Accelerating Towards Light Speed

[00:08]: [Music]

[00:12]: welcome back to science click today what

[00:14]: would we see if we approach the speed of

[00:19]: light let's Embark together on a

[00:21]: spaceship which accelerates away from

[00:24]: Earth moving faster and

[00:27]: faster what would we see when con L

[00:31]: accelerating what Optical effects would

[00:34]: occur and if we tried to exceed light

[00:36]: speed by bending SpaceTime like in

[00:39]: Sci-Fi movies what would we

[00:42]: see answering these questions will lead

[00:44]: us to explore many fascinating phenomena

[00:47]: of our universe and immerse ourselves at

[00:50]: the heart of special and general

[00:58]: relativity our spacecraft raft is

[01:00]: initially at

[01:02]: rest once launched we begin to

[01:06]: accelerate we imagine that its

[01:08]: propulsion system allows us to

[01:10]: accelerate constantly as long as we

[01:14]: wish as we gain speed we move away from

[01:19]: Earth by accelerating more and more one

[01:22]: would think that the speed would

[01:23]: eventually become

[01:25]: unbearable however this is not the case

[01:27]: speed has no effect on our body

[01:30]: when two trains cross it is impossible

[01:33]: to feel whether it is our train or the

[01:35]: other which is at

[01:37]: rest similarly inside the spaceship we

[01:40]: cannot feel its speed but only its

[01:42]: acceleration the thrust of the reactors

[01:45]: which presses us against our seats if

[01:48]: this thrust is reasonable it will be

[01:50]: perfectly bearable all along our journey

[01:53]: even When approaching the speed of

[01:58]: light there is nonetheless a risk

[02:00]: associated with very high speeds a

[02:03]: collision at High Velocity a mere Speck

[02:07]: of dust would cause colossal damage if

[02:09]: it hit us fortunately our ship is

[02:12]: equipped with a force field that repels

[02:15]: dangerous objects and allows us to roam

[02:17]: freely through

[02:21]: space as we accelerate a first Optical

[02:25]: effect appears the stars in front of us

[02:28]: which we get closer to seem to gradually

[02:30]: move away the sky contracts before

[02:35]: us to understand think of this common

[02:39]: situation you are in a car driving at

[02:42]: high speed and it starts to

[02:45]: rain the rain falls vertically from top

[02:48]: to bottom but as the car moves forwards

[02:51]: you receive raindrops on the windshield

[02:53]: from face on the rain does not appear to

[02:56]: fall from above but from the front as if

[02:58]: its trajectory were tilted and the more

[03:01]: you accelerate the more the rain seems

[03:03]: to come from face

[03:05]: on in our spaceship the optical effect

[03:08]: is perfectly analogous the stars's light

[03:11]: comes from a certain direction but as we

[03:14]: accelerate the light rays appear to come

[03:16]: more and more from the front their

[03:18]: Direction seems different when we are in

[03:22]: motion this is the aberration of

[03:26]: light as light focuses in front of us

[03:29]: its intensity

[03:32]: increases while behind us the sky seems

[03:35]: to widen and becomes

[03:40]: darker the aberration of light results

[03:43]: in another strange

[03:45]: phenomenon let's imagine we are moving

[03:47]: through an immense grid representing the

[03:49]: fabric of

[03:51]: space ordinarily we would see a straight

[03:54]: grid made up of straight lines

[03:56]: converging in front of us due to

[03:58]: perspective

[04:02]: however by speeding up the aberration

[04:04]: phenomenon distorts the image of the sky

[04:07]: around us as the days go by the grid

[04:10]: seems to contract forward and the

[04:12]: straight lines

[04:14]: Bend if we were to go past an object it

[04:18]: would appear to be slightly angled in

[04:20]: our

[04:21]: Direction This is called Terell Penrose

[04:25]: rotation as we move very quickly images

[04:28]: of objects seem contracted in front of

[04:30]: us and perspective is strongly

[04:35]: [Music]

