
20221208: How Are Quasiparticles Different From Particles?
 00:03: ... watching this video on is best understood by thinking about positive and negative charges moving around a circuit of diodes and ...
 00:12: But the only elementary particle actually flowing in the circuit are the negatively charged electrons.
 03:21: This is an ntype semiconductor  n because the flowing charge is negative.
 10:50: At the same time, the negatively charged electrons in a metal lattice attract the positive nuclei.


20221109: What If Humanity Is Among The First Spacefaring Civilizations?
 18:50: ... quantum field theory, with the timereversed signals corresponding to negative frequency ...


20220824: What Makes The Strong Force Strong?
 00:28: ... orbitals by the electromagnetic force  opposite charges attract, so the negativelycharged electrons are attracted by the positively charged ...
 04:31: With electromagnetism you have one type of charge, which can be positive or negative, with the Strong Force you have three charge types.
 09:05: You would have to get really close to an atom to feel the positive electric field of the nucleus, or the negative electric field of the electrons.
 10:37: ... like the 3 charges of the strong force, each of which can be positive or negative, and transform into each other additively in the same way, or cancel out ...
 11:51: ... in fact they carry two charges at the same time  a positive and a negative of different colours  or more accurately, a superposition of multiple ...
 18:36: Those fields have positive and negative frequency modes that cancel each other out to leave a vacuum.


20220817: What If Dark Energy is a New Quantum Field?
 01:41: ... that the antigravitational effect of dark energy is due to it having negative pressure. That’s counterintuitive because negative pressure is an ...
 02:31: ... dark energy is that, on top of the antigravitational effect of its negative pressure, it also produces regular attractive gravity due to its ...
 12:31: ... omega is between ⅓ and 1 then the outwardpushing negative pressure still dominates over the inward pull of regular gravity, so ...
 13:04: ... to get omega less than 1 is for the kinetic energy of the field to be negative. These sorts of negative energy scenarios break the rules in general ...


20220630: Could We Decode Alien Physics?
 00:00: ... through the alien circuitry is the lightest lepton and has negative electric charge. Obviously the ...
 01:25: ... electric charge in the opposite way  electrons positive, positrons negative. But how could we make such a blunder? Surely, a quick glance at the ...
 02:25: ...  bring together two positive charges or two negative charges and you get a net positive force  which means a repulsive ...
 11:52: ... progression of positive times like we do, and the past as negative? Or do they do it the other way around? The laws of physics ...


20220622: Is Interstellar Travel Impossible?
 19:30: Hawking’s original argument talked about perturbing the positive and negative frequency modes of the quantum vacuum.


20220420: Does the Universe Create Itself?
 08:42: ... Wheeler’s version is a little different. He called it “negative 20 questions,” and it works like this. Unbeknownst to Bob, Alice never ...
 09:48: ... came to the view that the universe was one giant game of negative 20 questions, in which the reality we observe is the only reality that ...
 15:24: ... because it’s a positive energy density, just like matter. But it’s negative pressure can allow even a closed universe to expand forever. Thanks, ...


20220323: Where Is The Center of The Universe?
 04:14: ... on whether the average curvature is positive, negative, or zero, we get one of three shapes which we call ‘closed’, ‘open’, and ...
 11:01: That’s a topic worth it’s own video  although as a spoiler, the answer isn’t as obviously in the negative as you might think.


20211210: 2021 End of Year AMA!
 00:02: ... there a required a type of energy that doesn't exist and that is uh a negative energy density so as far as we know energy can only be positive um ...


20211005: Why Magnetic Monopoles SHOULD Exist
 00:35: ... cut the bar in half and you get a pair of electric charges  one negative and one positive, both of which have electric fields that radiate ...


20210921: How Electron Spin Makes Matter Possible
 05:59: ... Mathematically, a halfcycle phase shift corresponds to putting a negative sign in front of the wavefunction. That makes sense right  because ...
 06:49: ... of phase compared to its starting point. So a 360 rotation introduces a negative sign to the spinor ...
 07:17: And that little negative sign is ultimately what drives the difference between fermions and bosons.
 07:22: ... is the same as doing a 360 degree rotation  so that should put a negative sign in front of the combined wavefunction. And that’s the last piece of ...
 12:04: ... that is just the negative of the original. So Psi(B,A) = Psi(A,B)  swapping electrons flips the ...


