I defended my PhD nearly 5 years ago, and have been working as a research scientist since then. I lecture a little, and have a couple of PhD students.

This week, i'm working on central Australian time, so i'm going to go to bed soon. But i'll plan to spend my morning answering your questions, so i'll be back on line starting from when this post is about 10 hours old.

Proof: Here is some of my comment history from askscience as evidence.

Edit 1. (23.45 CST) Okay, really going to bed now, having (i hope) sorted out proof. Talk to you in the morning.

Edit 2. (11.30 CST) I've got to go and do some stuff, but i'll come back in about three hours and answer whatever questions are left.

Comments: 58 • Responses: 26  • Date: 

slowlike_emu4 karma

What was your PhD on? Also, what is the most misunderstood thing you've come across in relation to your field?

doctorBenton6 karma

The first part of my PhD was focussed on adding near infrared data to a very well studied 'deep field'. Infrared is helpful, because it probes the ordinary starlight from very distant galaxies in the early universe. The particular thing that i wanted to look at was how the most massive galaxies grow from when the universe was only 1 or 2 billion years old to now.

In the second half of my PhD, i focussed more on nearby galaxies, using the Sloan Digital Sky Survey. First i looked into how well it is possible to estimate a galaxy's mass, using just the observed colours.

Then the last paper was confirming that there really are no 'early type' galaxies that are as massive and as compact as those found in the very early universe. This implies that galaxies have to keep evolving, even after they stop forming stars. This is really weird: they have to grow in size without increasing their masses.

Here's a lay summary if you're really keen.

Reddit-Mark3 karma

What are some of the topics you have been researching?

With so much of our galaxy unexplored, what are the chances that some sort of life exists on another planet?

doctorBenton7 karma

I'm currently working on a new way to use the physical phenomenon of weak gravitational lensing to probe the dark matter halos that surround individual galaxies. If it works, it'll be really pretty cool.

I'm also particularly interested in the main stages of a galaxy's life. For some reason, it seems that all of the most massive galaxies are finished. They don't appear to have formed many or any stars in the past few billion years, and they definitely aren't forming any new stars just now. They are also mostly elliptical, rather than disks. But we're really not sure how or why galaxies undergo these major evolutionary changes.

As to your second question, from what we know, it seems virtually certain that life is common in our galaxy. The thing you want to look into is called the Drake equation. Also, just today, there was a widely reported press conference laying out a roadmap to discovering life (or at least habitable planets) in the 2020s, if not before.

Reddit-Mark2 karma

Wow, truly fascinating! Thanks for the reply!

doctorBenton1 karma

Thanks for saying thanks!

LilyoftheRally3 karma

How did you get interested in the field?

doctorBenton3 karma

This is hard to answer, because there are a bunch of different ways to tell the story.

I was definitely okay at maths and science as a kid, but i never really felt that inspired by anything there. In the last couple of years of high school i discovered history, and art, and this was the first time that i was really exposed to ideas. So when i went to Uni, i studied history and philosophy.

In my second year, i did mostly epistemology (nature of knowledge) and metaphysics (nature of existence), and i came around to the idea that if you want to make sense of the universe, then it makes sense to ask the universe what it thinks. It seemed to me that empirical and experiential knowledge was what matter. And so i found my way back to science.

Originally, i started physics wanting to demystify quantum mechanics, and then i wanted to be an astronaut. That's obviously not why i stayed in the field! But that is (part of) the story of how i found my way to astro.

whatzen2 karma

Did you ever manage to demystify quantum mechanics even a little bit? I ask as I find it incredibly fascinating and are contemplating a career changing move to study physics full time.

doctorBenton2 karma

I made my peace with it, or at least with some of it.

I think that going in, what i really wanted to do was to get involved in the philosophy of quantum mechanics. In particular, when i made up my mind to go back into science, i was really caught up in questions of free will and determinism. But after a few years of studying physics (and, to be honest, some growing up!) those questions just kind of lost their urgency for me. Answers like "if free will is just an illusion, then it's an utterly compelling illusion, and we have no choice but to carry on as if we have free will" became enough for me. Also, i think i came around to the view that quantum doesn't really have much to say on the issue!

