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	2022-11-16: Are there Undiscovered Elements Beyond The Periodic Table? 17:35: ... says that all other things equal, an observer should reason as if they are randomly selected from the set of all ...
	2022-11-09: What If Humanity Is Among The First Spacefaring Civilizations? 01:46: ... in a location in the universe capable of forming and supporting observers. ... 13:25: The emptiness of our skies and the constraint of making us typical observers is enough to determine all of the parameters of this model. 01:46: ... in a location in the universe capable of forming and supporting observers. ... 13:25: The emptiness of our skies and the constraint of making us typical observers is enough to determine all of the parameters of this model.
	2022-09-21: Science of the James Webb Telescope Explained! 05:57: Most of what the telescope will look at over its hopefully long life will be through the General Observer or GO program.
	2022-07-20: What If We Live in a Superdeterministic Universe? 00:57: Others - perhaps most - prefer to think of the universe as having a concrete existence independent of the observer. 01:12: Pilot wave theory, objective collapse models, and even the Many Worlds interpretation all seek to describe a reality that exists sans observers. 14:01: The latter has philosophical implications for our own dreams of being detached observers, independent of our subjects. 02:03: Along with Boris Podolsky and Nathen Rosen, he proposed the EPR paradox which was meant to deal a swift death-blow to this observer-centric nonsense. 01:41: ... ad absurdum designed to highlight the ridiculousness of extrapolating observer-dependent indeterminacy to large or macroscopic ... 01:12: Pilot wave theory, objective collapse models, and even the Many Worlds interpretation all seek to describe a reality that exists sans observers. 14:01: The latter has philosophical implications for our own dreams of being detached observers, independent of our subjects.
	2022-06-01: What If Physics IS NOT Describing Reality? 01:41: ... - the most fundamental existence is between the relationship between the observer and the ... 02:20: ... he was careful to note that observer in this context didn't necessarily need to be conscious. Wheeler may ... 11:24: ... does not have a physical   existence independent of the observer. Rather,  the wavefunction and the math that governs it   ... 12:09: ... is.” To learn about something necessarily involves  an observer who is acquiring this knowledge,   so all we can ever know ... 11:24: ... does not have a physical   existence independent of the observer. Rather,  the wavefunction and the math that governs it   describe our ... 12:09: ... famously once asked whether proponents of these   observer-centric interpretations truly believe  the moon isn’t there when nobody ...
	2022-04-20: Does the Universe Create Itself? 00:30: ... to the extreme and we imagine the scientist as this perfectly detached observer of the world, capable of monitoring and modeling physical reality ... 00:59: ... in physical particles and quantum fields, nor solely in the mind of the observer, but rather in the interaction of the ... 01:59: ... plays a role in defining reality, how is it possible for separate observers to share a single, consistent reality? Solutions to this seeming paradox ... 04:14: ... Although he started out as a pure realist, he came to believe that the observer must in some way be ... 04:44: ... he saw as the solipsistic view of von Neumann and Wigner, in which the observer was in a sense the primary causal agent and center of its own ... 05:27: ... felt that reality must have its primary existence not in the observer nor in the subject of observation, but rather in the connection between ... 09:48: ... only reality that is consistent with the questions asked of it by all observers. The universe, according to Wheeler, was a giant, closed, “self-excited ... 10:30: ... the present day, the other side of the U. An eyeball here symbolizes the observer. All the astronomers with their telescopes and observatories are aimed ... 12:27: ... about each other. Crucially, these acknowledge the importance of the observer in defining the observed, while still allowing that there’s some kind of ... 13:57: It may be that we live in a participatory universe that’s self-generated by its observers. 14:02: ... with certainty that PBS Space Time is generated by participation of its observers - that’s you guys - and most especially by the participation of our ... 01:59: ... physicists John von Neumann and Eugene Wigner doubled down on the observer-centric view, in which the act of observation in some way creates reality. ... 00:59: ... until it is an observed phenomenon”. Bohr led the charge with this observer-dependent view, encapsulated in his Copenhagen interpretation of quantum ... 01:59: ... plays a role in defining reality, how is it possible for separate observers to share a single, consistent reality? Solutions to this seeming paradox ... 09:48: ... only reality that is consistent with the questions asked of it by all observers. The universe, according to Wheeler, was a giant, closed, “self-excited ... 13:57: It may be that we live in a participatory universe that’s self-generated by its observers. 14:02: ... with certainty that PBS Space Time is generated by participation of its observers - that’s you guys - and most especially by the participation of our ... 01:59: ... objective collapse interpretations. Still others sought to explain the observer’s influence as a sort of selection bias - for example, Hugh Everett’s many worlds ...
	2022-03-23: Where Is The Center of The Universe? 12:19: ... found that an observer in a sufficiently large cloud that was expanding or contracting would ... 17:22: ... asks whether chirality depend on the observers reference frame, given that it’s defined in reference to the momentum ...
	2022-02-16: Is The Wave Function The Building Block of Reality? 04:36: ... they’re measured. But the collapse has nothing to do with a conscious observer or any subjective explanation. The wave function and the collapse are ... 16:15: ... point of view of falling matter, but from the point of view of a distant observer. Only the distant observer sees matter approach a state of frozen time, ... 16:48: ... that note Pesila Ratnayaje asks if an outsider observer sees matter slow and freeze at the event horizon, what happens when the ... 18:37: ... nonsensical noise. Which of the two is the case might be relative to the observer. For example Siderite Zackwehdex loves how we ask questions a kid would ... 16:15: ... but from the point of view of a distant observer. Only the distant observer sees matter approach a state of frozen time, and you’re right that from that ... 16:48: ... that note Pesila Ratnayaje asks if an outsider observer sees matter slow and freeze at the event horizon, what happens when the event ... 16:15: ... but from the point of view of a distant observer. Only the distant observer sees matter approach a state of frozen time, and you’re right that from that ... 16:48: ... that note Pesila Ratnayaje asks if an outsider observer sees matter slow and freeze at the event horizon, what happens when the event ...
	2022-02-10: The Nature of Space and Time AMA 00:03: ... showed us i guess that my perception of now could be some other observer's future and yet another observer's past and this leads us to the notion ...
	2022-01-27: How Does Gravity Escape A Black Hole? 04:32: The “speed of space” is just the speed of a free-falling, or inertial observer. 04:38: Falling from very far away, an observer and the patch of space that they occupy reach light speed at the event horizon of the black hole.