[04:37]: distorted we often hear that the further

[04:40]: we look into space the further we look

[04:43]: into the

[04:44]: past indeed the light we receive from a

[04:47]: distant star must travel billions of

[04:50]: kilometers this journey is not

[04:52]: instantaneous and it takes him time for

[04:54]: light to reach

[04:56]: us as a result we see the star as it was

[04:59]: in the past when it emitted this light

[05:02]: perhaps several thousand years ago who

[05:04]: knows how it might have changed since

[05:08]: then looking out from our spaceship the

[05:10]: same phenomenon is at work as we move

[05:13]: away from Earth the planet's light takes

[05:16]: longer and longer to reach

[05:20]: us if we could zoom in with a telescope

[05:23]: we would see people on Earth evolving in

[05:25]: slow

[05:26]: motion this is the Doppler effect each

[05:30]: tick of the clocks on Earth takes longer

[05:32]: and longer to reach us we receive light

[05:35]: rays in slow motion and their intensity

[05:37]: seems to weaken as the image of the

[05:40]: planet shifts towards red

[05:44]: color in front of us it's the opposite

[05:47]: effect the ship catches up with the

[05:49]: light and the Stars seem to get brighter

[05:52]: they shift towards blue color and their

[05:54]: clock seem to tick faster

[05:59]: with precise enough equipment we could

[06:02]: measure the aberration and Doppler

[06:04]: effects since the start of our

[06:05]: acceleration in

[06:07]: space however if we decided to end our

[06:10]: journey and return to Earth these

[06:12]: effects would have had no impact they

[06:14]: are only optical illusions due to the

[06:17]: way we receive light while

[06:21]: moving after several 100 days however as

[06:25]: the reactors push us ever faster the

[06:27]: ship begins to approach light speed

[06:30]: at such speeds some phenomena will no

[06:32]: longer be mere optical illusions real

[06:35]: physical effects will kick in with

[06:37]: irreversible

[06:39]: consequences special relativity comes

[06:41]: into

[06:46]: play a first consequence of Relativity

[06:49]: is known as time

[06:51]: dilation our universe is a huge

[06:54]: four-dimensional fabric called SpaceTime

[06:57]: made up of three dimensions of space and

[06:59]: one dimension of time inside SpaceTime

[07:03]: all bodies trace a trajectory since they

[07:06]: all Advance towards their

[07:08]: future before takeoff our ship was

[07:11]: moving in the same way as Earth towards

[07:14]: the future but by picking up speed and

[07:17]: zooming away from Earth faster and

[07:19]: faster our trajectory gradually deviated

[07:22]: from that of the

[07:23]: planet the axis we call time is no

[07:26]: longer aligned with the time axis of

[07:29]: people on Earth if we decided to turn

[07:32]: around and come back our clocks would

[07:34]: have measured a different time and we

[07:37]: would have aged less than earthlings

[07:39]: because our trajectories in SpaceTime

[07:41]: would have been

[07:43]: [Music]

[07:44]: different another key consequence of

[07:47]: special relativity is called length

[07:50]: contraction when a body moves at close

[07:52]: to light speed its length gets

[07:54]: contracted in the direction of its

[07:57]: motion from the space ship's perspective

[08:00]: the entire universe is moving backwards

[08:03]: for us the universe is contracted along

[08:06]: our direction of motion as such the

[08:10]: length of the journey to reach our

[08:11]: destination is shorter than

[08:14]: expected we are not talking about

[08:16]: Optical effects anymore but of a

[08:18]: physical and concrete phenomenon if we

[08:21]: tried to reach a distant star the

[08:24]: journey would genuinely seem shorter to

[08:26]: us it would take less and less time the

[08:28]: faster we move

[08:29]: move thus contrary to popular

[08:32]: misconception it is in fact possible to

[08:35]: travel thousands of light years in just

[08:37]: a few seconds and in the case where we

[08:40]: would move almost at light speed the

[08:42]: journey would even seem instantaneous to

[08:45]: us for the people on Earth however

[08:48]: several thousand years would have

[08:51]: [Music]

[08:53]: passed length contraction is a very real

[08:57]: phenomenon however visually it would be

[09:00]: difficult to

[09:01]: see to understand why imagine that we

[09:04]: cross the orbit of a

[09:07]: planet as it moves very quickly towards

[09:09]: us the planet's length is

[09:12]: contracted but remember the farther we

[09:15]: look into space the farther we look into

[09:17]: the past and from our perspective the

[09:20]: back of the planet is located further in

[09:23]: space than the front of the planet the

[09:26]: image that we receive from the back of

[09:28]: the planet has taken longer to reach us

[09:31]: so we see it as it was in the

[09:34]: past even if it is contracted the planet

[09:37]: does not look like it because we receive

[09:39]: the image of its back with a

[09:43]: delay rather than contracted it appears

[09:46]: to be

[09:47]: rotated this is a phenomenon we have

[09:50]: already seen Terell penrose's

[09:53]: rotation when we move very fast and look

[09:56]: to our side the edges of the object

[09:58]: around us seem shorter in the direction

[10:01]: of motion this can be interpreted either

[10:04]: as length contraction or as a rotation

[10:08]: as a change in

[10:11]: perspective for the same reason a person

[10:14]: watching us soon by would not really see

[10:16]: our ship contracted but rather rotated

[10:19]: on

[10:21]: [Music]