I guess that for me, the key to making my peace with quantum was the realisation that you have to conform your language to your understanding of physics/the universe, rather than expecting the universe to conform to your intuition. So the big change for me came when i understood that the standard explanations that you get about things quantum mechanical are valiant attempts to hang words on equations, and that its these equations that are really the heart of quantum. So that trying to understand quantum in words is the wrong way to go about it -- the explanations and pictures that you encounter outside a physics lecture theatre are more interpretations than they are explanations.

I'm not sure that i've said that quite right, but i hope you see what i mean? I'll think about it some more...

EATTEXT2 karma

How many chin-ups can you do?

doctorBenton2 karma

Not as many as you, i think. : (

heathbar242 karma

With your Ph.D, do you make a lot of money? Like over 6 figures?

doctorBenton1 karma

Not over 6 six figures, no. In absolute terms, i guess i'm not paid badly -- better than most friends who work for NGOs, say, and i think about as well as someone who works for state or federal government. But given the amount of time spent in training (compared to, say, a surgeon or a trial lawyer) it's really not great. Definitely we don't do this for the money.

escherbach2 karma

In a previous AMA, Sean Carroll (in response to my post) said that cosmic inflation is a unitary process (quantum mechanically speaking). Someone added a comment later, disagreeing, since this would imply that it was a reversible process.

What do you think?

doctorBenton2 karma

So, this is waaay outside my area of expertise, because i'm not that kind of cosmologist. But my very vague and dim understanding (from a course that i took nearly wince ten years ago) is that it is a unitary process.

But i guess i'm not sure that i understand reversibility would be a problem for inflation? The cosmic expansion is, i think, reversible in principle.

In physical terms, reversibility is a thermodynamic idea, which is based on whether or not the entropy in the system changes. And i'm not sure how/why inflation could/would alter the large-scale entropy of the universe.

But like i say, i am absolutely not an expert in this area!

window52 karma

on the expanding universe ... is the universe defined by the space in which light particles have traveled since the big bang? That is, after the first second the universe was 186K * 2 miles wide? Is the space that those first light particles are traveling into any different than the space that the earth moves in? That is, I assume the space these outer edge light particles are traveling into does not exist until they arrive there.

doctorBenton1 karma

You've hit on the idea of the observable universe. This is the (bounded) region of space and time that is observationally accessible to us.

I have to explain a diagram. Draw a set of x-y axes. Call the x axis space - just one dimension of space. Call the y axis time. You sit at the origin of the axis; that is your here and now. All the way down the bottom of the diagram is the beginning of the universe, which is something like 13.67 billion years ago.

Now, because light travels at the speed of light, it takes time for light to travel from one place to another. If you trace the light rays back in space and time, they will follow a straight diagonal line in your diagram, and so you'll draw something that looks like a triangle. This is what defines your observable universe.

The most important point that i want to make with this is that as you look a long distance away, you are actually seeing parts of the universe that are younger than the here and now. In other words, the time separation between a point somewhere on that triangle and the beginning of the universe -- the y coordinate -- is less than that for your here and now.

Using light, the edge of the observable universe is the cosmic microwave background. This is because beyond (or before) this point, the universe is a plasma. A plasma is what you get from a cigarette lighter. In a plasma, light is constantly being absorbed and reemitted, so instead of travelling a straight path, light kind of gets bounced around from place to place. It's only once the universe expands and cools and stops being a plasma (about 300000 years after the big bang) that photons can free-stream throughout the universe.

The next thing is that causality travels at the speed of light. You'll sometimes here people say things like 'because the sun is 8 lightminutes away it could blow up and we wouldn't know about it for 8 minutes'. (But you should now understand that we don't look back into the past, we see parts of the universe that are younger!) But the fact is that physics works on where the sun appears to be right now, rather than where it may or may not be in eight minutes. The gravitational acceleration of the earth is towards where we see the sun to be, not to some point ahead of or behind the sun, if you see what i mean.

So, in the same way that the region of space and time that we can see is bounded by the CMB, the region of space and time that we are in causal connection with is also bounded.

So, the universe may well be infinite, but it may as well be finite. Because anything that may or may not lay outside that region is of no consequence; there is no causal connection between it and anything else inside our observable universe; there is no experiment that you can do to learn anything about whether or what there is there; it is literally immaterial.

