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Language: en
this video was sponsored by kiwo more
about kiwo at the end of the
show I know what you're thinking
clickbait no one has measured the speed
of light that's ridiculous the speed of
light is exactly
299,792,458
m/s we are so sure of it that since 1983
we've actually used the speed of light
to Define how long a meter is it's just
the distance light travels in a vacuum
in 1 over 299792458
e of a second that definition ensures
that the speed of light is exactly this
number no decimals but hear me out in
this video I will prove to you that
light may never actually travel at this
speed and I can say that because no one
has actually measured it we can't
measure the speed of light the same way
we measure the speed of anything else
I think we're recording everywhere what
are we doing this is a video about
measuring the speed of stuff okay tell
me about how you measured the speed of
the baseball fired out of your Cannon
well to get the speed of the baseball
you need to know two things you need to
know the distance between two points and
you need to know the time that it takes
the baseball to travel between those
points so basically we took distance
divided by time and that's the speed of
the baseball and in our case we were
shooting with a high-speed camera so you
basically just count the frames and then
your clock is internal oh you're going
relativity you're going to do something
weird aren't you you saw it coming I
can't believe it oh man the thing I want
to ask you about is the speed of light
okay could you measure the speed of
light like
this imagine you have a laser that can
fire a beam through a perfect vacuum for
1 km start a timer the instant you fire
the laser beam then exactly when it hits
the end stop the clock
except how do you know when Light
reaches 1 km if you and the clock are at
the starting point okay so you need two
clocks one at the laser and one at the
end which stops automatically when it
detects the laser light but now how do
you make sure your two clocks are
synchronized well you could connect them
via a wire and send a pulse from one to
the other but that pulse will travel at
the speed of light so it will arrive
with a time delay you might think you
can just subtract that time delay but it
is equal to the time it takes for light
to travel 1 km that's what we don't know
and are trying to
measure okay new plan start with the
clocks together and sync them up first
and then send one of the clocks down to
the end now what could possibly go wrong
well I'll tell you the clock at the
finish line was moving with respect to
the one at the start and special
relativity tells us moving clocks tick
slow relative to stationary observers so
by the time the clock reaches 1 km it
will no longer be in sync with the clock
at the start can I tell you the only
solution to this problem ditch the
second clock put a mirror at the end to
reflect the light back and use a single
clock at the start to time the full 2 km
round trip wasn't this actually done
before he was on a mountain and there's
a wagon wheel with a lantern and there's
something like a mirror on the other
side of the mountain always wanted to do
this so H that sounds a little like how
the speed of light was first
experimentally measured by epilite feo
in
1849 he sha a beam of Light Between the
teeth of a rapidly spinning gear to a
mirror up on a hill 8 kilomet away and
then by increasing the speed of the gear
he reached a point where the reflected
light passed through the next gap on the
gear and so it was observed so he
measured the speed of light to be
313,000 km/s
which is within 5% of the presently
accepted value so someone has measured
the speed of
light or have they what has been
measured is the round trip or two-way
speed of light but no one has measured
the one-way speed of light one thing I'm
going to throw at you and I'm I'm just
gonna just going to come out and tell
you it's like what if the speed of light
in this direction is different from the
speed of light in this direction then
that's that sounds like a veritasium
video the question is could you figure
it out the kind of crooks of this
problem is that the only way people have
managed to measure the speed of light is
for a round trip no one's ever managed
to measure the speed of light in Just
One Direction it's possible that the
speed of light is half of C in One
Direction and then instantaneous on the
return Journey what that's that's
possible are you
serious think about communicating with
an astronaut branded on Mars let's call
him Mark we send out a signal and get a
response 20 minutes later so we imagine
our signal takes 10 minutes to get there
and the reply takes 10 minutes to come
back but it's possible that our message
took all 20 minutes to get there and the
reply came back
instantaneously there's no way we could
tell the difference between these two
scenarios but why would the speed of
light be different well it's possible
that there is some preferred direction
through SpaceTime I mean our universe
has a lot of symmetries but there is
also some asymmetry for example why is
there so much matter relative to
antimatter and physicists have worked
out internally consistent theories of
physics in which the speed of light is
different forwards and in reverse the