	2021-12-20: What Happens If A Black Hole Hits Earth? 16:33: ... the same as inertial motion in free space. That means that an observer falling through a black hole event horizon shouldn’t notice anything ...
	2021-11-02: Is ACTION The Most Fundamental Property in Physics? 07:09: ... to understand. Action is now just how much time is perceived by an observer in their own frame of reference. All objects moving through space time ...
	2021-07-21: How Magnetism Shapes The Universe 17:10: ... for each quantum outcome - because in the many worlds interpretation, an observer is more likely to see the most probable outcomes simply because there ...
	2021-06-23: How Quantum Entanglement Creates Entropy 16:39: ... The distances have to depend on the velocity of the observer. Moving fast should then collapse the   grid in one direction - ...
	2021-05-25: What If (Tiny) Black Holes Are Everywhere? 02:17: To a distant observer it would look like the black hole is radiating particles.
	2021-03-09: How Does Gravity Affect Light? 02:49: The spider still observes the laser traveling at the speed of light - because the speed of light is invariant to all observers. 05:28: Let’s look at it from the perspective of a non-accelerating observer outside the ship. 02:49: The spider still observes the laser traveling at the speed of light - because the speed of light is invariant to all observers.
	2021-02-24: Does Time Cause Gravity? 00:34: ... relativity theory: that the speed of light is constant for all observers, and that the weight induced by acceleration is fundamentally the same as ...
	2021-02-10: How Does Gravity Warp the Flow of Time? 00:25: ... it himself - he described his happiest thought as the following: “For an observer falling freely from the roof of a house, the gravitational field does ... 04:48: ... - that the speed of light is always measured to be the same for all observers, no matter their personal ... 05:25: For an observer in the moving lab, it appears that the stationary clock is ticking slow. 06:52: Both observers see the other’s time has slowed. 06:55: But after a full revolution, both observers ask each other how many ticks their clock ticked. 07:14: The summary is this: two observers moving in straight lines to each other do perceive the other as time-dilated - slowed. 07:22: But as soon as one of those observers changes direction, the symmetry is broken. 09:49: You do have to be careful to choose the right relative distances between observers. 00:25: ... it himself - he described his happiest thought as the following: “For an observer falling freely from the roof of a house, the gravitational field does not ... 04:48: ... - that the speed of light is always measured to be the same for all observers, no matter their personal ... 06:52: Both observers see the other’s time has slowed. 06:55: But after a full revolution, both observers ask each other how many ticks their clock ticked. 07:14: The summary is this: two observers moving in straight lines to each other do perceive the other as time-dilated - slowed. 07:22: But as soon as one of those observers changes direction, the symmetry is broken. 09:49: You do have to be careful to choose the right relative distances between observers. 07:14: The summary is this: two observers moving in straight lines to each other do perceive the other as time-dilated - slowed.
	2021-01-26: Is Dark Matter Made of Particles? 13:52: Cezar Catalin asks what if the ladder traveling through the barn stops when the outside observer sees both doors closed?
	2021-01-19: Can We Break the Universe? 00:17: ... that the speed of light was the fastest speed possible, and that all observers should measure the same speed of light, regardless of their ... 00:26: But from that can the inevitable conclusion that space and time themselves were relative - depended on the velocity of the observer. 01:09: ... will appear to tick more slowly from the point of view of a stationary observer back on the ... 01:42: ... seeming contradictions only become paradoxes if the different observers - on the spaceship and on Earth - can compare the results of an ... 01:52: ... example, there IS a disagreement between the astronaut and an observer back on Earth about the relative passage of time and the distance ... 02:08: ... observer on Earth thinks the astronaut’s clock ticked slow, but the astronaut ... 02:54: But in the frame of a stationary observer, the ship is just moving ridiculously fast, and doesn’t destroy itself. 04:27: One of the consequences of special relativity is that different observers give different accounts of what events are simultaneous. 04:33: ... the spacetime diagram, the set of simultaneous events for a motionless observer lie on a horizontal line - all events corresponding to your notion of a ... 09:09: One observer says the ladder fits, the other says no. 09:18: In this case, the different observers disagree on when the ends of the ladder enter and exit the barn. 09:41: The ladder’s observer perceives the front of the ladder exiting the barn before the base enters. 04:33: ... the spacetime diagram, the set of simultaneous events for a motionless observer lie on a horizontal line - all events corresponding to your notion of a ... 09:41: The ladder’s observer perceives the front of the ladder exiting the barn before the base enters. 00:17: ... that the speed of light was the fastest speed possible, and that all observers should measure the same speed of light, regardless of their ... 01:42: ... seeming contradictions only become paradoxes if the different observers - on the spaceship and on Earth - can compare the results of an ... 04:27: One of the consequences of special relativity is that different observers give different accounts of what events are simultaneous. 09:18: In this case, the different observers disagree on when the ends of the ladder enter and exit the barn. 01:42: ... seeming contradictions only become paradoxes if the different observers - on the spaceship and on Earth - can compare the results of an experiment ... 09:18: In this case, the different observers disagree on when the ends of the ladder enter and exit the barn.
	2020-12-15: The Supernova At The End of Time 14:17: The point here is that even the idea of "it has happened" is a relative concept - relative to the observer. 14:24: And WE observers ride the wave of time in a particular direction. 14:35: John Ring correctly summarizes that observers in a reverse-time universe wouldn't know the difference. 15:49: ... is a time-reversed observer from a Many Worlds multiverse, and complains they actually can remember ... 14:24: And WE observers ride the wave of time in a particular direction. 14:35: John Ring correctly summarizes that observers in a reverse-time universe wouldn't know the difference. 14:24: And WE observers ride the wave of time in a particular direction.
	2020-11-11: Can Free Will be Saved in a Deterministic Universe? 00:36: In it, the past and future have a sort of eternal timeless existence from the point of view of some god-like observer outside both space and time.
