Thanks to Warframe for inspiring me to make today’s video.
Can information travel backwards in time?
It’s the sort of thing that would be really useful, if it were true.
You could tell your past self not to eat that burrito that didn’t agree with you, or could
reveal to yourself the winning lottery numbers.
But it just doesn’t happen; the resulting paradoxes alone make the whole thing laughable.
In our universe, time always seems to flow in one direction – forward.
The idea of travelling backwards in time, or even simply communicating with your past
self, seems so outlandish, it can’t possibly be true.
So, why is it that on the quantum level, information seems to be doing just this?
What?
You’ve not noticed particles communicating backwards in time?
Well, perhaps we need to talk about the strangeness that is quantum mechanics.
It is an effect that, if understood, could one day bring us technologies like faster-than-light
communication, or faster than light travel.
At least, if we can somehow harness it.
But even if we can’t, it’s an undeniably strange insight into the unseen world around
us.
I’m Alex McColgan, and you’re watching Astrum.
You’re about to see some real-world experiments that are mind-bendingly weird.
And if by the end of this video you enjoyed what you learned, feel free to give this video
a like, and subscribe to the channel.
Just, please don’t break causality when you do.
So, what do I mean by particles travelling backwards in time?
By all accounts, it doesn’t seem possible.
In previous videos I’ve mentioned that objects would require infinite energy to even go fast
enough to reach the speed of light.
So how could something go so fast as to reverse the usual direction of time, and arrive at
a destination not just instantly, but before they left?
Not even light can do that, and it’s the fastest thing we know of.
Well, this rule about nothing travelling faster than light is mostly true, for the macro-scale
universe that we live in.
And by macro-scale, I mean everything significantly larger than an atom.
But physicist John Stewart Bell noticed an exception to this rule when it comes to quantum-entangled
particles.
Ok, so let’s start there.
What is a quantum-entangled particle?
In quantum physics, it’s possible to hit two particles together in such a way as to
link them together, so that by measuring the one particle, you learn things about the other.
For instance, if you know that the particles originally had a total of 0 momentum, and
you learn the momentum of one of the new quantumly entangled particles, you know the momentum
of the other particle will be the exact reverse – making sure that the total remained 0.
Effectively, by measuring the one particle, you can learn things about the other.
This works for other particle properties too, such as position, polarisation, or spin.
On the surface, there’s nothing too weird about this.
It’s no different from me meeting up with a friend, and discussing our plans for the
evening.
We agree to go out, and we agree that I will pay for the evening and my friend won’t.
Then, no matter how far we go on our night out, or even if we at some point separate,
I know that I will be paying, and my friend will know that he won’t.
This is how Einstein thought it worked.
Only, it turned out that Einstein was wrong.
Because as it happens, me and my friend did not discuss in advance who would be paying.
And strangest of all, we still both agree with each other anyway, 100% of the time,
no matter how far apart we are.
This is the strange thing about quantum entanglement, and quantum physics in general.
We like to think of particles as having fixed properties.
However, our first mind-bending experiment shows that particles only have properties
when you detect those properties.
Until then, they’re kind of vague about the whole “properties” thing, instead
only relying on probabilities, as defined by a quantum wave equation.
This doesn’t make sense intuitively.
Looking at a thing shouldn’t be what gives it properties…
right?
Well, how would you know?
If a tree falls in the woods, does it make a sound?
According to quantum physics, not necessarily.
Let’s talk about that first mind-bending experiment, the Bell experiment.
The maths for this is pretty complicated, but bear with me, it’s worth the ride.
This experiment was instrumental in our modern day understanding of quantum physics, and
closing off its loopholes earned Alain Aspect, John F. Clauser and Anton Zeilinger the nobel
prize for physics in 2022.
The experiment was first conceptualised by John Stewart Bell, who wanted to know if particles
really did have secret properties that they carried around with them, known as hidden
variables, or whether they really were making some of it up on the spot.
He noticed an interesting mathematical fact about the spin of particles.
Before we go any further, I should probably mention that quantum spin isn’t the same
as normal spin.
Misleadingly, quantum spin actually defines whether a particle is influenced – pushed
or pulled – by a magnetic field.
The name isn’t important, but it is important to note that these particles aren’t actually
spinning, and so can have different “spin” values in almost any given direction.
Now let’s take two quantum-entangled particles, and let’s say that we’ve arranged it so
that their spin adds up to a total of 0 between them.