speed of light could vary by just a few
percent up to at the extreme going C
over2 in One Direction and infinitely
fast in the other
direction okay so let me let me let me
figure this out so yeah I I kind of
don't believe you I kind of don't
believe you I don't believe you that
light is a different speed in One
Direction but in the other but I know
you well enough to know that you
wouldn't you wouldn't call me and and
put a camera on me unless you knew you
were right and that's what scares me
about this conversation that scares
me now you might think it is just
simpler that light should travel at the
same speed in all directions but the
truth is that is a convention rather
than an experimentally verified
fact Einstein himself pointed this out
in his famous 1905 paper on the
electrodynamics of moving bodies he
spends the first couple of pages on the
problem of synchronizing clocks at
different locations A and B and he says
there is no way that we can meaningfully
compare the times that measure unless we
establish by definition that the time
required by light to travel from A to B
equals the time it requires to travel
from B to
a he's essentially defining that the
speed of light in opposite directions is
the same and he puts by definition in
italics to remind us that this is only a
convention it's known as the Einstein
synchronization convention so the idea
that the speed of light is the same in
opposite directions as Einstein would
later write is neither a supposition nor
a hypothesis about the physical nature
of light but a stipulation that I can
make of my own free will to arrive at a
definition of
simultaneity that sounds a lot more
subjective than how I think most people
would imagine the speed of light is
defined dude this is
Hardcore I've never thought about this I
didn't think about this before either I
always assumed that when we said the
speed of light is C we we meant the
oneway speed of light there's no way to
define the oneway speed of light so the
only thing we can really Define is the
two-way speed of light just look at the
way Einstein defines C it's for the
round trip from A to B and back I don't
know if you saw in your physics classes
but whenever there was a light clock it
would always bounce the light up and
then back you would never see a light
clock Just Bounce light one way and this
is why the only thing we can be certain
is constant for all inertial observers
is the two-way speed of light
for over a hundred years scientists have
tried to find a way around this to
measure the one-way speed of light by
itself here is a paper published in the
American Journal of physics in 2009 that
claims to measure the one-way speed of
light and here is the paper debunking
this study pointing out that these
authors were actually measuring the
two-way speed of light but I'm imagining
you might have some ideas for how to
measure the oneway speed of light so
let's go through some of them I mean
can't we just use a high speed camera
that shoots at a trillion frames a
second so we can actually see light
passing through an object the problem is
you're not only seeing the light pass
through the object you're also seeing it
bounce back to the camera measuring the
two-way speed not
oneway here you go get a spool of fiber
optic cable you know I don't know like
186,000 miles and then and you could
Shine the Light here and you have the
the other end of the fiber here and you
could shine here and then wait and see
the delay see if it's one second later
over here the thing is like that fiber
is going around and around and around so
it could be that when the light goes
this way over the top of the loop it
goes slower and then when it goes on the
bottom it goes faster it all averages
out in the fiber and you're you're
essentially getting lots of round trips
in that fiber so you're never getting a
one way what if you Center a
synchronizing device between your two
clocks and send out simultaneous pulses
well if the speed of light is the same
in both directions this perfectly
synchronizes your clocks but if the
speed of light is different in each
Direction one of the clocks will be
ahead of the other and it will be ahead
by just the right amount so that when
you measure the speed of light you'll
find the value to be C even though that
was not the speed the light was
traveling this is the same reason GPS
synchronized clocks won't work the whole
GPS system is based on the assumption
that the speed of light is the same in
all directions if the speed of light is
different in different directions the
light Pulses from satellites will travel
at different speeds so the clocks won't
be properly synced by that I mean they
will always measure C for the one-way
speed of light whether it is or
isn't how about starting with
synchronized clocks in the middle and
moving them apart with equal and
opposite speeds that way the time
dilation for each clock will be the same
and they'll still be synchronized when
they reach the end points but again this
only works if the speed of light in each
direction is the same if the speed of
light depends on Direction then so does
time dilation you might think you could
move the clocks really really slowly so
that time dilation is negligible but if
the speed of light is different in
different directions do you can't just
