1

Physics, Astronomy & Cosmology / Re: Is time dilation rigorously predicted from first principles?

« on: 02/01/2019 05:39:44 »

It was predicted first, but since has been experimentally verified. In fact, we rely on it every day: gps calculations rely on time dilation calculations to be correct. The satellites that send the GPS signals are moving so fast -- and are so high up -- that time dilation from both special and general relativity needs to be taken into account so your phone can tell you where you are.

2

General Science / Re: How can science be correct if theories are always changing?

« on: 13/11/2016 05:59:47 »

Can you give examples?

As an (ex) physicist, I don't see that in my field. There is often disagreement on what is important to look at next which might be confused as to disagreement on what is known to be a good theory. Progress in Physics certainly sometimes happens because you prove a previous theory wrong; It mostly happens because you *refine* an existing theory.

3

General Science / Re: Do we live in an electrical universe?

« on: 13/11/2016 05:47:07 »

We live in an electrical universe in the sense that that force exists. But it is not the only force, and while the effect of that force might qualitatively describe some aspects of how the world works, the whole reason physics has added other forces is because just having electrical forces doesn't work *quantitatively*.

That being said, your second point is sound. Theoretical physicist tend to work in "natural units": mass is just energy, length is just inverse energy.

4

Physics, Astronomy & Cosmology / Re: Why do photons travel at the `speed of Causality'?

« on: 13/11/2016 05:11:22 »

A photon has no rest mass, but it does contain energy. We could use E=mc^2 to ascribe a mass to this, but this would be what's called the *relativistic mass*, and is basically just a book-keeping concept. 

5

Physics, Astronomy & Cosmology / Could Dark Energy Be a 5th Force?

« on: 04/09/2010 04:55:04 »

there have been attempt to explain both dark matter and dark energy as modification of gravity (so, not a fifth force, just a modification of what is there; in this sense gravity means the force due to whatever you are considering having mass). So far all such modifications have contradicted other experimental observations. Right now the dark energy and matter assumptions are the best bet, but it's far from decided....

6

New Theories / What is string theory?

« on: 25/08/2010 05:21:48 »

A definition of string theory is hard, as what counts as string theory evolves quite quickly with time. Basically, we have two well tested physical theories: General Relativity, which describes gravity on a large scale, and Quantum Field Theory, which describes the interactions of particles due to the rest of the forces at the small scale. The predictions of both these theories agree very well with experiment; there is no experimental evidence to suggest they are wrong, and lot's to confirm their predictions.

The problem is that it's hard to come up with a Quantum Field Theory that includes Gravity which is not logically inconsistent, so based on our current understanding of Quantum Field Theory and General Relativity, both can't be correct. As such, a search should be made for a theory which looks like (but isn't) Quantum Field Theory at short distances, and looks like (but isn't) General Relativity at large distances, and is logically self consistent.

String Theory is an effort to find such a theory. Quantum Field theory works with particle (which exist at a specific point in space and time). String theory started by saying "what if, when you probe deeper, that point is actually two points connected by a line/string". This turned out to give a massive win: General Relativity can be work in this case, but only as long as you work in 24 dimensions (rather than the 4 -- 3 space, 1 time --  we experience). This solves one problem (combining GR and QFT), but adds one massive one: how you explain our four dimensional experiences in terms of those 24 dimensions. String theorists have striven to solve that problem for 20 years, but -- to be honest -- have yet to make significant progress.








       

7

Physics, Astronomy & Cosmology / Do dimensions expand when space expands?

« on: 16/08/2010 06:36:09 »

Short answer: no.

When people talk about space expanding, they usually refer to the a foxe dimension of
space expanding (i.e. everything is getting further away from everything else, but no new dimensions are added)

8

Physics, Astronomy & Cosmology / Are heavy partlicles made of combinations of lighter particles?

