- #1

Gecko

- 63

- 0

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter Gecko
- Start date

- #1

Gecko

- 63

- 0

- #2

zefram_c

- 259

- 0

- #3

Gecko

- 63

- 0

- #4

RandallB

- 1,550

- 0

Gecko said:i mean tell mathmatically, why the speed of light is always the same from any reference point

Mathematically you can not "Tell" anything - You can show or extend from known information to new conclusions. Conclusions that can be tested to prove the original "known information" was correct.

As an example:

Let us assume we "know" that sound travels only at 1 foot per millisecond regardless of the speed of the observer!

Well then we can make a 'sound clock' that bounces sound between two sound mirrors 3ft vertical apart - moving or not it will always take 6 msec for one round trip. But have an observer watch it move horizontally fast enough that goes 8ft before the round trip is completed. That means the sound "BEAM" traveled 10 ft in 10 msec for the external observer but still only 6ft for the moving clock for 6 msec. 4 msec slower!

Just simple math here where 3*3 + 4*4 = 5*5 for a 3 by 4 by 5 right triangle OK.

But boy can we start cranking out some great formulas now - we are off and running & going to be famous!!

One problem - all the math is right, even looks good, but I can not get any of the resulting formulas to explain why I'm hearing Sonic Booms out there. Say it isn't so - pilots catching up with and going though there own sound! BUMMER for what I thought I knew about sound being the same for all observers (maybe the speed is wrong too). Should I loose confidence in the math or just my idea?

BUT if you want to make a bold leap - and use the same mirrors, and one nano second per foot for the speed of a bouncing light beam - - now your cooking up some really nutty conclusions. Extend them to even more truly nutty ideas and you'll be telling folks light will bend around the gravity of the sun!! And clocks need to be adjusted in orbit!!

The math doesn’t prove it but helps you create the formulas that need to be tested and how to test them. We still need to convince people to perform the tests that confirm them.

Draw confidence from never looking up into the sky and seeing any "sonic" light booms.

I think we have a winner here.

But for being able to build the foundation of the formulas yourself from your own 'Thought Experiments' the light clock is a good one and the geometry and math is easy.

Randall B

- #5

Gecko

- 63

- 0

- #6

jcsd

Science Advisor

Gold Member

- 2,101

- 12

Gecko said:

What your missing is that the second postulate of relativty is "the speed of light is constant in all inertial refrence frames", so the formulas are derived from this fact not vice versa. The second postulate of SR come from experimental observation.

- #7

jcsd

Science Advisor

Gold Member

- 2,101

- 12

[tex]w = \frac{u + v}{1 + uv}[/tex]

where u and v are the velcoties of the two observers in the original frame.

- #8

Gecko

- 63

- 0

ok, thanks for clearing that up. and in the formula you posted, [tex]w = \frac{u + v}{1 + uv}[/tex], would you put .999c and c for u and v? i saw one similar to this but it looked like [tex]u = \frac{v + w}{1 + \frac {vw}{c^2}}[/tex]. i was told that one only worked for speeds under the speed of light. thanks.

Last edited:

- #9

Fredrik

Staff Emeritus

Science Advisor

Gold Member

- 10,875

- 420

Let's use his version of the formula. Let u be the speed of the rocket ship relative to Earth, and v the speed of the beam of light relative to Earth (i.e. v=1). w is the speed of the beam of light relative to the rocket ship.

[tex]w=\frac{u + v}{1 + uv}=\frac{u+1}{1+u\cdot 1}=\frac{u+1}{u+1}=1[/tex]

(u would be 0.999 here, but as you can see, it doesn't really matter what u is).

The others who answered you told you that the constancy of the speed of light is a postulate of SR, and not a consequence of the postulates, and I agree, because I think that what zefram c called his favorite postulate is by far the best way to formulate SR mathematically. But I think it should be mentioned that it's

Share:

- Replies
- 17

- Views
- 635

- Replies
- 36

- Views
- 1K

- Replies
- 11

- Views
- 374

- Last Post

- Replies
- 3

- Views
- 643

- Last Post

- Replies
- 34

- Views
- 2K

- Last Post

- Replies
- 27

- Views
- 1K

- Last Post

- Replies
- 5

- Views
- 750

- Last Post

- Replies
- 1

- Views
- 330

- Replies
- 11

- Views
- 752

- Last Post

- Replies
- 4

- Views
- 223