This means that if one particle would be pulled by a field, the other will be pushed by it
an equal amount along that direction (with the understanding that this doesn’t tell
you anything about their spin in other directions).
One of the features of quantum spin is that if we measure an entangled particle’s spin
in a given direction, let’s say up and down, it will have a 50% chance to be spinning up,
and an equal 50% chance to be spinning down.
But remember, once you measure the other entangled particle, it will have a 100% chance to be
spinning in the opposite direction to the first particle.
On this fact alone, there’s no way to tell if the two particles already knew their spin,
or are somehow deciding it on the spot and conferring with each other now that they’ve
been asked.
But Bell noticed a clever thing, by asking a clever question.
If you measured two quantum-entangled particles from two randomly selected directions, what
are the odds that their spins for different directions would match?
Let’s define that any time a particle is spinning towards a detector, its spin is “up”,
and any time it is spinning away from a detector its spin is “down”.
What are the odds that both particles would be spinning “up-up”, or “down-down”
when tested, and what are the odds they would contrast?
Let’s formalise this with a little experiment.
Here, we have two entangled particles, with three detectors reading their spin in different
directions.
If particle A and particle B are both read with the top detector, then one of their spins
will be up and the other will be down.
They are entangled, and this is what we looked at previously.
However, if Particle A is read using the top detector, while particle B is read with one
of the other two, these two directions of spin aren’t opposites, so Particle B has
more flexibility in which way it goes.
Quantum physics claims the particles are making up their attributes on the spot, so once you’d
measured the spin of particle A using the top detector, it was a 50:50 whether the spin
on the other particle, using one of the other detectors, would match or contrast.
But this is not what classical physics predicted.
Let me show you what I mean.
Classical physics claims that particles each carry around secret information defining their
spin in any given direction.
So, for our 3 tested directions, each particle would have a value already.
They aren’t making it up on the spot.
Let’s say hypothetically our particles hidden information states “Up Up Down” for particle
A, and “Down Down Up” for particle B, as B must be opposite to A for each of the
directions 1, 2, and 3.
Let’s pick out a random detector for A.
We select detector 1.
Detector 1 tells us that Particle A is spinning Up.
Now let’s select a random detector for particle B.
We select 1 there too.
This detector gives us a reading of Down.
1-1 Up/Down We can actually map out all the possible outcomes of this process of random
selection in this graph.
There are 9 possible outcomes if you were to only measure from two detectors at a given
time: 1-1, 1-2, 1-3, 2-1, 2-2 and so on.
For each of these possible selections, we have fixed hidden variable results that we
know already, because we hypothetically defined them earlier.
Let’s fill them in now.
Of course, if you test particles using the same detector on both particles, you’ll
get a contrasting result because they’re entangled, but we’re not interested in these
results.
Classical physics and quantum physics both agree on this.
So, let’s remove them.
What are the odds that two different detectors for Particle A and B will see the same result,
and what are the odds they’ll differ?
Remember, quantum physics expected it to be 50:50.
Particles are making up their values on the spot, and so it’s perfectly random which
they’ll choose, as they aren’t confined by the opposites rule.
But in this table, classical physics says that contrasting results only happen a third
of the time.
The other times, they’re either both up, or both down.
If we do this many times, assigning different directions each time, and ignore exceptions,
for instance where the spins of the particles are all Up-Up-Up or Down-Down-Down - once
you crunch the numbers, the important thing to take from all of this is that according
to this maths, classical physics predicts a matching outcome 55% of the time, while
quantum physics continues to simply predict 50%, pretty table be damned.
This percentage difference was key.
By quantumly entangling particles, and running this test over and over again, you could now
see which percentage was correct.
And it turned out the winner was quantum physics.
Particles were just apparently making up their spin results on the spot.
Which is spooky.
Because not only does that call into question our perceptions of reality itself, but that
also means that the moment one particle decided on its spin result, its quantum-entangled
partner instantly knew that that decision had happened.
You could test both particles at once, no matter the distance, and this same result
would come back.
Somehow information had travelled from the one particle to the other in no time at all,
far faster than light itself.
So already something strange was going on here.
This result disproved Einstein’s predictions, and showed that some information does seem
to go faster than light.
But we can take this one step further, and have information going back in time.
There is another experiment, known as the “delayed choice” test.
Its primary purpose was to explore the fundamental nature of light – whether it was a wave,
or a particle, and to figure out when it decided to be one or the other.
