How Does Gravity Affect Light?

we know that gravity exerted pull on lights and we have an explanation for why well actually we have multiple explanations that all predict the same thing and at first glance these explanations seem to describe completely different causes so what is the true connection between light and gravity or is truth in fact entirely relative [Music] gravity bends the path of light that fact is guaranteed by einstein's general theory of relativity but for some reason we scientists have been convinced of the fact with no good evidence for hundreds of years in 1783 the english clergyman john mitchell proposed that a particle of light gripped by the gravitational field of a sufficiently massive star would slow down stop and fall back and so was the first to predict the existence of black holes mitchell wrote to his good friend henry cavendish on the subject and cavendish followed similar reasoning to predict that a particle of light would be deflected in its path as it passed near a massive object mitchell and cavendish used the same utterly wrong assumptions to do their calculations they assumed that light could be slowed down and that light experiences a force of gravity in the same way that a massive object does they assume that isaac newton's theory of gravity was the full picture and that light behaves like any other particle in response to newtonian gravity but the weird thing is that despite these incorrect assumptions the effects that these guys predicted have proved very real we now know that both gravity and light are much weirder than newton thought we explored some of this in recent episodes when we saw that what we experience as a force of gravity is mostly due to the way mass warps the flow of time but the photon doesn't experience the flow of time it doesn't even have any mass and yet general relativity demands that gravity does affect light in ways eerily close to the predictions of mitchell and cavendish the really hard part is understanding the why of it what is really happening when light interacts with gravity let's see if we can't figure it out in general relativity the best place to start is always the equivalence principle this is einstein's great insight that there's no experiment that can distinguish between the backwards pull due to being in an accelerating reference frame and the downward pull of gravity of the same strength or of the sense of weightlessness in free fall in a gravitational field versus the weightlessness felt in the absence of gravity imagine that you're in a rocket ship accelerating rapidly i'd know to escape giant alien spiders or something a spider breaks through the front hull and you fire your laser at it by the time the beam reaches its target the ship is moving a little faster than when it fired the spider still observes the laser travelling at the speed of light because the speed of light is invariant to all observers but something else does change see light is a wave the distance between the peaks of that wave is its wavelength after the front peak of your laser pulse reaches the spider the ship continues to accelerate which means the second peak has a little further to travel than the first and the third peak further still the distance between the peaks gets drawn out wavelength increases which means frequency and energy drop your laser power rating goes from kill to tickle good news for the giant alien spider not so much for you the equivalence principle tells us that we must experience all the same physics if at rest in a gravitational field say in a fake rocket ship in a hollywood set that for some reason is also being attacked by giant alien spiders light emerging from a gravitational field is stretched out it experiences gravitational red shift and we get exactly the same prediction if we use the fact that time runs slow in gravitational fields any process that generates a photon of light can be thought of as a clock fiat an electric charge pulsing up and down in a radio antenna or an atom vibrating back and forth in a glowing light filament due to its heat these are the ticks of a clock and the frequency dictates the frequency and the wavelength of the photon produced by that motion but from a great distance away those clocks appear to run slow and so the frequency of light emerging from within a gravitational field is lower and if the density of the gravitating body is large enough light emerging from it can be zapped of all energy redshifted so the wavelength is effectively infinite at the event horizon of the black hole gravitational time dilation is so strong that clocks stop and the frequency of photons trying to escape is brought to zero and the bizarre thing is that the density of mass required to produce this infinite redshift is exactly the same as is required to turn a light speed particle around and have it fall back as calculated by mitchell from totally wrong assumptions okay let's tackle an even trickier problem the solving of which shot einstein to his international fame i'm talking about the deflection of the path of light by gravity let's start with the good old equivalence principle again and a spaceship attacked by giant alien spiders this time you fire your laser across the deck perpendicular to the direction of acceleration if you aim straight do you hit let's look at it from the perspective of a non-accelerating observer outside the ship they have to see light travel in a straight line in the absence of any gravity but for that to be the case light must travel a curved path from the point of view of the accelerating frame you aim straight but miss your mark these giant alien spiders are no joke and the equivalence principle tells us that we must see the same bending of the light ray in our stationary rocket ship set in our gravitational field okay so that's the prediction in the case of gravitational redshift we could come up with a physical explanation for the prediction the difference in the flow of time changes the frequency of outgoing light can this gravitational time dilation also explain the bending of a light ray traveling horizontally well tricky because there's no change in clock speed in the horizontal direction only in the vertical but we saw in our previous episode that a gradient in clock speed across the vertical extent of an object is enough to drag it downwards to get technical any massive object has a component of its four dimensional space-time velocity its for velocity in the