Physicists Claim They Can Send Particles Into the Past

Summary

In this video, the speaker clarifies the misconceptions around the idea of sending particles back in time, as suggested by a New Scientist article. They explain that particles don't inherently move forward or backward in time, and the concept of retrocausality doesn't imply actual time travel. The discussion then shifts to a quantum mechanics experiment involving entangled qubits, where post-selection improves measurement accuracy without altering the past. The video concludes by highlighting the practical benefits of this approach for detectors with dead times, debunking the notion of time-traveling particles.

Takeaways

• 🔬 The concept of sending a particle into the past is considered meaningless in physics as particles don't inherently move forward or backward in time.
• 🌀 The speaker expresses a fondness for retrocausality and uses humor to discuss the idea of warning their past self about modern technology.
• 📈 The script discusses the technical aspects of space-time diagrams and the lack of inherent directionality in the path of a particle.
• ⚛️ It explains the concept of entanglement in quantum mechanics, where two particles are correlated in a property but the value of that property is unknown until measured.
• 🧲 The script uses the analogy of 'sniffing a T-shirt' to describe the process of making a precise measurement in quantum systems by observing changes in a quantum particle's spin.
• 🔍 The idea of post-selection in quantum mechanics is introduced, where certain outcomes are selected after many have been produced, which is likened to retroactively determining the spin of a particle.
• 🤔 The speaker argues against the notion that post-selection equates to time travel, using the example of blocking disagreeing comments on social media to illustrate the point.
• 🛠️ The practical application of the discussed quantum mechanics concept is highlighted, showing how it can improve the accuracy of measurements and reduce the frequency of necessary measurements.
• 😅 A humorous comparison is made to standup comedians and the 'dead time' of detectors, emphasizing the practical benefits of selecting only the best probes in measurements.
• 💡 The script concludes by emphasizing that while the discussed quantum mechanics concept is useful, it does not involve sending particles back in time.
• 🎓 A promotional offer for Brilliant.org is presented, suggesting it as a resource for learning more about the science behind the discussed topics.

Q & A

What is the main topic discussed in the video script?

The main topic discussed in the video script is the concept of retrocausality and the idea of sending particles into the past, with a focus on a recent article published in New Scientist and an experiment related to quantum entanglement and post-selection.

What is the speaker's stance on the possibility of sending a particle into the past?

The speaker argues that the statement of sending a particle into the past is meaningless, as particles don't go into the past or future; they simply exist at certain positions at certain times in a quantum state.

What is the concept of retrocausality?

Retrocausality is the concept that cause may sometimes follow effect, challenging the traditional understanding of causality where effect follows cause.

What is the significance of entanglement in the context of the discussed experiment?

Entanglement is significant because it allows for the correlation of properties between two particles without knowing the value of those properties for each individual particle, which is used to improve the accuracy of measurements in the experiment.

What is the standard interpretation of quantum mechanics regarding the measurement of a particle's property?

The standard interpretation of quantum mechanics suggests that a particle's property, such as spin, does not have a definite value until it is measured.

What is the practical advantage of the procedure discussed in the script?

The practical advantage of the procedure is that by selecting only the best probes, the frequency of measurements needed to reach a certain accuracy is reduced, which is beneficial due to the 'dead time' of many detectors after a measurement.

What is the concept of post-selection in quantum mechanics?

Post-selection is the process of selecting certain quantum particles after producing many more, which can be used to improve the accuracy of measurements but does not imply changing the past.

How does the script relate the concept of post-selection to social media interactions?

The script uses the analogy of posting a comment on social media and then blocking those who disagree, which is similar to post-selection in that it does not change the past but only affects the present perception.

What does the script suggest about the practicality of the discussed experiment?

The script suggests that while the experiment does not involve sending particles into the past, it is a useful idea with practical applications in improving measurement accuracy and efficiency.

What is the speaker's opinion on the sensational headline about particles going back in time?

The speaker considers the headline to be nonsense, as it misrepresents the actual scientific findings and concepts discussed in the paper.

What is the role of Brilliant.org in the script?

Brilliant.org is mentioned as a resource for learning more about the science behind the topics discussed in the video, with a special offer for users of the channel to try out the platform.

Outlines

00:00

🔬 Particles and Time: A Misunderstanding

This paragraph discusses the misconception of particles traveling through time, as suggested by a New Scientist article. The speaker clarifies that particles do not inherently move forward or backward in time; they simply exist at various positions at different times. The concept of time direction is more philosophical than physical, with forward and backward time travel being meaningless for a single particle. The speaker also explains the technical aspects of space-time diagrams and the role of entropy in defining time's arrow. The paragraph concludes by debunking the headline's claim and introducing the actual subject of the paper, which involves entangled qubits and the concept of post-selection in quantum mechanics.

