The theory of relativity

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Jan 18, 2009
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I'm sure it was on this site but I cant find it. Is there a link or Web Page that explains the theory of relativity in layman's terms? I remember something about a train. My daughter asked me about it yesterday and I wanted to show it to her.
Thanks in advance.
Well if you want to use the train reference, here goes;
Imagine your standing on top of a train that is traveling at 30 miles per hour. How fast are you going?
It depends. "Relative" to the ground, you are going 30 mph. "Relative" to the train, you are going 0 mph.
Now suppose you start running across the top of the train at 10 mph. How fast are you going? Again, it's "Relative". Relative to the train you are going 10 mph. Relative to the ground you are going 40 mph. It all depends on your point of reference.

Hope this helps.

There was an outstanding article in Discover magazine a few months ago that had very easy-to-understand pictoral illustrations of the principles of relativity. Almost the entire issue was dedicated to Einstein, with many of the articles getting into some of the headier stuff, but that 2-pager with all the pictures may be what you were looking for.

and catches far did the ball travel?
depends on where you are standing, of course.. in the reference frame of the tosser ( ;) ), the ball goes about 4 feet up and down...but to the guy stnding along the tracks, the ball might appear go 100 yards...(if he was Superman and could see through the walls of the coach, of course).
...and if you throw the ball forwards at say 20mph, it seems to you as though it travels at 20mph, but relative to anyone on the ground, it's travelling at 50mph.

Now shine a torch in the direction of travel.
Then you have the problems with light...

First a bullet in a gun, same train. Your train is traveling 100MPH east. You fire your gun east and it has a muzzle velocity of 200 MPH, relative to a ground observer, what is the speed of the bullet coming out of the barrel of the gun? 300 MPH.

Now face West and fire the gun again and ask the ground observer. He sees 100MPH, just as was discussed in some of the earlier replies.

Now repeat with a flashlight. Shine your light east with a "muzzle velocity" of 670,471,200 MPH (c) and what muzzle velocity does the ground observer see?

670,471,300 MPH? Nope, he sees 670,471,200 (the exact same muzzle speed that the shooter sees)

Now have him face west on the train and fire. Same thing. Both the shooter and observer see a muzzle speed of 670,471,200

ARGH! :)
AHHH but here is another conundrum!

Say the train is traveling at a much higher speed say...600,000,000 MPH

The speed of the flashlight is still 670,471,200MPH

BUT!!! to an observer on the ground with the train traveling TOWARDS him the light is STILL traveling at the same speed but is shifted toward the blue end of the spectrum.

An alternate observer looks at the light as the train goes AWAY from him the light is shifted towards the red end of the spectrum!
Gets weirder than this, too. We're not even getting into time and/or space dilation yet and the apparent paradoxes that brings up. There's a great book out there that my girlfriend loaned me. It's called "Simply Relativity". I forget who wrote it, but it does a really great job of explaining all this stuff.
Originally posted by LocalX
I'm sure it was on this site but I cant find it. Is there a link or Web Page that explains the theory of relativity in layman's terms? I remember something about a train. My daughter asked me about it yesterday and I wanted to show it to her.
Thanks in advance.

Don't know which link you mean. I found lots.

Here's an explanation that came from a physics course at Purdue. It requires only a basic understanding of the Pyhtagorean theorem. Look at the attached drawings.

Special relativity says that time is a dimension just like space has dimensions. It is at a right angle to the 3 dimensions we know. You can't visualize that, so "collapse" our 3 dimensions into one (because what direction you're moving doesn't matter to realtivity). Now draw a line at a right angle to that line. Call the vertical one space and the horizontal one time. That's the top left drawing.

You move through time just as you move through space. In fact, all you do is move through spacetime. Not space time, but spacetime. It's one thing. And you must always move at a constant rate through spacetime. Let's call it One "C". That's the red line in the second drawing.

When you're sitting still in space, you're still experiencing time, so you're moving in spacetime at the rate of One "C", but only in the time direction. You experience the time at that rate. Let's call this "normal". This is the third drawing; all the red "movement" is along the time direction.

When you start to move through space, your total velocity must remain constant, so you move less through time. In the fourth drawing you'll see that One "C" is drawn between a tiny increment in the space direction, to a point on the time direction slightly less than before. The One "C" remains constant. (Pardon my poor drawing skills and pretend the picture does show this).

If you go fast, the One "C" line has to connect a point well up the space direction with the time direction's line, but does so at a much shorter distance. As you go faster, you experience less time, so that your total remains constant. This is the fifth drawing/

The sixth drawing represents moving at the speed of light, or One "C". You always were, but now you're doing it entirely in the space direction. Since all your velocity is there, you do not move in time.

That's special relativity. There's a precise relationship between space and time. The pcitures describe it, but the equation is a little more difficult. There is also a similar precise relationship betwen mass and energy. The equation for this is a little more straight forward even though it derives from the other. It says that the mass is energy, and the amount of {E}nergy (measured in ergs) represented by a certain mass is equal = to that {M}*** (measured in grams) times 898751347000000000000. Which happens to be One "C" squared.