use the standard formula to calculate
what that time dilation would be I mean
it could be a lot worse than you think
so the reality is we're stuck we need
synchronized clocks to measure the
oneway speed of light but we need to
know the One Way speed of light in order
to synchronize our clocks now this might
sound like just an academic concern so I
want to go through an example to
illustrate just how differently the
universe works if the speed of light is
not the same in all
directions let's say on Mars Mark is
trying to synchronize his clock with the
Earth at noon Mission Control sends out
a message that says this signal was sent
at exactly 12:00 when Mark receives this
message he uses the Einstein
synchronization convention to set his
clock he knows the roundtrip time delay
is 20 minutes so he assumes the signal
must have taken 10 minutes to reach him
he programs his clock to 12:10 p.m. and
sends a return message this reply is
sent at
1210 the message is received on Earth at
12:20 p.m. so both parties know the
synchronization was successful when the
clock reads 12:20 on Earth it
simultaneously reads 1220 on
Mars but now consider what happens if
the speed of light is not the same in
both
directions let's say it is C over2 from
Earth to Mars and then instantaneous
from Mars back to Earth no one knows
this of course so they continue to use
the Einstein convention the message is
sent from Earth but now it takes a full
20 minutes to reach Mars but Mark
doesn't know this and as before he
assumes the signal took 10 minutes to
reach him so he sets his clock to 12:10
p.m. even though on Earth it is now
12:20 Mark then sends this reply sent at
12:10 p.m. which is instantaneously
received on Earth at 12:20 Earth
time the experience for the two
communicators is the same the same
messages were received with the same
local time delays but their clocks are
out of sync by 10 minutes what they
think is the same moment for the other
Observer actually isn't and there is no
way they can ever recognize or correct
this error imagine if someone on Earth
Earth immediately responded how long did
this message take to reach you it's now
12:20 well the message would take 20
minutes to reach Mars but due to the
clocks being out of sync it would arrive
at 12:30 Mars time so mark would reply
10 minutes a message that would
instantaneously reach the Earth at 12:40
Earth
time the SpaceTime diagram shows how
there is flexibility in what you
consider to be the same moment at two
different locations and in how you
define the one-way speed of light
Einstein chose the convention where the
one-way speed of light is always the
same but from an experimental
perspective any other convention is just
as valid up to and including one where
the speed of light is c over2 one way
and instantaneous the other
way and in that case it's interesting to
think about what each Observer is seeing
when they look at the other mark would
be seeing the Earth as it was 20 minutes
ago but Earth is seeing Mars in real
time exactly as it is right now and this
effect wouldn't stop at Mars look Beyond
it and you could see stars hundreds of
light years away not as they looked
centuries ago but exactly as they are
right this
instant one of the things is you only
know about that light when it reaches
you and you don't know anything about
what Journey it took to get to you you
just see it and it's there so like it's
instantaneous so an instantaneous
interpretation of that light is just as
good as one where it takes C to reach
us this is breaking my brain yeah yeah I
mean it's it's kind of unknowable isn't
it it is unknowable that's the whole
point of the video is to say we've all
agreed and this is based on something
Einstein wrote in 1905 we've all agreed
to just say it's C in every direction
but the truth is the physics works the
same whether it's C or it's C over2 and
instantaneous or anything in between
as long as the round trip works out to
be see none of physics breaks and that's
the crazy thing so if we can never
measure the one-way speed of light and
if it makes no difference to any of the
laws of physics then what's the point in
even talking about it well that is
certainly one valid perspective in a
debate that has been ongoing since 1905
you know some physicists appeal to aam's
Razer isn't it just simpler if light
travels at the same speed in all
directions most working physicists just
accept the convention and move on with
their lives but I think it's important
to point out that it is just a
convention not an empirically verified
[Music]
fact personally I find it fascinating
that this is something about the
universe that is hidden from us sure the
round trip speed of light is C but does
the oneway speed even have a
well-defined value and if it doesn't
what does that mean for the con concept
of simultaneity when is right now on
Mars and does it even make sense to talk
about things happening at the same time
if they're separated by distance you
know maybe this is an odd Quirk of the
universe and there's no good reason for
it or maybe when physics takes the next
paradigmatic leap our inability to
measure the one-way speed of light will
be the obvious clue to the way general
relativity quantum mechanics space and
time are all connected and we'll wonder
why we didn't see it
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