But of course, the size of this region grows by one light second per second. And of course, your region is just slightly different (by a couple thousand kilometres, at a guess) than mine.

bengrf2 karma

I really hope your still answering questions because this has been bugging me for the past 3 months. If we know the universe is expanding faster because the further away objects are the faster they go away from us, then wouldn't the universe be expanding slower because those objects are travailing at that speed further back in time? For instance it takes 3 billion years for light from a galaxy to reach us so it is travailing at that speed 3billion years ago while the one that is 2 billion years from us is moving at a slower velocity.

doctorBenton1 karma

Hmm... i'm not sure that i really understand your question. Let me try to describe how i think about the expansion, and then maybe if that doesn't help, you could try rephrasing it?

Picture a map of Australia. You are sitting at Uluru, which is pretty much the dead centre of the map, and you measure the distances to the cities of Perth (to the West), Adelaide (to the South), Brisbane (East), and Darwin (North). Now say that the universe doubles in size. When you measure the distances to all of those places again, they have doubled. They are all still off in the same directions as before, they're just twice as far away.

Now imagine that you're sitting in Melbourne, which is in the Southeast, and which is really (obviously) the centre of the universe. If the universe doubles in size, you will see the same thing. The distance to everywhere else doubles, but the directions to all the different cities stays the same.

Now, the important thing is that nothing is actually moving. Instead, the separations between distant points is getting larger because the space between them is expanding.

Now, because light travels at the speed of light -- ie. at a finite, constant speed -- it takes time for light to travel from one place to another. As it travels, it gets caught up in the expansion. The wavelength of light will expand along with the universe, making the light shift to redder and redder wavelengths.

This is similar to what happens for a moving object: the Doppler effect will cause a redshift for something that is moving away from you, or a blue shift for something that is moving towards you. But this is actually a different effect to the cosmological redshift. (Sidenote: just to confuse things further, there is also the gravitational redshift, but forget that for now.)

If you see a redshift, you might interpret it as being a Doppler shift but that's not necessarily the right thing to do. In general, the observed redshift will be a combination of the Doppler redshift (from the object's motion through space) and the cosmological redshift (from the expansion of space itself). In general, it is very hard to disentangle these two effects.

So, getting back to your question. Let's think only of very, very distant things, so that the cosmological redshift is much larger than the Doppler redshift. Because light travels at the speed of light, if we know the distance to an object, then we know the time of flight for the photon. Now if we know the expansion history of the universe, then we know how much the universe has expanded over the time that that photon has travelled from its origin to us, and so we work out the redshift. In other words, redshift, time of flight, and distance are all connected via the speed of light, and the expansion history of the universe.

This means that if you know (or can measure) two of these things, then you can infer the others. So if you can measure the relation between redshift and distance, then you can infer the expansion history of the universe. Or once you know the expansion history of the universe, you can use redshift as a measure of distance.

Does that help your understanding? Does it answer your question at all?

bengrf1 karma

Thanks for the reply. I am reasonably scintificly literate and know about the expansion of the universe however my question lies in the accelerating explansion of the universe. Now when that photon is makeing his marry little way towards Earth the universe expands incresing the wavelenght of the light, now if light is emmited 3 billion light years away 3 billion years of expanding universe take effect on the photon now wouldn't the light naturaly have a greater redshift than one emmited 1 billion light years away. Furthermore, how do we know at what pace this expansion has taken place for instance how do we know that the redshift is takeing place more rapidly and therefore that the universe is expanding faster?

doctorBenton2 karma

Okay. Here goes.

Part of the difficulty with this question is that you have to be careful about how you measure/define distance. As a simple example, take two points that are 1 billion ly apart in a static universe. Clearly, it takes 1 billion years for light to traverse this distance. But if the universe is expanding, and the two points are 1 billion light years apart at the time the photon was emitted, then it will take longer to traverse this distance. If you froze time at some point while the photon was in flight, you would know that the distance between the photon and its source is going to expand, and so the distance between the photon and its source is going to increase at faster than the speed of light. And for similar reasons, you know that the photon is going to close the distance between itself and its destination at slightly less than the speed of light.