Now all this moving more slowly in time is as seen from the outside. To you, in your spaceship or whatever (your "frame of reference") everything seems normal inside. It's outside that's getting weird.

The light coming at you from in front is moving at C. You're moving fowards at say 90% of C. The light can't move 1.9C. It must move C total. So, some of what would be greater than One "C" is coverted to energy. Light with more energy has a shorter wavelength -- it's "bluer". The opposite happens to light catching up to you from behind.

There is also strange shortening of space itself in the direction of fast travel, called Lorentz contraction. And an increase in mass as you go faster. This is all because spacetime and massenergy are inextricably linked into one cohesive universe and we only perceive things about it as separate.

Einstien also put forth the equivalence principle, saying you can't tell from inside a box whether you're in a gravity field or accelerating. They're equivalent because bent spacetime due to gravity (the old elastic sheet idea) is an acceleration. "There is no special frame of reference".

General relativity combines special relativity and equivalence and ends up explaining strange things light light "bending" due to gravity as well as more exotic things like the fact thatblack holes can exist.

If that hasn't bent your head enough consider: anything moving at the speed of light experiences n0 time. In comparison it would observe the outside world as moving extremely fast. In fact, at One "C" everything outside happens immediately.

Imagine a photon created in the original big bang, zipping off into the cosmos. It never slows down and never hits anything. It continues at that speed until all matter and ebergy exhuast themselves, some 10(^200) years from now. But to that photon, it will have seemes as though the entire history of the universe from beginning to end happened immediately in a time of zero duration. Stranger yet, it would be every bit as correct as you who sat still and watched the universe unfold over all that period of "normal" time, never moving an inch.
Another interesting tidbit...

The speed of light limit only applies to the staionary observer, *not* the traveler...

If I have a ship that can travel (for the purposes of simplicity) exactly the speed of light (c) and I send that ship to alpha centari (4.3 light years away), and right back, then I will see my ship return 8.6 years from now.

I am the "stationary observer", as it were.

However, the Captain of the ship sees something much different. While traveling at C, time has stopped for him, the traveler. So, from his point of view, he goes to Alpha Centari and back in *zero* time (do the math, that is a LOT faster than the speed of light)... :)

It gets better too :)

NOW my head hurts!:(

I hate time travel!

Whenever the writers on Star Trek use time travel to get out of a situation (Like ending the Voyager series) I feel its a "cop out".

They couldn't think of anything else.
The beauty of it is, Time Travel is possible. Not only "possible" but "inevitable"...

Check it out. Look at your watch and record the time.

Now, 30 minutes from now, check your watch again and record the time.

Subtract one from the other and you will notice that you have traveled 30 minutes into the future!

kewl, eh?

It's a slow process, but you will also notice that it's very low-tech (no warping around a sun or nothin'!)

Now, this doesn't mean we can *see* into the future. Nope, only travel. Likewise, we can't *travel* into the past, but...

... we can *see* into the past. In fact, that is ALL we can do (not possible to see the present, only the past) Check it out...

the sun is 8 light minutes away (give or take). Therefore, when you look at the sun you do NOT see it as it appears right now, rather you see it as it appeared 8 minutes ago (in the past). If it were to go supernova *right now*, you wouldn't know it for 8 more minutes... Even if you are looking at the food on your plate... that's at least 2 light nano-sconds away...

Now, let's carry this a bit further... let's intantaniously go 42 light years away and grab a *very* powerful and sensitive telescope and look back at earth...

I would see the Kennedy Assassination... ...LIVE! (hey, turn the scope over to that grassy knoll there...")

things that make you go hhhmmmmmmmmm
What makes me say Hmmm is the fact that light in a vacuum travels at constant c. It enters an interveining medium, say ice, water or even air, and it slows down. After it leaves the medium, it resumes its previous speed. Where does that energy come from? How do I get some?
Originally posted by rbeckey
What makes me say Hmmm is the fact that light in a vacuum travels at constant c. It enters an interveining medium, say ice, water or even air, and it slows down. After it leaves the medium, it resumes its previous speed. Where does that energy come from? How do I get some?

You must be constantly and repeatedly coming out of water. It is due to that extra energy that you are able to get from the pool to your towel in 0.2 seconds when there is a cold breeze blowing.

Originally posted by rbeckey
What makes me say Hmmm is the fact that light in a vacuum travels at constant c. It enters an interveining medium, say ice, water or even air, and it slows down. After it leaves the medium, it resumes its previous speed. Where does that energy come from? How do I get some?

It doesn't lose the energy when it enters. It becomes "compacted".

It's still travelling at C in every case. Same velocity, same amount of energy. It only looks like a different velocity to you because you're not in its frame of reference. The energy seems to be acting different because it's interacting with other energy which happens to be "frozen" on the M side of the equation.