20210915: Neutron Stars: The Most Extreme Objects in the Universe
 05:48: ... into the iron nuclei in a process called electron capture. The negatively charged electrons merge with positively charged protons ...


20210818: How Vacuum Decay Would Destroy The Universe
 02:52: ... and magnetic fields rise and fall between positive and negative values, but the average field value is zero. And after the ...


20210803: How An Extreme New Star Could Change All Cosmology
 17:08: ... earthism  it’s just dressed up a bit nicer. But I don’t want to get all negative at the end of the video  so instead I”ll direct you to the Professor ...


20210721: How Magnetism Shapes The Universe
 01:29: The nuclear forces are short range, and the electrostatic force never adds up to much because its positive and negative charges tend to cancel it out.


20210421: The NEW Warp Drive Possibilities
 04:48: We can summarize this by saying that it requires a negative energy density, which should be impossible except perhaps on the tiniest, quantum scales.
 04:59: ... people talk about the drive requiring negative mass  also called exotic matter  and yeah, that would do the trick too ...
 05:30: ... studies demonstrated that any superluminal warp drive MUST use negative energy ...
 06:58: ... the authors: Warp drives are inertially moving shells of positive or negative energy material which enclose a `passenger' region with a flat ...
 08:44: ... note that even a subluminal reactionless warp drive would still require negative energy and an enormous amount of it for any decent sized ...
 09:11: Lentz claims to have found an actual superluminal warp field solution that does NOT require the impossible negative energy densities.
 09:54: When you calculate the total energy with only this component than it’s alway negative.


20201027: How The Penrose Singularity Theorem Predicts The End of Space Time
 05:01: ... matter always produce this convergence. It would take negative mass or negative pressure to cause light rays to diverge. ...


20200908: The Truth About Beauty in Physics
 08:35: ... allowed the electron to have states with negative energy levels  not technically possible, but we now understand these as ...


20200810: Theory of Everything Controversies: Livestream
 00:00: ... what chose the asymmetry to to break left or to the right positive or negative that's not a problem no that's not a problem why is that a problem ...


20200708: Does Antimatter Explain Why There's Something Rather Than Nothing?
 02:12: ... should be unchanged. We have charge conjugation, where positive and negative charges are swapped; we have parity inversion, where the universe is ...
 08:00: ... to a number of different experiments, including ALPHA. There, the negativelycharged antiprotons are trapped by a combination of electric and magnetic ...


20200630: Dissolving an Event Horizon
 04:10: ... the central singularity  which pointlike in this case  results in a negative pressure that again resists the inward pull of ...


20200622: Building Black Holes in a Lab
 05:25: ... and are separated  one escapes and one falls in, somehow converting to negative energy and so decreasing the mass of the black hole. A more technical ...
 06:33: ... hole because the infalling particlesslashvibrations themselves acquire negative energy. This effect on the black hole is called the backreaction of the ...


20200527: Does Gravity Require Extra Dimensions?
 11:58: This is the negative pressure due to the exclusion of quantum vacuum modes, or virtual particles, between two very closely separated plates.


20200518: Mapping the Multiverse
 07:34: For one thing, the ring singularity becomes entirely repulsive  as though it had negative mass.
 10:37: ... not the bizarro negative time traveling universe either  here, the laws of physics are the same ...
 12:06: ... time carries a current of positive energy, while the backwards carries negative ...
 13:13: ... black holes the electromagnetic field within causes massive tension, or negative pressure that produces an antigravitational ...


20200504: How We Know The Universe is Ancient
 14:36: ... that does not have an event horizon, but you need the nonexistent negative energy to do so and keep it open long enough to traverse. The reality is ...


20200428: Space Time Livestream: Ask Matt Anything
 00:00: ... the fact that as the universe gets bigger it acquires more and more negative gravitational potential energy okay and so if energy can be negative ...


20200422: Will Wormholes Allow Fast Interstellar Travel?
 08:42: ... common way to think of exotic matter is anything with negative energy density. Something with negative mass could produce the required ...
 09:44: ... existing in that region. The gap between the plates will then have a negative energy density relative to surroundings. Unfortunately, the Casimir ...
 10:15: ... if you could create a Casimir negative energy wall in the center of a wormhole that should keep it open. ...