	2020-10-20: Is The Future Predetermined By Quantum Mechanics? 00:47: But different observers will slice the block at different angles that depend on their velocities. 00:52: ... means that for every observer it's possible to imagine another observer who lives in their definition ... 01:10: ... if we accept a unique reality for those other observers in our present and we accept that their perception of the present is as ... 02:00: ... or everything outside our past light cone is undefined, including other observers and the room around you in what you would normally think of as the ... 06:09: But now let's say we believe that other observers in the universe can also collapse the same universal wave function with their observations. 06:17: Well, no problem so far, you can imagine this fleet of observers collapsing the universe all the way up to what you perceive as the present. 06:34: A fast moving observer defines the present very differently to you. 06:38: In fact, any part of spacetime, not in your future light cone is potentially the past for another observer in your present. 06:46: ... that observer can imagine a third observer on their different present for whom your ... 06:58: ... to define an absolute definition of the past, and if we believe in other observers there's no way to keep the wave function of your future from being ... 07:20: It could be observers as in the Von Nuemann-Wigner Interpretation. 07:24: That's the hardest to gel with a non-deterministic universe, unless you are the only observer in the universe. 10:59: There are still other observers in your slice of reality for whom you are now is their past or future. 12:54: ... maybe, but two observers can certainly construct a map of events across the universe that they ... 13:27: ... can ascribe actual existence to one slice of the block universe, to one observer's definition of the present, then you should ascribe existence to the ... 13:44: The alternative seems to be to not ascribe reality to any observer's present but then what exists? 13:50: Just the one observer? 06:34: A fast moving observer defines the present very differently to you. 00:47: But different observers will slice the block at different angles that depend on their velocities. 01:10: ... if we accept a unique reality for those other observers in our present and we accept that their perception of the present is as ... 02:00: ... or everything outside our past light cone is undefined, including other observers and the room around you in what you would normally think of as the ... 06:09: But now let's say we believe that other observers in the universe can also collapse the same universal wave function with their observations. 06:17: Well, no problem so far, you can imagine this fleet of observers collapsing the universe all the way up to what you perceive as the present. 06:58: ... to define an absolute definition of the past, and if we believe in other observers there's no way to keep the wave function of your future from being ... 07:20: It could be observers as in the Von Nuemann-Wigner Interpretation. 10:59: There are still other observers in your slice of reality for whom you are now is their past or future. 12:54: ... maybe, but two observers can certainly construct a map of events across the universe that they ... 13:27: ... can ascribe actual existence to one slice of the block universe, to one observer's definition of the present, then you should ascribe existence to the ... 13:44: The alternative seems to be to not ascribe reality to any observer's present but then what exists? 06:17: Well, no problem so far, you can imagine this fleet of observers collapsing the universe all the way up to what you perceive as the present. 13:27: ... can ascribe actual existence to one slice of the block universe, to one observer's definition of the present, then you should ascribe existence to the slice of the ...
	2020-10-13: Do the Past and Future Exist? 02:08: Newton assumed that all particles, all observers, all points in space were ruled by a single, constantly ticking clock. 08:14: The upshot is that the moving observer builds an entirely different map of that time slice - they define the present differently. 08:21: In fact, it’s possible for another observer to be in your slice of the present, but for you not to be in theirs. 08:29: ... relativity - space and time tilt into each other, so that different observers will slice up block time at different angles depending on their ... 10:03: Consider the time-slice of our present; we can imagine other observers on that time slice that we surely must “exist”. 10:19: ... them. Just like it does for us. So now fill our present time slice with observers and their remains no part of the block universe that couldn't be ... 08:14: The upshot is that the moving observer builds an entirely different map of that time slice - they define the present differently. 02:08: Newton assumed that all particles, all observers, all points in space were ruled by a single, constantly ticking clock. 08:29: ... relativity - space and time tilt into each other, so that different observers will slice up block time at different angles depending on their ... 10:03: Consider the time-slice of our present; we can imagine other observers on that time slice that we surely must “exist”. 10:19: ... them. Just like it does for us. So now fill our present time slice with observers and their remains no part of the block universe that couldn't be ...
	2020-07-28: What is a Theory of Everything: Livestream 00:00: ... that leave the speed of light um the same value for all different observers in that case for special relativity or symmetries under any change of ...
	2020-06-30: Dissolving an Event Horizon 13:11: ... between atoms in one aeon would be infinite from the point of view of observers from the previous ...
	2020-06-15: What Happens After the Universe Ends? 03:28: Remember, the universes contain only light - no observers and no clocks. 04:57: Lines representing constant distance or simultaneous times shift with the velocity of the observer. 05:12: They show how time will tick for any constant-velocity observer passing through this point. 03:28: Remember, the universes contain only light - no observers and no clocks.
	2020-05-18: Mapping the Multiverse 15:51: No indication has ever been found that the speed of light depends on the speed of the device or of the observer.
	2020-05-11: How Luminiferous Aether Led to Relativity 07:06: ... - depends in a simple way on the velocity and direction of motion of the observer. This is Galilean relativity - after Galileo ... 11:04: ... The speed of light appeared to be independent of the motion of the observer. ...
	2020-04-28: Space Time Livestream: Ask Matt Anything 00:00: ... light comes into it when we think about sort of the size scale of the observer the size scale of a human being relative to that propagation okay so ...
	2020-03-31: What’s On The Other Side Of A Black Hole? 01:53: ... horizon, time appears to freeze from the point of view of a distant observer. And the Schwarzschild metric is defined in terms of that observer’s ...
	2020-03-03: Does Quantum Immortality Save Schrödinger's Cat? 09:36: ... probability arguments including Bayesian analysis or thinking in “observer years” rather than observer ... 12:03: ... configurations between experimental aparatus and the brain of different observers means decoherence should proceed differently along those ... 12:14: So why do different observers always agree on the result of the experiment? 12:19: Well, Vampyricon answers this partically, saying that each observer will be on one decohered branch of the wavefunction. 12:29: Each decohered branch will have its own set of observers. 12:33: In other words, not all observers agree on experimental outcomes - it's just you never meet the ones who don't agree with you. 12:46: ... have sets of observers who make consistent observations, and who are unaware of observers on ... 09:36: ... including Bayesian analysis or thinking in “observer years” rather than observer lifetimes. ... 12:03: ... configurations between experimental aparatus and the brain of different observers means decoherence should proceed differently along those ... 12:14: So why do different observers always agree on the result of the experiment? 12:29: Each decohered branch will have its own set of observers. 12:33: In other words, not all observers agree on experimental outcomes - it's just you never meet the ones who don't agree with you. 12:46: ... have sets of observers who make consistent observations, and who are unaware of observers on ... 12:33: In other words, not all observers agree on experimental outcomes - it's just you never meet the ones who don't agree with you.
	2020-02-24: How Decoherence Splits The Quantum Multiverse 10:01: ... of signals traveling from those pixels ultimately to the brain of the observer. ... 12:31: So you shouldn’t think of yourself as this gods-eye observer, capable of seeing the whole wavefunction and causing it to collapse.