That's not entirely true because it can interact with the matter and change it, such as heating it. But that's the photons that get stopped. The ones that don't act as you say. The ones that do but later escape come out with the same speed (because they have to travel at C) but their energy is changed. They've lost energy (become "redder") and are being re-transmitted as "cooler" photons. They've interacted with mass, which is energy, which is bent spacetime, the result being exactly the same as if there were a speed differential or an intervening gravity field, all of which are one in the same thing.

The real miracle of sorts is that all of this that appears to be different effects are really all the same thing, and all are summed up in a single equation that describes the behavior of that one thing, the universe.

What's missing is a "theory of everything" which incorporates all the various forces. Since these are probably all manifestations of the same one single thing, the universe, the ToE probably won't supersede relativity, but rather fill in the gaps that prevent us from seeing how it applies to all of them.

Einstein was smarter than he gave himself credit for. He created the "cosmological constant" to explain the behavior of the universe as a whole, and when it wasn't found, he called this is "greatest blunder". His greatest blunder was being too far ahead of everyone. What wasn't found then has been since. It's what we're calling dark matter/dark energy, and are seeing in the changing speed of expansion of the universe. Just don't call it anti-gravity, because physicists have been saying there's no such thing for so long that now that they've found it they have to call it anything but.

Smilin' Uncle Albert isn't done with us yet. Here we are exactly 100 years from the time he dumped more spectacular science on the world than anyone else ever in such a short time (

We're still catching up to him, 50 years after he died. And I'll betcha he's still sticking his tounge out.
Originally posted by KermieD
Wow. That's an excellent explanation.

For my next trick I will transport Captain Kirk.

Let's say he weighs 150 lbs. (he *is* a little guy, through muscular). That's 68038.8555 grams mass. When he's converted entirely to energy in order to beam down, he becomes 61150013028963358500000000 ergs of energy.

That's a big number, but ergs are pretty small. Let's do a conversion.

61150013028963358500000000 ergs
= 4.5101935 × 10(^18) foot pounds or
= 1.46152039 × 10(^18) calories or
= 5.79590293 × 10(^15) BTU.

Still too big?

One ton of TNT = 4 X 10(^16) ergs. Captain Kirk converted to energy would be about 15287.5 megatons. All the nuclear weapons on Earth total about 12000 megatons. The Indian ocean tsunamis (not the earthquake, just the tsunamis) totalled about 5 megatons.
The speed of light limit only applies to the stationary observer, *not* the traveler...

The same time-space geometry that makes the speed of light constant also means there is no absolute position in space, so technically, there's no such thing as a stationary observer.

Fortunately, time isn't constant either. The closer you get to the speed of light, the slower time goes, so the relative speed of light appears to be the same as it was before. At the event horizon of a black hole, light slows down to a halt, but effectively time becomes infinitely slow to compensate, resulting in *very* weird stuff happening.

It's been posited that it's the acceleration of the observer that causes different relativistic effects (like asymmetric aging), not the velocity.

But before you think this is all too weird to believe, consider this: there's software in the GPS satellites to compensate for the fact that time isn't the same outside our gravity well than it is on the surface (different for different reasons, but related to space-time geometry).
If theres amatuer nuclear fusion and theory of relativity discussions in the Coffee House, what are you talking about in the Advanced Topics forum!?!?!? Warp Drives!? Human Teleportation!?

"beam me up, Scotty!!"

;) ;)

(just kidding, of course)
I need to do some research, but there was an experiement on one of the (i beleive) early shuttle missions (though it may have been a skylab experiement) to test the theory of relativity. From what I recall, some of the folks working on it saw it as a way to prove that it was wrong.

They make matching atomic clocks. Kept one on earth and sent the other aboard the spacecraft. Traveling at 17,500+ MPH for an extended period of time should (if the theory is correct) produce a measurable time-dialation effect.

Sure enough, when they returned with the subject clock, the time difference between it and the ground clock were off by exactly what the theory predicts it should have been.

There was a nice write up in one of those science periodicals but I can't recall the details.

kewl stuff though.
Very cool indeed!

"During a Shuttle mission, the orbital speed is only a tiny fraction of the speed of light (namely, 1/42857th). So, the "time dilation," as the effect is called, is also tiny, but it is there, nevertheless, as Shuttle experiments have proven. For example, a highly precise atomic clock flying in an experiment called NAVEX on STS-61A/Challenger in 1985 measured a slowdown of 0.000,000,000,295 seconds for each second of flight, almost exactly what Einstein’s formulas predicted."
Jim, you just had to go and start all this didn't you. ;)
I'm just gonna stay here on my train and read a book.
And if my train doesn't arrive on time, I'm blaming all you guys. :D

So I guess we were ALL wrong when we were answering those questions back in grade school. You know, the ones that go something like, "Train A leaves Cleveland at 9am going 30 mph, and Train B leaves Toledo at 11:30am going 40mph. What time do they collide and kill all the passengers in a scene of bloody carnage?"

We should have answered that it depends on whether or not you were on one of the trains, and which train! :D