20200324: How Black Holes Spin Space Time
 09:37: ... Kerr metric of the ergosphere allows one half of the object to acquire negative energy, which is transferred to the black hole, while the other half ...


20200211: Are Axions Dark Matter?
 01:05: ... axes. Another example is flipping the charges of particles  positive to negative and vice versa  most of the equations of physics hold under that flip. ...
 03:02: ... exhibit an electric field like you’d get from a pair of positive and negative charges  an electric dipole field. Our very sensitive measurements have ...


20191209: The Doomsday Argument
 14:02: If it turns out to the universe is negatively curved then well it's infinite.
 14:06: ... finite or infinite because it may be that there's a slight positive or negative curvature just beyond the sensitivity of our ...


20191202: Is The Universe Finite?
 03:36: Then there’s 2) the negatively curved universe, analogous to a hyperbolic plane  an infinite saddle shape.
 04:26: Rapid expansion tends to give negative curvature and open the universe  make it infinite.
 04:56: ... curvature COULD have been very slightly positive or very slightly negative  just as the surface of the Earth appears flat if you’re standing on ...


20191021: Is Time Travel Impossible?
 05:17: We need to counter gravity, and to do that we need another probablynonexistent form of mass – negative mass – also referred to as exotic matter.
 05:27: As far as we know, mass can only take on positive, real values, so a requirement of negative mass seems a nonstarter.
 05:37: Really what we need to open the wormhole is a negative energy density.
 05:51: However there’s no clear path to translating this to a largescale negative energy distribution that could keep a wormhole open.
 05:59: And in fact we'd need entire planets – perhaps entire stars converted to negative energy to do this.
 06:05: Some other time travel options also involve using negative energy densities  for example the Alcubierre warp drive, which we already covered.
 06:19: But is negative massenergy as much of a nonstarter as imaginary mass?
 06:36: They’re a set of requirements that do things like prevent negative energies and enforce energy conservation.
 06:58: And as it turns out, there may be other ways to build time machines without either negative or imaginary masses.
 07:55: Stephen Hawking showed that unless the cylinder is infinitely long this doesn’t work – unless you also modify the spacetime with negative energy.


20190826: How To Become an Astrophysicist + Challenge Question!
 10:46: ... increased by a factor of 10 to the power of 26 Every 10 to the power of negative 32 seconds and an extra credit question at that rate How close to our ...


20190806: What Caused the Big Bang?
 03:48: ... by a factor of 10 to the power of 25 in less than 10 to the power of negative 30 ...


20190718: Did Time Start at the Big Bang?
 01:56: ... observational evidence that the time before around 10 to the power of negative 32 seconds Included a period of extremely rapid expansion called cosmic ...


20190701: Thorium and the Future of Nuclear Energy
 15:29: ... that this whole black hole killing star formation thing seems like a negative feedback Interaction more gas equals more active black hole equals more ...


20190410: The Holographic Universe Explained
 11:15: While the original space was flat, the new space had negative curvature – it was a hyperbolic, antide Sitter or AdS space.
 13:41: ... represent THIS universe, because our universe doesn’t appear to be negatively curved AdS space, nor does it have 4 spatial dimensions as in ...
 16:40: KI9 asks whether the things we learn from AdS/CFT are applicable to the universe we live in given that our universe doesn't have negative curvature.
 16:50: ... we don't know for sure that it doesn't have negative curvature  just that any curvature  negative or positive  is very ...


20190403: The Edge of an Infinite Universe
 03:25: Finally there’s the universe with negative curvature, and the 2D analog of that is the hyperbolic surface, like an infinite saddle or pringle.
 09:16: For the holographic principle we need the infinite boundary of a negativelycurved universe – an antide Sitter, AdS universe.
 17:01: ... energy is conserved and that dark energy is created from the increasing negative potential energy of the cosmic gravitational field, but I think at that ...


20190328: Could the Universe End by Tearing Apart Every Atom?
 03:05: ... density and no real pressure leaves the right side of the equation negative so negative acceleration matter on its own can only cause deceleration ...
 04:32: ... definition of the cosmological constant. So now the pressure term is A, negative and B because of this factor of 3 it's a bigger influence than the ...
 06:03: ... the limit: What if we make the equation of state parameter smaller than negative ...
 11:35: ... first, it seems too much of a coincidence that it should be so close to negative one without being negative ...
 12:11: ... violates the same energy conditions of general relativity that prohibit negative mass and time ...
 12:40: ... quasars hints at an equation of state parameter slightly less than negative 1 well they're not yet with enough significance to overturn all other ...