	2020-02-18: Does Consciousness Influence Quantum Mechanics? 07:38: So there’s the conflict - different observers say the wavefunction collapses at different times. 11:37: ... maybe you are the only observer and you’re inventing your friend and, well, the rest of reality and ... 11:46: ... for the consistency of experimental results between different observers seems to be that the result of the experiment - and reality - exists ... 07:38: So there’s the conflict - different observers say the wavefunction collapses at different times. 11:37: ... your friend and, well, the rest of reality and there are no other observers in the universe to give conflicting ... 11:46: ... for the consistency of experimental results between different observers seems to be that the result of the experiment - and reality - exists ...
	2019-12-09: The Doomsday Argument 03:24: He realized that we should instead assume that we are the most common type of observer. 04:15: ... refinement of the anthropic principle to talk about observers rather than environments is essentially using the Self-Sampling ... 08:39: ... likely to observe - so reference class was an astronomer - or really any observer capable of measuring the cosmological ... 09:03: ... example, if you define our reference class as all observers in our past and future evolutionary chain, then we could still be in the ... 08:39: ... likely to observe - so reference class was an astronomer - or really any observer capable of measuring the cosmological ... 04:15: ... refinement of the anthropic principle to talk about observers rather than environments is essentially using the Self-Sampling ... 09:03: ... example, if you define our reference class as all observers in our past and future evolutionary chain, then we could still be in the ... 01:20: ... can use: our existence selects for a particular type of potentially-rare observer-supporting environments, but we should expect to find ourselves in the most typical ...
	2019-12-02: Is The Universe Finite? 14:35: ... mean by "reference class" - as in, how do you choose the sample of observers from which you consider yourself randomly ... 15:04: ... anthropic seletion only demands that our universe be able to produce observers who think about the nature of the ... 14:35: ... mean by "reference class" - as in, how do you choose the sample of observers from which you consider yourself randomly ... 15:04: ... anthropic seletion only demands that our universe be able to produce observers who think about the nature of the ...
	2019-11-18: Can You Observe a Typical Universe? 02:35: ... we must live in a place and time in the universe capable of supporting observers - in our case, a habitable biosphere, and the strong anthropic ... 03:10: ... we necessarily observe from an environment capable of producing observers; be that environment a planet within a universe or a universe within a ... 04:37: ... the strong anthropic principle to mean that the evolution of observers somehow had some causal influence on the initial formation of the ... 05:00: The principle is NOT causal - it just tells us to account for an observer selection bias when interpreting the nature of our environment. 07:32: ... anthropic principle tells us we must account for our status as observers when we interpret our environment - including the probability of being ... 07:42: ... in a typical region of the cosmos that is consistent with us being observers. ... 08:48: ... one galaxy to spawn a life-bearing planet - so there should be many more observers in small entropy fluctuations than in large ... 10:08: ... self-sampling assumption, which states that “All other things equal, an observer should reason as if they are randomly selected from the set of all ... 10:33: That means you’re most likely a common type of observer, and in a common environment in which observers can exist. 10:55: The prior is that we are an observer. 10:59: But proper Bayesian thinking requires careful definition of priors - for example, Bostrom talks about “observers in their reference class”. 13:03: ... about your universe, given your privileged status as a typical conscious observer of space ... 14:21: ... they say that the universe isn't really fine-tuned for life or for observers because there could be many types of observer very different to ... 14:43: ... can probably assume that for an intelligent observer to emerge in any universe, that universe must be capable of forming ... 15:16: ... while there may be many small parts of that parameter space where observers can arise, most of it - hence most universes - should be devoid of ... 16:43: ... self-sampling assumption says we should assume we're a typical observer - so maybe the most typical observers are in relatively barren ... 17:53: In other words, find all possible combinations of constants that can produce observers. 16:43: ... self-sampling assumption says we should assume we're a typical observer - so maybe the most typical observers are in relatively barren universes, ... 05:00: The principle is NOT causal - it just tells us to account for an observer selection bias when interpreting the nature of our environment. 07:58: That allows us to be in a rare, observer-friendly environment, but tells us that we should be in the most typical of such environments. 06:32: We certainly don’t observe the universe in a typical, observer-hostile state, and and so it’s tempting to use the anthropic principle here. 02:35: ... we must live in a place and time in the universe capable of supporting observers - in our case, a habitable biosphere, and the strong anthropic ... 03:10: ... we necessarily observe from an environment capable of producing observers; be that environment a planet within a universe or a universe within a ... 04:37: ... the strong anthropic principle to mean that the evolution of observers somehow had some causal influence on the initial formation of the ... 07:32: ... anthropic principle tells us we must account for our status as observers when we interpret our environment - including the probability of being ... 07:42: ... in a typical region of the cosmos that is consistent with us being observers. ... 08:48: ... one galaxy to spawn a life-bearing planet - so there should be many more observers in small entropy fluctuations than in large ... 10:08: ... as if they are randomly selected from the set of all actually existent observers (past, present and future) in their reference class." If there are a ... 10:33: That means you’re most likely a common type of observer, and in a common environment in which observers can exist. 10:59: But proper Bayesian thinking requires careful definition of priors - for example, Bostrom talks about “observers in their reference class”. 14:21: ... they say that the universe isn't really fine-tuned for life or for observers because there could be many types of observer very different to ... 15:16: ... while there may be many small parts of that parameter space where observers can arise, most of it - hence most universes - should be devoid of ... 16:43: ... we should assume we're a typical observer - so maybe the most typical observers are in relatively barren universes, and there are just way more of those ... 17:53: In other words, find all possible combinations of constants that can produce observers. 02:35: ... we must live in a place and time in the universe capable of supporting observers - in our case, a habitable biosphere, and the strong anthropic principle, ...
	2019-11-11: Does Life Need a Multiverse to Exist? 02:50: We must find ourselves at a place and time in the universe capable of producing observers. 03:05: Well, Carter also has a strong version. The universe must be such as to admit the creation of observers within it at some stage. 03:27: ... strong anthropic principle tells us that there’s an observer selection bias that may help us understand why we live in such a ... 02:50: We must find ourselves at a place and time in the universe capable of producing observers. 03:05: Well, Carter also has a strong version. The universe must be such as to admit the creation of observers within it at some stage.
	2019-11-04: Why We Might Be Alone in the Universe 00:38: The anthropic principle tells us that we shouldn’t expect to find ourselves in some random corner of the multiverse - there’s an observer bias. 00:57: ... principle tells us that an observed universe must be able to produce observers - and we’ll get to the implications of that soon - including the ... 01:29: ... may seems tautological, but accounting for this observer selection bias is important to understanding why the universe looks the ... 01:54: To get to this, let's think about what it means to be an intelligent observer. 00:38: The anthropic principle tells us that we shouldn’t expect to find ourselves in some random corner of the multiverse - there’s an observer bias. 01:29: ... may seems tautological, but accounting for this observer selection bias is important to understanding why the universe looks the way it ... 00:57: ... principle tells us that an observed universe must be able to produce observers - and we’ll get to the implications of that soon - including the ...
