwell's equations of electro-magnetic theory show the speed of light in a medium to be determined by 2 properties of that medium. For vacuum, those two properties are fundimental constants - thus the speed of light is fixed.

Now, if I take a squirt gun with a fixed exit velocity and squirt it at you, the water will be moving slower if I am backing away from you and quicker if I am running at you. That fits with our day-to-day experience.

But for light in a vacuum that does not happen - if I now use a light-gun, you will measure the speed of all three beams of light (me backing away, me standing still relative to you, me running at you) as the same.

And curiously, so will I - I will measure the speed of light leaving my light-gun as the same, no matter what.

Now, the ONLY way you can get both my measurements and yours to agree is if things like length, mass, and time change based upon my motion relative to you - hence the need for the Lorentz transformation.

Then, you get into the "twins paradox" - Take 2 twins. Kick one of them up to nearly the speed of light. Wait till the other one has aged 10 years. Bring the high-speed twin back.

From the stationary twin's perspective, the high speed twin slowed down. From the high speed twin's perspective, the stationary twin (who wasn't stationary from the high speed twin's perspective) slowed down. Yet both cannot be true.

So Einstein reasoned out that the ONLY difference between the twins was who felt the acceleration - that twin would slow down.

But if I lock you in a box, you cannot tell if you are setting on a planet or in free space being accelerated - so gravity must be like acceleration.

That's GR in a nutshel.

## Twins Paradox (Score:5, Interesting)

You can't just wave it off saying the one that experienced the accelleration will have their clock slow down. If you want to calculate how much less one person aged, you go by how long (time and distance) they were travelling at that speed. For example, if I accellerate to 0.999999c in about a year (I think that's about 1g accelleration) and travel 10 light-years and back, then 24 years will have passed on Earth (20 travel + 4 for accelleration), but I will have aged only about 4. If I undergo the same accelleration but travel 10,000 light-years and back, then 20,004 years will have passed on Earth, but I will still have aged only about 4. The accelleration didn't make time pass more slowly, it was the period while I was tavelling at high speed that made it pass more slowly. Take both examples together and the one would seem to have 4 years pass and the other would seem to have 19,984 years pass, even though they experienced the same accellerations.

This also leads to an absurd result from my point of view. I will have only seen 2 years go by, but I will have travelled 20,000 light-years. From my point of view I would have been travelling 10,000 times the speed of light. How can this be?

I think it has to do with contraction. Lorentz contraction is one thing I haven't understood, how you can measure the length of something that is going nearly the speed of light? Apparently, when you are going nearly the speed of light, everything else contracts in the direction of your travel. For instance, if you were going a certain speed and passed a meter stick, it would appear to be only 1 millimeter long, although a stationary observer by the meter stick would see it as 1 meter long.

Now as for how fast you are going, that is all relative as well. If I take off from earth and accellerate to 0.999999c for about a year and travel 10 light-years, I don't think I'm going 10 light years. Space and the galaxy will seem to contract along the direction of my motion. When I get 10 light-years in space, it will appear to me like I have travelled a much shorter distance.

Here's a more concrete example. Let's say that I pass Earth going at velocity V, which slows down time for me to 1/10th normal. Then I travel to a space buoy that you have measured from earth as 10 light years away. Not only will I reach that buoy in about a year, but I will think I have travelled much less than 1 light-year because space along my direction of motion has contracted. During that time, an earth-based observer thinks 10 years have passed. The reason that his clock doesn't appear to slow down for me is because I don't think he's travelling that fast. To me, he has travelled much less than 1 light-year because space contracted and I think it was in 1 year, so he is travelling much slower than the speed of light and subject only to minor relativistic effects.

