# 24.1 Introducing general relativity  (Page 4/6)

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## The paths of light and matter

Einstein postulated that the equivalence principle is a fundamental fact of nature, and that there is no experiment inside any spacecraft by which an astronaut can ever distinguish between being weightless in remote space and being in free fall near a planet like Earth. This would apply to experiments done with beams of light as well. But the minute we use light in our experiments, we are led to some very disturbing conclusions—and it is these conclusions that lead us to general relativity and a new view of gravity.

It seems apparent to us, from everyday observations, that beams of light travel in straight lines. Imagine that a spaceship is moving through empty space far from any gravity. Send a laser beam from the back of the ship to the front, and it will travel in a nice straight line and land on the front wall exactly opposite the point from which it left the rear wall. If the equivalence principle really applies universally, then this same experiment performed in free fall around Earth should give us the same result.

Now imagine that the astronauts again shine a beam of light along the length of their ship. But, as shown in [link] , this time the orbiting space station falls a bit between the time the light leaves the back wall and the time it hits the front wall. (The amount of the fall is grossly exaggerated in [link] to illustrate the effect.) Therefore, if the beam of light follows a straight line but the ship’s path curves downward, then the light should strike the front wall at a point higher than the point from which it left.

However, this would violate the principle of equivalence—the two experiments would give different results. We are thus faced with giving up one of our two assumptions. Either the principle of equivalence is not correct, or light does not always travel in straight lines. Instead of dropping what probably seemed at the time like a ridiculous idea, Einstein worked out what happens if light sometimes does not follow a straight path.

Let’s suppose the principle of equivalence is right. Then the light beam must arrive directly opposite the point from which it started in the ship. The light, like the ball thrown back and forth, must fall with the ship that is in orbit around Earth (see [link] ). This would make its path curve downward, like the path of the ball, and thus the light would hit the front wall exactly opposite the spot from which it came.

Thinking this over, you might well conclude that it doesn’t seem like such a big problem: why can’t light fall the way balls do? But, as discussed in Radiation and Spectra , light is profoundly different from balls. Balls have mass, while light does not.

Here is where Einstein’s intuition and genius allowed him to make a profound leap. He gave physical meaning to the strange result of our thought experiment. Einstein suggested that the light curves down to meet the front of the shuttle because Earth’s gravity actually bends the fabric of space and time . This radical idea—which we will explain next—keeps the behavior of light the same in both empty space and free fall, but it changes some of our most basic and cherished ideas about space and time. The reason we take Einstein’s suggestion seriously is that, as we will see, experiments now clearly show his intuitive leap was correct.

## Key concepts and summary

Einstein proposed the equivalence principle as the foundation of the theory of general relativity. According to this principle, there is no way that anyone or any experiment in a sealed environment can distinguish between free fall and the absence of gravity.

