Do Now: • TRUE OR FALSE?• Two twin sisters say goodbye to each other at the

space station.• One leaves on a space ship flying at a speed close to

the speed of light.• The other stays on Earth.• When the two brother reunite

– The brother who stayed on Earth is 50 years older.– The one you traveled through space is only 10 years

older.

Relativity[special and otherwise]

“It’s all relative!”

• What is the WORST pain you have ever felt in your life?

• Turn and discuss with a small group.• Everyone had a different idea of the “worst

pain” …

• Pain is “relative”, it’s not the same for all observers!• Breaking a bone hurts, so does a paper cut, so

does giving birth ….

What does “relative” mean …

What does “relative” mean …• “Relative” infers that something is not “absolute”

• Turns out that most things are “relative”.– Bug on the ceiling, kid on the ceiling.– Being in pain from breaking a finger, being in pain from breaking

a leg.– Being sad your boyfriend lives in a different city. Being sad your

boyfriend lives in a different country!– Thinking walking 3 miles to school is bad, thinking walking 3

days to school is bad!

• In physics: time, displacement, velocity … most things are relative!!!

• Relativity leads to “dilation”, time, velocity and displacement dilation.

Unsolved MysteriesWho Killed Him? • Why is this whole “dilation” stuff important?

• http://dsc.discovery.com/tv-shows/other-shows/videos/other-shows-stephen-hawking-videos.htm

• “Mad Scientist Paradox”

What is a frame of reference?

• A frame of reference is…– A system for specifying the precise location of

objects in space and time.

• Frame of reference ≈ point of view.• The bug’s ceiling … the kid’s ceiling …

Read page 102 from Holt “Physics”

Relative Motion

• “Mom! Why are you rolling backward!”• Demonstration

– Does everyone see motion the same?– Who is really moving?– Stationary vs. Moving Observer

Relative MotionReview

• Time, velocity, distance, it is all relative.– It changes depending on your point of view

or “frame of reference”

• So what if you are moving … really - really fast?

How about another

Paradox?The Twin Paradox (Part I)

http://www.livephysics.com/forums/viewtopic.php?t=23

http://www.yteach.co.za/page.php/resources/view_all?id=p5_time_distance_relativity_muons_lorentz_fitzgerald_contraction_journey_star_t_page_5

Ok … so that wasn’t quiet as

deadly…

BUT…

How is the Twin Paradox Possible???

• It has do to with relativity and dilation.

• Time, displacement, velocity ARE relative.

• BUT the speed of light IS NOT!• The speed of light is constant!

Speed of Light (c) c= 300,000,000m/s [186,000 mi/hr]

NEVER CHANGES! Stays constant!

•The speed of light is NOT relative! •It is constant, regardless of what is in motion and what is stationary!

How does time dilation work?• http://www.learner.org/vod/vod_window.html?pid=611

– Motion is relative– So light can travel differently for difference observers. [8:20-

10:12]– Specifically, the same light beam can travel different distances

and different amounts of times for different observers. [12:00-14:00]

– Again … but more simply …• http://www.phys.unsw.edu.au/einsteinlight/

– Literally, it means a 1 meter ruler … doesn’t always have to be 1 meter long! [23:00]

Clock at rest

w

Clock moving at v

ct

vt

Two identical light clocks: one at rest, one moving relative to us. The light blips in both travel at the same speed relative to us, the one in the moving clock goes further, so must take longer between clicks. t is the time from one mirror to the other.

8.5m

9.5m

If the speed of light is constant … and one light blip has to travel more distance … then it most take less time to do so.

Time must move slower for the moving observer!

A simple version of the “Time Paradox” and “Time Dilation”

• http://gizmodo.com/5765603/this-animated-video-simplifies-einsteins-complicated-twin-paradox

The faster you go, the more time, displacement or velocity

dilates…

Literally the baseball WILL BE shorter. It won’t just

“look” shorter, it WILL BE shorter!

So lets revisit that Twin Paradox

• http://www.learner.org/vod/vod_window.html?pid=613 – The Twin Paradox is possible because the two

observers would have different frame of reference and different amounts of “time” relative to the other person. [20:36-24:10]

– Therefore, literally and completely, upon meeting again, one would be older than the other!

No hold on … this isn’t something we “think” might be true…

•We have shown time dilation to be true!!!!

“It’s all relative!”

“Put your hand on a hot stove for a minute, and it seems like an hour. Sit with a pretty girl for an hour, and it seems like a minute. THAT'S relativity.” Einstein

Don’t you just LOVE physics!?