The Philosophy of Relativity

brontidegrrrMechanics

Nov 14, 2013 (3 years and 7 months ago)

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The Philosophy of Relativity






Dr Cormac O’Raifeartaigh




The

Special Theory of Relativity

(1905) underpins much of modern
science and technology, from our understanding of the cosmos to the
harnessing of nuclear power. However,
the theory remains controversial
among the wider public because many of its predictions are hard to
believe. A short description of the theory and the supporting evidence
will be presented. The philosophical implications of relativity will then be
explored
, with particular emphasis on the prediction that time and
distance are not absolute quantities, but depend on the motion of
observers.




“It’s terribly difficult…”

The theory is not complex, but its implications are
hard to accept


“It’s been disputed…”

The theory has survived thousands of tests



It’s all relative…



Most things are relative, but some are invariant



It’s just a theory..”



It underpins much of modern science, providing an

explanation for the birth of the Universe, the sun’s
energy,

radioactivity and many more…






Venue : F 06

Time: Wed 27/04, 4.15 pm



Special Theory of Relativity




-


One of the world’s most successful scientific theories


-

100 years old this year


-

Dramatic effect on 20
th

century science and technology




-

c
hanged our view of time and space







What is relativity ?



“Modification of traditional physics required to

describe bodies moving at tremendously high speed”




Special Relativity

-

bodies in uniform (non
-
accelerated) relative
motion



General Rela
tivity

-

bodies in non
-
uniform (accelerated) relative
motion






Relativity





Galileo : Object released from a height on the deck of a ship at
rest experiences the same drop irrespective of motion of ship






Principle of Relativity



‘laws of moti
on indifferent as to whether a system is at rest or
moving at constant velocity’



‘no absolute frame of reference for the laws of motion’





Note :

motion of ship cannot be detected inside the ship


In tune with Copernican view of earth


Relativity an
d the Ether





Maxwell
-

electricity and magnetism one phenomenon
-

EM




calculated speed of
EM

= measured speed of light
c






light =
EM

wave



wave must travel through medium : ether


c

= speed of
EM

relative to ether




Absolute frame of reference for
EM

?



Conflict with Galileo’s relativity




-

Experiments undertaken to measure earth’s motion relative

to ether


o

null r
esult















Einstein




-

unsuccessful attempts to detect earth’s motion relative to
the ether


-

induction of an electric current in a loop of wire by a
magnet depends only on the relative motion of magnet and
loop



no absolute frame of reference

for
EM?






The Special Theory of Relativity





Galileo’s relativity must be replaced by new relativity principle
that includes
EM



“…the laws of
all physics

are the same for observers in uniform
relative motion”



From
EM
, Einstein added a second po
stulate



“…
the velocity of light in empty space has a constant value,
independent of its source, for all observers in uniform relative
motion”





EINSTEIN


‘the phenomena of electrodynamics as well as of
mechanics possess no properties corresponding to

the idea of
absolute rest.’


Note : if this could not be predicted by traditional mechanics,
then traditional ideas about time and distance must be revised!




Consequences







L
'

=

L
0
2
2
/
1
c







m
'

=

2
2
0
/
1
c
m





t
'

=

2
2
0
/
1
c
t








Object is longest in its rest frame


Mass is smallest in its rest frame


Time interval is smallest in its rest frame




No absolute reference frame for the me
asurement of

distance, mass or time!




Other

Implications





1.
ULTIMATE SPEED LIMIT





2. Energy assoc with mass increase



∆E = ∆m.c
2



Energy assoc with all mass


E = mc
2


Implications for Time




-

observers moving at high speed relative to one another
would see time running slowly in each other’s reference
frame (
time dilation)



-

two observers in relative motion c
ould measure elapsed
time differently and not even agree that two events were
simultaneous.



-

no absolute reference frame for the measurement of time





time is a 4
th

dimension


Evidence for SR



Mass increase


B
-
rays


Electron atomic orbits


Mass of h
igh
-
energy particles



Ultimate Velocity


Speed of high
-
energy particles





Light emitted by high
-
energy particles



Mass
-
energy



Nuclear fission


Nuclear fusion


Particle anti
-
particle creation



Time Dilation


Atomic vibration


Particle lifetimes


Gen
eral Relativity




1915

extend theory


-

motion of bodies with accelerated relative motion.


-

equivalence principle


-

gravity = geometric curvature of space
-
time.


-

origin and evolution of the Universe



-

incomplete….


-