Why does a Neutron star know as the strongest magnet in the universe instead of a Black hole?
that would be a magnetar- a type of neutron star just like pulsars but with very high B fields.
Kevin
what is the cause of blackhole
a collapsed massive star
donot
A dead star with intense gravity
Saaketh
star collapse remnant greater than 1.4 solar masses
Kevin
what is the difference between An Astronomer and a Cosmologist?
cosmologist only learn cosmology right. an astronomer is learn all about astronomy, including cosmology
donot
Cosmology Is The Study Of The Universe As A Whole, Origin And Development Of The Universe.....
astronomy include cosmology, astrophysics, astrodynamic and astrometry
donot
what is astronomy
A Branch Of Science That Deals With Celestial Objects And Space.....
Joecy
Yes Outer Space.....
astronomy is the study of the universe beyond the borders of the planet.
rizza
what object can protect my pyramid being affected through doorways and interaction with others?
what is big bang
This is the blast of extremely huge sphere contains some unique particles from which our lovely universe begins
Can we say that there is constant gravitational force in outer space instance of 0 gravity
i think garvity is not constat in every place in space,depends on the cosmic that u r near for...
Israel
I think I am right if we imagine a universe in a completely circular sphere or as an atom of some unknown element ... ....
if you think this then think about them who use material made up of that
Ashish
element
Ashish
This is not so interesting
I also think our universe as complex as our brain and as a computer program
yes bro we have not that powers or that is a different dimension which a human brain does not think yet but may be possible after some time
Ashish
a group of seven stars is called
saptarishi
Suresh
Why our earth or other celestial bodies is approx or completely spherically bounded why this is not square ,rectangle..... .I want to know that universal force which bound them
The most stable 3 dimensional shape which keeps the spinning material of that celestial body together in an equilibrium is a sphere. So the shapes are spheres and not squares or something different.
Ujwala
gravity. this force favors spheres and spheroids because spheres allow gravity to act equally in all directions whereas a rectangle or square wouldn't.
Mark
what do you mean by flint glass?
Suresh
which glass is called flint glass?
Suresh
who said Flint glass?
Mark
Stars, planets and moons can be made of gas, ice or rock. Get enough mass in one area, and it’s going to pull all that stuff into a roughly spherical shape. Less massive objects, such as asteroids, comets, and smaller moons have less gravity, so they may not pull into perfect spheres.
Kevin
the mass needed goes by a funny name: the potato radius
Mark
where you heared
what?
Grisha
i mean when u enter to a black hole it will take u a billions of light-years away...
Israel
yes because time slows down in more gravity
Ashish
and they said they are not always black
Israel
i don't understand this
Ashish
what do wanna say
Ashish
are black holes always black? what if there is a black hole with a blue color...
Israel
I guess black hole is called black hole because it was sucking matters that around them, including dark matter.
donot
what do u mean by "dark matter"
Israel
How man.. We don't even know what's inside the Black hole.. Just know that even light can't overcome against the gravity of BH.. So BH don't reflect any type of light..
Pavan
in universe black is not colour like other colours you can't change colour of universe because it is its nature given colour if light reflects and then come to our eyes then we saw the colour but a black hole absorbs light and never reflects it back
Ashish
are you never heard of it? well, when you look at space it never illuminated by star's lights right. space itself always look black or you can say dark although there more than zillion stars around
donot
ok then what happens to the materials which are trapped in BH when it dies
Israel
that is why scientist named them by dark matter
donot
Just quit knows about the event horizon,,,.. We don't know that what's beyond the Singularity of BH.. If any others BH singularity joins other singularity then possible well.. Don't know
Pavan
oky thanks guys...
Israel
what begin the black hole? or what is the original form of black hole? its a exploded star and black hole is just making a new star by collecting matters around them If they die, well they just became a material
donot
in our universe real and natural colour is black and light is a form of energy which emit photons colour is of that objects which are in 3 dimensions and when light reflects from these objects it comes back to our eyes space is not 3d so it is not our dimension
Ashish
Its just my thought, okay. if you wanted to know more ask some professor or search it or read a book about BH
donot
i cant know more about it becaus i had just passed my 12th class
Ashish
Well even scientists are still researching about universe we cannot sure yet, if we don't prove it
donot
its my own thinking i can't prove this now but may be in future
Ashish
really, you want to prove it too
donot
yes i wamt but at present i am a student and i have no contact to reserchers if you are then plz help me
Ashish
am also a student in 11th grade...do u some advice for me...
Israel
from which country you are
Ashish
How much u got in 12?
Pavan
from Ethiopia
Israel
i got 80 percent
Ashish
Ashish
astronomy is world wide..ok which subjects should i pay more attention?
Israel
physics >maths >chemistry
Ashish
ok then thanks again.. stay safe..
Israel
which acid is also called chamber acid?
Suresh
sulfuric because a lead chamber was used to manufacture and process it.
Mark
thank u very much.
Suresh
i heard that black holes are not always black and they can take u to another coordinate...is this true?
They have no color, and no
Cyclone
black holes have no hair.
hair?... what do you mean by that..
Israel
***en.wikipedia.org/wiki/No-hair_theorem
Black holes size gets increase by Eating massive stars and black hole are made from high massive stars who were exploded in supernova.when a black hole ears a star A quasar is formed thus a black hole is not always black
Mr
Black holes can change its shape by attracting all planets and even light
Saaketh
what is hisboson particle
Higgs Boson Is An Elementary Particle In The Standard Model Of Particle Physics, Permeates The Higgs Field Which Gives All Particles Thier Mass.....
Simply.. A god particle..
Pavan
The God Particle Is A Little Miss Leading.....
what you mean by a god particle?
vasu
God Particle Was A Nick Name They Gave It When They Had No Clue What It Was Or Did.....
so they know what it is now?
James
A field call higgs field is one of the field which are present in vacuum, just like electric, quark and gravitational fields.. So basically befor interaction of photos to this field, all are mass less.. But after interaction, electrons get some mass.. Just like a quack.. But there is very less
Pavan
Yes, It Has Been Confirmed By All The Physicists And Engineers At Cern, Large Haldron Collider.....
Interaction between field and hisboson.. So they remain mass less..
Pavan
Simply call.. God Particles.. 😊
Pavan
why space is black?
Hubble's Law, the Doppler Effect, and the lack of objects to reflect light
Cody
Kevin
because it night there man
Grisha
Dark matter!
Pavan
what is the best book in order to study astronomy.
personally i don't use a book, but any book is good
daniel
I know it sounds funny but I picked up a few books including Astronomy For Dummies. I also got a couple from Neil Tyson, he has some simple into books.
josh
tell me any lectures that are available on YouTube for astronomy as a beginner.
Crash course- Astrophysics. that's the beginner course on YouTube.
Kevin
And anything from the brilliant Director of the Hayden Planetarium, Neil deGrasse Tyson
Bill
there is alit on you tube luke bill said NGT has alot of into stuff. Brian Green is good at makin tv thing simple and when your ready Lawernece Krauss is good too.
josh
Amy Mainzer is especially good.
Because Space is Not a Object light can't reflected but it.light simply passes thoughtout the space and about 90%of the space is Dark because their is No star or quasar to Emit light
Mr
could get a suggestion to become a good astronomer
Monisha
monisha sure
Mr
Well You should be curious about space and Astronomy and then you would must become a good Astronomer
Mr
oh thank you
Monisha
you are very welcome
Mr
can i know which course i'll choose to study in higher studies
Monisha
well it requires pHd in the field of Astronomy which would take 5-6 years to complete it
Mr
hello I am Piyush from india, I am a student of class 8th and I want to become an astronomer in future. I am confused that what course I do in college to become an astronomer. please answer me
There are many courses, and they all depend on where you are, and what college or university you go to. I hope this helps!
Cody
Thank you sir
Piyush
Math, Geometry,Trigonometry Calculus, Algebra.....
don't worry bro you would become an Astronomer in NASA
Mr
what is cosmos
a space active evaluation to other sorts of things
Olivia
Cosmos is the whole space of everything happening or destroying creating cosmology which is the study of the whole Cosmos deals with the happening inside the universe but not of astrophysical bodies but of the beginning and the end of the universe
Amritansh