Why can't we travel faster than light? c is the limit!
Ladies and Gentlemen welcome to the first international space-racing championship!
Please fasten your seatbelts because we are going to start in a few minutes!
I see that you are coming from different part of the world, and this is beautiful.
Before we start, we would like to remind you that there is only one rule for space racing:
you can't go faster than the speed of light. The c constant. It's a limit.
Also, everybody knows that in trying to reach that enormous speed, some weird things would happen,
and I'm pretty sure nobody here tonight wants to be shrunk in a tiny volume.
This is not our fault: it's just how things are.
Everything was well described and explained by Einstein's Relativity Theory and,
although it seems counterintuitive, we all accept the fact that the speed of light is
the same in all reference system and that nothing can go faster than c.
Why is that? Why can't we travel faster than light?
Follow me on this journey to know more about this topic, but remember: c is the limit!
It has long been believed that light has an infinite speed. Daily experience would seem
to confirm this: as soon as a lamp is turned on, the light instantly floods the space. However,
as early as the 17th century, the Danish astronomer Ole Rømer hypothesized that
light had an enormous, but not infinite, speed. Before him, Galileo Galilei had also tried his
hand, but without success. Galileo's experiment involved placing two lanterns at a distance of one
mile and calculating the time it took the light to get from one point to another: together with an
assistant he took a shielded lantern and went to the top of two hills that they were a mile away.
Galileo discovered his lantern, and the assistant on the other hill, as soon as he saw the light,
discovered the lantern in his turn. The Pisan scientist would therefore
have had to measure the time needed to see the light from the other hill and at that point,
it would have been enough to divide the distance by the time to obtain the speed of light.
The experiment did not lead to any results: to travel a mile, light takes about 0.000005 seconds,
an immeasurable value with the tools at Galileo's disposal.
Roemer, working at the Paris Observatory, was not looking for the speed of light
when he found it. Instead, he was compiling extensive observations of the orbit of Io,
the innermost of the four big satellites of Jupiter discovered by Galileo in 1610.
By timing the eclipses of Io by Jupiter, Roemer hoped to determine a more accurate
value for the satellite’s orbital period. Such observations had practical importance
in the seventeenth century. Galileo himself had suggested that tables of the orbital
motion of Jupiter’s satellites would provide a kind of “clock” in the sky.
Navigators and mapmakers anywhere in the world might use this clock to read the absolute time
(the standard time at a place of known longitude, like the Paris Observatory). Then, by determining
the local solar time, they could calculate their longitude from the time difference.
This method of finding longitude eventually turned out to be impractical and was abandoned after the
development of accurate seagoing timepieces. But the Io eclipse data unexpectedly solved another
important scientific problem—the speed of light. The orbital period of Io is now known to be
1.769 Earth days. The satellite is eclipsed by Jupiter once every orbit, as seen from the Earth.
By timing these eclipses over many years, Roemer noticed something peculiar.
The time interval between successive eclipses became steadily shorter as the Earth in its
orbit moved toward Jupiter and became steadily longer as the Earth moved away from Jupiter.
These differences accumulated. From his data, Roemer estimated that when the Earth was nearest
to Jupiter (at E1), eclipses of Io would occur about eleven minutes earlier than predicted
based on the average orbital period over many years. And 6.5 months later, when the Earth was
farthest from Jupiter (at E2), the eclipses would occur about eleven minutes later than predicted.
Roemer knew that the true orbital period of Io could have nothing
to do with the relative positions of the Earth and Jupiter. In a brilliant insight,
he realized that the time difference must be due to the finite speed of light.
That is, light from the Jupiter system has to travel farther to reach the Earth when the two
planets are on opposite sides of the Sun than when they are closer together. Roemer estimated that
light required twenty-two minutes to cross the diameter of the Earth’s orbit. The speed of light
could then be found by dividing the diameter of the Earth’s orbit by the time difference.
The Dutch scientist Christiaan Huygens, who first did the arithmetic, found a value for the speed
of light equivalent to 131,000 miles per second. The correct value is 186,000 miles per second. The
difference was due to errors in Roemer’s estimate for the maximum time delay (the correct value is
16.7, not 22 minutes), and also to imprecise knowledge of the Earth’s orbital diameter.
