Why LIGHT Doesn’t Actually Move | Leonard Susskind

What if I told you that the light from your screen, the light from the sun, the light that lets you see everything around you, has never actually moved, not even a little bit, not even once.

That sounds impossible, right? We

literally call it the speed of light. We

measure it. We know it travels at exactly 299,792,458 m/s.

Scientists have used this speed to define the very meaning of a meter.

Light clearly moves. We watch it travel from flashlights, from stars, from everywhere to everywhere else. But

here's the thing. From light's own perspective, from the uh photons point of view, something utterly strange happens. Something so counterintuitive

happens. Something so counterintuitive that when you truly understand it, you'll never look at a sunset, a glowing screen, or even a candle flame the same

way again. Light doesn't experience

way again. Light doesn't experience traveling anywhere. For a photon, the

traveling anywhere. For a photon, the moment of its creation and the moment of its absorption are the same moment, not similar moments, not nearly

simultaneous, the exact same moment. The photon

doesn't experience the journey because from its perspective, there is no journey. There's no time for a journey

journey. There's no time for a journey to happen in. And this isn't science fiction. This isn't speculation. This is

fiction. This isn't speculation. This is

what Einstein's special relativity has been telling us for over a century. We

just don't usually talk about it because it's so mindbendingly weird. Today,

we're going to understand this properly.

We're going to follow the approach that Leonard Suskin champions building genuine intuition step by step until

this bizarre claim makes perfect sense.

No handwaving. No, just trust the math.

real understanding. Suskin always

insisted that physics education should create what he called physical intuition, the ability to genuinely feel why something must be true, not just

calculate that it's true. As he often reminded his Stanford students, equations are just the language we use to describe the universe. The

understanding must come first. So, let's

build that understanding together. Start

with something completely ordinary.

You're sitting in a car traveling down a highway. How do you experience motion?

highway. How do you experience motion?

Well, you feel the vibration of the engine. You see telephone poles whipping

engine. You see telephone poles whipping past the window. Time passes seconds, minutes, hours as you cover distance.

That's what motion means to us. Distance

covered over time elapsed. It's so basic we don't even think about it. Now

imagine the car speeds up, then speeds up more. You're going faster and faster.

up more. You're going faster and faster.

Distance accumulates more quickly, but time still passes normally, right? A

minute still feels like a minute.

Whether you're driving at 60 mph or sitting still at a red light, that's what our intuition says. That's what

Isaac Newton would have told us. Motion

changes your position in space, but time just flows along on changing indifferent to how fast you're going. But uh

Einstein showed us something remarkable.

Newton was wrong, not a little wrong, fundamentally wrong about the relationship between space, time, and motion. As you move faster through

motion. As you move faster through space, you actually move slower through time. This isn't a metaphor. This isn't

time. This isn't a metaphor. This isn't

philosophical musing. This is measured, verified, confirmed reality. Clocks on

airplanes tick slightly slower than clocks on the ground. GPS satellites

have to account for this effect or your navigation would drift by kilometers.

Particles in accelerators live longer than the stationary twins because their internal clocks are running slow. Time

dilation is real and it gets more extreme as you approach the speed of light. Here's where it gets truly

light. Here's where it gets truly strange. Imagine you could somehow

strange. Imagine you could somehow accelerate to 90% of light speed. Your

time would pass significantly slower than time for someone standing still. If

they measured an hour passing, you might experience only about 26 minutes. You'd

age more slowly. Everything about your internal experience would be stretched out relative to the outside universe.

Now pushed to 99% of light speed. That

hour for your stationary friend might feel like only about 8 and 1/2 minutes to you. Go to 99.9%

to you. Go to 99.9% then 99.99%.

The closer you get to light speed, the more extremely your time dilates. A year

for the stationary observer might be just days for you. Then hours, then minutes. Do you see where this is

minutes. Do you see where this is heading? What happens when you reach

heading? What happens when you reach exactly the speed of light? Time

dilation becomes infinite. Time stops

completely. This isn't an approximation.

This isn't basically zero. At exactly

the speed of light, at exactly 299,792,458 m/s, no time passes whatsoever. Now, you

and I can never reach light speed. We

have mass. To accelerate mass to light speed would require infinite energy, which is impossible. This is actually one of the uh profound insights of

special relativity. The speed of light

special relativity. The speed of light acts as a cosmic speed limit for anything with mass. But photons,

particles of light, have no mass. They

don't accelerate to light speed. They're

born traveling at exactly light speed.

It's the only speed they ever know. They

come into existence at sea and cease existence at sea. There's nothing else for them. And for something traveling at

for them. And for something traveling at exactly light speed, time simply does not pass. Think about what this means.

not pass. Think about what this means.