20190306: The Impossibility of Perpetual Motion Machines
 09:54: ... to come up with a perpetual motion machine based on the funky notion of negative ...
 10:06: According to some interpretations, negative mass should be attracted to positive mass, but positive mass should be repelled by negative mass.
 10:15: The result is that a positive and negative mass should accelerate indefinitely, potentially powering an infinite energy device.
 10:58: We have a rare nondynamo device from Adrien Romeo, who uses cogs with negative and positive mass teeth.


20190220: Secrets of the Cosmic Microwave Background
 07:42: ... effect On the other hand an expanding universe with no energy would have negative curvature So a flat universe must have exactly the right amount of ...


20190207: Sound Waves from the Beginning of Time
 13:19: Today, it's the 'Crisis in cosmology' and 'Negative mass perpetual motion' episodes.
 15:16: Ergo, time will prove negative mass' existence.
 16:03: flux_capacitor notes an alternate model for how negative mass might behave.
 16:08: In socalled bimetric gravity, you can have positive and negative masses, but each is described by its own set of Einstein field equations.
 16:18: That's kind of like having parallel spacetimes, one with positive and one with negative masses, which can still interact gravitationally.
 16:26: ... get the crazy runaway motion that occurs if you put both positive and negative masses in the same ...


20190130: Perpetual Motion From Negative Mass?
 00:13: Because you could use negativemass apples to build warp drives, travel in time, and construct a perpetual motion machine.
 00:31: Matter with negative mass  has long been the pipedream of science fiction writers, futurists, and certain rather.
 00:59: And we also recently covered a very new use for negative mass: as “dark fluid”, a proposed explanation for both dark matter and dark energy.
 01:10: That episode really got me thinking about the subtleties of negative mass and how it should really behave gravitationally.
 03:19: OK, so how do negative masses work in Newtonian mechanics?
 03:28: In fact, in Newtonian gravity, any like masses – both positive or both negative  should produce a mutually attractive force.
 04:37: Newton’s second law seems to say that negative inertial masses respond oppositely to the applied force.
 04:45: A repulsive force on a negative mass becomes an attractive force and vice versa.
 04:51: ... this in the dark fluid episode, but to recap: this suggests that two negative masses produce an attractive force which actually drives them ...
 05:02: Even weirder, a positive mass should attract a negative mass while at the same time being repelled by it.
 05:08: A negative mass apple would still fall to the Earth, and you wouldn’t notice Earth’s infinitesimal repulsion from the apple.
 05:17: But put equal positive and negative masses next to each other and they should accelerate uniformly forever.
 07:13: That should mean that a negative mass behaves the same in a gravitational field as a positive mass.
 07:50: This suggests that a positive gravitational field attracts everything, including negative masses.
 07:57: So what about negative gravitational fields?
 08:01: Negative mass causes negative curvature, which in our sheet analogy looks like pulling the sheet up.
 08:13: That suggests that everything, regardless of mass, should be repelled from a negative mass.
 08:21: It suggests that positive masses attract everything and negative masses repel everything, including each other.
 08:29: A positive mass attracts and is repelled by a negative mass.
 08:51: So a positive and a negative mass apple placed side by side should chase each other across the cosmos, accelerating forever.
 09:06: ... mass apple gains positive momentum and energy as it speeds up, while the negative mass apple balances that with negative momentum and negative ...
 09:37: They are a set of conditions against negative energies that seem necessary in order for general relativity to describe a sensible universe.
 09:46: ... fact the mere existence of negative mass breaks certain energy conditions, but the prospect of a bottomless ...
 10:03: In particular, with the idea of negative and positive masses accelerating each other to infinite energies?
 10:18: In this case, the basic nature of the positive versus negative gravitational fields – the way the fabric of spacetime gets stretched has to be right.
 10:26: Much less clear is the way a negative mass responds to an applied force.
 10:39: We then happily plugged our negative inertial mass into Newton’s 2nd law to get acceleration.
 11:42: The real nonsense seems to be the idea of negative inertial mass.
 11:48: By comparison, negative gravitational mass is kind of ok.
 11:52: ... Negative inertial mass implies flipping the sign of the acceleration when any ...
 12:28: ... if we conclude that negative inertial mass can’t exist and we want to save the equivalence principle ...
 12:55: Some say negative mass can’t exist, some say it can.
 12:59: ... say that, if they do, then positive mass always attracts while negative mass always repels, while others say that like mass signs always attract ...
 13:18: ... it is: assuming that positive mass both attracts and is repelled by negative mass, come up with a way to use a pair of infinitely accelerating ...
 13:59: Make the subject line negative mass challenge question.