More important than the exact answer, however, was the fact that Roemer’s data provided the
first quantitative estimate for the speed of light, and it was in the right ballpark.
Roemer returned to Denmark in 1681, where he pursued a distinguished career in both
science and government. He designed and built the most accurate astronomical instruments of
his time and made extensive observations. He later served as mayor and prefect of police of
Copenhagen and ultimately as head of the State Council. Roemer is remembered today of course
not for his high political office but for being the first person to measure the speed of light.
Now, we have a question to answer: why can't we go faster than the speed of light?
To answer this question, we have to recall some of Einstein's relativity.
In 1905, Einstein proposed two postulates. The first is that the laws of nature are the
same in all “inertial” reference frames – i.e. those frames moving at a constant velocity with
respect to one another, in which an object with no net forces on it will appear at rest.
The second postulate states that an observer
in any inertial reference frame will measure the speed of light in a vacuum to be the same.
From his theory, Einstein derived length contraction as well as time dilation,
the equivalence of mass and energy, and more. In particular, he realized that an
observer at rest on a high-speed train will measure the length of a passing high-speed
train as shortened in the direction of its motion, by the so-called “Lorentz factor”
(1–v^2/c^2)^1/2 where v is the relative
velocity between the observer and the other train. (Paradoxically, an observer on that
train will measure the first as shortened by the same factor. Isn’t special relativity fun?)
You can see by yourself what would happen if you reached the speed of light: the Lorentz factor
would become 0, that is, your length would shrink to zero in the direction of motion!
Time dilation indicates that, for an observer in an inertial frame of reference, a clock that
is moving relative to them will be measured to tick slower than a clock that is at rest in their
frame of reference. This case is sometimes called special relativistic time dilation.
The faster the relative velocity, the greater the time dilation between one another,
with the rate of time reaching zero as one approaches the speed of light (299,792,458 m/s).
"We are almost done, but we would like to give you a more precise explanation.
If you surfed the net googling why nothing can go faster than light,
you’ll probably get lots of answers saying “mass would increase to infinity which would require
infinite energy to accelerate and so on...” But you can ignore those - somehow the only
part of relativity that sticks for non-physicists is the “relativistic mass increase” concept,
which causes so much confusion that real physicists usually don’t even use it any more.
The real answer is that you can go as fast as you want.
Do you want to go to Andromeda (2.5 million light-years away) in an hour? Go for it!
You’ve got some incredible engineering problems to conquer, but relativity won’t get in your way.
However, some odd things will happen along the way:
1. No matter how fast you go, light still acts the same:
it speeds away from you at exactly c in every direction, as if you’re standing still. Because
in your reference frame, you ARE standing still! 2. In your reference frame, you’re standing still
and the universe is moving past you at some incredible speed. Due to length contraction,
the universe gets “compacted” in the direction of travel and so Andromeda is NOT 2.5 million
light-years away any more. In fact, it’s less than a light hour away (about 670 million
miles) - which means you can get there in an hour without ever exceeding the speed of light.
3. For the folks back home, your time seems “slowed down”. YOU may think it only took an hour,
but to THEM it took you over 2.5 million years! So again, you never exceeded the speed of light.
So as you can see, no one ever sees anything moving faster than the speed of light.
It’s not that something keeps you from going as fast as you want - it’s
that no matter how fast you go, time and space rearrange themselves in such a way
that no one (you or the folks back home or anyone else) ever sees anything going faster than light.
And it has nothing to do with “mass increase” or “needing infinite energy” or anything like that.
As you can see, we are living in an amazing universe, where often
reality seems to exceed fantasy. And we think it's beautiful that,
making science, we can go deeper and deeper in our understanding of the universe.
All the effort we put in understanding and making science, it's always well repaid:
science will never disappoint us.
"This video ends here! Thanks for watching everyone! What
do you think about the speed of light? Does it amaze you?
Let us know in the comment below! See you next time on the channel!