ask a physicist

[Umbran]

However, my question just now shifts to what causes the "freefall". I imagine that if you take a ball and funnel into space, and push the ball around the edge, that without some other force, (like blowing on it), the ball would just spin out of the funnel all together.

I will try one small bit that wasn't put so clearly that might clarify it, and then respect the "take it to another thread" request...

Consider space away from anything large. It is open and flat, right? Say you have an object. It got a push some time ago, and is now just cruising along. If nothing else pushes on it, it'll keep cruising along in a straight line, right?

Well, now put that object cruising along through a curved spacetime. Imagine, what does a "straight" line look like in a "curved" space?

Think about driving in your car, in a straight line down the road. Now, zoom out in your head. Way out, until you see the curve of the Earth - and you realize that "straight" line you were driving in is, from teh point of view of someone not on the globe, curved!

This is what is happening in your example - you keep thinking that the spacetime is flat, and straight lines are straight - but near a massive body, the spacetime is curved, and the "straight" lines near it - the lines that are parallel and wont' intersect - are *curved* from the point of view of a distant observer.

 

[garnuk]

I will try one small bit that wasn't put so clearly that might clarify it, and then respect the "take it to another thread" request...

Consider space away from anything large. It is open and flat, right? Say you have an object. It got a push some time ago, and is now just cruising along. If nothing else pushes on it, it'll keep cruising along in a straight line, right?

Well, now put that object cruising along through a curved spacetime. Imagine, what does a "straight" line look like in a "curved" space?

Think about driving in your car, in a straight line down the road. Now, zoom out in your head. Way out, until you see the curve of the Earth - and you realize that "straight" line you were driving in is, from teh point of view of someone not on the globe, curved!

This is what is happening in your example - you keep thinking that the spacetime is flat, and straight lines are straight - but near a massive body, the spacetime is curved, and the "straight" lines near it - the lines that are parallel and wont' intersect - are *curved* from the point of view of a distant observer.

Ok thanks. I'll just attribute it to some sort of ethereal "friction" and "surface tension" then. I'm trying to understand what keeps things attached to the flat or curved space, that they stick to the contours at all.

 

[Umbran]

Ok thanks. I'll just attribute it to some sort of ethereal "friction" and "surface tension" then. I'm trying to understand what keeps things attached to the flat or curved space, that they stick to the contours at all.

Oh, I see. Here's the thing you aren't getting - it isn't "attached to" the space. Despite the analogies, the curved space is *not* a 2-dimensional sheet that the thing has to stick to. The thing isn't resting on a sheet - it is embedded in a volume all around it. All three dimensions of space, and time, are *all curved together*. If it went "up", it'd still be in the space.

 

[freyar]

Always good to see another Physicist here. My study was astrophysics, specifically exoplanet detection with the Kepler Satellite data as well as modeling the planet and it's system.

Neat stuff! Feel free to chime in with any appropriate answers if you like.

Oh, I see. Here's the thing you aren't getting - it isn't "attached to" the space. Despite the analogies, the curved space is *not* a 2-dimensional sheet that the thing has to stick to. The thing isn't resting on a sheet - it is embedded in a volume all around it. All three dimensions of space, and time, are *all curved together*. If it went "up", it'd still be in the space.

Thanks, Umbran! That does seem to have gotten to the question!

 

[freyar]

A common trope in science fiction is using a type of propulsion which "bends" spacetime. Some have the idea that a ship could move at normal speed while contracting the space in front of it. Effectively reducing the distance it has to travel.

Is this theoretically possible (with exotic matter)?

Yes, this is the way the "warp drive spacetime" discovered by Miguel Alcubierre works (along with an expansion of space behind the ship). In fact, the shop doesn't have to move at all --- the bending of space does everything! But don't get your hopes up: the exotic matter necessary is has much more unusual properties than, for example, dark matter (which is in most models like normal matter in many respects). Specifically, it needs negative energy density. That is possible, but most hypothetical forms of exotic matter with negative energy density end up causing mathematical inconsistencies in physics. From a quick literature search today, I'm not finding anything more specific than that for the case of the warp drive spacetime.

Also, to give you something from a true authority on the subject, I saw a talk by Professor Alcubierre a couple of months ago. On his slides, he wrote "please, please, please don't believe the hype" about NASA's Eagleworks lab's work on warp drive. In short, he doesn't find that work terribly promising.

What would happen to any objects in or near the space being contracted?

That's a little more complicated, and I haven't seen anything analyzing this directly in the literature. But here are my thoughts on it. Generally, objects are ok with being in expanding or contracting space, as long as the expansion or contraction is gentle enough. For example, our universe is expanding, but galaxies are able to remain as coherent objects. On the other hand, one paper I did find argues that the "warp bubble" (contracting region) has to be really very thin. In that case, an object passing through the bubble would be contracted (and perhaps stretched) different amounts in different places. That can certainly destroy the poor object, most likely by crushing part of it. Slightly more technically, I'd expect the victim to feel strong tidal forces that stretch or crunch it because there's a different gravitational action on different parts of the object. In a much less extreme form, the gravitational pull of the moon is more on the close side of the earth than the far side, which causes tides in the oceans. Another example of tidal forces that's come up on EN World before is near the event horizon of a black hole --- essentially your feet want to move toward the black hole so much faster than your head does that you stretch out like spaghetti. But for large enough black holes, the stretching of space is actually gentle enough to leave you alive until you're well inside the event horizon.

 

[fuindordm]

One more fast one...

