# Space, Time and EinsteinPage 2

#### WATCH ALL SLIDES

“Absolute Space, in its own nature, without regard to any thing external, remains always similar and immovable. Relative Space is some moveable dimension or measure of the absolute spaces; which our senses determine, by its position to bodies; and which is vulgarly taken for immovable space . And so instead of absolute places and motions, we use relative ones”

There exists absolute space, but we measure space only relative to other objects in space

Isaac Newton (1643-1727)

Slide 11

## Time for Sir Isaac Newton

“Absolute, True, and Mathematical Time … flows equably without regard to any thing external … Relative, Apparent, and Common Time is some sensible and external (whether accurate or unequable) measure of Duration by the means of motion, which is commonly used instead of True time; such as an Hour, a Day, a Month, a Year.”

There exists an absolute time, but we measure relative time only

Isaac Newton (1643-1727)

Slide 12

## Einstein’s genius

Change the definition of time:

“It might appear possible to overcome all the difficulties attending the definition of ‘time’ by substituting ‘the position of the small hand of my watch’ for ‘time’ ”.

Example:

By “ ‘That train arrives here at 7 o’clock,’ I mean something like this: ‘The pointing of the small hand of my watch to 7 and the arrival of the train are simultaneous events’ ”

- famous 1905 Einstein paper

Slide 13

So what?

From the point of view of Jack, lightening struck both train cars at the same time

From the point of view of John, lightening struck car B first and then car A later

Simultaneity is relative - things occurring at the same time at one place may occur at different times in another place!

Slide 14

So how do we measure things?

John sends out a signal to mark the ends of the train that he’s on

From the point of view of someone on the ground, the signal takes longer(shorter) to reach the right(left) side of the train

Relativity of simultaneity -> relativity of distance! A moving train appears to be shorter!

Need signals to talk about space

and time!

Slide 15

How do we measure time?

Imagine a clock that measures time by how long it takes light to bounce back and forth …

Relativity of distance -> relativity of time!

c=speed of light

Slide 16

## Putting it all together

The only way we can measure time and distance is by sending signals

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