You are currently hurtling through time at the speed of light.
But be careful.
If even a tiny bit of your breakneck temporal velocity leaks into one of the dimensions
of space.
And you’re standing in the wrong place at the time, you will rapidly accelerate to your
doom.
You think I’m kidding?
I just described the true source of gravity.
Don’t look down.
Clocks run slow in gravitational fields.
Our GPS satellites tick faster by a factor of 1-in-a-billion - enough to thrown their
position accuracy off by 11km per day.
In our recent episode, we saw why this gravitational time dilation is inevitable - it follows as
surely as 1+1=2 if we accept the two axioms of Einstein’s relativity theory: that the
speed of light is constant for all observers, and that the weight induced by acceleration
is fundamentally the same as that induced by gravity - the so-called equivalence principle.
But none of this is very satisfying.
We know that gravity must cause clocks to run slow on the basis of logical consistency.
And we know that gravity DOES cause clocks to run slow based on many brilliant experiments.
But I never explained WHY or HOW gravity causes the flow of time to slow down.
And I’m not going to explain it now - because in a sense it’s not true.
Gravity does NOT warp the flow of time.
It’s the other way around - the warping of time causes gravity.
That’s what I’m going to show you right now.
If you didn’t watch the previous episode - do it, though it’s also ok if you watch
this one first.
Just don’t forget.
So how is it that time causes gravity?
Let’s start with … a teapot.
I lA nice china teapot hanging in space, minding its own business.
Absent a gravitational field or any forces, if the teapot starts motionless it stays that
way.
At least, it stays motionless with respect to the three dimensions of space.
Everything is moving through the dimension of time.
We can show this with our old friend the spacetime diagram.
Let’s have just two dimensions of space and so we have space … for time.
We show progression through time as the object moves up.
You could say that it has a positive velocity through time, and zero velocity through space.
OK, now let’s add a second object - something nice and massive … the planet Earth will
do.
We know that the presence of mass and energy warp spacetime - and the most intense part
of that warping is in time - our gravitational time dilation.
Things closer to the Earth move through time more slowly.
We can show this as a bunch of identical clocks.
They tick as they move up.
Clocks closer to the Earth take longer to tick for every tick on a distant clock.
Velocity through time increases away from the Earth.
If we move particles through time according to those velocities, we have this sense of
time flowing in a gradient - faster streams distant from the Earth, slower streams near
it.
Kind of like how water in the center of a stream flows more quickly than the edge, where
the shallower stream drags on that flow.
It’s almost like Earth’s mass creates a drag on the flow of time around it.
So what happens to an object sitting in this stream of time - parts further away from the
Earth age faster, right?
Well, yes, but that’s not all.
We can think of any object as being made of many tiny clocks.
Each atom, each subatomic particle trying to tick at its own rate.
And each of those clocks has a velocity vector in time.
So what’s the temporal velocity of the entire object?
In Einstein’s relativity you have to remember that time and space are not independent of
each other.
Objects don’t just have a velocity through space or through time - they have a velocity
through spacetime.
We call this their 4-velocity.
To get at this, let’s move away from our teapot for a second and talk about boats.
Imagine two boats on an actual stream.
One near the edge moves slow and one near the center moves fast.
The slow boater reaches out an oar which the fast boater grabs.
What happens?
Instinct tells us that the fast boat is pulled tow ards the shore.
We can think of the two objects as becoming one object, and the difference in velocities
across its length causes a torque that rotates the overall velocity vector towards the shore."
It’s the same with the 4-velocity of an object in a gravitational field.
The gradient of velocities cause the overall 4-velocity of the object to be rotated.
All individual 4-velocities start out being purely in time, but the sum is rotated partially
into space.
And it’s always rotated in the direction of decreasing flow - which in a gravitational
field is downwards.
So this is the motion of any object in a gravitational field - it gradually picks up velocity in
the down direction - it accelerates - and it pays for that acceleration by losing velocity
- Decelerating in the time direction.
There's a certain way of interpreting the math of relativity that says that everything
travels at the speed of light.
Light travels at the speed of light through space - obviously enough - and we know that
nothing with mass can reach that speed traveling through space.
But if we interpret time as a dimension like space, then a stationary mass really is moving
at the fastest possible speed in the temporal direction.
This is something we can come back to another time - for now let's go with it.
The 4-velocity of a massive object is pointed almost entirely in the time direction.
On the other hand, light itself travels at the speed of light through space only, and
not at all through time - a photon’s clock is frozen.
