Score
Title
9439
Megathread: 2017 Hurricane Season
47
Earthquake Megathread
5027
Nutrition Facts: Why is sodium listed instead of salt?
5
If NASA's mission to Mars is successful, will Mars become American Territory?
6
How heavy is fire? If something catches on fire is it heavier or lighter?
9952
Duck fat melts at 57 degrees Fahrenheit. So on a 90 degree day, is a living duck's fat just... sloshing around?
15
How does deodorant work?
15
What gas is inside of unopened peppers? Or is it just air?
6
How much Asteroid mining/extra mass until it has an impact on earth's orbit?
1
What is a kilowatt hour, and why do electric companies charge based on this instead of kilowatts?
4
Why is drinking milk after spicy foods better than drinking water?
192
If natural fruit juices contain large amounts of sugar, why do we only seem to refine sugars from a select few plants (sugarcane, sugar beets) instead of from fruits in general?
152
What have we learned from Cassini's dive into Saturn so far?
78
Why do hospitals have heart clinics specifically for Women? Aren't all hearts the same?
279
How does computer memory work when the computer is turned off?
45
Do ape's toenails grow slower than their fingernails, like humans?
8399
What have been the implications/significance of finding the Higgs Boson particle?
1
If we want to colonise mars, why don't we colonise it first with Cyanobacteria and then plants in order to create a habitable atmosphere?
97
Can microwaves work without using water molecules to heat up food?
7
Why does the fourth power show up in the Stefan–Boltzmann law?
7
When I scratch a piece of metal, do small amounts of atoms break off from it?
5
Do lactose intolerant people absorb the same amount of calories from milk as regular people?
3
What has kept land animals from evolving to enormous sizes, i.e. the size of a mountain?
14
How do they prevent the ISS from crashing into satellites and space junk?
29
Do small songbirds - a finch, say - ever get stung by bees/ wasps? If so, is it typically fatal?
45
It's been about 5 years since the Mochizuki's ABC Conjecture proof was originally published. What's its current status?
1
How do vaccines fail?
8
Does Quinine glow even after you remove it from a black light?
7
Can we forecast the northern and/or southern lights?
240
On a planet with more than 1 sun, what would a rainbow look like?
6
What can layers and swrils in rock indicate?
5
How do insects protect their eyes from direct sunlight?
4
Why can't you count the number of things touching you in a certain spot?
216
We are carbon based life forms, however, is it possible for life to be based off another element?
124
Is there a maximum size for a raindrop?
6
In a coronary bypass surgery, why do doctors use veins instead of arteries? Is there an advantage to this?
2110
Are there any challenges for parasites living in animal blood?
14
How real is the threat of human extinction by gamma ray bursts?
3
What happens when wind / a fluid is put through a T-shaped tube, where the bottom of the T is closed off, but the two sides are open? What happens to the fluid in the closed, vertical tube?
46
Is learning another language simply additive to your mother tongue, or is the second language "separate" in your brain?
8
What is actually happening when an electric current flows through an a salt solution or a molten salt?
3
How can Burning wood (carbon) generate UV radiation?
21 mofo69extreme The energy-time uncertainty principle is different from the others. Usually, you have observables in quantum mechanics which have some sort of probabilistic distribution, and the normal uncertainty relation is given as σ*_A_* σ*_B_* ≥ something where σ*_A_* is the standard deviation of the distribution describing A, and "something" depends on the properties of the observables A and B. ("something" can be zero, where the uncertainty principle just tells you that standard deviations are positive; it can't be negative). But time is not an "observable" in the technical sense I gave above, because there is no probability distribution for the time of the system, so there's no standard deviation. The time is just a parameter which the probability distributions of all the observables depend on. This basically answers your question: time cannot "tunnel," it doesn't have a distribution and can't "collapse" etc. So what is the correct statement about the energy-time uncertainty principle? It is the following: Consider any observable (once again, in the sense I have in the first paragraph) B. Now consider the energy, E. The following is true: σ*_E_* σ*_B_* ≥ (ħ/2)|d<B>/dt| Here, <B> is the average value (mean) of the observable B. If the average value of B doesn't change with time, then there's nothing interesting to say here. But if B is changing with time, then there will always be an uncertainty relation between E and B. Let's think about what this means physically. If you have a system where some observable B is changing, then the energy of that system must not be well-defined - instead there is some spread. This must be true, in particular, for any unstable system, since an unstable system is defined as one which is changing w.r.t. some observable. So unstable systems have some distribution of energies which must satisfy σ*_E_* Δt ≥ ħ/2 where Δt = σ*_B_*/|d<B>/dt| is sometimes called the "time uncertainty." You can think of Δt as the approximate time it takes for <B> to change by an amount σ*_B_*. If this amount of time is large (which happens for stable systems with long lifetimes), then the energy uncertainty is small. Vice-versa, if this amount of time is small (e.g. in unstable systems), the energy will have a very large spread.
21 0 mofo69extreme The energy-time uncertainty principle is different from the others. Usually, you have observables in quantum mechanics which have some sort of probabilistic distribution, and the normal uncertainty relation is given as σ*_A_* σ*_B_* ≥ something where σ*_A_* is the standard deviation of the distribution describing A, and "something" depends on the properties of the observables A and B. ("something" can be zero, where the uncertainty principle just tells you that standard deviations are positive; it can't be negative). But time is not an "observable" in the technical sense I gave above, because there is no probability distribution for the time of the system, so there's no standard deviation. The time is just a parameter which the probability distributions of all the observables depend on. This basically answers your question: time cannot "tunnel," it doesn't have a distribution and can't "collapse" etc. So what is the correct statement about the energy-time uncertainty principle? It is the following: Consider any observable (once again, in the sense I have in the first paragraph) B. Now consider the energy, E. The following is true: σ*_E_* σ*_B_* ≥ (ħ/2)|d<B>/dt| Here, <B> is the average value (mean) of the observable B. If the average value of B doesn't change with time, then there's nothing interesting to say here. But if B is changing with time, then there will always be an uncertainty relation between E and B. Let's think about what this means physically. If you have a system where some observable B is changing, then the energy of that system must not be well-defined - instead there is some spread. This must be true, in particular, for any unstable system, since an unstable system is defined as one which is changing w.r.t. some observable. So unstable systems have some distribution of energies which must satisfy σ*_E_* Δt ≥ ħ/2 where Δt = σ*_B_*/|d<B>/dt| is sometimes called the "time uncertainty." You can think of Δt as the approximate time it takes for <B> to change by an amount σ*_B_*. If this amount of time is large (which happens for stable systems with long lifetimes), then the energy uncertainty is small. Vice-versa, if this amount of time is small (e.g. in unstable systems), the energy will have a very large spread.