Leonard Susskind - How does Dark Energy Drive the Universe?

leonard when you first heard the data from the cosmological uh observations that uh dark energy may be 70 of reality and there was this potential cosmological constant what did it do to your world view well it was completely stunning um the tsunami just uh just something overwhelming the reason though is not the fact well the reason was a little bit confused and confusing we were all so confused that to look back on it now it just seems like we were thinking all the wrong things uh the question is not why there is dark energy theorists and physicists have understood for many years that quantum mechanics and quantum field theory create dark energy dark energy being the fluctuations in the vacuum and so forth the real question is why is there so little of it what happened was theoretical physicists realized that it was almost zero that there was a negligible amount of it so they convinced themselves that someday they would have an explanation of why it was exactly zero which is easy to explain a zero is easy well they thought that easy that zero should be easy to explain they never explained it they never explained it they thought oh it must be exactly zero and there must be some principle that we don't know about probably in string theory that will make it be exactly zero so string theory will have the solution that's the solution aha we've explained it that was the logic that was the logic uh then the real world then the real world came and said there is a little bit of dark energy all right the physics and the science of it said that there should be dark energy but as we understood that physics and science it should have said there's a huge amount of dark energy not this terribly little bit so what's mysterious is not that this dark energy what's mysterious is that there is so little of it so how do we deal with that mystery well the only thing that we can connect it to this terribly tiny number is that if it were very much bigger we wouldn't be here to ask the question if there were much more dark energy and remember what dark energy does it has a repulsive character to it makes everything repel the part of our own existence is dependent on the existence of planets and stars for example how did they form they formed by gravity pulling them together if there was this repulsive character to this cosmological constant it would have the condensation of the galaxies and the stars and the earth and so forth if it was strong enough it would completely uh prevent it well the answer was that if the cosmological constant was just a little bit stronger than the amount that we detected it would have prevented stars galaxies earth and presumably us from forming and being here to ask the question so that's the only thing we know how to connect it to our own existence that's a a radically different approach to to science yes of course it's not mine this was a point that was made many years ago by stephen weinberg and it is a radically well yes and no yes and no if we were to ask the question why is the temperature of the earth what it is why is it in the very very narrow range where life is possible i think most of us would say that's a stupid question of course it's in the narrow range where life was possible if we were on a planet where life was not possible we wouldn't be there to ask the question and i think we would agree that that's the answer i hope anyway so i would say it's not all that radical what's radical is the use of it in this particular context well the difference conceptually as we start is that we know for a fact that there are other planets we see them yeah yeah we see them and now we know there are hundreds of extra solar planets and no doubt it's a virtually an incredibly large number so i mean this is something part of our world we know that they exist but we only know that one universe exists by the same observation right so you put your finger on exactly what the problem is for physicists and cosmologists at the present time that yes we know about all this diversity that's out there we see it and we have no hope whatever of seeing the diversity of the multiverse that may be out there we have no hope of seeing it it's just too far it's moving away from us it's beyond horizons and so forth and that's what we have to contend with why is it so obvious that there should be dark energy what is it about the laws of quantum mechanics that would cause energy in absolutely empty space right it's the uncertainty principle for example supposing you had a particle on the end of a spring we call those harmonic oscillators particles which could just vibrate back and forth um classically without quantum mechanics you can imagine that particle just standing still with the spring uh being as short as possible and just not moving it would have no potential energy it would have no kinetic energy standing still at rest no velocity no displacement well the uncertainty principle says that's impossible it says you can't know both the position and the velocity they have to fluctuate because they have to fluctuate that little oscillating thing has to have some energy the same is true in many contexts so one context would be the electromagnetic field the field that makes light electric fields and magnetic fields also have uncertainty principles and they have to fluctuate well that little bit of fluctuation has energy how much energy well that depends on how big meaning how big in space the fluctuation is the smaller in space the fluctuation is a little tiny fluctuation in space has more energy than a big overall fluctuation right so physicists calculate how much vacuum energy how much energy should be there in the oscillating electric magnetic and all the other fields of physics and they calculate that each different size scale would produce a certain amount of energy if you start at very long scales not much energy you start going towards smaller and smaller scales more and more fluctuation on smaller and smaller distances you discover more and more energy if you only took into account for example up to wavelengths or up to sizes about as big as an atom that would give you enough uni energy in the universe to create a cosmological constant which was vastly bigger than we can tolerate well an atom is not very big we think we know the laws of physics down to atomic size we think we know how to calculate how much vacuum energy is there and it's much too much we can go past atomic size we go to nuclear size we still think we know what the physics is we start calculating then we find out that the uh that the vacuum energy is enough to completely explode every atom every molecule in nature just by blowing it apart and if we push that logic all the ways down to the shortest distances which we think we understand the energy stored in those fluctuations would be vastly vastly bigger than uh than anything we can tolerate what does that mean is there some inconsistency with the laws of quantum mechanics um let me tell you first of all if you were to calculate these things in ordinary quantum field theory the answer would come out just plain infinite and the reason would be infinite is you can go down to smaller and smaller wavelengths there's a story about this i don't know if anybody knows it but me i wasn't there but i heard the story paul dirac who was one of the greatest quantum physicists of the world ever said well if it comes out infinite i don't believe in it probably doesn't mean anything and wolfgang paulie who was also a very very great physicist said paul just because it's infinite doesn't mean it's zero and that's exactly the way it sort of worked out physics evolved paul dirac thought it didn't mean anything because it was mathematically came out to be infinite as we now understand quantum field theory there's some small distance that we should do these things down to it's called the plunk distance and not beyond that and we would get a finite answer the finite answer would be very much bigger than uh than what we see so uh what do we do about it well when i say it was very much bigger than it should be you have to be a little bit careful there are various contributions to it the electric field contributes to it the magnetic field contributes to it the higgs field whatever that is contributes to it the electrons field contributes to it they all contribute some with positive signs some with negative sign conceivably they could all add up and cancel out but that would require a fine tuning and that would require a degree of precision and fine-tuning far beyond anything we ever imagined so one possibility is that for reasons unknown the parameters of physics happen to be tuned in precisely the right way so that all of this vacuum energy just happens to cancel out but we don't have any reason to believe we don't have any theory that explains why that's so so the answer is that without all this tremendous fine tuning the calculations that physicists do would expect you would expect a very large dark energy many orders of magnitude bigger than what you see

Channel: Closer To Truth

Views: 133,007

Keywords: closer to truth, robert lawrence kuhn, Leonard Susskind, How does Dark Energy Drive the Universe, Dark Energy, what is dark energy, where does dark energy come from, dark matter, big bang, black holes, inflation theory, dark matter and dark energy, quantum physics, astronomy dark energy, astrophysics, dark energy explained

Id: mMgRCwzEkBQ

Length: 10min 32sec (632 seconds)

Published: Fri Nov 06 2020