20190124: The Crisis in Cosmology
 13:26: ...about negative mass dark fluid...
 14:06: ...will give the exact opposite results if dark matter is due to this negative mass fluid...
 14:48: Marik Zilberman's distaste for negative masses...
 16:31: And a negative mass particle, moving backwards in time,...


20190116: Our Antimatter, Mirrored, TimeReversed Universe
 03:02: ... how does this work in our Antion on a clock well antimatter atoms have negatively charged nuclei which means their nuclear magnetic fields point in the ...


20190109: Are Dark Matter And Dark Energy The Same?
 00:32: And it’s pretty wild: negative mass particles continuously popping into existence between the galaxies.
 00:58: Farnes 2018, “A unifying theory of dark energy and dark matter: Negative masses and matter creation within a modified LambdaCDM framework”.
 02:53: But here’s the punchline: if empty space has a constant, positive energy density then it also has a negative pressure.
 03:02: For obscure reasons that we delve into those episodes, negative pressure produces an antigravitational effect.
 03:10: ... positive energy density gives it a positive gravitational effect but its negative pressure is ...
 03:58: He tried to do this in very different way: with negative mass.
 04:15: So what about negative masses?
 04:18: The answer is complicated  it’s not always obvious how weird stuff like negative masses translates from general relativity to Newton’s laws.
 04:45: Two negative masses should cancel each other’s signs so also give you an attractive force.
 04:51: But with one negative and one positive mass, the final force of gravity has the opposite sign  that makes it repulsive.
 05:29: For a negative mass, this equation suggests that acceleration is in the opposite direction to the applied force.
 05:36: Push a negative mass away from you it’ll move towards you, pull it and it’ll move away.
 05:45: So Farnes argues that the “attractive” gravitational force between two negative masses should actually drive them apart.
 05:56: At the same time, the repulsive force between a negative and positive mass should repel the positive and attract the negative mass.
 06:15: The author uses these ideas about the interactions of negative and positive mass particles to create an Nbody simulation.
 06:22: ... he programs a virtual universe into his computer with both positive and negative mass particles, along with his interpretations of Newton’s ...
 06:32: Those simulations showed that galaxies do indeed spin more quickly when surrounded by negative mass particles.
 06:39: ... is because the positive mass in the galaxy attracts a halo of negative mass, but at the same time that positive mass is repelled inwards by the ...
 06:59: To make the same negative mass stuff also emulate dark energy requires an extra gigantic assumption.
 07:07: The problem is that our negative masses will dilute away as the universe expands.
 07:12: To fix this, Farnes proposes that these negative masses are constantly created as the universe expands.
 07:20: That keeps the density of negative masses constant, even as the density of positive masses falls.
 07:26: The result is a very diffuse negative mass fluid that fills the universe and constantly replenishes itself.
 07:57: And that a positive Lambda results in an antigravitational negative pressure?
 08:02: Well a constant NEGATIVE energy density – like the one proposed by Farnes, gives a negative cosmological constant.
 08:18: ... really the analog of Newton’s law of gravity for the whole cosmos, this negative mass fluid – as a negative cosmological constant – has the same ...
 08:35: The direct effect is repulsive – antigravitational – which I guess was the original motivation for using negative matter.
 08:50: ... Farnes acknowledges that his proposal gives a negative cosmological constant that ultimately decelerates, but there’s some ...
 09:06: And yet a negative cosmological constant gives you an extremely different universe.
 09:10: Dr. Farnes does a couple of calculations that are consistent with a negative lambda.
 09:15: ... – that’s this thing  to correctly conclude that a universe with a negative cosmological constant should have a sinusoidal scale ...
 09:43: The negative cosmological constant is the source of that slowdown and for the subsequent accelerating collapse.
 09:55: Plugging negative masses into general relativity allows you to break causality.
 10:07: This is a strong indication that negative mass can’t exist.
 10:16: Does a universe with a constant negative energy density fit the observations?
 10:48: ... a negative cosmological constant and this sinusoidal expansion, any slowdown ...
 11:00: ... an age for his universe of 13.8 billion years assuming a very low negative energy density, but that really just corresponds to the very first ...
 11:57: If you replace both dark energy and dark matter with negativeenergy stuff, then the universe becomes negatively curved.
 12:59: ... all, it’s easy to get confused by the compounding negatives in what is probably not a negative mass, antigravitational, positive ...