It makes much more sense to think about the distance between us and a distance galaxy now, rather than at the time that the light that we are seeing was emitted. But how would we measure this distance? The most physically meaningful way to describe this distance is called the 'proper distance' (or also the 'comoving distance'). If we could somehow freeze time (stop the expansion), and lay a series of rulers end to end from here to the galaxy in question, then this is the distance that we would measure would be the proper distance. This is actually the distance that goes into the laws of physics (like Newton's gravitational law, or the Coulomb force, or whatever).

So if the proper distance between us and a galaxy is currently 1 billion light years, then we know that the time of flight for the photons that we are seeing right now is less than 1 billion years. Or, conversely, if we know the time of flight, we know that the proper distance is more than that time times the speed of light. This is why the universe is 13.7 billion years old, but nearly 90 billion light years in diameter.

The actual quantitative relationship between the proper distance and what i will call the 'lookback distance' (ie., the time of flight divided by the speed of light) depends on the exact expansion history. But there will always be a one-to-one relationship between the two distances, such that if you know the expansion history and one of those distances, then you can exactly work out the other distance. I hope that that takes care of the first part of your question.

As for the second question, about how we know the expansion history of the universe. In order to determine the expansion history of the universe, what we need to do is to measure the distance--redshift relation. We could phrase in terms of the proper distance, or in terms of that 'lookback' distance. But there are other distance measures that make a bit more sense.

The one that we use most often is called the luminosity distance. Imagine a light source that sends light in all directions. If it were to emit a single bright flash, then that light would propagate outwards like a thin spherical shell, with surface area 4 pi R2. This means that as time goes on, and the spherical shell grows in size, the light per unit area on the sphere drops like L/(4 pi R2), where L is the total amount of light emitted, and R = c t is the size of the sphere. So if you had a detector that was 1 sq. metre in size, the brightness that you would measure would be Lobs = Ltot / (4 pi R2), where R is in metres. This is, in fact, one way to define distance; it is the definition of the luminosity distance.

The trouble is, of course, that as the universe expands, this contributes to the R that is the luminosity distance. And what's more, the effect on the luminosity distance is different to the effect on the proper distance. But this time, the relationship between the two is relatively easy to write down: for a redshift z, the relation is that D_Lum = (1 + z) D_Prop. (If you're really keen, you can read this to find out why.)

So. Now. If we have a class of objects that we can somehow know their total brightnesses, then we can use their apparent brightness to estimate their distances. If we can also measure their redshifts, then we can use this information to determine the present expansion rate, and even the expansion history.

If the universe were expanding at a constant rate, then the distance--redshift relation would be linear. If the expansion were slowing down, then it was expanding faster in the past, and the redshift at a certain distance would be greater than if the universe were expanding at a constant rate. If the expansion were speeding up, then we would see the opposite, the redshift at great distances would be a little bit less than we might otherwise expect.

Then, when we go out and use supernovae (which all have roughly the same intrinsic brightness), we see that the expansion of the universe is actually accelerating. Hence dark energy. Which is weird.

everymop2 karma

I feel so stupid...

doctorBenton1 karma

All it takes is time and effort. Hard work will get you a lot further than brilliance. (And the good news is, you can decide how hard you want to work!)

rohitu2 karma

What does your day to day work mostly consist of? Is it a lot of coding and dealing with large sets of data?

doctorBenton2 karma

Yup. Unless i'm teaching (or checking email, or on reddit), i spend just about my whole day tinkering in a terminal (emacs and python).

I work on primarily on large galaxy surveys; mostly one called GAMA. Over the past six or seven years, we have collected spectra and redshifts (distances) for about 220,000 galaxies, which makes us the second largest galaxy redshift survey. So, go us, i suppose! I have been particularly looking at a set of 30,000 of those galaxies, which are relatively nearby (~ 1.6 billion light years, or 10% of the universe ago).

So, 30000 points isn't really a super-large dataset. I mean, it is for extragalactic astronomy, but not so much for truly 'big data' ventures. I guess that while i do is more 'big calculations'. My particular niche is that i'm good with Bayesian statistics, which makes me really good at fitting complex models to the data.

So, say that i have a library of 10 million spectra; i do things like compare each of these spectra to the data that we have on a particular galaxy, and use this to infer the mass of that galaxy. Or say that i have some model which is defined by maybe 40 parameters, and which is intended to describe the distribution of the data in some observeable parameter space, then i can use the data to figure out what are the allowed values for the model parameters. So, what i do, in essence, is a bajillon variations on a relatively simple calculation to explore possible and likely explanations of the observed data -- that's what i mean by 'big calculations'.

