# Explaining special relativity

1. Aug 16, 2004

### VantagePoint72

Hi everyone,
Lately, a few of my friends have been arguing with me about special relativity, not because they've analyzed the theory and have found some issues with it, but because they haven't got a clue how special relativity and reject because, well, to quote the most frequent statement, "Einstein was a fraud! Time doesn't slow down as you go faster! It's so stupid, who'd believe that?" So, I've tried to explain the principles to them, with them as willing students, however I've been working with SR for so long that I'm having a very difficult time with coming up with simple explanations, and they're just not getting it, further strengthening their resolve that special relativity is the stupidest thing in the world. Can any of you think of perhaps some easy analogies or simple explanation of the concepts of SR? I'm looking for something beyond the light clock. It's driving me crazy, their disrespect for the good doctor more than anything else Any help would be appreciated. While you're at it, perhaps some suggestions for explaining GR too, as they'll inevitably ask. Thanks!

2. Aug 16, 2004

### robphy

What's wrong with the "light clock"? In my opinion, if done well, it's one of the clearest physical motivations.

Using a minimal amount of algebra, the following relativists do a great job motivating the physics operationally:
• Bondi's "Relativity and Common Sense" (k-calculus)
• Geroch's "General Relativity from A to B" (unique interpretation of the square interval)
• Ellis and Williams' "Flat and Curved Space-Times" (vivid spacetime diagrams)

For a mathematical motivation, one could pursue the geometric analogy.

Finally, one would have to appeal to experimental evidence to support the theory.

Last edited: Aug 16, 2004
3. Aug 17, 2004

### ArmoSkater87

Yea...the light clock is the easiest way I can think of explaining time dilation. Its also pretty easy to algebraicly derive the time dilation equation from using the triangle in the light clock diagram. Just explain the fact that the speed pf light is constant to ALL obsevers...and since the speed of light cant change, then time must change because you see the light travel a greater distance on the clock that is moving relative to you.

4. Aug 17, 2004

### pmb_phy

Its not really useful to participate in such arguements since they reject them regardles of anything that they hear. They probably don't even understand what is meant by time dilation. They might think that if they travel near light speed then when they look at their watch on their wrist it will be running slow. So don't start with the results. Start with the postulates. Show them what Einstein actually said instead of what they think he said. I think you can find, and print, the first few pages of his 1905 paper online somewhere. Show them the first few pages of that. Einstein explained why he made those postulates so start by explaining the postulates. When it comes to the time when the claim that the postulates are wrong, and they will, then explain that any experiments have been performed which are consistent with those postulates. Describe the experiments that were done on Mt. Washington with muons. Tell them the results and tell them to explain why the results of relativity describe the time dilation formula preciscely. If you need a page which describes the light clock then see
http://www.geocities.com/physics_world/sr/light_clock.htm

Keep in mind that Einstein himself was looking for ways (i.e. experiments which were performed, including, Einstein later claimed, the Michelson-Morely experiment) to detect the ether and when he failed it was then that he made those two posultates.

Pete

5. Aug 17, 2004

### VantagePoint72

Hey everyone,
Thanks Pete, I'll give the "proving the postulates" approach a try.
Robphy and ArmoSkater87, I know the light clock is best explanation- that's why them still rejecting it is driving me crazy! They refuse to accept that just because one observer will see the light as taking longer to complete one circuit, that therefore the same would happen for the ticking of any other clock. They see the lengthened tick for the light clock as just a strange quirk of using a light clock. That's why I'm looking for another way of going about it...perhaps along the lines of all objects travel at c through spacetime and that speed must stay constant. Can you think of any illustrations to show how increasing your spatial velocity must neccessarily divert some of your speed through time? Of course, proving that all objects are travelling through spacetime at c will be the trick then...

6. Aug 17, 2004

### pmb_phy

And that's their problem. They're focusing all their attention on what they "see" and are paying no attention to what is actually being measured by clocks. Try to get them to pay attention to the readings on the clocks. The light is simply a tool used to analyze the problem. Explain to them that you are using identical clocks which are at rest in two different frames of reference. Ask them to prove that the difference between measured differenences between the times of events is anything but real. I'd like to see them put some meaning behind the term "quirk" if they still can't see it.