Really think about it. A photon leaves the surface of the sun. From our

perspective, it takes about 8 minutes and 20 seconds to reach Earth. We can

measure this. We can calculate it. It's

a perfectly valid description from our reference frame. But from the photon's

reference frame. But from the photon's perspective, there is no 8 minutes.

There's no time at all. The moment of leaving the sun and the moment of arriving at your eye happen simultaneously, not almost simultaneously.

Literally the same moment. The photon

doesn't experience the 93 million miles of space between the sun and earth because it doesn't experience the time it would take to cross that space.

Without time, distance becomes meaningless. You can't travel through

meaningless. You can't travel through space if there's no time in which to do the traveling. Leonard Suskin would

the traveling. Leonard Suskin would remind us at this point that uh we need to be careful about our language. The

photon doesn't experience anything in the way you and I experience things. Uh

it has no consciousness, no internal clock, no awareness. But the physics is unambiguous.

In the reference frame of something traveling at light speed, zero time elapses between any two events along its path. This is what I mean when I say

path. This is what I mean when I say light doesn't actually move. Not from

its own perspective. Movement requires

time. Movement is change of position over duration. Without duration, the

over duration. Without duration, the concept of movement dissolve. Let me

give you another way to think about this uh that uh might help solidify the intuition. Instead of imagining space

intuition. Instead of imagining space and time as separate things the way we naturally do physicists since Einstein have understood them as one unified

entity. Spacetime, not space and time.

entity. Spacetime, not space and time.

Spacetime. Four dimensions woven together into a single fabric. In this

picture, everything that exists traces out a path through spaceime. Not just

through space. Through spacetime, your path starts at your birth and extends through every moment of your life, every location, every instant, creating what

physicists call your world line. Right

now, as you watch this video, you're moving through spacetime. Even if you're sitting perfectly still in your chair, you're still moving through time. Time

is passing. You're getting older. Your

world line is extending forward through the time dimension. Even as your space coordinates stay fixed, here's the key insight. There's a fundamental

insight. There's a fundamental relationship between how much you move through space and how much you move through time. If you're stationary in

through time. If you're stationary in space, all your motion is through time.

Time passes at its maximum rate for you.

If you start moving through space, some of your motion gets redirected from the time dimension into the space dimensions. Time passes more slowly. The

dimensions. Time passes more slowly. The

faster you move through space, the more of your motion is spatial rather than temporal. Time passes slower and slower.

temporal. Time passes slower and slower.

And at the speed of light, all your motion is through space. None remains

for time. Time stops completely. This

isn't just a description. It's a deep geometric truth about the structure of spaceime itself. Imagine you're walking

spaceime itself. Imagine you're walking north. You can redirect some of your

north. You can redirect some of your motion toward the east, but that means less of your motion is actually carrying you north. There's a fundamental

you north. There's a fundamental tradeoff. Spacetime works similarly

tradeoff. Spacetime works similarly except the trade-off is between motion through space and motion through time.

Everything in the universe is always moving through spaceime at the same speed, what physicists call C, the speed of light. But that motion can be

of light. But that motion can be directed entirely through time if you're stationary in space or entirely through space if you're a photon. For everything

else with mass, it's a mix. This

perspective reveals something profound about light. A photon isn't really

about light. A photon isn't really moving through spacetime the way we are.

Its entire motion is spatial. It has no motion through time at all. Its world

line in spaceime isn't a line stretching forward through time. It's something

else entirely. Physicists call the path of a photon a null geodessic. null

because the space-time interval along it equals zero. Let me explain what that

equals zero. Let me explain what that means because uh it's crucial. In

ordinary space, we measure distance using the Pythagorean theorem. If you

walk 3 m east and 4 m north, you've traveled 5 m total distance, the hypotenuse of that right triangle. In

spacetime, we have a similar measure called the space-time interval. But

there's a strange twist. Time and space contributions enter with opposite signs.

When you calculate the space-time interval, you're essentially subtracting the spatial distance from the time distance. For ordinary objects moving

distance. For ordinary objects moving slower than light, the time contribution dominates. The interval is positive,

dominates. The interval is positive, which physicists call timelike. More

time passes than space is covered. For

something moving exactly at light speed, something remarkable happens. The time

and space contributions exactly cancel each other out. The interval equals zero. This is a null interval. Zero

zero. This is a null interval. Zero

doesn't mean nothing happened. Zero

means that for a photon, the space-time separation between its emission and its absorption is zero. From the geometry of spacetime itself, those two events, the

photon leaving the sun and the photon hitting your eye are zero distance apart in the full fourdimensional sense.

That's why light doesn't really move.

