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. If you are already a patron or a member, join
in Astrum’s new discord server! We’ll be doing a livestream every month
talking about interesting stuff we just couldn’t fit into some choice episodes. If you’d like to join us too, check the
links below. All the best, and see you next time.