	2019-07-18: Did Time Start at the Big Bang? 06:02: ... there is no universal clock time is relative Clocks are attached to each observer each moving frame of reference to see what time does at the Big Bang We ...
	2019-04-10: The Holographic Universe Explained 03:03: ... the point of view of outside observers, its contents is smeared into 2-D on the surface, but from the PoV of ...
	2019-01-09: Are Dark Matter And Dark Energy The Same? 12:07: Which is good for string theorists, as Farnes notes, but not for observers because that’s not consistent with what we see.
	2018-10-18: What are the Strings in String Theory? 14:03: Roman R. asks whether computation at an event horizon would experience massive time dilation relative to an outside observer.
	2018-06-20: The Black Hole Information Paradox 06:07: The motivation for this idea is the fact that, from the point of view of an outside observer, nothing ever actually crosses the event horizon. 07:25: From the point of view of an observer falling into the black hole, they aren't frozen at the horizon. 07:55: No observer can ever see both. 07:25: From the point of view of an observer falling into the black hole, they aren't frozen at the horizon.
	2018-06-13: What Survives Inside A Black Hole? 13:30: More likely is that the observer and the observation are a small part of a global wave function that continues to evolve in a unitary manner.
	2018-04-11: The Physics of Life (ft. It's Okay to be Smart & PBS Eons!) 10:57: Last week, we talked about the mysterious Unruh effect, in which accelerating observers find themselves bathed in a sea of particles. 11:07: ... Diagrams points out that from the point of view of an inertial observer, an accelerating particle detector emits particles instead of absorbing ... 11:23: ... in short, the inertial observer sees the accelerating particle detector click as though it registered a ... 11:49: ... inertial observer sees that there's a type of friction between the accelerating observer ... 11:59: But the accelerating observer doesn't directly see that friction. 12:05: The answer is that the accelerating observer perceives themselves to be plowing through a bath of Unruh particles, and these produce the drag. 12:13: The accelerating observer must expend more energy to produce the same acceleration. 11:59: But the accelerating observer doesn't directly see that friction. 12:05: The answer is that the accelerating observer perceives themselves to be plowing through a bath of Unruh particles, and these produce the drag. 11:23: ... in short, the inertial observer sees the accelerating particle detector click as though it registered a ... 11:49: ... inertial observer sees that there's a type of friction between the accelerating observer and ... 10:57: Last week, we talked about the mysterious Unruh effect, in which accelerating observers find themselves bathed in a sea of particles.
	2018-04-04: The Unruh Effect 00:44: They were independently studying how the nature of quantum fields appears to change depending on whether or not an observer is accelerating. 01:14: It tells us that accelerating observers find themselves in a warm bath of particles. 02:16: ... of light, so the world line of a massive object, which includes any observer, has to be less than 45 degrees from the ... 02:26: ... light ray world lines backwards from our observer defines what we call the past light cone, the region of space-time that ... 02:38: ... because photons fired from anywhere in the past light cone can reach our observer either at the current point or at some point in their past world ... 02:47: ... our observer moves forward in time, as long as they don't travel faster than light, ... 03:13: That's the world line of an observer undergoing constant acceleration. 04:50: This means that any events happening to the left of that diagonal line will never affect the accelerating observer, which sounds pretty horizon-like. 05:06: ... after the coordinate system we use to describe a constantly accelerating observer in special relativity, Rindler coordinates, devised by Austrian ... 05:20: The Rindler horizon flows at a fixed distance behind a constantly accelerating observer. 05:25: Let's call them Rindler observers from now on. 05:36: All parts of the universe beyond that horizon are out of causal connection with the Rindler observer as long as they continue to accelerate. 06:47: In the case of Hawking radiation, an inertial observer far from the black hole sees the radiation. 07:00: The only observers who don't see Hawking radiation are those plummeting in freefall towards the event horizon. 07:06: ... if an accelerating Rindler observer is in the same location as an inertial observer, the former will see ... 07:23: What if the Rindler observer accelerates fast enough that they are burned to a crisp by Unruh radiation? 07:29: Does the inertial observer see some sort of spontaneous combustion? 07:37: A little less gruesomely, imagine the Rindler observer has a particle detector. 07:45: And the inertial observer would agree that it clicked, but they wouldn't see the particle that triggered it. 09:13: An inertial observer sees the charged particle itself radiating, its energy extracted from the magnetic field. 09:19: But an observer accelerating with that charged particle sees it absorbing Unruh particles and then spitting them out again. 09:27: The Rindler and inertial observers disagree on the source of the energy even if they agree on the final result. 10:29: ... event horizon and the particles of Hawking radiation seen by a distant observer? ... 07:23: What if the Rindler observer accelerates fast enough that they are burned to a crisp by Unruh radiation? 09:19: But an observer accelerating with that charged particle sees it absorbing Unruh particles and then spitting them out again. 02:26: ... light ray world lines backwards from our observer defines what we call the past light cone, the region of space-time that can have ... 02:47: ... our observer moves forward in time, as long as they don't travel faster than light, their ... 09:13: An inertial observer sees the charged particle itself radiating, its energy extracted from the magnetic field. 03:13: That's the world line of an observer undergoing constant acceleration. 08:58: The upshot is that the very existence of particles is observer-dependent. 01:14: It tells us that accelerating observers find themselves in a warm bath of particles. 05:25: Let's call them Rindler observers from now on. 07:00: The only observers who don't see Hawking radiation are those plummeting in freefall towards the event horizon. 09:27: The Rindler and inertial observers disagree on the source of the energy even if they agree on the final result.
	2018-03-15: Hawking Radiation 05:44: A distant future observer sees radiation coming from the black hole. 09:12: In fact, an observer in freefall through the horizon sees nothing. 09:22: This radiation is visible only to distant observers. 05:44: A distant future observer sees radiation coming from the black hole. 09:22: This radiation is visible only to distant observers.
	2018-01-24: The End of the Habitable Zone 10:49: Last week we talked about how slicing a piece out of space time caused different observers to disagree on the nature of the vacuum. 11:14: The vacuum state of all fields are redefined in the vicinity of a black hole or for an accelerating observer. 10:49: Last week we talked about how slicing a piece out of space time caused different observers to disagree on the nature of the vacuum.
