232

Things are not empty space. Our classical intuition fails at the quantum level. Matter does not pass through other matter mainly due to the Pauli exclusion principle and due to the electromagnetic repulsion of the electrons. The closer you bring two atoms, i.e. the more the areas of non-zero expectation for their electrons overlap, the stronger will the ...


134

This is a tricky question because it asks about the meaning of words. People use the word "particle" to refer to various, not always well defined, notions in physics. In the end, I think the simplest and more correct single way to categorize the terms is to interpret "particle" as "excitation of a field". For example, if someone says There are two ...


113

The Higgs field (note it is the field that is important here, not the Higgs boson itself, which is just a ripple in the Higgs field) gives particles mass in the same sense that the strong force gives the proton mass (context: $99\%$ of the mass of the proton comes not from the mass of its constituent quarks, but from the fact that roughly speaking the quarks ...


109

The word photon is one of the most confusing and misused words in physics. Probably much more than other words in physics, it is being used with several different meanings and one can only try to find which one is meant based on the source and context of the message. The photon that spectroscopy experimenter uses to explain how spectra are connected to the ...


108

The total amount of antimatter ever created on earth is not even sufficient to be visible by eye, so it is hard to answer. However, if a bunch of antimatter was available as stable solid or liquid material, there is no reason to think it would look different. Indeed, its interaction with visible light is pretty much exactly the same as usual matter, so it ...


101

That's a really great question. The 'replication crisis' is that many effects in social sciences (and, although to a lesser extent, other scientific fields) couldn't be reproduced. There are many factors leading to this phenomenon, including Weak standards of evidence, e.g., $2\sigma$ evidence required to demonstrate an effect Researchers (subconsciously or ...


97

The answer "because we do not need more" by @rubenvb is fine. Studying physics, you must realize that physics is not answering fundamental "why" questions. Physics uses mathematical tools to model measurements and these models have to fit new data, i.e. be predictive. As long as the models are not falsified, they are considered valid and useful. Once ...


93

The photon is a construct that was introduced to explain the experimental observations that showed that the electromagnetic field is absorbed and radiated in quanta. Many physicists take this construct as an indication that the electromagnetic field consists of dimensionless point particles, however of this particular fact one cannot be absolutely certain. ...


92

Isn't the universe full of Higgs bosons, making up the Higgs field? No. In particle physics, it is understood that the underlying (more fundamental) object is the field, not the particles. Particles are excitations of the fields that can be measured, and always carry certain properties like charge, mass, spin etc. The field that you are most familiar with ...


92

Quantum mechanical particles have well-defined masses, but they do not have well-defined sizes (radius, volume, etc) in the classical sense. There are multiple ways you could assign a length scale to a particle, but if you think of them as little balls with a well-defined size and shape, then you're making a mistake. de Broglie Wavelength: Particles which ...


90

There are many competing limits on the maximum energy an accelerator like the LHC (i.e. a synchrotron, a type of circular accelerator) can reach. The main two are energy loss due to bremsstrahlung (also called synchrotron radiation in this context, but that's a much less fun name to say) and the bending power of the magnets. The bending power of the magnets ...


87

$7\ \mathrm{TeV}$ is not that much kinetic energy, that has been covered by your question and previous answers. However, in the context of a proton, with a rest mass of $1.672\times10^{−27}~\mathrm {kg}$ (very, very little mass), when a single proton has $7\ \mathrm{TeV}$ then it is traveling at a specific speed: $$E= mc^2$$ \begin{align}E& = E_0 + E_\...


84

The energy of a bullet is around 735 joules (see bullet details here). This is about the same energy that I have when I'm running at about 4.6 m/s. Would you rather be hit by me or the bullet? The bullet kills you because it concentrates all the energy onto a small impact area while my impact area is rather larger (and sadly getting even larger as ...


79

There are multiple reasons why protons are heavier than electrons. As you suggested, there are empirical and theoretical evidence behind this. I'll begin with the empirical, since they have important historical context associated with them. As a preface, this will be a fairly long post as I'll be explaining the context behind the experiments and the theories....


77

Fairy Physics It is entirely possible to construct a theory of the universe which states: "All effects are caused by fairies. Each effect has its own fairy, and every fairy is unique. When two fairies produce the same outcome, that is just a happy coincidence." Unfortunately, it is basically impossible to disprove this theory. Also, this theory ...