Uh, its entire existence traces out a path of zero space-time length. The

beginning and end of its journey are the same point, not in ordinary space, but in spacetime. Now you might be wondering

in spacetime. Now you might be wondering if uh photons don't experience time, if their journey has zero length in spaceime, why do we measure light as

taking time to travel? Why does the light from the sun take 8 minutes to reach us? The answer reveals something

reach us? The answer reveals something important about perspective and relativity. The 8 minutes is real for

relativity. The 8 minutes is real for us. We're stationary or moving slowly

us. We're stationary or moving slowly compared to light. Our reference frame is different from the photon's reference frame. In our frame, time passes

frame. In our frame, time passes normally. We wait, we measure, we count

normally. We wait, we measure, we count the seconds, and indeed the light takes 8 minutes to cross that distance. But

the photon has no rest frame in the usual sense because it can never be at rest. It's always going at exactly C.

rest. It's always going at exactly C.

And the math of special relativity tells us unambiguously zero time elapses along its world line.

Both descriptions are correct. Both are

real. This isn't a contradiction. It's

the nature of spaceime. Different

observers can measure different uh times, different uh distances, even disagree about whether events are simultaneous and all of them are right within their own reference frames. What

remains invariant the same for all observers is that space-time interval and for light that interval is always zero. Zuskund often emphasized that

zero. Zuskund often emphasized that truly understanding relativity requires giving up our stubborn attachment to absolute time and absolute space. These

concepts are hardwired into human intuition by millions of years of evolution in a slowmoving world. But

they're not how the universe actually worked. The universe runs on spacetime

worked. The universe runs on spacetime and uh in spacetime light traces out paths of zero length. It doesn't travel

through spaceime. It connects across it.

through spaceime. It connects across it.

Let's push this understanding further and see where it leads. Consider the

most distant light we can observe, the cosmic microwave background radiation.

These photons have been traveling across the universe for 13.8 8 billion years.

From our perspective, they've crossed billions of light years of expanding space. They're among the oldest

space. They're among the oldest messengers in the cosmos. But for those photons themselves, no time has passed.

The moment of their creation in the hot plasma of the early universe and the moment of their absorption in our radio telescopes today are the same moment.

13.8 8 billion years of cosmic history collapse to an instant. This isn't a paradox. It's just what spaceime looks

paradox. It's just what spaceime looks like from light's perspective. Or

consider photons escaping from near a black hole. To a distant observer, light

black hole. To a distant observer, light emerging from just outside the event horizon appears massively redshifted, stretched to longer wavelengths by the

intense gravitational field. From the

distant observer's perspective, those photons take an extraordinarily long time to climb out of the gravitational well. But for the photons themselves,

well. But for the photons themselves, same answer as always. Zero time, no journey, just existence stretched across

spaceime with zero interval. This has

profound implications for how we understand information and causality in the universe. Light doesn't just carry

the universe. Light doesn't just carry information from one place to another.

In a deep sense, light is the structure that connects events in spaceime. The

paths light can take define what physicists call the causal structure of spacetime. Which events can influence

spacetime. Which events can influence which other events. If a star explodes 100 light years away, we won't know about it for 100 years. Not because

information is delayed, but because that's the geometry of spaceime itself.

The null geodessics the paths of zero interval define the boundaries of what can be known and when. This is why light speed acts as a cosmic speed limit.

Nothing can exceed it because doing so would require traveling along paths of negative space-time interval which would mean traveling backward in time from certain perspectives. The geometry

certain perspectives. The geometry simply doesn't allow it. When Suskin

teaches relativity, he always stresses the geometric nature of these insights.

We're not talking about arbitrary rules or mysterious prohibitions. We're

talking about the shape of reality itself. Spacetime has a geometry and

itself. Spacetime has a geometry and that geometry has consequences. Light

not experiencing motion is one of those consequences. But uh let's not stop

consequences. But uh let's not stop here. This rabbit hole goes deeper

here. This rabbit hole goes deeper still. In quantum field theory, the

still. In quantum field theory, the framework that unifies quantum mechanics with special relativity photons aren't quite particles in the classical sense.

They're excitations of the electromagnetic field. When we say a

electromagnetic field. When we say a photon travels from the sun to your eye, we're using particle language for something that's fundamentally a wave phenomenon. From the quantum

phenomenon. From the quantum perspective, the electromagnetic field exists everywhere throughout space. A

photon isn't a little ball flying through the void. It's a ripple of excitation in an everpresent field. The

wave propagates at the speed of light, but the field itself is already everywhere. This doesn't contradict what

everywhere. This doesn't contradict what we've discussed. It enriches it. Whether

we've discussed. It enriches it. Whether

we use particle language or field language, the core insight remains. The

nature of light is fundamentally different from the nature of massive objects. Light doesn't move through

objects. Light doesn't move through space and time the way we do. It

connects events across spaceime in a unique way that our intuitions built for a world of slowmoving objects can barely grasp. There's a famous thought

grasp. There's a famous thought experiment that uh illuminates just how strange this is. Imagine a photon created in a galaxy billions of light

years away. From our perspective, that

years away. From our perspective, that photon travels for billions of years to reach us. During those billions of

reach us. During those billions of years, the universe expands. Stars are

born and die. Galaxies merge. Life

evolves on at least one small planet.