	2018-01-17: Horizon Radiation 00:17: Even so, every observer agrees on whether a particle exists or not, right? 00:26: [MUSIC PLAYING] Both theories of relativity, special and general, tell us that many things are observer dependent. 00:37: Different observers might disagree about speeds, lengths, or times, but the laws of physics should be the same for everyone. 00:46: And for two observers with very different, but constant speeds-- inertial observers, the vacuum itself should appear the same. 01:24: It limits an observer's causal connection to a part of the universe. 01:45: As it turns out, what gives is the nature of the vacuum, and in fact, the notion of what a particle is becomes observer dependent. 01:53: ... the Unruh effect, both of which are very similar and are results of this observer dependent ... 02:22: ... get at this idea of observer dependent particles and vacua, we're going to need some quantum field ... 03:00: For the laws of physics to be consistent, the fundamental properties of these fields must be the same for all observers. 03:14: ... observers, be they floating in empty space or accelerating or orbiting a black ... 03:33: There's no conflict for constant speed inertial observers. 03:55: ... when an observer who sees a horizon tries to write down these equations, in order to ... 11:23: It appears to be bathed in thermal particles-- particles that don't exist for an observer who doesn't see that horizon. 00:17: Even so, every observer agrees on whether a particle exists or not, right? 00:26: [MUSIC PLAYING] Both theories of relativity, special and general, tell us that many things are observer dependent. 01:45: As it turns out, what gives is the nature of the vacuum, and in fact, the notion of what a particle is becomes observer dependent. 01:53: ... the Unruh effect, both of which are very similar and are results of this observer dependent ... 02:22: ... get at this idea of observer dependent particles and vacua, we're going to need some quantum field theory, and ... 01:53: ... the Unruh effect, both of which are very similar and are results of this observer dependent vacuum. ... 00:37: Different observers might disagree about speeds, lengths, or times, but the laws of physics should be the same for everyone. 00:46: And for two observers with very different, but constant speeds-- inertial observers, the vacuum itself should appear the same. 01:24: It limits an observer's causal connection to a part of the universe. 03:00: For the laws of physics to be consistent, the fundamental properties of these fields must be the same for all observers. 03:14: ... observers, be they floating in empty space or accelerating or orbiting a black ... 03:33: There's no conflict for constant speed inertial observers. 01:24: It limits an observer's causal connection to a part of the universe.
	2017-12-06: Understanding the Uncertainty Principle with Quantum Fourier Series 02:06: The uncertainty principle exists alongside this observer effect.
	2017-11-29: Citizen Science + Zero-Point Challenge Answer 02:20: ... American Association of Variable Star Observers, founded in 1911, has generated an archive of variable star data taken ...
	2017-08-30: White Holes 03:24: ... the perspective of an outside observer, any events occurring at the event horizon, including folding into it, ...
	2017-08-24: First Detection of Life 01:29: But first, let's talk about what life on Earth looks like to an observer in space.
	2017-08-16: Extraterrestrial Superstorms 02:09: Any object moving over a spinning surface will appear to follow a curved path relative to an observer moving with that surface.
	2017-07-07: Feynman's Infinite Quantum Paths 13:40: That should cause different observers to measure a different speed of light.
	2017-06-21: Anti-Matter and Quantum Relativity 01:56: ... and only one clock, typically the clock in the reference frame of the observer. ...
	2017-05-03: Are We Living in an Ancestor Simulation? ft. Neil deGrasse Tyson 08:47: We must observe a universe or a part thereof that can have observers. 08:51: ... tells us that we should be the most typical, the most common type of observer, that could possibly be having our current ... 09:03: ... our own experience, then it's more likely that we are those more typical observers. ... 12:11: Just for now, I strongly recommend that we proceed as though we are real life observers, part of the original space time. 08:47: We must observe a universe or a part thereof that can have observers. 09:03: ... our own experience, then it's more likely that we are those more typical observers. ... 12:11: Just for now, I strongly recommend that we proceed as though we are real life observers, part of the original space time.
	2017-04-26: Are You a Boltzmann Brain? 06:02: We can only observe an environment capable of producing observers. 07:05: So, aren't there more probable, smaller dips in entropy that could lead to conscious observers? 07:23: And so should the conscious observers that evolve in them. 06:02: We can only observe an environment capable of producing observers. 07:05: So, aren't there more probable, smaller dips in entropy that could lead to conscious observers? 07:23: And so should the conscious observers that evolve in them.
	2017-03-29: How Time Becomes Space Inside a Black Hole 01:22: Different observers may report that two events are separated by different distances delta x and by different amounts of time delta t. 01:30: However all observers record the same spacetime interval. 01:22: Different observers may report that two events are separated by different distances delta x and by different amounts of time delta t. 01:30: However all observers record the same spacetime interval.
	2017-03-22: Superluminal Time Travel + Time Warp Challenge Answer 01:54: ... now, and that all points move forward in time at a constant rate for all observers, governed by one global ...
	2017-03-08: The Race to a Habitable Exoplanet - Time Warp Challenge 00:51: In that episode, we talked about the spacetime diagram and how it transforms between observers traveling at different speeds. 01:04: In fact, there's only one possible way to map between different observers' reference frames. 01:11: It lets us figure out what spacetime looks like for every observer, no matter what his or her velocity is. 01:17: If two events happen in spacetime, observers with different velocities will report different separations between them, in both space and time. 00:51: In that episode, we talked about the spacetime diagram and how it transforms between observers traveling at different speeds. 01:04: In fact, there's only one possible way to map between different observers' reference frames. 01:17: If two events happen in spacetime, observers with different velocities will report different separations between them, in both space and time. 01:04: In fact, there's only one possible way to map between different observers' reference frames. 00:51: In that episode, we talked about the spacetime diagram and how it transforms between observers traveling at different speeds.
	2017-02-02: The Geometry of Causality 01:31: An observer I leave behind with an amazing telescope, observes me traveling the entire original distance but will perceive my clock as having slowed. 01:41: ... length contraction and time dilation allows both moving and stationary observers to agree on how much older every one looks at the end of the ... 02:00: ... time measured by a moving observer on their own clock is called proper time, but counting those clock ticks ... 02:13: ... this thing called the spacetime interval that relates observer dependent perspectives on the length and duration of any journey that ... 07:27: They will always land on the same hyperbola, no matter the observer's reference frame. 02:13: ... this thing called the spacetime interval that relates observer dependent perspectives on the length and duration of any journey that all ... 01:41: ... length contraction and time dilation allows both moving and stationary observers to agree on how much older every one looks at the end of the ... 02:13: ... perspectives on the length and duration of any journey that all observers will agree on, even if they don't agree on the delta x and delta t of ... 07:27: They will always land on the same hyperbola, no matter the observer's reference frame.