Does that answer your question?

mercifromage2 karma

What do you think the answer 42 means?

reddit_fungus1 karma


doctorBenton2 karma

I honestly don't know. And i'm not even really sure if i want to know. And even if i did know, i'm not sure that i'd want others to know. I fear that it might end up ruining the books, kind of like finding out how a magic trick is done. You should listen to this.

doctorBenton1 karma

Oops. Meant to reply to your comment, but replied to the one under your comment... duhdoi.

mercifromage2 karma

Thanks for the link... having a listen now.

doctorBenton1 karma

Rad. It's the second story that i was thinking of -- starts i guess 20ish minutes in.

window51 karma

The Sun was formed 5 billion years ago. And the universe started 13.67 billion years ago. What was the stuff that went on to form the Sun doing for the billions of years before it all came together and ignited? What did our solar system look like 9 billion years ago?

doctorBenton1 karma

When the universe begins, basically all the matter is either hydrogen or helium. Everything else gets manufactured in the centres of massive stars, either during their lifetimes, or in their final explosive death throes. Either way, before these massive stars burn out, they expel a large fraction of that material back out into the galaxy.

This is where all the oxygen in the air, the silicon in the soil, the calcium in your bones, the iron in your blood -- literally every single molecule in your body, and on earth -- comes from. All of this is the ashes of a long dead star.

So, 9 billion years ago, the material that now finds itself here in the solar system was distributed elsewhere, but probably most of it was at the centres of dozens, maybe hundreds of massive stars. The rest of it would have been as gas and dust in free space.

It's highly likely that this wasn't all in the same galaxy, either. The Milky Way grew to its present size through a series of mergers between smaller galaxies. So it may even have been that 9 billion years ago, the material that we find here and now was distributed across several dwarf galaxies.

elvis20121 karma

What did you have to defend your phd against?

doctorBenton3 karma

I did my PhD in the Netherlands, where the PhD defence ceremony is quite a production. The whole thing takes place in a tiny room in the original building of the university, which dates back to 1660something, i think. The walls are literally tiled with a couple centuries' worth of oil portraits of past professors. The candidate has to wear tails and white tie (if they're male). You sit at this massive table, flanked by two friend/supporters. I think that, historically, they are supposed to hold your sword, but i'm a bit hazy on that. The committee sit across from you, wearing ceremonial hats and gowns. After an hour's worth of questions, a guy comes in carrying the university mace and declares in Latin that 'the time has come', and everyone from the committee shuffles out into a little anteroom to deliberate. While they do, everyone but you stays standing. Then they come back in and give you your degree and everyone goes out and gets very drunk. It's a bit Harry Potter, but in an awesome way.

elvis20122 karma

This sounds bad ass. I would get a phd if it involved ceremonial weapons. Thanks for the detailed reply.

doctorBenton1 karma

No worries. Thanks for saying thanks! Makes it feel less like firing messages into the void...!

herpberp1 karma

gah, why can't you be Sean Carroll?

doctorBenton1 karma

None taken! But, seriously, why can't i be Katie!

herpberp1 karma

here's my crazy question about black holes. if i throw a brick into a very dense object, such that it all collapses into a black hole... then wait for a while for it to evaporate, when it loses the mass of the brick and light can escape, do i see anything?

doctorBenton1 karma

There's a lot to this question.

First, i have to go through what happens near a black hole. Let's say you send a small probe into a black hole. For the sake of argument, let's say that you engineered this thing so that it will actually survive the journey (which is physically impossible, but that's another story). What this probe does is it transmits a regular signal that says "i'm good!"

As it nears the black hole, it starts to experience the relativistic effect of time dilation. The special theory of relativity ('special' like 'special case') predicts time dilation for fast-moving objects. This can be summed up as 'fast clocks run slow'. But the general theory of relativity ('general' like 'general case') also predicts time dilation near to a massive body.