20181220: Why String Theory is Wrong
 03:43: Shrink it down to around 10 to the power of negative thirtieth of a meter, so it's only visible to things equally miniscule.


20181212: Quantum Physics in a Mirror Universe
 00:02: ... transformation followed by a flipping of electric charge positive to negative and viceversa and a reversal in the direction of the flow of time will ...


20180920: Quantum Gravity and the Hardest Problem in Physics
 05:57: ... to an accuracy better than a Planck length around 10 to the power of negative 35 of a meter, the amount of energy you would need to put into that ...
 06:46: Try to measure any time period shorter than 10 to the power of negative 43 seconds, the Planck time, and boom black hole.


20180912: How Much Information is in the Universe?
 02:48: ... smallest meaningful measure of distance, at around 1.6 times 10 to the negative 35 ...


20180830: Is There Life on Mars?
 06:12: Three of the tests were negative, but the fourth was positive.
 12:16: With enough of this stuff, the freezing point of water could be as low as negative 75 Celsius.


20180704: Will A New Neutrino Change The Standard Model?
 03:20: So an electron has a charge of negative 1 and an antielectron has a charge of plus 1.
 05:05: ... example, both left and right chiral negatively charged electrons have their own positively charged antimatter ...


20180620: The Black Hole Information Paradox
 13:20: ... electric charge within the black hole produces a negative pressure that actually halts the cascade of space within the black hole ...


20180613: What Survives Inside A Black Hole?
 08:15: ... quickly attract particles with the opposite charge until positive and negative charges within the black hole balance out and the black hole becomes ...


20180411: The Physics of Life (ft. It's Okay to be Smart & PBS Eons!)
 04:19: ... as a struggle for entropy, well, more accurately, against entropy or for negative ...
 04:29: Erwin Schrodinger, in his 1944 book, "What is Life," describes life as a process feeding on negative entropy.


20180404: The Unruh Effect
 06:20: ... here they also lead to a mixing of positive and negative frequency modes in the accelerating frame of reference, which leads to ...


20180315: Hawking Radiation
 03:32: One way that quantum fields are very different to guitar strings is they can have both positive and negative frequencies.
 03:39: A negative frequency can be thought of as a mode that travels backwards in time and can be interpreted as corresponding to antimatter.
 03:51: ... field is in a vacuum state, there's a balance between positive and negative frequency modes, which you can crudely think of as a balance between ...
 06:30: They describe a sort of mixing of the positive and negative frequency vibrational modes that are caused by that curved space.
 08:13: So Hawking's math describes splitting or mixing of these pure positive and negative frequency modes.
 10:05: We can think about positive and negative frequency modes being mixed due to scattering, perhaps, by the as yet undiscovered graviton.


20171108: ZeroPoint Energy Demystified
 05:02: ... is that the reduction of energy between Casimir plates can be considered negative energy and so could be useful for all of those wonderful things that ...
 05:18: Well, if we define the average vacuum energy as 0, then the Casimir Effect produces negative energy.
 05:36: That means the vacuum energy between Casimir plates still produces positive spatial curvature, not the negative curvature required for warp drives.


20171102: The Vacuum Catastrophe
 06:07: ... positive zero point energies, then perhaps others have extremely large negative zero point energies that cancel them ...
 07:31: Assuming vacuum energy is to blame, dark energy weighs in at 10 to the power of negative 8 ergs per centimeter cubed.
 07:59: ... or a vacuum energy of exactly 0 if we assume symmetry of positive and negative zero points between different fields, but a very small nonzero vacuum ...
 08:19: Gigantic positive and gigantic negative zero point energies would need to cancel each other out down to a very tiny nonzero value.