	2017-01-19: The Phantom Singularity 05:17: The Schwarzschild metric allows us to compare two points or events in space time around a massive object from the perspective of different observers. 05:50: Every inertial, so non-accelerating observer, will agree on the same space time interval for every pair of events and for every world line. 05:17: The Schwarzschild metric allows us to compare two points or events in space time around a massive object from the perspective of different observers.
	2016-12-21: Have They Seen Us? 14:32: So a distant immortal observer, with a ridiculously good telescope, will detect photons from the falling monkey at all future times. 16:31: ... the black hole's stationary frame of reference as recorded by a distant observer. ...
	2016-12-14: Escape The Kugelblitz Challenge 03:41: Any observers within this extended event horizon are cut off from any future causal connection with the rest of the universe. 03:51: OK, in the case of the collapsing start, that's still a core, is going to be an insanely hot, dense place and not great for observers. 03:41: Any observers within this extended event horizon are cut off from any future causal connection with the rest of the universe. 03:51: OK, in the case of the collapsing start, that's still a core, is going to be an insanely hot, dense place and not great for observers.
	2016-10-26: The Many Worlds of the Quantum Multiverse 05:23: ... extends beyond the radioactive decay, beyond the cat, and includes the observer and, indeed, the entire universe, ... 06:20: ... trajectories, of histories, merges into the single timeline of the observer's ... 10:23: It explains the apparent randomness of quantum mechanics with a sort of observer bias. 06:20: ... trajectories, of histories, merges into the single timeline of the observer's ...
	2016-09-29: Life on Europa? 11:08: Alex Trusk very reasonably asks me to define what I mean by observer. 11:18: The definition of observer sort of depends on what interpretation of quantum mechanics you're going with.
	2016-09-21: Quantum Entanglement and the Great Bohr-Einstein Debate 00:08: Or is that reality an illusion in the eye of the observer? 01:19: This notion that the universe exists independent of the mind of the observer is called realism in physics.
	2016-08-17: Quantum Eraser Lottery Challenge 02:25: ... or a single pile distribution, the information regarding whether the observer knows the path appears to travel back in time, even if was only a tiny ...
	2016-08-10: How the Quantum Eraser Rewrites the Past 07:22: The observer lost the info of which slit we went through. 09:43: Perhaps this thing we call observation is just entanglement between the observer and the experiment. 07:22: The observer lost the info of which slit we went through.
	2016-06-15: The Strange Universe of Gravitational Lensing 12:13: But from the standpoint of us, the observer, the effect is the same. 13:09: However, one view that's not really favored is the idea that a conscious observer is needed to collapse a wave function.
	2016-02-17: Planet X Discovered?? + Challenge Winners! 04:30: ... a clock that first moves towards the observer and the stationary clock, overtakes, and then moves away, will have ... 04:48: To understand that, you need to draw lights like photon paths between the moving clock and the stationary observer.
	2016-01-27: The Origin of Matter and Time 06:02: It's defined locally for any observer, or indeed, thing.
	2016-01-13: When Time Breaks Down 03:46: All observers, regardless of their own speed, will report seeing the same speed for any particle of light-- any photon.
	2015-12-16: The Higgs Mechanism Explained 07:55: ... although an outside observer can never witness anything cross the event horizon, as something falls ... 08:06: ... infalling stuff does vanish, and the event horizon that an outside observer sees does grow because anything falling into the black hole adds to its ...
	2015-12-09: How to Build a Black Hole 09:37: From the point of view of an outside observer-- so, us-- this never happens.
	2015-11-05: Why Haven't We Found Alien Life? 02:01: ... we're touching on the anthropic principle, which states that an observer will always observe a universe that can make observers or a planet that ...
	2015-10-07: The Speed of Light is NOT About Light 00:14: ... speed but 300,000 kilometers per second in a vacuum-- according to any observer-- and, two, keep anything from traveling faster than that ... 00:51: Causal connections give us the only ordering of events that all observers will agree on. 08:48: In fact, it's the maximum speed at which any observers can see two parts of the universe talk to each other. 00:51: Causal connections give us the only ordering of events that all observers will agree on. 08:48: In fact, it's the maximum speed at which any observers can see two parts of the universe talk to each other.
	2015-09-23: Does Dark Matter BREAK Physics? 01:01: Place a strong gravitational field on an axis between a light source and an observer and voila, you basically have a lens. 08:02: ... the monkey were to calculate the clock time of an external observer as it fell, then that calculated time would approach infinity as the ... 08:59: ... is eternally frozen on the event horizon with respect to a distant observer, shouldn't the black hole evaporate beneath ... 10:07: The distant observer does witness the instance that the black hole evaporates, with a huge burst of Hawking radiation. 10:21: It never even happens in the distant observer's universe, either before or after the black hole's evaporation. 08:59: ... is eternally frozen on the event horizon with respect to a distant observer, shouldn't the black hole evaporate beneath ... 10:21: It never even happens in the distant observer's universe, either before or after the black hole's evaporation.
	2015-08-19: Do Events Inside Black Holes Happen? 02:24: In fact, so would any observer, inertial or otherwise, who is always outside the black hole's edge. 03:24: According to observers like the monkey who are at those events, those events take place at spatial locations inside that black blob we see in the sky. 03:37: It's all the events that have ever or will ever take place there, according to observers who are physically there. 03:58: ... temporal terms when you delete entire occurrences from every external observer's self-consistent record of the history of the ... 06:38: So a freefalling observer inside that cutoff, like the monkey, will go radially inwards. 08:20: That means that to external observers, black holes are black because light that gets emitted just outside the horizon is redshifted into invisibility. 09:52: So to external observers, most of the matter never crosses the horizon. 02:24: In fact, so would any observer, inertial or otherwise, who is always outside the black hole's edge. 06:38: So a freefalling observer inside that cutoff, like the monkey, will go radially inwards. 03:24: According to observers like the monkey who are at those events, those events take place at spatial locations inside that black blob we see in the sky. 03:37: It's all the events that have ever or will ever take place there, according to observers who are physically there. 03:58: ... temporal terms when you delete entire occurrences from every external observer's self-consistent record of the history of the ... 08:20: That means that to external observers, black holes are black because light that gets emitted just outside the horizon is redshifted into invisibility. 09:52: So to external observers, most of the matter never crosses the horizon. 08:20: That means that to external observers, black holes are black because light that gets emitted just outside the horizon is redshifted into invisibility. 03:58: ... temporal terms when you delete entire occurrences from every external observer's self-consistent record of the history of the ...