« on: 16/08/2010 06:09:11 »

The general viewpoint wouldn't be that the heavier particles are made up of the lighter
ones (it's not like you add two downs to get a strange). However, quarks come in several
flavours ( up, down, strange, charm, bottom, top) and the Weak force can mediate the
change in flavour of a quark. The force carriers for the weak force (the W and Z bosons)
are are massive (as opposed to photons and gluons which mediate the electromagnetic
and strong nuclear forces, which are massless). This means it takes *energy* to make them
(E = mc^2, after all). So in a high energy accelerator you have enough energy density to
make the transformation of an up (say) into a larger mass quark likely. It is an *transformation" however, and not two quarks coming together to form a composite particle.

Due to "Quantum Weirdness", however, systems are allowed to make transformations that require higher energy than they have as long as it is for a very short time (a form of the Heisenberg uncertainty principle). As such, even the "low energy" particles such as the proton and neutron, which we naively think of as being made of up and down quarks,  will also have some strange quark content on average (the amount is an area of active current research).

9

Physics, Astronomy & Cosmology / does time really exist as a fourth dimension of sp

« on: 16/08/2010 05:44:34 »

yes.

10

Physics, Astronomy & Cosmology / Does gravity really exist?

« on: 16/08/2010 05:43:54 »

There is a good understanding of gravity as it applies to human sized objects and
the solar system. It can not be explained as a residual effects of the other forces that
we currently know of (they are either too weak, short range, or cancel over large
distances).

We may get some understanding that gravity "unifies"  with the other forces at a large
enough energy (as JP mentions), but such unifications are only interesting if you have
a guiding principle to predict (correctly) where such a unification will happen. In the
case of gravity such a unification has the problem that we can only formulate theoretical
models for the low energy limit of gravity, while a full theory is needed to make such
a unification (string theory could be considered a full theory of quantum gravity, but they
have yet to show that the rest of the forces can be found within it).

11

Physics, Astronomy & Cosmology / ? Time travel

« on: 16/08/2010 05:30:36 »

There are two factors which effect the passage of time for a fast moving plane (as
measured by you; assumed to be stationary on the surface of the earth):

1) The plane is moving fast. Special relativity tells you that this slows the rate of
time for the plane.

2) The plan is high. General Relativity tells you that time moves slower the higher
the gravitational field; this contributes a speed-up of time for the plane.

For a plane, both these effects are tiny (and the Special Relativity effect is the
largest, so time moves slower). They have been measured, however.

An example of a situation where an understanding of relativity is really needed
for something in everyday life to work is the GPS network. The GPS satellites travel
very fast and very high, and are basically just sending out a clock signal.  The onboard
computer correct for both special and general relativity effects (and if they didn't
GPS wouldn't work).


http://www.phys.lsu.edu/mog/mog9/node9.html

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Physics, Astronomy & Cosmology / Do protons at the LHC collide faster than the speed of light?

« on: 14/12/2008 04:35:15 »

No.

> "When two cars going 50 kph run into each
> other, it's said that it's the
> same as a single car running into
> a wall at 100 kph."

It isn't; it's just *almost* the same. So, close that
at that speed you couldn't tell. However, it will be
very slightly less than 100 kph. At 500kph + 500kph
it would be even more less than 1000kph, to the point
that as you get faster and faster, you will never
get a combined speed faster than the speed of
light. From this basic fact you can work out a lot
of the crazy consequences of special relativity: to
keep things consistent things that move faster get
shorter and time moves slower for them, for example.

Since everything we deal with in our lives moves
way slower than the speed of light, all this seems
very counterintuitive. However, it is true. For example:
the gps satellites orbit fast enough that you need to
take into account for the fact that time moves slower
for them, for them to give you an accurate position.

13

Physics, Astronomy & Cosmology / Why is the universe asymmetrical

« on: 25/11/2008 05:14:20 »

It's a little complicated: Anti-matter certainly exists, and for every type of
particle there is a corresponding anti-particle. For many years it was thought
that particles acted just like anti-particles, thus leading to the puzzle of why
we seem to see mostly matter. In the early 60's it was discovered -- at around
the same time -- that both charge conjugation symmetry ( particles are equivalent to
anti-particles ) and parity symmetry ( our world and mirror image world look the same)
are not respected. However, for a few more years it was though that the combined symmetry
(CP) was respected. I this were true then for every particle you would have
an anti-particle of opposite parity, so still we would expect equal amounts of matter
and anti-matter.