In fact, GPS satellites, which are basically just accurate clocks, have to account for this fact. Because they are a little further from the earth (and because they are moving rather quickly in their orbits, but this is less of an effect), they experience time a bit quicker than we do. Even though from earth they seem to tick off one second every time our atomic clocks tick off one second, if you have a GPS satellite in the lab, it would appear to be running just a little bit fast. It would get ahead of your lab clock by about 45 microseconds each day. But i digress.

So, as your probe goes towards the black hole, it feels the effect of this time dilation. The interval between signals gets longer and longer the closer in it gets. The length of each signal gets longer and longer, too. In fact, the frequency of the light that carries the signal changes too, since the electromagnetic oscillations slow down, and so the wavelength of the light gets longer and longer. That is, the energy of each photon drops. Also, the time between photons gets longer and longer. If you had a telescope to let you watch the probe, the same would go for the light that goes to making its image.

In short, what you see is the probe fading away until, at the moment it reaches the event horizon of the black hole, it simply disappears.

That is, from your perspective, it never actually reaches the black hole, it just stops at the event horizon. What happens then is that its mass gets added to the black hole, and the event horizon expands a bit to accommodate it. So that's the sense in which the black hole swallows the probe. (Cool...)

Next i have to do black hole evaporation.

Let me start with the quantum mechanical side of things. You might be familiar with the uncertainty principle as it pertains to position and momentum: the position and momentum of a body cannot be simultaneously and precisely determined. There is a similar uncertainty principle that pertains to energy and time, but it is harder to put into words. The basic idea is that you can ""borrow some energy from the universe, but only for a short amount of time. Energy is conserved in the long run, but for very short times a particle to act as though it has a lot more energy than it ""really does. This is how quantum tunnelling works.

If you marry this idea with E = m c2 (from relativity), then you get this truly ridiculous fact: in empty space, the universe is constantly creating particle-antiparticle pairs by ""borrowing energy/mass. If these particle-antiparticle pairs lived long enough for you to detect them, then energy conservation would be violated and bad things would happen. So the universe has to make these particle/antiparticle pairs annihilate again immediately, thus paying back the energy debt.

This is happening everywhere all of the time, as evidenced by the Casimir effect and the Lamb shift. Still with me?

This also happens around the event horizon. But here, one of the pair might be just outside the event horizon for a bit, while the other just goes straight in. In this case, you have an accelerating charged particle (the acceleration comes from it falling into the black hole), which means that you have some radiation.

So long as the particle is outside the event horizon, this radiation escapes the black hole. If the black hole is losing energy, then it must also be losing mass -- E = m c2, after all. So this Hawking radiation is the mechanism whereby the black hole evaporates.

The important point, though, is that the mass that gets lost is not the same as the mass that is lost. It goes through a pretty mindblowing change of forms, where it goes from being the massenergy of the probe/brick, to somehow being stored in the curvature of spacetime (the relativistic effect that defines the event horizon), to somehow being borrowed by the spontaneous virtual particle-antiparticle pair (the quantum uncertainty bit), to then radiation. It's pretty wild.

AlexBerghe1 karma

Do you watch Cosmos series by Neil Degrasse Tyson ? :D

doctorBenton1 karma

I haven't yet, no, but i do remember reading the book from Sagan's Cosmos series when i was a kid, and then again at age 19 or 20, when i was getting seriously interested in astro. Is it good? My worry is that it'll be schmultzy like the guy from Top Gear's one about How to Make a Universe... It's horrible: literally half the show is just filler.

AlexBerghe1 karma

The Cosmos series by Neil Degrasse Tyson are really good and informative totally worth watching and much better than How to Make a Universe :D

doctorBenton1 karma

Good to know. I'm thinking i'll buy the series to watch with my nieces.

Padre_of_Ruckus1 karma

What are you having for breakfast this morning? How are you liking Australia, and what are you getting up to down under?

doctorBenton1 karma

So far, coffee. But i might have a muesli bar in a bit. So, y'know, it's a wild time. (I have hit the hotel buffet a few times in the past week, and i think i've well and truly filled my bacon and hash brown quota for a good long while...)

I'm at Uluru (Ayer's Rock) for two weeks, as part of an astronomer in residence programme, organised by an organisation called CAASTRO. Uluru and Kata Tjuta (which is another rock formation a ways away) are incredible. I have to admit that i find it a bit problematic to be at a resort that's sited so close to such a truly magical site -- it's just weird to have so much traffic around the place, to see how much damage we have done in just the past few decades to a site that has been continuously used for 50000 years or more. But i've also had some great time to reflect on myself, my profession, my role in culture... it's been pretty great.