But you may retreat and first get them to precisely define "time" and what it means. Tell them that in physics that time is operationaly defined as that which a clock measures. See also - http://www.wfu.edu/~brehme/time.htm [Broken]

Pete

Last edited by a moderator: May 1, 2017
7. Aug 17, 2004

### robphy

You have to appeal to the principle of relativity here.
Imagine a Timex wristwatch (or a even a pendulum) riding along with each inertially travelling light-clock. If the light-clock ticks didn't coincide with the ticks of any other clock riding along with it, then one could distinguish one's inertial frame from one at rest.

If you think about it, the Michelson Morley apparatus is essentially a pair of differently oriented light-clocks, which experiment has shown their ticking to agree.

This is essentially follows a mathematical motivation. To proceed along these lines, one should start drawing and interpreting spacetime diagrams, while drawing analogies to Euclidean geometry. This is probably not an easy road without some careful thought.

Last edited: Aug 17, 2004
8. Aug 17, 2004

### VantagePoint72

A lot of great ideas, I hope they work. *sigh* I think know how Einstein must have felt when he first introduced the theory and had to face the inevitable group of people who labelled him a crank.

9. Aug 17, 2004

### Janus

Staff Emeritus
Try this then. With your light clock have a spool of film running across its path, Everytime the the light travels back and forth it puts a spot on the film. The film is running at a constant speed, for instance, 1 in per sec, according to a clock (not connected to the light clock) and observer sitting next to the film spool. If the pulses of the light clock were 1 sec apart in the frame of the light clock then the spots will be one inch apart.

Now note that to the observer to which the light clock is moving will see the light pulses as occuring further than 1 sec apart. Thus unless the separate clock controlling the film speed also ticks slower, according to this observer, the spots on the film will be more than 1 inch apart.

Since this film permamently records the distance between the spots, you can stop the motion of the light clock system and bring both observers together with the film spool. If the light clock and all other clocks in that reference frame did not remain in sync, you would have two observers, now in the same frame, both looking at the same piece of film and each saying that the spots were different distances apart, an obvious contradiction.

However, if the clocks all do remain in sync, then the film will feed through slower for the one observer and he too will measure that the spots were 1 in. apart and when everything was brought back together both observers agree as to what the film recorded.

You don't have to use this exact example, but any method that shows that unless all clocks in the light clock frame behave the same as the light clock, a contradiction will be produced when you bring two relatively moving frmaes back together.

10. Aug 17, 2004

### Janus

Staff Emeritus
Or another example:

Imagine that there is a mirror on the end of a clock's second hand. The light pulse is fired off such that is takes 60 secs to return, during which the second hand makes one revolution. The returning pulse is aimed such that it it will hit a photoreceptor that will set off a explosive device which will destroy the whole mechanism. The only thing that prevents this is the fact that the sec hand is timed so that it covers this receptor with the mirror when the pulse arrives, reflecting it back to the other mirror. As long as the sec hand stays in sync with the light pulse diaster is averted.

But. From the perspective of the observer which which the light clock is moving, the pulse takes more than 60 secs to return. If the hand of the clock does not also take more time to make its revolution, it will be past the receptor when the ligth pulse returns, uncovering the receptor and allowing the light to set off the explosive. Thus this observer will see the device explode. Thus we get a contradiction again, unless both the light clock and mechanical clock both run slow by the same amount

11. Aug 17, 2004

### VantagePoint72

Thanks Janus, that's a great illustration!

12. Aug 17, 2004

### omin

As far as explanations, I found something that has aided, but not solved my dilemna of not understanding SR. Universal time and perceptual time.

Universal time is when things happen, and they happen simultaneously, and physcis operates the same in each coordinate even when acceleration occurs.

Perceptual time is when things appear to happen and depend upon the speed of the medium that sends the information and the distance between the object where the information comes and the perciever.

At this point, I am convinced, (at this point), that SR confuses a perceptual time as being physical, thus claiming since time travel is perceived to happen it physically does. And, claim that is hard to swallow is that an object that is moving has no effect upon the lights velocity that emits from it. Everything else in human experience that moves effects the velocity of what emits from it, what makes light so special? The answer that supposed to be abracadabra logic is that it's constant.

And if things could time travel, they could actually exist in the same place under certain drawn up scenarios, which is a physical violation. If you understand Relativity, an example can be easily drawn.

13. Aug 17, 2004

### Tom Mattson

Staff Emeritus
SR does not confuse the two. They are clearly distinguished in any physical theory, relativistic or otherwise.

14. Aug 17, 2004

### robphy

The Ellis-Williams book I referenced above was a great help in clarifying aspects of such notions. (See p. 48.)