## Re:Twins Paradox - Hogwash (Score:3, Insightful)

From this explanation [physicsguy.com]. Twin A stays on Earth and Twin B sets off in a spaceship going 0.995 c (time and space will dilate to 1/10th). He reaches a point C that is 9.995 light-years away and heads back at the same speed. Let's assume accelleration is instantaneous. When Twin B leaves earth, both twi

## Re:Twins Paradox - Hogwash (Score:2)

In relativity, "simultaneous" means an event occurs at the same time in the same reference frame. Where a (inertial) reference frame is one that is not accelerating -- all objects in the frame are at the same velocity. If you add acceleration, you hit general relativity which is beyond my understanding at present.

For every reference frame, we have a set of coordinate axes (x_frame12345, y_frame12345, z_frame12345). An event can be described as a coordinate and a time in a given frame (x_frame12345, y

## Re:Twins Paradox - Hogwash (Score:2)

In relativity, "simultaneous" means an event occurs at the same time in the same reference frame.Actually, no. In relativity, "simultaneous" doesn't mean anything. That's half the point.

Before relativity, we had several ways of describing causality - we could say that "event A happened before event B", "event A happened at the same time as event B", "event A happened after event B" - and those definitions seemed absolute, just like the distances between objects were absolute - the distance between point

## Re:Twins Paradox - Hogwash (Score:2)

"Simultaneous" is meaningful in special relativity; it just depends on the reference frame.I was clarifying, not disagreeing, with the simultaneous point. The definition he gave for simultaneous was basically correct well before special relativity, and didn't much change with special relativity. If you define your "dt" as being "the time from when the light emitted from when event 1 struck my eye to when the light from event 2 struck my eye", then even in classical mechanics, "simultaneous" depended on yo

## Re:Twins Paradox - Hogwash (Score:2)

Allow me to demonst

## Re:Twins Paradox - Hogwash (Score:2)

## Re:Twins Paradox - Hogwash (Score:2)

## Re:Twins Paradox - Hogwash (Score:2)

From this explanation. Twin A stays on Earth and Twin B sets off in a spaceship going 0.995 c (time and space will dilate to 1/10th). He reaches a point C that is 9.995 light-years away and heads back at the same speed. Let's assume accelleration is instantaneous. When Twin B leaves earth, both twins agree their clocks read zero. When Twin B reaches point C, Twin A sees that his clock reads 10 years and Twin B's clock reads 1 year. Twin B thinks his clock reads 1 year and Twin A's clock reads 0.1 year. As s## Re:Like Dirty Harry said: (Score:2)

From the stationary twin's perspective, the high speed twin slowed down. From the high speed twin's perspective, the stationary twin (who wasn't stationary from the high speed twin's perspective) slowed down. Yet both cannot be true.

So Einstein reasoned out that the ONLY difference between the twins was who felt the acceleration - that twin would slow down.

It's not about who felt the acceleration, well it is but in a round about way...

It's about whose frame of reference they meet in. For either twin to

## Re:Like Dirty Harry said: (Score:1)

## Re:Like Dirty Harry said: (Score:2)

The issue is, if we examine the object from a local frame of reference, we can find no difference between pulling and pushing. If we pull on the object, we may observe the object being

## Gravity (Score:1)

The curvature of a section of space is defined by its metric. (A metric can be thought of as the generalization of the dot product between two vectors in some space, which therefore relates the two vectors in either a co

## Re:Like Dirty Harry said: (Score:2)

except that the article is about describing SPECIAL RELATIVITY which is a great deal harder to comprehend, let alone explain in a 5 minute presentation. Your discussion, while being quite exceptional is on GENERAL RELATIVITY.Actually, this seemed wrong so I looked it up. The grandparent described SPECIAL relativity (time warps and all that) but called it GENERAL relativity. His entry would be acceptable. My attempt (look right above this post) described GENERAL relativity.

Everyone always gets confused b

## Re:Like Dirty Harry said: (Score:1)

## Re:Like Dirty Harry said: (Score:1)

"test it, just math"

## Re:Like Dirty Harry said: (Score:5, Insightful)

I have a poster of Einstein on my wall, with a quote:

"Do not worry about your troubles in mathematics. I assure you, mine are much greater."

