stupid physics questions

[pogo]

The whole point of relativity is everything is relative to the observer. So you might be moving but as far as what you experience you consider yourself to be at rest, everything else moving relative to you.

So if your speed relative to a target is large enough to have relativistic effects (which is shorthand for going so fast that time dilation etc. become measurable) then you can and should think of it more as if you were at rest and it is moving towards you at large speed. Then yes, it would gain mass and its length towards you would shorten, so it would appear to have higher density.

In practice though this is only “appear”. To actually measure its density you would have to get close to it, and so get into its inertial frame. At which point it is not moving relative to you and it has just normal density, for whatever material it is made out of.

[gumboot]

But if everything is expanding as the universe expands, then doesn't a bend in time imply a change in the rate of expansion?

[pogo]

I have no idea what you mean. The universe is expanding yes but very very slowly. It is not detectable on our physical or time scale, we only can measure it by looking very far, so outside our galaxy, and very long ago. You need general relativity for it. Relativistic effects like the one you described in your previous post need only special relativity which is much simpler.

[gravy]

Except for Einstein’s own development of GR to extend SR no-one has found any flaws in his theories, despite a century of applying them to everything from particle accelerators to the large-scale universe.

The flyby anomaly is interesting. The few things we have sent at a bit of pace and distance through the gravitational field of the solar system and can measure accurately don't end up moving in the way we expect them to.

That could be down to a number of things but there is an undeniable problem.

[pogo]

Wasn’t aware of that so had a look at it, on WP of course. Seems there are a bunch of possible explanations, some more interesting in the sense of challenging current physics than others. That’s probably why it’s not that well known (as in I had not come across it): it does not directly contradict any one area of physics.

Seems like the sort of thing that will be solved eventually, if only as over time we should accumulate more and more of this data, and so patterns and relations should become more obvious. It’s just too small a data set at the moment. Looking at the data the anomaly is greatest in earlier measurements, before 1998, and almost absent since 2005. That suggests to me that better data collection could have eliminated the problem. Again, I’m confident we will if we keep collecting data on this get to the bottom of it.

[Absolute_Zero]

There's also the Pioneer anomaly. That was eventually 'explained' away using a complex thermal emission model. I regard it as a weak [move along] accounting in that it relies on 1000s of finite elements and two adjustable parameters. Nor does it explain periodic variations in the anomaly and its distinct onset near the orbit of Saturn.

Additionally, another craft (Ulysses) exhibited a similar anomalous acceleration that doesn't fit the Pioneer explanation, as it orbits the Sun out of the plane of the ecliptic.

If all else fails, I expect dark matter will be invoked... in whatever freely-adjustable quantities as are required to match the observations.

The black hole information paradox became a bit of a proxy war between those that emphasise the principles of relativity over those of quantum mechanics. Susskind won [Hawking lost] the argument. The debate led to the Holographic Principle... that the information isn't lost, it's stored on the boundary of the system.

[pogo]

Dark matter is in many ways a pretty well understood phenomena. We know something is out there and can measure its effects, map it and characterise its distribution. We just can’t detect it directly and don’t know exactly what it is. But the broad consensus is it has some physical form, we just need to identify precisely what and fit it into the Standard Model.

As for the black hole information paradox I don’t accept there is one as I don’t accept the premise of the paradox. The main basis of it is information is never created or destroyed. As every physical reaction is reversible you could in theory run them in reverse to recover the previous state and so the information before.

But to me that seems highly theoretical and suspect. We have no way of actually doing this for even the simplest reactions. No way for example to undo a single radioactive decay. The energy required to restore a previous state, to overcome entropy and so recover information, is simply implausible. Certainly at a black hole recovering information by e.g. reversing something falling into it is simply impossible. And as time increases entropy increases so it just gets harder.

There’s also uncertainty, which among other things says if you try and do something twice you will get two different results, at the quantum level. Which also rules out reversing a reaction to recover the previous state, as you cannot know what you will get, even if you could overcome the laws of thermodynamics etc.

edit: and according to WP “ The holographic principle is a property of string theories ”, i.e. from the realm of entirely speculative physics. No-one has yet seen a string, devised an experiment which would confirm they exist, even eliminated all the variations and versions of string theory and so determined which of them is the correct one. One day it might become part of mainstream physics but it is too abstract, theoretical and uncertain at the moment.

[Absolute_Zero]

Take it up with Hawking... he now accepts the information non-paradox re boundary storage but proposes his own [unconvincing, imo] mechanic: “supertranslations” on the event horizon.

[Absolute_Zero]

Likely redundant but I was struck by the serendipitous synchronicity.

http://motls.blogspot.co.uk/2016/01/exp ... perry.html

[zip accident]

Likely redundant but I was struck by the serendipitous synchronicity.

http://motls.blogspot.co.uk/2016/01/exp ... perry.html

Designed for 640 * 480 pixel screens, which made it tough to read.

[Absolute_Zero]

Ctrl + mouse-wheel is your friend.

[KlaxonCow]

Equally, the fact that HTML can dynamically format a page and allows for sizes to be specified as percentages ought to be a web designer's friend as well.

But, well, "the line of least resistance" always wins in nature and in human nature, doesn't it?

[zip accident]

Ctrl + mouse-wheel is your friend.

Thanks, that is useful.

Doesn't mean that designing for 640 * 480 is optimal though, now I can make a UHD screen behave as if it were 640 * 480, that's hardly what I want though, is it.

I guess this needs a thread. I will make one.

[gumboot]

If the universe is expanding then potential energy must be constantly growing as gravitationally-attracted objects are pushed apart from each other.

How much energy is that?

[pogo]

If the universe is expanding then potential energy must be constantly growing as gravitationally-attracted objects are pushed apart from each other.

How much energy is that?

Take some big number and add a lot of zeroes. But energy is not being created from nothing. The universe is expanding because of the Big Bang. That explosion gave everything a lot of kinetic energy to start it off flying apart. And so in theory the kinetic energy from the Big Bang is what is fuelling the expansion of the universe, and so where the potential energy is coming from.

This should be accompanied by things losing energy and slowing down. Except it is a bit more complicated than that. The expansion of the universe is though to be actually accelerating, and we don’t know why. The thing usually associated with this expansion is “Dark Energy” but that is just shorthand for we really do not know what is causing it. Unlike Dark Matter we cannot detect Dark Energy even indirectly.

Back to the original question I think it is impossible to even hazard an answer. Potential energy is not an absolute, like kinetic energy, it is always relative. E.g. As something falls to Earth it loses potential energy. But it does not have zero could in theory lose more by falling even further, down a shaft. Even to the centre of the Earth. If the Earth were so somehow collapse to a black hole its gravity well would get much deeper and something could lose even more potential energy.