# Tag Info

7

The metric expansion of space is a fundamentally different phenomenon than the relative motion of two objects in the flat spacetime of Special Relativity: While special relativity constrains objects in the universe from moving faster than the speed of light with respect to each other, it places no theoretical constraint on changes to the scale of ...

6

Did he knew about the Michelson-Morley experiment? He just knew the name of the experiment not any details. The experiment didn't play any role in the formulation of STR by Albert Einstein. The context is taken from the book: Special Theory of Relativity by V. A.; Atanov, Yuri (Trans.) Ugarov (Author) Art: Was Michelson's experiment "decisive" for ...

5

Another way to say this: Speed of photon, graviton, gluon all equal to c? or Whether all massless particles necessarily have the same speed? You must not have been introduced to the concept of a virtual particle: In physics, a virtual particle is a transient fluctuation that exhibits many of the characteristics of an ordinary particle, but that ...

5

With the first question you are correct. Any "thing" with nonzero mass cannot achieve light speed. From this equation you can see why $$m=\frac{m_{0}}{\sqrt{1-\frac{v^2}{c^2}}}$$ where $m_{0}$ is the rest mass of the body (i.e. the mass it has when its speed is zero). As you can see from the equation, when $v=c$, the right hand side will blow up to ...

5

It is nothing but a problem with real quadratic forms. You have a pair of vectors $v,v' \in R^4$ with, respectively, components $(\Delta t, \Delta x, \Delta y, \Delta z)$ and $(\Delta t', \Delta x', \Delta y', \Delta z')$. Actually these components describe the same vector in spacetime (describing the difference of events) but referring to two different ...

4

In general, uniform motion in one reference frame implies uniform motion in a different reference frame. Suppose that frame $K'$ is moving at a constant velocity $\mathbf{v}$ relative to frame $K$. The transformation from frame $K$ to $K'$ must be linear, so it must be true that $$ds'^2=a\,ds^2\tag{1}$$ where $a$ depends on the relative motion of $K$ and ...

4

Because space itself was expanding faster than the speed of light. Which is perfectly acceptable by the way with Special Relativity, because the speed of light is only a barrier for matter traveling through space. There is no such limit for the speed with which space itself can expand (or contract), as far as I know.

3

If basic symmetry and homogeneity assumptions about the Universe hold, then yes, all massless real particles (see Anna V's answer for virtual particles must travel at a universal constant $c$, the speed of a massless particle, in all frames of reference. Given these basic symmetry and homogeneity assumptions, one can derive the possible co-ordinate ...

3

This is a more complicated question that you probably realise. This first point to make is that the speed of light is always locally $c$, that is, if you measure the speed of light at your location you will always get the result $c$. The problem comes when you measure the speed of light at some location distant from you. To measure the speed of light ...

2

The corpuscular model says that light is composed of tiny discrete particles. It can explain reflection if we assume that the particles are so small that they're very unlikely to collide with each other, or that they don't interact with each other for some other reason. For example, if the corpuscles in a beam of light take up only one trillionth of the ...

2

Acoustic and electromagnetic waves are totally different. There is no overlap whatsoever. You can obviously have very long EM waves, but best example I can think of would be old long wave radio. In my country they still transmit at 225 kHz which is quite easy to achieve with sound wave too, but their nature is different. EM wave propagates in vacuum while ...

2

Is there any significance in saying an observer as an imaginary entity? Yes. From Wikipedia: Physicists use the term "observer" as shorthand for a specific reference frame from which a set of objects or events is being measured. Speaking of an observer in special relativity is not specifically hypothesizing an individual person who is ...

2

It always helps to draw the right picture. This picture assumes that Boxguy is standing next to the lamp, and that the flash leaves the lamp just as it passes PlatGirl. (If, for example, BoxGuy were standing next to the mirror, the picture would look a little different.) The black vertical line is Platgirl's worldline, and any black horizontal line is ...

1

I thought the same thing for a long time. I wondered why gluons don't fly out of the nucleus at the speed of $c$. The difference is that photons don't interact with other photons and gravitons don't interact with other gravitons. They can move around and pass through each other. On the other hand, gluons do interact with each other. In fact, gluons form ...

1

Yes, the spot on the receiver will "move" faster than the speed of light, and this does not break any physical principles. Why? Because no real thing is moving. Consider a black box on a white background on your computer screen. Now suppose you animate it such that the box moves at 1cm/s. However, the box is just an image created by turning on and off ...

1

"Superluminal Interactions" is a fancy way (and, IMO if you look at the Latin roots, a not too accurate way) of saying "faster than lightspeed interactions". It is the notion that objects or events at a spacelike separation in spacetime from one another can influence each other's physics or transfer information. In flat, Minkowskian spacetime, the "proper ...

1

I wonder if you're getting mixed up with propagation of waves in a physical medium like a string. If you have a wave travelling on a string then it has a velocity along the string, but the string is also oscillating normal to its length. So if you stretched the string along the $z$ axis, as the wave travelled along the string (i.e. the $z$ axis) the string ...

1

In the specific case of slowing light with a Bose-Einstein condensate there will be a limit because the slowing of the light is due to an interaction of the light with the BEC to form a polariton. If you put too much energy in you'll destroy the BEC and it will stop slowing the light. Offhand I don't know what the limit is, but it will be a very small amount ...

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