time direction that temporal component is rotated into the downward spatial direction which translates into falling but light is frozen in time from its point of view it has no time component to its velocity it has in fact no time to lose so if we imagine light as a perfectly narrow ray or even as a massless timeless particle none of our intuitive explanations say that it should be deflected by gravity so let's be smart and do what the smartest guy in the world did let's think about light in an entirely different way einstein was in the business of overturning the ideas of newton so it's not surprising that he turns to one of newton's great rivals the dutch physicist christian huygens disagreed with newton on many things including the idea that light is a particle huygens wave theory of light advanced the field of optics enormously a big part of that was his huygens or huygens fresnel principle the idea is that any wave can be described as an infinite number of point-like oscillations each of which produces new waves the sum of all those waves perfectly describes the future evolution of the original wave for example imagine a single circular ripple on a pool of water at any point in time the expanding ripple can be thought of as an infinite number of sources of new circular ripples or wavelets those wavelets also expand outwards but they cancel each other out everywhere except in the outward direction of the original ripple the result looks like the continuation of the original ripple we can model light in the same way a plane wave of light is just an infinite number of new sources of light that generate the next step in the plane wave this explains why light can turn corners around opaque objects in the phenomenon of diffraction and why the path of light is bent when passing between substances refraction our plane wave reaches the boundary to a new medium with a slower speed of light if it arrives at some angle then the next set of wavelets forming at the boundary will be more closely packed together the fronts of those ripples don't travel as far in the time it takes to make a new wavelet so now if we connect the wavelets to reconstruct the overall wave front we see that the path of light has bent so that's refraction but in a sense light gets refracted by gravitational fields or at least you can model it that way you just need two completely crazy assumptions first the light acts like a very classical 17th century style plane wave so you can use huygens principle and also that the speed of light changes in gravitational fields which sounds counter to everything i've told you but this is exactly what einstein did let me explain why this isn't so crazy after all as you might expect from a theory called relativity stuff is relative the only thing that's not relative is the speed of light everyone observes the same local speed of light 300 000 kilometers per second in a vacuum but notice that i said local speed of light that means everyone measures the same speed of light passing through their own local patch of space but viewed from a distance the speed of light can at least appear to change okay imagine you're looking at the earth from a distance a photon passes by and the amount of time it takes to cross that space is larger than if earth wasn't there that's because of two effects your clock is ticking faster than clocks in the earth's gravitational field and space within the gravitational field is stretched the photon has further to travel through a region of slowed time and both of these effects conspire in the same direction to slow the apparent speed of light of course for someone actually inside the gravitational field the photon is still traveling at the speed of light as it whizzes past them and even if this change in velocity isn't real it's enough to let us use huygens principle at each location perpendicular to a gravitational field the wavefront of light can be thought of as a vertical column of new wavelets but for you tracking this from a distance the effect of speed of light decreases downwards because time slows and space stretches so those lower wavelets become bunched up from your perspective if you track the path of the wavefront by connecting the ripples you see that it bends einstein used this approach to calculate the deflection expected when light passes a massive object the number he got was exactly twice the deflection calculated by cavendish based on newton's gravity and einstein's deflection angle was famously verified by sir arthur eddington who voyaged to the west coast of africa to watch a solar eclipse that enabled him to measure the slight offset in the apparent positions of stars around the sun due to their light rays being refracted in the sun's gravitational field so i guess the question i'd be asking at this point is why did einstein's calculation even work light isn't really a simple plane wave it's a much weirder quantum wave particle thing the fact is none of these pictures really represent the ultimate reality if you solve einstein's equations in a different way you can show that gravity is due to free-falling waterfall of space rather than a gradient in time it's a matter of perspective and how you define your reference frames relativity is relative because so is our universe we come up with our physical explanations and then find that seemingly contradictory explanations work just as well light is a wave and a particle time slows or space flows in gravitational fields but our guiding star is that the universe is deeply self-consistent our explanatory stories are all true in their own regard but are really just narrow slices of some more fundamental reality underlying this generally relative [Music] space-time [Music] you

Channel: PBS Space Time

Views: 315,511

Rating: 4.940258 out of 5

Keywords: Space, Outer Space, Physics, Astrophysics, Quantum Mechanics, Space Physics, PBS, Space Time, Time, PBS Space Time, Matt O’Dowd, Astrobiology, Einstein, Einsteinian Physics, General Relativity, Special Relativity, Dark Energy, Dark Matter, Black Holes, The Universe, Math, Science Fiction, Calculus, Maths, Holographic Universe, Holographic Principle, Rare Earth, Anthropic Principle, Weak Anthropic Principle, Strong Anthropic Principle

Id: OHdV9aO6jaE

Length: 13min 25sec (805 seconds)

Published: Tue Mar 09 2021

Reddit Comments

[u/Hothr]

Giant alien spiders are no joke to the Eagle 5. Hope you don't owe Pizza The Hut any money.

[u/Scizo1]

They’re no joke.