I'm actually Australian, but i find our political situation all a bit grim at the moment. It seems to me that the current government (less than a year old) doesn't really have a coherent plan, and is introducing a lot of potentially disastrous reforms that haven't been properly thought through. And the tenor of the public political discourse is truly toxic.

But other than that, Australia in general, and Melbourne in particular, really does have to be one of the best places in the world to live. No question i'd rather live here than anywhere in the US or even the UK.

dazedlights1 karma

hi, im really interested in the field of astronomy and meteorology. my question is, if in any given circumstances that a wormhole suddenly just appears out of nowhere and a person travels (or 'slips') into the future, do you think they will get back to the present time? as stephen hawking did say that time travel to the future is possible but the past isn't, does that mean that person will stuck there in the future forever?

doctorBenton1 karma

Again, this is a pretty long way outside of my field of expertise, but i'll give it a go.

I think that it's fair to say that wormholes are fantasy physics -- maybe possible in principle, but practically-speaking they are completely impossible. To make a wormhole (or for a wormhole to appear) you have to do something insane like focus the equivalent of the total energy output of the sun across its entire lifetime into a region the size of a sugar cube. (But i can't instantaneously find a reference for that statement, so i might be wrong...?)

In any case, for me, the most important point to be made here is that it doesn't really matter what i think or what i believe. That's not how physics works. Instead, physics asks questions like 'under what conditions is it possible to make a traversable (two-way) wormhole between two points in time?'. Then, you have to come up with a plausible physical/mathematical argument that lets you answer that question quantitatively.

My understanding is that the creation of a traversable wormhole would require a large and stable amount of negative energy, and my understanding is that that would violate the uncertainty principle.

But then i go back to where i started: my understanding is that wormholes are really fantasy physics, and i guess i'm glad that there are people who think about this stuff, but it really doesn't feel like ""real physics to me.

Daymanahaaah1 karma

What is a theory or idea that you have that would blow my mind?

Thoughts on time travel? Quantum Thruster Physics? Making contact with other intelligent life forms?

doctorBenton1 karma

Time travel is, i think, almost certainly impossible. I have no knowledge of quantum thruster physics, and i think that's because there's no such thing. Contact with other intelligent life ... well, maybe we will be able to eavesdrop on another civilisation (more in a second), but we won't in our lifetimes see any communication with another civilisation: we already pretty much know that there's no one using radio communications within 80 lightyears or so of us.

But to try to blow your mind. I think that it was Arthur C Clarke who said something along the lines of -- there are two possibilities: either we are alone in the universe, or we are not. Both propositions are equally terrifying.

But within the next ten or twenty years we will know whether or not there are any other intelligent/communicative civilisations in our galaxy.

There is a project called the Square Kilometer Array. Remember the scene in Contact when Jodi Foster is in front of a bunch of radio telescopes? That is the Very Large Array (VLA) in New Mexico, which is a collection of 27 telescopes that are each 25m in diameter, for a total collecting area of around 13250 m2. The SKA will have a square kilometer of collecting area -- 1 million m2, or nearly 100 times that of the VLA. It will be partly spread across Southern Africa, and partly spread across the Australian continent. Maybe a better name would be the fuck-off-large array, but i think that that might've affected the funding situation...

Anyway, once its built this thing will be able to detect a single aircraft control tower at a distance of something like 300 lightyears. It would be able to detect a collection of a million mobile phones at a distance of about a third of the galaxy away. So if there is anyone on this side of the galaxy that is using radio communications, we'll see them.

Imagine the change in humanity if we do find another civilisation. It would be an intellectual watershed of greater scale and significance than the Copernican revolution. Imagine how that affects understandings of religion, or of nations, or what 'global' means. We are not alone.

Now imagine what happens if we don't. We are alone. Imagine the impact; how precious and precarious our presence on this planet is.

The SKA is due to be up and running in 10 years. Even if it ends up being 15 or 20 years, the chances are that in our lifetimes (touch wood), we will know whether or not we are alone in the galaxy. We will see that revolution play out.