Physics

### Does GR explain cosmological time dilation?

There is plenty of evidence for a cosmological time dilation effect. For instance a supernova that takes 20 days to decay will appear to take 40 days to decay when observed at redshift $z=1$ (see Ned Wright's cosmology FAQ (http://www.astro.ucla.edu/%7Ewright/cosmology_faq.html#TD), Ned Wright on Tired Light (http://www.astro.ucla.edu/%7Ewright/tiredlit.htm)). I understand how time .... Piper: 7 hours ago

Physics

### Naive question - Is Magnetic generator (like this) not a 'perpetual motion machine'?

Perhaps a question that is too naive to be on Physics.SE but I let the experts here decide (and downvote at will), but there seem to a certain company that claims to have created what it calls a "magnetic generator" where, if the machine is powered externally long enough (which .... Bella: 7 hours ago

Physics

### Mathematical description of the curve of a straight line in optics

If you look at a square object from far away, all the edges look perfectly flat, but if the square is something like 2 feet by 2 feet, for example, and you get close enough to it, you can see that the edges all curve due to perspective. If you .... Kinsley: 8 hours ago

Physics

### Can there be an acceleration when no net force acts on a massless object?

If no net force acts on a massless object can there be any acceleration of the object? My attempt: $$F=ma\\ \implies a=F/m\\ \implies a=0/0$$ $\implies a$ can be anything. .... Adrian: 8 hours ago

Physics

### Does an induction cooktop consume as much energy when it's heating something as it does when it isn't?

If you plug in an electrical charger or an adapter of an electronic device such as a mobile phone or a laptop into the power socket, but you don't actually attach the device into the charger, the charger does not consume significantly as much electrical energy as it would if .... Beau: 9 hours ago

Physics

### Chemical Potential with Pressure at Constant Temperature

The above picture shows how the Gibbs free energy of a solid, liquid and gas vary with pressure. I have found no similar graphs on the internet that match the above. Instead I often find these graphs : My questions are : 1.) How can both the two graphs correct .... Lincoln: 10 hours ago

Physics

### In neutrino oscillation experiment, what are the Feynman diagrams involved for the *detection of the neutrino* in the detector?

In neutrino oscillation experiment, for example NOvA , what are the diagrams involved for the detection of the neutrino in the detector ? I have explored inside many PhD theses to understand a bit, but none are very clear, and the authors seem not really to master the physics of .... Riley: 10 hours ago

Physics

### Confusion on conservation of angular momentum

Can someone explain how angular momentum is conserved while at the same time precession in freely rotating objects happens? It seems contradictory. .... Muhammad: 10 hours ago

Physics

### How do Hubble Units work?

I'm reading through a document - Briel et al (1992) (http://adsabs.harvard.edu/abs/1992A%26A...259L..31B) (pdf (http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1992A%26A...259L..31B&data_type=PDF_HIGH&whole_paper=YES&type=PRINTER&filetype=.pdf)) - and I've come across Hubble units with which I'm not familiar. How do you interpret something like $$h^{1/2}_{50} \:\mathrm{cm}^{-3}$$ or similar constructs? This one appears to be the square root of some unit-less value per cubic .... Luca: 11 hours ago

Physics

### Calculate the distance between two points from iPhone Camera

I want to calculate the distance between two objects using an iPhone camera. Suppose I'm standing with my iPhone 10 feet away from the objects. Now, I want to calculate the distance between those two objects. Suppose these two objects are on a straight horizontal line. So, how can I .... Matthew: 11 hours ago