You might imagine it’s 4-velocity is entirely rotated out of the time direction into space
- although technically photons and other massless particles don’t have a 4-velocity, which
is defined according to the ticking of your own clock - your proper time - which is zero
for the timeless photon.
In this picture, a falling object trades some of its enormous velocity through time to pay
for a small velocity through space.
To us currency exchange looks favorable for space - a teapot gains a rapid plummet to
its doom for an imperceptible slowing of its clock.
We see the same favorable exchange when we try to convert mass into energy via Einstein’s
most famous equation, E=mc^2 - the speed of light is the exchange rate, and the speed
of light is very large.
By the way, two of my favorite physics channels have great, slightly different explanations
of this effect.
Check out Nick Lucid on Science Asylum and Eugene Khutoryansky’s channel to get their
takes.
So is that it?
Do we now perfectly understand the source of gravity?
Well speak for yourself - I’m still confused.
This raises thorny questions.
Like - what about a particle with no size - supposedly point-like particles like electrons,
quarks, etc.
Well, actually, nothing truly occupies only a single, perfectly defined position in space
- quantum uncertainty means that everything is always at multiple places at once, and
so experiences the gradient of time flow.
But actually general relativity doesn’t need quantum mechanics to explain gravity.
It’s enough to imagine clocks that are infinitesimally separated and we still have our time gradient.
The other thorny question is about light itself.
If photons are already fully rotated into the spatial direction, how is it that they’re
also affected by gravitational fields?
They have no “velocity through time” to trade.
But light DOES bend in a gravitational field - astronomers see it happening all the time
in the effect we call gravitational lensing.
In fact, the imaginary paths of light rays were one the most important tools that helped
Einstein develop both special and general relativity.
So we’d better understand the effect of gravity on the path of light.
To do so we’re going to need to shift our perspective in a couple of mind-bendy ways
to see how the flow of time determines the path of even timeless particles.
And with those new perspectives we’ll get a new insight into the source of gravity that
seems weirdly at odds with everything I just told you - and yet is simultaneously exactly
as correct.
But first you’re going to need to some time to think on everything I’ve just told you
and let it settle, and we’re going to need some time to make that new episode of space
time.
Last time we talked about the gravitational wave background - the ambient buzz of gravitational
waves from the distant and ancient universe.
Which, by the way, we may have detected using a pulsar timing array.
Ivan Kilmoc asks where he can find the audio files of the pulsars that we played in the
episode.
Sorry, I should have linked those in the description.
I’ll link them in the description for this video.
They’re from a number of different radio telescopes, but were collated by the Parkes
Observatory in Australia.
Stu Lora asks if I can elaborate on my comment “the time before the big bang” - which
I mentioned in reference to a potential component of the gravitational wave background.
I was referring to the extremely energetic events during the inflationary epoch - fluctuations
in the so-called inflaton field, or in the final decay of those inflatons at the end
of inflation.
I call this “before the big bang” because many physicists are moving away from the picture
where you have the big bang, then you have an instant of inflation, then regular expansion.
For example, in the eternal inflation model, inflation may have lasted for a very very
long time and still be continuing almost everywhere - but it ceased in isolated bubbles - corresponding
to the formation of a new universe.
It makes more sense to talk about the end of inflation as the beginning of such a universe,
rather than the beginning of global inflation.
So in that case the last instant of inflation IS the instant of the big bang, and gravitational
Kinkusnacht asks whether gravitational waves can be used to test ideas in quantum gravity.
The answer is absolutely.
The most well known prospect is by detecting the signatures of primordial gravitational
waves - waves from the inflationary epoch.
These could be found in the gravitational wave background, but also indirectly through
their effect on the cosmic microwave background.
Interaction of those waves with matter right after inflation may have caused characteristic
patterns in the distribution of matter, which we might now see in the way the CMB light
is polarized.
The BICEP2 experiment claimed detection of these so-called “b-modes” but it turns
out they were wrong.
But the b-modes may be there, and we’re digging deeper to find them.
During inflation it’s believed that quantum gravitational effects would have been very
important, so if we can get any type of signal from then perhaps we can learn something.
No Mercy8008 says that the way I described the boats rocking on the ocean suggests that
I would be an awesome dungeon master.
Hm.
I'd say I'm at best an ok dungeon master.
I think I just rolled a nat 20 on my skill check to describe boats rocking on the ocean.
Or did you fumble your saving through versus being impressed by someone describing boats
rocking on the ocean?
Anyway, thanks for the compliment and for reminding me how much I miss the game.
So... who wants to play?