Anyway, this happens a lot. It requires pretty sensitive handling from the physicist, though the questioner isn't always respectful or may not realize how much time they're actually taking since they're typically more concerned about self-promotion than learning anything. But it does lead me to appreciate a forum like EN World, since the conversation here is respectful and genuinely curious.

I had several long conversations with one such author about general relativity and cosmology. The man had a law degree but no science degree. Eventually I realized that he was fixated on an antique cosmological theory (I forget which one--a deSitter variant, I think) as the basis of his new theory, not because it was better but because it had precedence. In his mind, the early cosmologists simply had more weight than the late cosmologists, and even if observations had disproved the early theory then it was a sounder foundation than general relativity. I was never able to convince him to drop that mindset.

My colleagues in biology tell me that they get similar submissions, especially alternative theories of evolution.

Anyway, thanks freyar for starting the thread. As a former observational cosmologist, I can say that your answers are spot on.

 

[freyar]

Anyway, thanks freyar for starting the thread. As a former observational cosmologist, I can say that your answers are spot on.

Thanks for stopping in! Feel free to chime in if you like!

 

[freyar]

What's the layman's explanation for String Theory these days?

Pretty much the same as it's always been.

More seriously, string theory starts out as studying the behavior of a quantum string moving in spacetime --- you can think of the string as something like a very tiny rubber band. In quantum mechanics, the string can only vibrate in particular ways. If you look at those vibrations closely, they look like certain types of particles, including photons (as you know, particles of light) and gravitons (hypothetical particles of gravitation). Remarkably, if you study the motion of the string even more carefully, you can (among other things) derive Einstein's equations for gravity. So, string theory predicts at least one thing: gravity!

The advantage of string theory vs normal particle physics lies in the detailed math. String theory is typically more complicated (more below, though), but it is complete. With normal particle physics, we are typically forced to say that there's some energy level beyond which we have no knowledge, but string theory in principle can explain everything. That's part of the appeal. Of course, part of the complexity includes things like extra dimensions, etc, etc. And most of the distinctively stringy effects are at such high energies we don't really have prospects for testing them, so string theory gets criticized a lot for that. On the other hand, other theories of quantum gravity which are less mathematically rigorous also don't make unique predictions, so we just have to accept that if we want to think about quantum gravity at this point. It's very hard for any quantum gravity theory to make currently testable predictions, and many of the predictions can be mimicked by more standard physics.

So string theory is, in principle, a theory of all physics, including quantum gravity. It is simple in its basics but extremely rich, and that richness comes along with complications (extra dimensions, for example, give us the chance to figure out where the Standard Model of particle physics comes from but at the cost of some very hairy math and a lot of other possibilities to choose from). But string theory is also very important in other ways. In 1997, Juan Maldacena made the startling discovery that string theory in certain spacetimes is actually the same thing as certain theories of normal particle physics which happen to be very similar to theories of the strong nuclear force, which are notoriously difficult to solve otherwise. This has been extremely well tested and in fact provided the first remotely reasonable explanations for some results of the RHIC experiment (which smashes large nuclei together). Many nuclear theorists have learned string theory for this reason. In more recent years, this correspondence has been generalized to other theories, and some physicists are using string theory to try to understand problems in materials science, such as superconductivity. So string theory is also very much another way to understand more normal theories of particle physics.

 

[Umbran]

Okay, so, let's go with a hard one...

What do you think of pilot wave theory?

For those who haven't heard of it - "plot wave theory" is an old interpretation and formulation of quantum mechanics. Instead of having wave-particle duality, you have a real physical particle, and a real physical wave. The two are connected, so that the movement of the wave impacts the particle, and vice-versa. The ultimately important bit is that this interpretation tosses out quantum indeterminacy - the universe is deterministic in pilot wave theory.

 

[freyar]

If you could unHiggs me, what would happen? Would I be less massive? What would that mean?

Well, I don't think you'd enjoy it very much.

But let's first deal with a misconception I see lurking under your question, since it's been popularized by the media who are either mis-quoting particle physicists or have been the victim of physicists who've gone for a sound bite without bothering to get things right. I imagine you've heard that the Higgs field is responsible for creating mass or some such thing. In a sense, that's true: all the fundamental Standard Model particles that have mass do have their mass because of their interaction with the Higgs*. So electrons, for example, gain mass due to the Higgs field.

However, matter is made of protons and neutrons as well as electrons, and protons and neutrons are about 2000x heavier than electrons. Protons and neutrons are not fundamental but are made up of quarks. While the quarks get their mass due to the Higgs, nearly all the mass of a proton or neutron is due to the energy of the quarks whizzing around relativistically inside (E=mc^2!).

What this means is that you wouldn't actually lose much mass if you "unHiggsed" (this is not a good diet plan!) since only your electrons would become massless, and they make up only about 0.05% of your mass (actually less; that's the maximum amount, which would be the case if you were pure hydrogen). The mass of your protons and neutrons wouldn't change much.

What would happen, however, would be rather nasty to you. If your electrons became massless, the electromagnetic force would no longer be able to bind them to protons. In other words, all your atoms would fly apart. Your electons would suddenly leave your body at the speed of light, leaving behind a cold plasma of atomic nuclei. In other words, chemistry would cease to exist for you.

Fortunately, it would take a ridiculous amount of energy to reset the Higgs field to zero ("unHiggs") a person-sized part of space, so you don't have to be worried about your electrons flying off. You'd probably be vaporized by the necessary high-energy particle beams first.


*except for neutrinos. In the Standard Model, they are massless, but we know that they must (in reality) have very small masses. We don't know what mechanism gives them mass yet.