05:02

📈 Practical Applications of Post-Selection in Quantum Measurement

The second paragraph delves into the practical advantages of the post-selection process in quantum measurements, as outlined in a scientific paper. By using entangled particles, researchers can improve the accuracy of measurements on quantum systems. The process involves sending one entangled particle past a probe and then measuring it to determine if it was a good probe. If not, the other particle is discarded; if it was, it is measured. This method enhances the efficiency of measurements by reducing the number of attempts needed to achieve a certain accuracy, which is beneficial given the 'dead time' of many detectors after a measurement. The speaker humorously suggests that if particles could truly be sent back in time, researchers would be trading bitcoin instead of writing papers. The paragraph ends with a promotion for Brilliant.org, a platform offering a variety of science and math courses with interactive visualizations and follow-up questions.

Keywords

💡Retrocausality

Retrocausality is the concept that cause can precede effect in time, challenging the traditional understanding of causality where effects follow their causes. In the video, the concept is mentioned as a fondness of the speaker and is used to set the stage for discussing the possibility of sending particles into the past, which is a central theme of the video.

💡Quantum Particle

A quantum particle refers to a particle that is described by the principles of quantum mechanics, exhibiting properties such as being in multiple states or positions simultaneously. The video script discusses quantum particles in the context of their position in time, stating that they do not inherently move 'forward' or 'backward' in time, which is a key point in the discussion of time travel.

💡Space-Time Diagram

A space-time diagram is a graphical representation that combines space and time into a single continuum, often used in physics to visualize the trajectory of objects in space-time. The script mentions this concept to illustrate that the path of a particle does not have an intrinsic direction, which is crucial for understanding the video's argument against the possibility of time travel.

💡Entropy

Entropy, in physics, is a measure of the number of possible microscopic configurations of a system, often associated with the degree of disorder. The video uses entropy as an example to explain a phenomenon that might appear to be moving 'backwards' in time, such as water droplets lifting from a puddle, to contrast with the behavior of a single particle.

💡Entanglement

Entanglement is a quantum mechanical phenomenon where two or more particles become interconnected such that the state of one particle is instantly correlated with the state of the other, regardless of the distance separating them. The script discusses entanglement in the context of improving the accuracy of measurements in quantum systems, which is a significant part of the video's explanation.

💡Post-Selection

Post-selection refers to the process of selecting certain outcomes after a quantum system has been observed multiple times. The video script uses the analogy of post-selection to explain how certain measurements can be discarded to improve the accuracy of quantum measurements, which is likened to retrocausality but is not actually time travel.

💡Measurement in Quantum Mechanics

In quantum mechanics, measurement refers to the process of determining the value of a quantum system's property, such as its position or spin. The script discusses how measurement affects the state of a quantum system and is central to the concept of post-selection and the debate over time travel.

💡Probe

In the context of the video, a probe refers to a quantum particle used to measure properties of another system, such as a material or a magnetic field. The script explains how the process of using a probe and then post-selecting based on its measurement can improve the accuracy of quantum measurements.

💡Dead Time

Dead time, in the context of detectors, is the period after a measurement during which the detector cannot make further measurements. The video script mentions this to explain the practical advantage of post-selection, as it allows detectors to have rest periods between measurements.

💡Bitcoin

Bitcoin is a digital currency and is mentioned in the script as a humorous aside to illustrate the point that if time travel were possible, people would likely use it for financial gain rather than writing scientific papers. It serves as a cultural reference to emphasize the video's stance on the impossibility of time travel.

💡Brilliant.org

Brilliant.org is an online platform for learning various subjects, including science and mathematics. The script mentions it as a resource for viewers to learn more about the scientific concepts discussed in the video, offering a special promotion for users of the channel to try out the platform.

Highlights

The concept of sending a particle into the past is discussed as a meaningless statement in the context of particle physics.

Retrocausality is mentioned as a fond topic of the speaker, with a humorous reference to warning oneself about future events.

The idea that particles do not inherently move forward or backward in time is introduced, challenging the common perception of time's directionality.

A space-time diagram is used to illustrate the lack of directionality in the path of a particle.

Entropy increase and the unusual phenomenon of water droplets lifting from puddles are given as examples to discuss the concept of time's arrow.

The paper on entangled qubits and their role in improving measurement accuracy is summarized, highlighting a recent experimental confirmation.

Entanglement is explained as a correlation between particles without knowing the value of the property for each individual particle.

The process of measuring a quantum system's property by using entangled particles is described, likening it to 'sniffing a T-shirt'.

The concept of post-selection in quantum mechanics is introduced, where certain particles are selected after many have been produced.

A social media analogy is used to explain post-selection, comparing it to blocking disagreeing comments to alter the perception of past interactions.

The practical advantage of the discussed procedure in reducing the frequency of measurements needed for a certain accuracy is highlighted.

The issue of 'dead time' in detectors after a measurement is explained, and how selecting good probes can mitigate this.