In the late 60's Cronin and Fitch (who won the Nobel prize in 1980 for this) discovered
that CP was not conserved when studying the decay/oscillation of a kaon to an anti-kaon.
Such CP-violation is now part of the Standard Model of particle physics. Within the
standard model this CP-violation is controlled by a single parameter. The aforementioned
attempts to study CP-violation on the b-meson system are not an attempt to discover
CP-violation (been there, done that), but to get another extraction of this parameter.
If it agrees with that from the kaon system (does so far), then the Standard Model
looks good, it if differs (let's hope) then we have a handle to move beyond the
standard model. 

That being said: The Standard Model does lead to matter/anti-matter asymmetry,
but, as observed, this effect comes entirely from the weak force and
is *tiny*; certainly not enough to naively explain the asymmetry we see.
However, this asymmetry could have been seeded in the early universe where
the rules are very different than we see now. For example, there are several
theories that use the fact that QCD (so the strong, not the weak force) can
break CP to explain this; while we see no evidence that this happens in
our experiments, this is an effect that should grow with energy. As such, it
might have been strong enough in the early universe to account for the
matter/anti-matter asymmetry, while still being weak enough at the
energies our current experiments probe that we couldn't measure it.

14

Geek Speak / Why is some text tiny on my screen?

« on: 20/08/2008 06:47:23 »

I have used several versions of Ubuntu, and haven't had this problem.
Naturally,this makes it hard to give advice... Even so, I would
suggest using control+ in firefox to increase the font size
whenever you have such a problem. 

15

General Science / Why does the LHC need to operate at close to absolute zero?

« on: 05/08/2008 05:44:08 »

I'm not an expert on experimental design, but I would guess the reason is
that they are producing dramatically large magnetic fields:
~8.36 Tesla which is quite close to the expected ~10 Tesla breakdown
point for this kind of super-conducting magnet. Previous magnets of the
same type (for the Tevatron) did work at ~4.2 K but only produced ~4 Tesla.

Also, I think they are specifically using helium in it's
superfluid state (which doesn't set in until ~2.2 K) because of it's
particularly friendly heat transfer properties (superfluids are weird),
so they may be cooling down to more than the magnets need just to
exploit this property.   
 
   

16

Geek Speak / Why does my PC keep running out of memory and how can I increase this, please?

« on: 05/08/2008 05:22:26 »

As somebody said previously, it sounds like you have too little RAM in your machine. Computers
have several different types of memory, basically because fast memory is expensive and slow memory is cheap; your computer will have a small amount of fast memory and a larger amount of slow memory. RAM will be held on a silicon chip inside you computer and is fast memory. When you're running a program your computer will try and keep that program and any data it is
using in it's fast memory at all times. If you are running several programs at once it's
quite likely that you will use up all the fast memory (especially as you only have 224MB of
ram). In this case the computer will move the program files it *thinks* you aren't using
onto slow memory (in this case onto your Hard Drives). Next time you try an use these it will have to read the information back off the hard-drives into the fast memory. Hard-drives
are dramatically slower than RAM so this is why your computer will seem to crawl.

17

Physics, Astronomy & Cosmology / Why don't pions explode?

« on: 23/07/2008 23:59:04 »

> Can't particles only change flavour if symmetry is unbroken?

The breaking of the electro-weak symmetry via the Higgs mechanism
doesn't change the fact that flavour changing interactions exist,
however it does lead to the gauge-bosons - the W bosons - that
mediate these interactions having a (heavy) mass; for an unbroken
symmetry the gauge boson mass would be zero (as it is for photons
and gluons). This mass is ~80GeV (GeV == Giga Electron Volts) which
is very large compared to the mass of the pions, ~140 MeV (Mega
Electron Volts), or even a proton (~1GeV). This disparity of
scales means that if you're looking at pion-scale energies the
probability of producing a virtual W-boson is quite small and hence
the force is weak; if you were looking at energies ~80GeV this
suppression mechanism wouldn't apply and the weak force wouldn't
actually be very weak.
 