72

The highest energy reached so far by an accelerator is $13\,\text{TeV}$ in the LHC. The Planck scale is $\sim 10^{19}\,\text{GeV}$, so we are $15$ orders of magnitude away. (Cosmic rays with a center-of-mass energy of about an order of magnitude above the scale of the LHC have been observed). If by Antiquity we mean a period of history in which there were no ...


69

It's impossible to say whether you are correct or Griffiths is correct a priori -- that is, before having any experience of how the world works. You need to do experiments, and Griffiths' version agrees with experiments better than yours. The basic experiment involves detecting the products of decayed particles. Suppose we have some process that happens ...


69

Strictly speaking, it is indeed incorrect that neutrinos travel at "close to the speed of light". As you said, since they have mass they can be treated just like any other massive object, like billiard balls. And as such they are only traveling at nearly the speed of light relative to something. Relative to another co-moving neutrino it would be at rest. ...


67

This is the elementary particle table used in the standard model of particle physics, you know, the one that is continuously validated at LHC despite hopeful searches for extensions. The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the ...


67

As noted "why" is a tricky question but we may ask what is the most fundamental view known concerning this question. Electrons and protons are very different beasts. Electrons as far as we can tell are elementary, participating in the electromagnetic and so-called weak interactions. On the other hand protons are known to consist of quarks. Quarks are very ...


65

You are seeing particles. However there's more to this than meets the eye so I need to explain exactly what I mean by this. Light is neither a particle nor a wave. Instead it is a quantum field. As a general rule while light is travelling it appears as a wave, but when the light quantum field is exchanging energy with anything it does so in quanta that ...


61

Although it's commonly said that fundamental particles are point particles you need to be clear what this means. To measure the size of the particle to within some experimental error $d$ requires the use of a probe with a wavelength of $\lambda=d$ or less i.e. with an energy of greater than around $hc/\lambda$. When we say particles are pointlike we mean ...


60

That a particle decays into other particles is completely disjoint from it having substructure/being fundamental or composite. Some examples: A highly energetic photon may "decay" into an electron and a positron in the presence of another object that takes the excess momentum. That doesn't mean a photon is a composite of electron and positron. A free ...


60

Those neutrinos come from the Sun. Fusion converts protons to neutrons, so that must produce neutrinos. One can calculate the number of nuclear reactions necessary for the power output, and get a number for the neutrino flux. One can also estimated the flux from the cross section of the detector. The two rates differ by a factor of about three. That was ...


59

Most electromagnetic radiation is of very high frequency - the magnetic field changes many times per second. This means that the compass just doesn't have time to "follow" the magnetic field changes. The only thing that does affect a compass is a DC magnetic field - usually this is a large piece of iron etc. that gets magnetized (e.g. by the earth's ...


57

Your friend is correct: there's only one type of proton. The proton is the lightest baryon. It has charge $+1$, spin $1/2$, and baryon number $+1$. These three quantum numbers are so fundamental that if you try to change any of them, the result won't be a proton. For example, if you change the charge to $0$, you get the neutron, and if you change the spin ...


52

Here is a quick & simple answer until the professionals arrive. In the Standard Model, it is zero. This $< 1\cdot 10^{-18} \frac{\mathrm{eV}}{c^2}$ is an experimental upper limit (i.e. if it has a rest mass, because of physics beyond the Standard Model, it must be smaller than this value). This value is very small, compared to the estimated rest mass ...


50

Under special relativity nothing can be incompressible: consider any object of nonzero size and finite mass in its rest frame; when you apply a force to it on one side it will start moving. If it were completely incompressible, the other end would start moving simultaneously. Since the ends are spatially separated, there is a frame in which the other end ...


49

The starting point to explain photons from a theoretical point of view should be the Maxwell equations. In covariant form, the equations in vacuum without sources are \begin{align} \partial_\mu F^{\mu\nu}&=0\\ \partial_\mu(\epsilon^{\mu\nu\alpha\beta}F_{\alpha\beta}) &=0 \end{align} It is well known that the second equation is automatically verified ...


49

Addressing misconceptions First, I address some misconceptions in your question. the decay indicates that the muon may be just a composite particle The fact that the muon decays at all is not evidence that it's composite. It's tempting to say that if a particle $A$ can decay into $B$ and $C$, then it must be "made of" $B$ and $C$. However, this doesn't ...


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