I might not have that exactly right, but as I understand it he was struggling with the math too.

## Re:Like Dirty Harry said: (Score:2)

You've got it right. Einstein was no slouch mathematically but his real abilities laid in physics and gedankenexperiments. That's why he usually had a mathematician as an assistant or collaborator. Simply because he wasn't enough of an expert in mathematics to be able to do all of the work on his own.

From what I recall he was really really stuck for a couple of years until a friend who was a mathematic

## Yeah, like his first wife... (Score:1)

## Re:Like Dirty Harry said: (Score:2, Interesting)

The speed of light remains constant regardless of the motion of the person observing it.

Let's compare how light is differnt than a physical object.

First, take an object (let's say a bullet). You have a friend standing still, while are you moving away from him at 500 glorps (arbitrary unit for velocity) per second. Someone fires a bullet in the same direction you are moving at 1000 glorps per second. Your friend will measure that bullet moving at 1000 glorps/s. YOU will measure it mov

## Re:Like Dirty Harry said: (Score:2, Interesting)

## Re:Like Dirty Harry said: (Score:2)

and velocity is distance over time...so make t go up, as v goes up, and you'll still get some sane answer.

I am not a physicist.

## Cos school books are kack? (Score:2)

Thats my excuse, its till on my list of things to learn.

Sam

## Twins paradox: Yes and No (Score:2)

The difference between general and special relativity: General relativity has acceleration. The twins paradox has a rocket ship turning around, which means its velocity changes, which means acceleration must occur. This makes it a general relativity problem.

But one thing physicists do well is neglect values which are insignificant. When doing the twins paradox problems, we make the reasonable (within the context of learning the stuff) presumption that the time to turn around is not significant. But

## Re:Twins paradox: Yes and No (Score:1)

First, the time dilation, is a ratio of the different time-passing rates. at a given velocity, the time delta among the two points will increase over time, which has no connection at all with the accelerations:

I mean that, you will take the same "time" or amount of energy to get from remative speed zero to relative speed v and back. but the paradoxical time delta w

## Why (Score:2, Interesting)

## Re:Why (Score:5, Funny)

## Re:Why (Score:5, Funny)

Light goes the same speed

For everyone everywhere

Funkiness ensues

## Re:Why (Score:2)

Light goes the same speedFor everyone everywhere

Funkiness ensues

Bravo! Bravo!

## Re:Why (Score:2)

## Poincaré and Hilbert (Score:1)

Einstein's work was already in very simple laymen's terms. I don't know what the point is in trying to make it into braindead powerpoint.Einstein's work on STR is "Poincaré for dummies", Einstein's work on GTR is "Hilbert for dummies".

STREinstein's paper "On the electrodymanics of moving bodies" contains nothing new. It was actually Poincaré who was the first to correctly state the special theory of relativity (the transformation formulas were found by Woldemar Voigt [ntu.edu.tw] in 1887, H.A. Lorentz

## Sure (Score:3, Insightful)

## Re:Sure (Score:2)

(i'm not familiar in doing them..)

## Re:Sure (Score:1)

## Hm (Score:4, Funny)

## Re:Hm (Score:3, Funny)

## Re:Hm (Score:2, Insightful)

## Re:Hm (Score:3, Informative)

## Re:Hm (Score:1)

Explain females to the slashdot crowd in 4 minutes, and I'll give the winner a copy of Duke Nukem ForeverFemales: Those shapely things that always move *away* from you.

## Re:Hm (Score:2)

> > Explain females to the slashdot crowd in 4 minutes, and I'll give the winner a copy of Duke Nukem Forever>

>Females: Those shapely things that always move *away* from you.

And to keep this on topic: ...turning redder as you attempt to engage them in a discussion of special relativity at keg parties.

## Re:Hm (Score:1)

(Of course, I have nothing against females.)

