# Reality and Relativity

1. Jan 21, 2005

### Moneer81

hey,

so most of us understand special relativity, and I also understand it and appreciate it. However, most people understand it from a mathematical point of view. But come to think about it, what is actually happening in relativity?

there are many examples we can look at. for instance the two observers, one on a fast train and another on the ground: a certain event's time will be shorter for the one on the train. Another example, the twin paradox, where one of the twins travels to a distant planet on a very fast spaceship and returns to find his twin brother has aged many more years than himself. Or for a more realistic example, the 1975 experiment where an atomic clock flown on an airplane showed a different reading than an identical one on the ground.

When I explain relativity to friends or other students, people tell me: this is all BS! They keep asking me about what is happening to these clocks, and whether the velocity at which they're moving has anything to do with they way these clocks function. I also wonder about those twins: does the one twin actually age more? i.e. does he show signs of older age like wrinkles and stuff before his brother does?

thanks

2. Jan 21, 2005

### Ich

In SR, there is absolutely NOTHING happening to clocks/people when they move at high speeds, as speed is relative. It´s always the others that age slower or get contracted. Only when people meet again and compare clocks the one with the straighter way to the meeting point will turn out to be older. Nevertheless none of them will experience anything unusual. It´s not that one twin is "ageing faster" in some weird way, it´s that he is actually older because he experienced more time elapsed.

3. Jan 21, 2005

### eNathan

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Ewo

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Hi. I am new to physicsforums. I have tried to tell many people about relativity, and what I have learned is that relativity contradicts common sense to the point of disbelief. Although, scientist do accept it as true, many cannot grasp concepts of space and time. When I was first introduced to relativity, it was hard for me to perceive time as an actual entity (dimension), let alone imagine that it can be relative. I was suggest these links to your friends.

http://science.howstuffworks.com/time-travel.htm

http://science.howstuffworks.com/relativity.htm

try to get them to grasp the general concepts of relativity first, then you can tell them about the lorenz transformation $sqr(1- \frac {v^2} {v^2} )$ :surprised

PS, sorry for my poor latex skill :tongue2:

4. Jan 21, 2005

### Rob Woodside

There is an old paperback by David Bohm called Relativity that developes Bondi's K-calculus (really just space-time diagrams) and worries about why the concepts of special relativity appear so strange at first sight. He talks about Piaget's experiments with developing children and points out that our "common sense" Newtonian ideas are really hard won. For instance, very young children presented with a brightly coloured object from behind one side of a screen, moved in front of the screen in full view, and then hidden at the other side of the screen, look to the source of the object rather than where the object disappeared. This is supposed to indicate that the notion of invariance or permanence has to be learned.

From my own experience, I don't know how many times I stood in front of a class and told them of the symmetric dilation and contraction between inertial frames, before it dawned on me that something had to give if all inertial frames had the same maximum speed. Now it is perfectly obvious to me. However I notice that if I teach a topic in a way that is obvious to me I like to think I have taught well, but lord knows what the students think. A confident presentation is certainly better than a fumbling one, but some fumbling around is required for learning.

I remember one prof who got all kinds of awards for teaching excellence. An excited anticipation would build as the class assembled as today would be the day that we would see a great technique or theorem. He would carefully lead us down the path, making sure that every 2 pi i had the right sign and that each line followed meticulously from the last. As the class ended and he disappeared, we would awake from our reverie and with puzzled faces would ask each other "Just What did he show???" The guy could have sold refrigerators to Inuit. None of these concepts are obvious or trivial and learning them requires hard work. There is no royal road to learning.

5. Jan 21, 2005

### pervect

Staff Emeritus
Bondi himself wrote a book on relativity from the k-calculus point of view, called "Relativity and common sense". I have a certain fondness for this book, and for the k-calculus approach, because it was one of the very first books I read on relativity back in high school. It's not very demanding mathematically, one needs only basic algebra (not calculus) to follow his derivation of the Lorentz transforms from the basic premise that the doppler shift depends only on the relative velocity between two bodies.

Amazon.com reviews on this book were not so favorable, many apparently found the exposition a bit tedious

6. Jan 22, 2005

### yogi

Like Dingle - When I first understood SR a really didn't. After pondering it more I puzzled more. What I think I have come to understand is why those that are not disturbed by its consequences claim to understand it. On the other hand, if I don't understand that, the whole effort has been wasted.

7. Jan 22, 2005

### Hurkyl

Staff Emeritus
For me, the light didn't click until I started drawing space-time diagrams, and realized geometrically how to make measurements in reference frames other than the one I had drawn.

8. Jan 22, 2005

### yogi

Hurkyl - well put

9. Jan 23, 2005

### Icebreaker

I thought the twin paradox was flawed because there's acceleration involved.

10. Jan 23, 2005

### Hurkyl

Staff Emeritus
Yes -- the usual formulation of the twin paradox mistakenly assumes that the spacebound twin is an inertial observer for the entire trip.

11. Jan 23, 2005

### Moneer81

hey again,

ok let me more specific about my question. Take the example of the 1975 experiment of the atomic clock that was flown on airplane. When the reading on that clock was compared to an idenctical clock left at the air base, the two readings did not match!

so now aside from all the physics and math jargon, how would you explain in english the fact that a clock, for some reason, malfunctioned? it seems that for some reason, the high velocity motion somehow affected the clock, because all the other clocks on the ground had the same reading EXCEPT the one on board that plane?

12. Jan 23, 2005

Staff Emeritus
The clock did NOT malfunction, the speed of the plane had no effect on it. It correctly measured the time in the plane. What happened is that time in the plane's frame of reference slowed down relative to time on the surface of earth due to the plane's speed relative to the surface of earth. There is also the fact that the plane then returned its speed to zero relative to the surface, breaking the relativistic symmetry.

13. Jan 23, 2005

### jdavel

moneer,

There's no malfunction!

Where I live my clock says 7:49pm. But on the east coast the clocks say 8:49pm. Is anybody's clock malfunctioning? Of course not; there's just a man made difference in what the clocks read. We decided to create that difference.

When the universe was created, nature decided how time was going to work. Our clocks (even when they're functioning perfectly) are locked to the system nature decided on. And that system is that a clock that goes away and comes back will read an earlier time than one that stayed home.

14. Jan 24, 2005

### yogi

Moneer - SR uses the idea that the spacetime interval between two events is the same in every inertial frame. Its called the invariance of the interval during transformation. The interval in each frame is composed of the square of the temporal length minus the square of the spatial distance. So between two events plotted on a Minkowsi spacetime diagram, there will be an invariant spacetime interval but the individual components that make up the interval in two different frames in motion wrt to one another will be different - thus the interval for a traveler going from earth to a distant planet at a high speed (near c) as measured in the earth frame will have a large time component and a large distance component, but the square of their difference will be small. If this same spacetime interval is measured by a clock and odometer in the frame of the traveler, there will be no space reading because the clock is comoving with the traveler - and therefore the time read on the clock will be much less (equal to the difference measured in the previous case).

To give an extreme example, a high speed particle traveling at near the velocity of light might require 5 years + a few usec to reach a planet 5 light years from earth as measured by a clock in the earth frame that is set in motion at the beginning of the trip. But the particle would only age by several usec as determined by a clock that escorts the particle.

15. Jan 24, 2005

### Hurkyl

Staff Emeritus
One thing that students might get as a general principle is:

"Between two events in space-time, a straight line takes the 'longest' to traverse".

The straight line is, of course, the path that requires no acceleration.

This means that if two observers start and end together, the one whose journey was the most like a straight line will have aged the most.

16. Jan 24, 2005