	2015-08-12: Challenge: Which Particle Wins This Race? 04:31: ... fluid whose energy density is the same when measured locally by an observer that's instantaneously at rest at any location in that ...
	2015-07-29: General Relativity & Curved Spacetime Explained! 01:54: ... think about it, on the flat spacetime diagrams of inertia observers, the world lines of other inertial observers are straight, indicating ... 02:03: This captures Newton's idea that inertial observers shouldn't accelerate relative to other inertial observers. 03:32: ... when an observer in deep space says that the falling apple is accelerating, he's pushing ... 03:43: However, global inertial observers do. 03:45: They're observers that have no forces on them. 01:54: ... think about it, on the flat spacetime diagrams of inertia observers, the world lines of other inertial observers are straight, indicating ... 02:03: This captures Newton's idea that inertial observers shouldn't accelerate relative to other inertial observers. 03:43: However, global inertial observers do. 03:45: They're observers that have no forces on them. 02:03: This captures Newton's idea that inertial observers shouldn't accelerate relative to other inertial observers.
	2015-07-22: SPECIAL ANNOUNCEMENT + Flat Spacetime Geometry Comments 02:48: Inertial observers will see each other dynamically, moving with a constant three-dimensional velocity. 02:54: But does that mean that inertial observers are really moving with a constant three-dimensional velocity? 03:05: ... moving at constant three-dimensional velocity, sort of-- which is those observers whose world lines in space time are straight-- i.e., that are geodesics ... 03:22: Those are the guys that, it turns out, corresponds to inertial observers in Newtonian physics. 03:27: And they are distinguishable from non-inertial observers in a geometric way in space time. 04:32: ... that I drew in the last episode were from the point of view of inertial observers-- Blue Gabe, Red Gabe, the ... 05:01: Imagine a family of inertial observers, all of whom are moving with different speeds relative to each other. 05:06: That accelerating car, at any given moment, from its point of view, is always stationary instantaneously relative to one of those inertial observers. 02:48: Inertial observers will see each other dynamically, moving with a constant three-dimensional velocity. 02:54: But does that mean that inertial observers are really moving with a constant three-dimensional velocity? 03:05: ... moving at constant three-dimensional velocity, sort of-- which is those observers whose world lines in space time are straight-- i.e., that are geodesics ... 03:22: Those are the guys that, it turns out, corresponds to inertial observers in Newtonian physics. 03:27: And they are distinguishable from non-inertial observers in a geometric way in space time. 04:32: ... that I drew in the last episode were from the point of view of inertial observers-- Blue Gabe, Red Gabe, the ... 05:01: Imagine a family of inertial observers, all of whom are moving with different speeds relative to each other. 05:06: That accelerating car, at any given moment, from its point of view, is always stationary instantaneously relative to one of those inertial observers. 04:32: ... that I drew in the last episode were from the point of view of inertial observers-- Blue Gabe, Red Gabe, the ...
	2015-07-15: Can You Trust Your Eyes in Spacetime? 04:18: ... to be present at two events represented by points on such a line an observer or a photon would have to be moving faster than light, which normal ... 04:34: ... like time dilation, or length contraction, or disagreements between observers about event ... 06:38: In Newtonian mechanics, we distinguish inertial and noninertial observers dynamically by using the floating ball test. 06:44: But in spacetime, we can also distinguish those classes of observers geometrically. 06:49: Inertial observers have geodesic world lines and noninertial ones don't. 08:30: In fact, every observer's 4-velocity always has a length of minus the speed of light squared, even the accelerating car's 4-velocity. 08:37: ... vector a spacetime speed, then the world line of every inertial observer is a constant-speed straight ... 08:49: And accelerated observer's world lines are constant-speed non-straight lines. 04:34: ... like time dilation, or length contraction, or disagreements between observers about event ... 06:38: In Newtonian mechanics, we distinguish inertial and noninertial observers dynamically by using the floating ball test. 06:44: But in spacetime, we can also distinguish those classes of observers geometrically. 06:49: Inertial observers have geodesic world lines and noninertial ones don't. 08:30: In fact, every observer's 4-velocity always has a length of minus the speed of light squared, even the accelerating car's 4-velocity. 08:49: And accelerated observer's world lines are constant-speed non-straight lines. 08:30: In fact, every observer's 4-velocity always has a length of minus the speed of light squared, even the accelerating car's 4-velocity. 06:38: In Newtonian mechanics, we distinguish inertial and noninertial observers dynamically by using the floating ball test. 06:44: But in spacetime, we can also distinguish those classes of observers geometrically.
	2015-05-20: The Real Meaning of E=mc² 02:59: ... for talking this way, among them that rest mass is a property all observers agree about, much like the space-time interval that we discussed in a ...
	2015-04-22: Are Space and Time An Illusion? 01:01: Suppose two observers are moving relative to each other, and particles count as observers. 01:08: Fact-- those observers don't agree about how much time passes between events. 01:25: And yet, each observer measures things properly and is entirely consistent, which means neither of them is wrong. 01:57: If two observers can't agree on the sequence of events, it means that at present, someone's past is in someone else's future. 02:40: And as luck would have it, all observers do agree about this thing. 02:52: ... though two observers in relative motion will measure different distances and different ... 03:18: When it's positive, nothing can get from one event to the other, and there are always observers who disagree about which one happens first. 01:25: And yet, each observer measures things properly and is entirely consistent, which means neither of them is wrong. 01:01: Suppose two observers are moving relative to each other, and particles count as observers. 01:08: Fact-- those observers don't agree about how much time passes between events. 01:57: If two observers can't agree on the sequence of events, it means that at present, someone's past is in someone else's future. 02:40: And as luck would have it, all observers do agree about this thing. 02:52: ... though two observers in relative motion will measure different distances and different ... 03:18: When it's positive, nothing can get from one event to the other, and there are always observers who disagree about which one happens first. 01:08: Fact-- those observers don't agree about how much time passes between events.
	2015-04-01: Is the Moon in Majora’s Mask a Black Hole? 08:49: With each passing moment of time, any observer sitting anywhere will see photons that were emitted from progressively more distant locations.

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