A humorous remark is made about what the researchers would do if they could truly send particles back in time, suggesting they would not be writing papers.

Brilliant.org is promoted as a resource for learning more about the science behind the discussed topics, with a special offer for channel users.

The offer includes a 30-day trial and a 20% discount on the annual premium subscription for those using the provided link.

Transcripts

Can you really send a particle into the past?  Someone asked me this last week because New  

Scientist published an article about this. You  all know I am fond of retrocausality and would  

really like to go back and time to warn myself  about this YouTube thing. So I’ve had a look.

Can you send a particle into the past?  The answer is that that’s a completely  

meaningless statement. Particles don’t go into  the past or into the future. Particles are simply  

at some position at some time, or if it’s a  quantum particle at several positions. You see,  

a particle that goes left forward in time  is the same as a particle that goes right  

back in time. Forward, backward, doesn’t mean  anything. Unless you're a philosopher. Then  

it means everything and also nothing at the  same time. Schrödinger’s meaning, if you will.

Somewhat more technically, if you draw the curve  of a particle into a space-time diagram. then that  

doesn’t have a direction. You can really only talk  about things going forward or backward in time if  

you have something that tells you which direction  is forward, for example from entropy increase. 

If you see water drops lifting up from  puddles and disappearing into clouds,  

you could say, well, those seem to be going  back in time. But a single particle doesn’t  

tell you anything about it. It's just a  particle, it doesn't know the difference  

between yesterday's burrito and  tomorrow's existential crisis.

Ok, so I hope that explains why the headline is  nonsense. But that doesn’t mean that the paper  

is nonsense, so let’s look at what they did  with the particle that supposedly went back  

in time. The paper in question is a general  idea laid out last year with an experimental  

confirmation that just recently appeared  on the arXiv. The general idea is this.

You create an entangled state of two qubits. That  they are entangled means that they are correlated  

in some property, but you don’t know the value  of the property for each individual particle.

A typical example is spin. Think of it an arrow  that points into a specific direction. Let’s  

say the spins are correlated in that they point  in the exact same direction, but you don’t know  

which direction this is. If you believe in the  standard interpretation of quantum mechanics,  

then you would say actually the spin doesn’t  have a direction until you measure it.

The situation they look at is that you want to  make a very precise measurement of some property  

of a quantum system. That could be for example  some property of a material, magnetic field or  

such. You do this by bringing a quantum particle  in the vicinity and then measuring how the quantum  

particle’s spin changes. It’s kind of like  sniffing a T-shirt to see if it needs changing.  

To get any useful results, you need to repeat this  many many times. So a long sequence of particles.

However, the issue is that this measurement  works best for certain values of the spin.  

And you don’t know this spin, so some of  your measurement results are not very good.

So what can you do? What you can do is to  create a pair of entangled particles and  

send one past the probe. Then you first make a  measurement on the one particle to figure out  

whether it was a good probe. If the first  measurement says it wasn’t a good probe,  

you throw the other particle  away. If not, you measure it. 

This way, you can improve the accuracy of your  measurement. And they say it’s something to  

do with going back in time, because after you  have made the measurement on the one particle,  

it’s like you retro-actively selected the spin  at the time it was created. Because remember, in  

quantum mechanics the idea is that the spin didn’t  actually have any value until you measured it.

This idea is known as post-selection. You select  certain quantum particles after you produce many  

more. Does this really have something to do  with going back in time? I would argue no.

Let me give you an example that’s a little easier  to digest than spins and probes and entanglement.  

Suppose you post a comment on X/Twitter and  get a lot of replies, some agreeing with you,  

some disagreeing. You then go and block everyone  who disagrees with you and hide their comments,  

like certain climate scientists are known  to handle such things. But I digress. 

If you look at your post and the comments  afterwards, it’s like you went into the past  

and magically made those critics disappear. This  is what post-selection means. Does this mean the  

comments you removed never happened? I regret  to inform you that they still happened because,  

you know what, you can’t change the past.  And it’s the same with these post-selected  

states. Just because you decided to throw away  some states doesn’t mean you changed the past.

That said, this doesn’t mean it’s a useless idea.  Because this procedure has a practical advantage,  

and this is really what the paper is about.  You see, by selecting only the best probes,  

you reduce the frequency with which you  need to make a measurement to reach a  

certain accuracy. That’s useful because  a lot of detectors have a dead time  

after a detection. Basically after you make  a measurement, there’s a certain period in  

which you can’t measure anything else. It’s  like a standup comedian who delivers their  

jokes too quickly. You just can't laugh that  much, you need a break in between. Same with  

these detectors. And if you select only the  good probes, that gives the detectors breaks.

So, this idea is actually useful  and has some practical purposes.  

But no, they’re not actually sending particles  into the past. Because if they could do that,  

they wouldn’t be writing papers  about it, they’d be trading bitcoin.

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