[10:24]: itself over time our field of view

[10:27]: continues to contract it intensifies in

[10:30]: front of us while going completely black

[10:32]: behind

[10:33]: us by accelerating more and more we

[10:36]: would perhaps expect to reach the speed

[10:38]: of light at some point what would we see

[10:42]: then this question might be one of the

[10:45]: most fundamental in

[10:47]: relativity the answer is no even if the

[10:51]: ship accelerates constantly we will

[10:52]: never reach the speed of

[10:55]: light that is because the speed of light

[10:57]: is absolute you may try to catch a light

[11:00]: Ray but from your point of view it will

[11:03]: always Escape at the same speed the

[11:05]: speed of

[11:06]: light you may accelerate as much as you

[11:09]: wish even if from Earth you would seem

[11:11]: to approach the speed of light from your

[11:14]: point of view you are still motionless

[11:16]: and light Escapes

[11:20]: [Music]

[11:21]: inexorably in short it is strictly

[11:24]: impossible to exceed or even reach the

[11:27]: speed of light at best

[11:29]: our ship will continue to accelerate

[11:31]: forever and our field of vision will

[11:34]: continue to shrink ever more until

[11:36]: forming an infinitely bright spot in

[11:38]: front of us surrounded by a completely

[11:41]: black

[11:42]: Sky almost at light speed all Optical

[11:45]: effects would become

[11:47]: Extreme as we catch up with light rays

[11:50]: before us they all seem to come from the

[11:52]: front meanwhile the Rays behind us can

[11:55]: never reach

[11:57]: us in our frame of reference the

[12:01]: universe would be extremely contracted

[12:03]: like a thin fabric that we would cross

[12:05]: from side to side in a fraction of a

[12:08]: second for a person on Earth we'd be

[12:11]: moving at the speed of light but for us

[12:14]: the trip seems instantaneous we move

[12:17]: infinitely

[12:21]: fast to conclude nothing can ever move

[12:25]: faster than light through

[12:27]: space but there might be way to

[12:29]: circumvent this

[12:31]: rule in fact nothing prohibits space

[12:34]: itself from moving faster than

[12:38]: light indeed the theory of general

[12:41]: relativity teaches us that the fabric of

[12:43]: SpaceTime is dynamic it can bend in

[12:46]: multiple

[12:47]: ways we could thus imagine creating a

[12:50]: bubble around our spaceship which we

[12:53]: would Propel faster than light this is

[12:56]: called a warp drive in reference to

[12:58]: science fiction

[13:00]: it would no longer be a matter of

[13:01]: zooming a vehicle through space but of

[13:04]: propelling the fabric of space itself

[13:07]: our ship would remain motionless surfing

[13:10]: on a wave of faster than light

[13:13]: SpaceTime such a structure may seem

[13:15]: absurd and indeed it is currently

[13:18]: thought to be impossible to achieve

[13:20]: producing a warp drive would require

[13:22]: bending SpaceTime with huge amounts of

[13:24]: negative Mass a form of matter that does

[13:28]: not seem to exist in our

[13:30]: universe that said mathematics still

[13:32]: allow us to model and calculate what we

[13:35]: would see in such a

[13:37]: situation if we looked from outside the

[13:40]: warp drive would seem to appear out of

[13:42]: nowhere the curvature of SpaceTime

[13:45]: deviates the trajectory of light rays

[13:47]: forming a moving lens which splits into

[13:50]: two one part moving forwards and the

[13:53]: other retreating in the opposite

[13:56]: direction light travels slower than the

[13:58]: warp drive itself when it approaches

[14:01]: light has not yet had the time to reach

[14:03]: us and after it crosses us we receive

[14:07]: both the light which had not yet reached

[14:09]: us and the rays which were emitted

[14:14]: later looking from the inside of the

[14:16]: warp drive we would see the sky

[14:19]: contracted in front of us and very

[14:21]: bright behind us the universe would look

[14:24]: extremely dark and a whole patch of the

[14:27]: sky would simply vanish from our view

[14:30]: the light emitted from there is too slow

[14:32]: to catch up with us a cone of the

[14:35]: universe remains permanently invisible

[14:39]: [Music]

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