## Special, General, or both? (Score:1)

Special's not so bad, but General gets tricky...

## Re:Special, General, or both? (Score:1)

## "special" relativity theory (Score:3, Informative)

## The best explanation of relativity (Score:3, Informative)

This explanation explain relativity in context with the faster than light travel. Needs elementary math and explains lucidly why the time dilation occurs. Highly recommened.

## damn filter (Score:2)

## Simple relativity (Score:5, Funny)

I don't think it can be said much better.

## Best relativity resource I've found: (Score:5, Informative)

real deal; read it slowly and carefully. Intro calculus should suffice to get you through chapter one (which took me about two weeks spare time in the evening to read and digest), after that it gets tougher, although I'm finding the subjects don't build on each other so much after that so you can skip something you can't follow and keep going. (On the other hand, I only just finished Chapter 2, of 9.)About the only thing I can tell you, short of linking the book as I did or quoting it more extensively than Slashdot will allow, is that nearly everything physics fanboys think they know is wrong. Don't rely on Star Trek for your physics, get the real deal; it'll only take as much time as a few episodes of Star Trek and you'll feel much better about your expanding horizons

## How stuff works... (Score:3, Informative)

How stuff works in Einstein's theory of special relativity? [howstuffworks.com] is a good starting point for the contesters.

## Special or General Relativity? (Score:2)

mucheasier to explain in layperson's terms.## The Speed of Light's the Thing! (Score:1, Insightful)

pageson the almost trivial vector math to determine the relative motion of an object from two different frames of reference (which IMHO, is just a huge, long-winded setup of the fact that trivial vector math doesn't work). BUT the fact that the speed of light is constant is the thing that breaks the model andno one ever explains that!It's just re## Re:The Speed of Light's the Thing! (Score:1, Interesting)

BUT the fact that the speed of light is constant is the thing that breaks the model and no one ever explains that! It's just referred to axiomatically. In the book I would like to find, the whole book would be about how and why the speed of light is constant and then in the appendix they could state 'And oh, by the way, the speed of light being constant means that time dilation occurs and Lorenz tranformations have to be used instead of trivial vector math in order to figure out the position of objects in s## Re:The Speed of Light's the Thing! (Score:2)

## Re:The Speed of Light's the Thing! (Score:1)

It is one of the postulates(def: A proposition assumed to be true without any appeal to evidentiary support, especially when it is then used to derive further statements in a formal system or general theory. ) of a new theory of physics.

In the old theory, these were the postulates:

length is constant

mass is constant

the passage of time is constant

the s

## Re:The Speed of Light's the Thing! (Score:2)

Doug

## Re:The Speed of Light's the Thing! (Score:1, Insightful)

## Re:The Speed of Light's the Thing! (Score:1)

## That's so easy (Score:2, Funny)

Explanation of the Theory of Relativity

Boarder

10-18-2004

*rocket clip art zooms across screen*

Slide 2:

The speed of light in a vacuum is

*flies in from left* CONSTANT

Slide 3:

*in really, really small font, appearing one per mouse click* all equations proving relativity

Slide 4:

Conclusion:

*line fade* Everything is relative

Slide 5:

Special Thanks to:

*flashing text* Einstein

and

*spinning 3d text* Maxwell

*tiny red text* copyright 2004 boarder

## Re:That's so easy (Score:2)

## Re:That's so easy (Score:1, Funny)

Profit!

## modders suck (Score:1, Offtopic)

So maybe it wasn't all that funny, but that isn't the same as overrated... if it were modded Funny and you didn't agree, then maybe you can mod it down. Modding is supposed to encourage good posts, not discourage average posts. If you had RTFRules for Moderation, you would understand that.

## So this is why.... (Score:2)

## I like Marilyn Monroe's explanation in... (Score:2)

## Read the fine print... (Score:1, Informative)

## It's special, not general (Score:2)

Generalrelativity, on the other hand...