10,852 reputation
22862
bio website arXiv.org
location Toronto, Canada
age 26
visits member for 1 year, 8 months
seen 5 hours ago

I'm a graduate student in cosmology. I did my undergraduate in space engineering (rocket science). I enjoy Dark Energy, curvature, and modified gravity, but I mainly focus on inflation.


1d
comment Is energy conserved in general relativity? Does $\nabla_aT^{ab}=0$ represent the conservation of energy and momentum?
@Rexcirus I'm saying that $\nabla T$ is the GR equivalent; the analog of the classical energy-momentum conservation laws. I don't want that interpreted as me saying it means non-gravitational energy-momentum conservation laws are valid, which they aren't globally.
1d
comment Is energy conserved in general relativity? Does $\nabla_aT^{ab}=0$ represent the conservation of energy and momentum?
The stress-energy tensor is conserved. $\nabla_{\nu}T^{\mu\nu}=0$ The non-gravitational energy and momentum are not always conserved globally in general relativity, which corresponds to a non-zero covariant divergence of the stress-energy pseudotensor. And what Kyle refers to is that there is always a way to express this pseudotensor such that it is conserved locally. This means non-gravitational energy-momentum conservation laws apply locally. But $T^{\mu\nu}$ (the tensor, not the pseudotensor) includes gravitational energy-momentum and is still conserved globally.
1d
comment How to start learning Physics?
That may be a bit harsh. What teenager has a mind that's wrapped up in something 24/7? And how many know how to effectively learn things independently?
1d
comment The mass of universe at the point of the Big Bang
@CuriousOne Don't kill the messenger. I didn't make inflation widely accepted. And regardless whether or not I personally believe it, I'm only here to say what one of the accepted viewpoints is. If you aren't buying inflation, arguing with me about its validity won't change anything. I have no better alternative to offer you yet.
1d
comment How to start learning Physics?
Start with projectile motion. Maybe do some electricity. Then move on to Newton's laws. Once you get a basic handle on calculus, do simple harmonic motion, Newtonian gravity, and , if there's time, electrostatics. Also optics, that can't hurt. Leave non-mechanical waves, quantum stuff, and anything more than special relativity to the after high school years
1d
comment How to start learning Physics?
My advice is to explore this site, read some interesting posts. Keep wikipedia handy and take a look at it when you want to know the basics of something. And if you're not worried about popularity, get to know a high school physics teacher; they can help a lot. But be patient, if you rush into this, you're liable to miss something important.
1d
comment How to start learning Physics?
that's one way to do it.... Starting in physics isn't easy. You should do some calculus, like derivatives and possibly integrals. But at your level, most of physics may be a bit had to get through. I never went through the Feynman lectures myself, but I wouldn't be surprised if they quickly become too complicated. Physics is 10% super interesting and 90% moderately boring. If you rush into it too fast and before you're ready, you might disenchant yourself with it.
1d
comment Newton's Bucket
"This is a very ingenius solution to the problem" Right or wrong, I have to disagree with this point. If it's wrong, then the reason for disagreeing is obvious, if it's right, then I would call it novel, insightful, maybe even inspired. However, for a solution to be ingenious, let alone very ingenious, you would have to have a way of formulating it mathematically and it would have to generate testable predictions. As it stands, this is an unpublished personal theory that seems more speculative than scientific. Be it right or wrong, it's poor science
1d
comment The mass of universe at the point of the Big Bang
@CuriousOne Indeed, inflation is very much like epicycles. Except, of course, it has a greater chance of being the correct explanation than epicycles does (thank you hindsight!). But the broad applicability and maneuverability of a solution does not make it any less a solution, it just means we should not use it as a standard that enables us to discount other potential solutions. Inflation was designed to take arbitrary, unknown, and possibly vastly inhomogeneous and/or chaotic initial conditions and make them homogeneous and orderly. Can we fault it then for being able to do just that?
1d
comment The mass of universe at the point of the Big Bang
@CuriousOne But that still means that the old Big Bang model is not valid. And just as the ether was accepted until we could idealize a better solution, why not accept that inflation solves the problems of the old model better than any other current theory? At least until a better one comes along. But I repeat that all this discussion is moot. This site is for expressing the accepted views. Inflation is a widely accepted resolution to the problems of the old model, therefore it has a place here.
2d
comment The mass of universe at the point of the Big Bang
@CuriousOne Indeed as Anna v pointed out, the horizon problem uses the extrapolation already present on the old big bang model and focuses on logical inconsistencies. Whether or not it is because GR is insufficient is irrelevant to the point that the old big bang model is problematic and that inflation is widely accepted as a way of resolving many of the problems
2d
comment The mass of universe at the point of the Big Bang
@CuriousOne the old big bang has the horizon and flatness problems to start with. And the magnetic monopole density (if they exist) is large according to the old big bang model. Large enough that we should see them. Whereas with inflation, that density drops to only a handful in the observable universe. General relativity was not the problem, the old big bang model clearly fails to explain things like the horizon problem
2d
comment The mass of universe at the point of the Big Bang
@CuriousOne That depends what counts as evidence. It solves numerous problems with the old Big Bang model as well as explains the scarcity of magnetic monopoles. People thought BICEP2 results were a promising source of direct evidence, but that's become more skeptical as of late. Does better fitting the theories to the data than previous models count as evidence? I wouldn't say inflation is proven. But it is more accepted than the old Big Bang model, which has many problems
2d
answered The mass of universe at the point of the Big Bang
Dec
12
comment What does a sample of the Sun look like? Does it look like fire or gas?
That makes a good point. The Sun is a stable generator. The massive energy production isn't because it undergoes a lot of fusion, but because it is so large. So a sample wouldn't produce enough to stay visible
Dec
12
comment What does a sample of the Sun look like? Does it look like fire or gas?
@DavidHammen It doesn't have negative value. It has zero value. But it has positive value to the non-visually impaired people, which gives it positive value overall. That is, the visually impaired can probably say "It's no different to me whether or not there is a picture. But other people enjoy it, so I think it's still good to have"
Dec
12
comment Entire Universe's Momentum
Momentum is not conserved globally, it is required to change because the energy changes and the energy-momentum tensor must be conserved
Dec
12
comment What does a sample of the Sun look like? Does it look like fire or gas?
I wouldn't suggest looking at the samples. They would be very bright. At least, the core sample would be. The other regions only emit light because of their temperature, which comes from the fusion reactions in the core. Remove them from that heat source and they'll cool and stop emitting light and become boring gas
Dec
12
reviewed Close Where can I buy LEDs with an accompanying spectrum document?
Dec
12
comment How would night sky look like if the speed of light was infinite?
How would it look and would black holes exist are two different questions. I recommend separating them as such