Most Popular
1500 questions
50
votes
13 answers
Why doesn't a braking car move backwards?
The net force on an object is equal to the mass times the acceleration, $F = ma$
When I brake on a (moving) car, the net force is negative, therefore causing the resulting acceleration to also be negative. This all makes sense, but if the…
Harnoor Lal
- 637
50
votes
9 answers
Why exactly does diffraction occur?
Why do waves that were traveling in a straight direction change direction when passing through an opening?
I thought that the waves (red arrow) when colliding with the wall bounce in the opposite direction (green arrow).
And the waves that pass…
jony alton
- 609
- 1
- 5
- 4
50
votes
3 answers
Why do physicists assume that dark matter is weakly interacting?
IceCube, XENON, etc, keep yielding negative results. If dark matter exists, it doesn't interact with baryonic matter at the energy ranges they can detect. The response is to build even bigger detectors to search for even fainter energy…
Foo Bar
- 988
50
votes
6 answers
Why don't all objects bounce like rubber balls?
Some things don't bounce like rubber balls do. For example, books don't bounce much when dropped.
Why is it that some things bounce while others don't?
user24225
- 617
- 1
- 5
- 5
50
votes
7 answers
Why don't molecules of a gas settle?
It is said that the constant interaction between the molecules of a gas (in the form of collisions) acts as a randomising influence and prevents the gas molecules from settling. But given the force of gravity, won't the gas molecules settle at some…
Kunal Pawar
- 1,918
50
votes
6 answers
Why can fuel economy be measured in square meters?
With help from XKCD, which says
Miles are units of length, and gallons are volume — which is $\text{length}^3$. So $\text{gallons}/\text{mile}$ is $\frac{\text{length}^3}{\text{length}}$. That's just $\text{length}^2$.
I recently realised that the…
AvahW
- 569
50
votes
5 answers
The Science of Tearing Paper-bag Handles
My mother came back from a market which bags the products in paper-bags with handles, and asked me to move the bags from the trunk of the car to the house. Being the lazy human I am, I hung a few bags on each arm so I could cut the number of trips…
Fine Man
- 1,493
50
votes
2 answers
Analog Hawking radiation
I am confused by most discussions of analog
Hawking radiation in fluids (see, for example,
the recent experimental result of Weinfurtner et
al. Phys. Rev. Lett. 106, 021302 (2011), arXiv:1008.1911). The starting
point of these discussions is…
Thomas
50
votes
3 answers
How would we tell antimatter galaxies apart?
Given that antimatter galaxies are theoretically possible,
how would they be distinguishable from regular matter galaxies?
That is, antimatter is equal in atomic weight and all properties, except for the opposite reverse charge of the particles,…
Soren
- 803
50
votes
3 answers
Why are LIGO's beam tubes so wide?
Gravitational wave detectors and particle accelerators have at least one thing in common -- they require long vacuum tubes through which a narrow beam is fired (a laser in the gravitational wave case, a particle beam in the accelerator case). In…
tcamps
- 1,022
50
votes
3 answers
What would a blackbody sound like?
If a blackbody has a temperature such that its peak frequency was well within our audible range, for example $1\ \mathrm{kHz}$, what would that sound like if we used Planck's law to plot its spectral curve in the frequency domain and performed a…
ayane_m
- 1,068
50
votes
7 answers
What is the difference between a white object and a mirror?
I was taught that something which reflects all the colors of light is white. The function of a mirror is the same, it also reflects all light. What's the difference?
Update:
But what if the white object is purely plain and does not scatter light?…
SMUsamaShah
- 5,307
50
votes
4 answers
What is needed to claim the discovery of the Higgs boson?
As I understand the Higgs boson can be discovered by the LHC because the collisions are done at an energy that is high enough to produce it and because the luminosity will be high enough also.
But what is needed to claim a real "discovery" ? I guess…
Cedric H.
- 4,818
- 4
- 30
- 52
50
votes
5 answers
A pedestrian explanation of conformal blocks
I would be very happy if someone could take a stab at conveying what conformal blocks are and how they are used in conformal field theory (CFT). I'm finally getting the glimmerings of understanding from reading Moore and Read's wonderful paper. But…
user346
50
votes
5 answers
Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
Why does the sky change color? Why is the sky blue during the day, red during sunrise/set and black during the night?
user68