20171019: The Nature of Nothing
 14:10: Flo Striker was wondering about the idea of negative kelvin temperatures.
 14:17: It's not very intuitive, though, because having a negative temperature means the substance is hotter than any substance with a positive temperature.
 14:38: But at negative temperatures, most particles are excited towards the highest possible energy states.
 14:44: This means that a negativetemperature substance can only lose thermal energy to a positivetemperature substance, not gain it.
 14:52: So why call them negative temperature?
 15:29: So if temperature is change in thermal energy over entropy, then temperature is negative.


20171011: Absolute Cold
 00:42: This state of absolute cold is the zero point in the Kelvin temperature scale corresponding to negative 273.15 Celsius.


20170810: The OneElectron Universe
 03:58: ... example, if a negatively charged electron is moving to the left, it produces some current, I. ...


20170726: The Secrets of Feynman Diagrams
 05:04: Similarly, if a photon creates a negatively charged electron, it must also create a positively charged positron.


20170628: The First Quantum Field Theory
 13:50: ... Mr. Diagrams points out, the resulting Klein Gordon equation gives negative energy solutions just like the Dirac equation does, but Schrodinger ...


20170621: AntiMatter and Quantum Relativity
 06:30: It allowed electrons to exist in states of negative energy.
 06:38: ... energy as light infinitely, and sink lower and lower, to infinite negative energy ...
 07:06: These electrons occupy all of the negative energy states, all the way from negative infinity, up to zero.
 09:13: Paul Dirac's negative energy solutions describe antimatter, not holes in the Dirac sea.
 10:06: They are two sides of the same coin, positive and negative energy solutions of the same type of vibration in the electron field.
 10:37: It's not negative mass despite this negative energy description.


20170405: Telescopes on the Moon
 11:33: ... singularity is expected to produce an antigravitational effect through negative pressure, in a way that's mathematically similar to dark ...


20170329: How Time Becomes Space Inside a Black Hole
 01:34: If one event causes a second event, the spacetime interval must be 0 or negative.
 02:00: In fact forward temporal evolution requires a negative spacetime interval.
 02:05: In flat space time, that negative sign in front of the delta t drives that forward evolution.
 03:48: A negative spacetime interval still means causal movement.
 04:01: Then both of these brackets become negative.
 04:04: The entire delta r stuff is now negative.
 04:36: In the mathematics the coordinate r, which once represented distance, now grants the negative sign needed to maintain your causal flow.
 04:50: Meanwhile the coordinate previously known as time, t, lost its negative sign and become spacelike.


20170322: Superluminal Time Travel + Time Warp Challenge Answer
 03:37: ... we write the spacetime interval for flat space with a negative sign in front of the time part, then changes in your space interval have ...


20170202: The Geometry of Causality
 09:08: ... way I define the spacetime interval, it becomes increasingly negative in the forward time direction, so we can represent this as a valley ...


20161012: Black Holes from the Dawn of Time
 12:16: ... also a dipole, meaning it has more positive charge on one side and more negative on the ...


20160525: Is an Ice Age Coming?
 12:01: Last week, we wrapped up our conversation on dark energy, talking about antigravity, negative pressure, and conservation of energy.


20160518: Antigravity and the True Nature of Dark Energy
 00:46: Two, such an underdense universe should be geometrically weird, a negatively curved hyperbolic hyperplane.
 03:58: That's this negative sign here.
 04:00: Positive acceleration outwards, negative inwards.
 06:01: So it looks like as long as there's anything in the universe whatsoever, that whole right side is negative.
 08:10: It's negative.
 08:12: A negative p here cancels this minus sign and can result in outward acceleration.
 08:24: It's negative pressure.
 08:26: What does negative pressure even mean?
 08:33: So negative pressure would pull inwards.
 08:49: Negative pressure pulls in.
 09:05: The direct effect of dark energy's negative pressure doesn't do anything, because that negative pressure is the same everywhere in the universe.
 09:14: But even though the negative pressure has no direct effect, it has its relativistic effect.
 09:19: Relativistically, negative pressure has to do the opposite of positive pressure and results in antigravity, because math.
 09:29: The relativistic effective of negative pressure is actually really, really hard to describe intuitively.
 09:38: Part of the problem is that negative pressure doesn't come from the motion of dark energy particles, whatever they might be.
 10:09: That's exactly how we define negative pressure.
 10:12: A volume has negative pressure if it takes work to expand, just like a volume with positive pressure takes work to compress.
 10:22: Negative pressure means energy is gained on expansion.
 10:35: That looks like negative pressure, and yet this abstractsounding negative pressure has a very real physical effect.
 11:01: But both agree that a constant energy density and the resulting negative pressure leads to accelerating expansion.