Experiments like the double slit experiment had done this in the past, to mixed results.
Sometimes light behaved in a wavelike manner, creating interference patterns on detectors
that could only happen if it was a wave interfering with itself.
But sometimes it behaved like a particle, hitting only a single point on a detector.
But most baffling of all, it seemed to change which it behaved like depending on whether
you were observing its path through space or not.
If it could go through multiple paths, and no one was watching to see which it did go
through, light simply went through both, like a wave.
But observed?
It went through just the one, like a particle.
This result was baffling enough, and deserves a video of its own, but in 2006 a number of
scientists took it one step further by asking an interesting question: what would happen
if you tried to observe the light after it had to pick a path?
Consider the experiment: A single photon is sent into a Beam Splitter, with a 50/50 chance
of either being allowed to carry on its way along path 1, or getting reflected up along
path 2.
Once on either path, the photon is bounced off mirrors, with both paths reconverging
here, where another beam splitter is inserted.
Once again, the photon has a 50/50 chance to go either way, with an even chance of arriving
at one of the two detectors.
If light were just a particle, sending a single photon into this experiment would give you
an even chance of it arriving at the one detector or the other.
You’d not be able to tell which way it went, as the two beam splitters make that impossible
to know, but you could see where it ended up.
However, this does not occur.
When the second beam splitter is present, the light produces an interference pattern,
indicating that the single photon went down both paths, ultimately bumping into itself,
before moving on to both detectors.
This seems like strong evidence that light is a wave; it certainly behaves like one here.
But what happens if you remove the second beam splitter?
Suddenly you know which path the light travelled down – if light arrives at the top detector,
it must have arrived from path 1.
If it arrives at the side detector, it must have come along path two.
And something about this knowledge spooks the light.
It stops going down both paths, and suddenly each photon only arrives at one detector.
Here’s the question – what happens if you insert the beam splitter after the photon
has already started down either one or both routes?
This is why the test is called “delayed choice”.
If you delay choosing how exactly you intend to detect the photon, whether by knowing which
path it came down or making that ambiguous to you, what happens to the light?
What happens is a very strange thing.
When this experiment was performed, it was done multiple times, with the beam splitter
randomly being inserted or not, but always being inserted after the photon had entered
one or both paths.
And yet, the results came back unequivocal.
If the beam splitter was present, the photon suddenly, and seemingly retroactively, stopped
picking a path.
If the beam splitter was removed, the photon seemingly knew it would later be detected
and picked a specific path to accommodate.
Somehow, the beam splitter being added or removed in the future changed what the photon
did in the past.
So, what is happening here.
? Is it really true that particles somehow saw the future?
Did the experiment cause information to be sent back into the past?
Or is there some other principle at play here that explains this whole thing; that accounts
for the instant transmission of information between quantum particles, and allows it to
be perfectly rational that light could travel down one path or both at the same time.
Personally, I’m inclined to think that this is more likely.
We clearly don’t understand what is happening here.
But it must be admitted; if we don’t understand what is happening, there’s nothing saying
that causality isn’t being ignored.
In some way, maybe on the quantum level time really is more fluid than it is up here in
the larger universe.
Maybe space and time simply do not apply down there.
And maybe one day someone will be able to come up with a theory that allows all these
strange phenomena to make finally make sense.
Until then, we’ll just have to keep asking the same question: Can information travel
backwards in time?
Information is not the only thing that would be useful if you could go back in time.
If you could rewind the clock, you could avoid past mistakes, and overcome challenges and
opposition – perhaps even avoiding your own death.
In the brand new open-world expansion for Warframe – the Duviri Paradox – such time-looping
abilities might be just what is needed to stay alive.
This is a game I’ve enjoyed for many years so I’m excited to see this upcoming expansion.
The Duviri Paradox brings a rogue-like element to Warframe’s slick space-ninja gameplay,
putting you in the shoes of The Drifter as you attempt to escape an enormous, open world,
that transforms based on the mood of its ruler - the Child King - Dominus Thrax.
New players can begin playing it straight away on PC, Xbox series X, PS5, Nintendo Switch
and more, and if you sign up and download Warframe using my link and promo code in the
description below, you can claim a free bundle of items that are ideal for getting started,
such as the Braton assault rifle and a 3-day affinity and credit booster.
I’d highly recommend that you give it a look!
Thanks for watching.
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All the best, and see you next time.