18

Physics, Astronomy & Cosmology / Why don't pions explode?

« on: 21/07/2008 06:00:56 »

> A pion consists of a quark and an antiquark. So why don't they annihilate each other?

They do. Pions aren't stable particles. The pi-zero (up and anti-up) has
a mean life-time of ~10^{-16} seconds (zero point ... fifteen more zeros ... some
number seconds). Pretty much all of the time they annihilate to produce two
photons.

The charged pions (pi-plus and pi-minus) are made up of one-up and one-down
quark and so need a flavour changing process to decay. These exist, but are
weaker than the electromagnetic processes at low energies (Hence the name
of the force that provides such interactions: the Weak force). Their mean
lifetime are ~ 10^{-8} seconds (most of the time they decays into a muon
and a neutrino).

19

Physics, Astronomy & Cosmology / Speed restriction for light?

« on: 08/04/2008 00:40:46 »

First, when you talk about traveling at a certain speed, you have to say
what you are traveling that speed relative to. Relativity tells us that
there is no fixed reference frame, so you can equally well think of being in
a spaceship moving away from a stationary planet at speed c-100km/h as
being a stationary spaceship and with the planet moving away at c-100km/h;
these two situation have identical physical laws from the point of view
of the person in the spaceship. This includes the speed of light
being the same; light will always travel away at the same speed.

Where it get complicated in when you think about somebody on the planet
and somebody on the space-ship both viewing the light from the
headlights. They both view the speed of light being the same. Intuitively,
this is inconsistent. Just one example of this is that light waves
oscillate at a given frequency and so at different points in space
are in a different phase; naively the two viewers would see these oscillations
spread out differently in space, which isn't possible.

To make these facts consistent requires the really weird bits of
special relativity: if you view somebody moving relative to you
at great speed, it will look to you like time moves slower for that
person (also, they will be heavier and smaller). While really weird,
the constancy of the speed of light, and time-dilation effects
have been measured experimentally to good accuracy. In fact, the
GPS satellites (which rely on very accurate clocks) are moving fast
enough that they have to account for time-dilation to work.

20

Physics, Astronomy & Cosmology / Forces and virtual particles

« on: 09/03/2008 00:53:13 »

The answer hinges on the difference between quantum mechanics and quantum field theory, the latter of which is used to describe the physics of elementary particles. In quantum mechanics you sum over all the paths a particle might take to calculate the probability that a particle might be in a given place; there is no concept of "the path the particle took". In quantum field theory, you sum over all the paths a particle might take *and* all  possible interactions it can have; there is no concept of "only interacted twice".

For the case of virtual particle momentum you talked about it gets a little more
complicated. The formalism requires you to sum (actually integrate) over all possible momentum, however high. Some such integrals will be ill defined, however (give infinite answers). In such cases, the sum/integral effectively restricted to a finite range of momenta, leaving out momenta over a certain value. The particular maximum 
is arbitrary, as long as it is much higher than any momentum you are dealing with, with the strength of the interactions (ans values of the masses, etc..) being adjusted, as this maximum momentum is adjusted to keep all the answers the same.

A second point is that the idea of "interacting with the Higgs giving mass" doesn't really tally with your picture of interacting particles. The picture you present is
what I would call "perturbative": particles move along and every so often they interact, with some fixed probability per interaction such that having "many" interactions is unlikely. From the quantum field theory point of view you can try and apply  perturbation theory for excitations about the vacuum (lowest energy particle/field configuration). It doesn't always converge (i.e. the case of interacting 10 times may be way more important than the case of 1; 100 more than 10 etc...), but it might be a good picture. For the standard model, it is a good picture for electromagnetism and the weak force, but not for the strong force (QCD) at low momenta. Adding the Yukawa couplings to the higgs (the way the W and Z are given mass), however, is a dramatic enough change to the physics that it cannot be described by perturbation theory, and instead should be thought of as a change in the vacuum; if you want to have a mental picture of a W moving along and rarely interacting with a higgs, it should be a W that already has mass.