ask a physicist

[fuindordm]

Ok. Quick question for you:
One of the big problems with the dark energy/cosmological constant component of the universe is explaining its value-- when I was in grad school the best models from particle physics proposing a nonzero vacuum energy were still 30 orders of magnitude off from the observed cosmological value.
Has string theory made any progress in the past 10 years on this problem: a reasonable value for the vacuum energy density?
Ben

 

[freyar]

This may be a bit silly, but is it theoretically possible to harden glass to the point that it could physically break through a brick wall? a.k.a. the Kool-Aid Man theory...

I'm going to answer this one the best I can in terms of more garden-variety physics in contrast to the Ant-man question. This is really more of a materials science or engineering topic than I usually deal with, but I'll do my best.

The answer, actually, seems that you don't really have to do much to make glass tough enough to break through brick. The property of a material to resist breaking (from a pre-existing microscopic crack) is called fracture toughness, and normal glass actually has a fracture toughness in the same ballpark as brick or concrete (see wikipedia, for example). So I think you'd just need thick enough glass, which actually does seem pretty hard to break to me. If you google "glass breaking brick," you can find a couple of youtube videos of martial artists breaking thin cinderblocks with drinking glasses (assuming they're not faked somehow), too.

Fortunately, someone else already did the math to figure this out. Amazing what kind of research people will do.

Of course, the Kool-Aid-Man-shaped pitchers I've seen have tended to be plastic, and I don't think that would break from hitting a brick wall, either. But then I guess he'd bounce off instead of breaking through...

 

[Scott DeWar]

Ok, Ask you anything, huh?

I have seen this subject come up every now and then, tht of the EM Drive. I read this article about it, but now I would like to hear of how you guys react on it. Can it work, why or why not.

If this requires a separate thread, so be it.

 

[Scott DeWar]

. . . . *lots of stuff* . . . . . 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.

re: Highlighted portion in quote . . . . Baring lack of survivability of hard vacuum or massive doses of hard radiation, of course.

 

[Umbran]

It only requires a separate thread if it goes into a repeated back-and-forth.

I'll refer you to something more like a reliable science blog on the topic: http://www.iflscience.com/no-em-drive-will-not-lead-warp-travel-any-time-soon

To summarize: "The EM Drive is a hypothetical form of propulsion that uses microwaves in an enclosed chamber to create forward thrust." There have been several experiments that have reported *tiny* amounts of thrust. And by tiny, I mean (if my envelope-back calculation is correct) enough force to hold up one-tenth of one grain of rice against the force of Earth's gravity. And that's not "the device + that bit of rice". It is *just* the rice.

The most recent experiment that I know of didn't verify the effect, so much as eliminate some of the possible sources for erroneous measurements of effect.

Can it work? I shrug. I don't know of any solid analysis saying it should. What I have seen includes some pointed hand-waving at particular areas, which doesn't sit well with me. There is a basic reason it should not work - conservation of momentum.

Thus, I am skeptical. If they've found some new form of interaction that makes it possible, that is awesome. But, I have severe doubts, and will wait until experiments confirm the effect is real before I worry all that much about it.

 

[Scott DeWar]

conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?

 

[freyar]

Ok, Ask you anything, huh?

I have seen this subject come up every now and then, tht of the EM Drive. I read this article about it, but now I would like to hear of how you guys react on it. Can it work, why or why not.

If this requires a separate thread, so be it.

The most recent experiment that I know of didn't verify the effect, so much as eliminate some of the possible sources for erroneous measurements of effect.

Can it work? I shrug. I don't know of any solid analysis saying it should. What I have seen includes some pointed hand-waving at particular areas, which doesn't sit well with me. There is a basic reason it should not work - conservation of momentum.

Thus, I am skeptical. If they've found some new form of interaction that makes it possible, that is awesome. But, I have severe doubts, and will wait until experiments confirm the effect is real before I worry all that much about it.

I almost put in "ask me anything except about the EM drive" in the OP but decided not to. Anyway, like Umbran, I am skeptical on general grounds, as follows: this violates one of the oldest principles of physics, that of conservation of momentum, which is another way of saying Newton's laws. Before I'm willing to believe that could happen, I'd need very very good experimental evidence ("extraordinary claims require extraordinary evidence" in the words of Carl Sagan). I think it's worth saying that the experimental groups who've looked at this haven't yet felt their evidence was strong enough for submission to peer review yet as far as I can tell (certainly the most recent group has not). Another good discussion of the results on this can be found here: http://io9.com/no-german-scientists-have-not-confirmed-the-impossibl-1720573809

The problem with the EM drive is that the predicted thrust is so small as to be nearly unmeasurable with current techniques. While there have been measured "thrusts," those thrusts also behave in ways consistent with a heating effect (see the link I gave; apparently the measuring devices don't work well at the temperatures reached by the apparatus). Furthermore, there is no credible theoretical work to motivate testing the drive in the first place.

I think it's instructive to compare to another recent episode when an experiment possibly indicated a violation of a important physical law: a few years ago, the OPERA experiment, which was timing the flight of neutrinos over hundreds of kilometers, found them arriving something like 60 nanoseconds faster than the speed of light would allow. In that case, the experiment's press release was a touch more cautious --- inviting the physics community to find problems with their experiment rather than saying their results warranted further investigation --- but just a touch. The reception by the physics community was also very cautious, but the consensus was that there was less of an obvious problem (both the recent EM drive experiments have been criticized for not analyzing the effects of heating on the apparatus). In the case of the OPERA experiment, there were a number of theory papers, either showing that FTL travel wouldn't look like what OPERA saw or trying to figure out a way neutrinos could move FTL, even though it would be fair to say that everyone would have been shocked if the OPERA results stood up. In the end, the problem was a faulty cable connection.

And, in the end, these EM drive results are probably due to something like a faulty cable or more likely measuring devices that can't take the heat. But the EM drive experiments haven't given as much a reason to be interested as OPERA did.

Back to more normal physics tomorrow....

 

[freyar]

conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?

Yes, conservation of momentum is equivalent to the statement "For every action there is an opposite and equal reaction." What that means is that, if there's a thrust on an engine, there has to be a push on some kind of exhaust going the other way as well. The EM drive claims not to have any exhaust to be pushed the other way.

And perhaps further discussion on the EM drive can go to a new thread in Misc Geek Talk, unless it's a very short follow-up question.

 

[Umbran]

conservation of momentum, would that be similar to "For every action there is an opposite and equal reaction" as thus for the claimed thrust, where is the energy to equal it . . . or something like that?

Pretty much. If the drive feels a thrust, it needs to be pushing off something. In a conventional rocket, it is the rocket exhaust. But, the EM drive has no known exhaust.

 

[Scott DeWar]

I see how that makes no sense whatsoever. Ok, Next questioner!