20160427: What Does Dark Energy Really Do?
 10:11: The universe could be positively or negatively curved below that level.


20160420: Why the Universe Needs Dark Energy
 03:43: And we call that a "closed geometry." If k is minus 1, the universe is the 3D version of a negatively curved hyperbolic plane.


20160413: Will the Universe Expand Forever?
 06:48: And if the lefthand side is negative, in that case there was never a highenough expansion rate to reach that extreme size.


20160406: We Are Star Stuff
 10:58: ... about the geometry of the universe and describe it as flat positive or negatively curved, I should say that the curvature of space at a single, constant ...


20160323: How Cosmic Inflation Flattened the Universe
 02:00: On a negative curvature hyperbolic plane a saddlelike structure they add up to less.
 02:48: It should be larger if the universe is positively curved, smaller if negatively curved.
 03:35: If the center of the alley represents a flat universe, then the gutters represent extreme curvature in the positive or negative directions.


20151105: Why Haven't We Found Alien Life?
 10:55: ... bubble can traverse the walls in any direction besides forwards and our negative mass matter may also end up on the ...


20151028: Is The Alcubierre Warp Drive Possible?
 03:24: ... do this for the warp field, you find that you need to produce a ring of negative energy density in a band around the ship to produce the right warp ...
 03:38: Unfortunately, it may not even be possible to make negative energy densities on large enough scales.
 03:44: We can create something like it, a negative pressure, on quantum scales via Casimir effect.
 03:49: ... on macroscopic scales, you'd probably need some sort of exotic negative mass matter, like element zero, which is tricky, because there may be no ...
 04:37: ... that you can even make negative mass matter, to make a warp field, some of it would need to go outside ...
 04:46: ... are some proposed solutions, one of which is to lay down the external negative energy conditions along the path before you leave, sort of like a warp ...
 05:02: Last tricky thing as Alcubierre devised the warp bubble, he figured it would take a lot of negative energy.
 05:08: In fact, it would take significantly more negative energy than there is positive mass/energy in the entire observable universe.
 05:25: Thicken the walls of the warp field, and you get the negative mass/energy requirement down to the equivalent of maybe the moon or even an asteroid.
 05:46: ... Tardis style, which could get us down to needing only milligrams of negative ...
 06:18: Now, this would be a field created by positive, not negative, energy density.


20150520: The Real Meaning of E=mc²
 05:29: It's because potential energy can be negative.
 05:48: So the potential energy of the electron and proton in a hydrogen atom is negative.
 05:58: ... as it turns out, the potential energy is negative enough that the sum of the kinetic and potential energies still comes ...
 06:22: ... and potential energies of those atoms once they form a chemical bond is negative. ...


20150513: 9 NASA Technologies Shaping YOUR Future
 08:46: ... might be long term negative effects to which women are more susceptible that we haven't even seen ...
 08:56: ... asks why I didn't bring up osteoporosis as a bigger negative for women, and the reason is that spaceflight basically gives everyone ...


20150506: Should the First Mars Mission Be All Women?
 02:37: So while it's a negative for women, radiation risk might be one of those things that you have to solve for everyone in order to send anyone.


20150429: What's the Most Realistic Artificial Gravity in SciFi?
 01:10: Or you need something exotic, like gravitationally repulsive negative mass, which doesn't exist.
 09:53: ... could have been present at both events, which corresponds to a zero or negative space time interval between those ...


20150422: Are Space and Time An Illusion?
 03:12: We'll notice that since it involves subtraction, a spacetime interval can be positive, zero, or negative.
 03:27: When it's zero or negative, signals or things can get from one event to the other and everyone agrees on their sequence.