But from the photon's perspective, none of that history happens. The moment of emission and the moment of absorption are identical. The photon doesn't

are identical. The photon doesn't experience the universe aging. It

doesn't witness the cosmic evolution that we can reconstruct from its journey. It simply

exists stretched across a null geodessic of zero length. In a sense, the photon connects the past to the present without experiencing either. It's a bridge in

experiencing either. It's a bridge in spaceime, not a traveler through it.

Suskin has spoken about how these insights changed his own understanding of reality when he first truly grasped them as a young physicist. The universe,

he realized, is stranger and more beautiful than any intuition built on everyday experience could predict. And

yet, once you understand the geometry, it all makes perfect sense. This is the power of physics done right. Not

memorizing formulas, but building deep intuition.

Not accepting strangeness, but understanding why it must be so. Light

doesn't move because time doesn't pass for light. Time doesn't pass for light

for light. Time doesn't pass for light because at light speed all motion is through space and none is through time.

This is because spacetime has a particular geometric structure where these tradeoffs are built in at the deepest level. Every piece connects.

deepest level. Every piece connects.

Every insight supports the others.

That's how you know you're understanding physics rather than just learning facts.

Let me leave you with one more implication that I find particularly beautiful. We often think of ourselves

beautiful. We often think of ourselves as isolated beings, separated from the distant universe by vast stretches of space and time. The light from a star a

thousand lighty years away. We feel

disconnected from whatever is happening there now. But uh consider this. The

there now. But uh consider this. The

photons reaching your eyes from that distant star don't experience the separation. For them, their birth in

separation. For them, their birth in that stellar furnace and their death in your retina are the same moment. The

connection is instantaneous. From the

photons's perspective, in some deep sense, light weaves the universe together. It creates threads of

together. It creates threads of connection across what appears to be an impossibly vast and ancient cosmos. And

those threads have zero length. They

connect without separating. We see light as traveling, as taking time, as crossing distances. But that's our

crossing distances. But that's our perspective. The perspective of beings

perspective. The perspective of beings made of matter, moving slowly through spaceime, trapped in the flow of time.

Light itself exists outside that flow.

It doesn't move through the universe. In

its own strange way, it holds the universe together. As Suskind reminds

universe together. As Suskind reminds us, the goal of physics is not just to predict what will happen, but to understand what the predictions mean

about the nature of reality. Today,

we've seen that reality includes something deeply counterintuitive.

The very thing that makes seeing possible, the very thing that illuminates our world, doesn't experience time, doesn't experience distance, doesn't move in any meaningful

sense from its own perspective. And yet

it connects everything. This is the universe we live in. This is what spacetime actually is. And the more deeply you understand it, the more

profound and beautiful it becomes. If

this journey has sparked your curiosity, and I hope it has so much more to explore. Suskin's book, Special

explore. Suskin's book, Special Relativity and Classical Field Theory from the Theoretical Minimum Series, develops these ideas with all the mathematical rigor you could want while

maintaining his signature intuitive approach. His original theoretical

approach. His original theoretical minimum lecture series, available free online, builds this understanding from the ground up. I'd also recommend

looking into null geodessics and the causal structure of spacetime if you want to see how these ideas connect to general relativity and black hole physics. The rabbit hole goes very very

physics. The rabbit hole goes very very deep. Um for now though, take a moment

deep. Um for now though, take a moment to look around you. The light filling your room, bouncing off surfaces, entering your eyes, none of it is

experiencing the journey you perceive.

Every photon exists stretched across spacetime with zero interval. Every beam

of light is a connection without separation. The universe is stranger

separation. The universe is stranger than we imagine. But it's also more beautiful. And understanding it, really

beautiful. And understanding it, really understanding it, not just accepting it, is one of the greatest joys a curious

mind can experience. If this video helped you see light and spacetime in a new way, consider subscribing to join us

on these explorations. There are many more mysteries to unravel, and we'll tackle each one the suskind way, building intuition step by step until

the strange becomes sensible and the complex becomes clear. Thank you for spending this time with me. time that uh for the photons illuminating your screen doesn't exist at