Fundamentals of Lightspeed: Questions & Answers

[Inertia_Squared]

TL;DR Summary

Why don't beams travel at different speeds on earth?
What is stopping a second particle from travelling at lightspeed from a faster initial velocity than the other particle and accelerating the first particle faster than the speed of light from the sender's perspective?

Hey there, I'm aware this is a bit of a stupid question, and I think that I understand the principle fundamentally, however, my intuition is still having a little trouble catching up, and I'm trying to figure out if it is because of an important detail that I have missed/misinterpreted.

I think the best way to ask is with a series of questions, and if there is anything assumed that is wrong and/or can be explained, it would be greatly appreciated, thanks!

1. Assuming that the speed of light remains constant for all observers since the Earth is orbiting around the sun, which orbits around our galaxy and so on (therefore assuming that we have a velocity, and somewhat substantial at that, as opposed to 0), why doesn't two beams of light traveling in opposite directions (we'll assume that Earth's atmosphere is a vacuum here) have a different velocity from one another relative to the point of emission? Since the Earth is traveling in some arbitrary direction, shouldn't one of the beams travel slower assuming the velocity of the Earth is greater than 0 and is not perpendicular to the beams?

2. Assuming (and expecting) that 1 does not occur, doesn't that imply that in order for the particle to travel at lightspeed in reference to the observer, and stay that way, that the particle must have some sort of knowledge of the initial velocity from where it was fired, otherwise, what is stopping another particle (or two sets of things that can interact) from traveling at lightspeed from a faster initial velocity (but identical direction) and accelerating the first particle faster than the speed of light from the sender's perspective?

3. If 2 is also 'false' due to the velocity being relative, or something to that effect, what frame of reference was decided for the maximum speed of things? Wouldn't that imply that we know how fast the Earth is traveling in reference to the universe and therefore calculate the origin of the big bang (I don't know if we've calculated this, and this was more of an afterthought which is why I haven't just searched it up yet)? And how does this concept remain true while still disproving q1?

If any clarification is needed on a question, please let me know and I'd be glad to elaborate.

I'm 99% sure I'm missing something conceptually here, I'm just unsure exactly what it is even after a good bit of research.
Thanks for your time, and I appreciate the help!

 

[Grasshopper]

Just with respect to (1), if “relative to the point of emission” is defining that point as the chosen rest frame, then intuitively, why should the situation not be identical to firing beams in opposite directions from any rest frame in empty space?

You seem to be rejecting the principle of relativity (“we have a velocity”) here.

 

[Drakkith]

1. Assuming that the speed of light remains constant for all observers since the Earth is orbiting around the sun, which orbits around our galaxy and so on (therefore assuming that we have a velocity, and somewhat substantial at that, as opposed to 0), why doesn't two beams of light traveling in opposite directions (we'll assume that Earth's atmosphere is a vacuum here) have a different velocity from one another relative to the point of emission? Since the Earth is traveling in some arbitrary direction, shouldn't one of the beams travel slower assuming the velocity of the Earth is greater than 0 and is not perpendicular to the beams?

One of the postulates of relativity is that light travels at c in all inertial reference frames. This means that even though two observers may be moving relative to one another, both will measure a beam of light to travel at c as long as they aren't accelerating. Note that this is a postulate, and there isn't an underlying explanation as to why.

2. Assuming (and expecting) that 1 does not occur, doesn't that imply that in order for the particle to travel at lightspeed in reference to the observer, and stay that way, that the particle must have some sort of knowledge of the initial velocity from where it was fired, otherwise, what is stopping another particle (or two sets of things that can interact) from traveling at lightspeed from a faster initial velocity (but identical direction) and accelerating the first particle faster than the speed of light from the sender's perspective?

What this implies is that our measurements of space and time (as in distance and time intervals) are not universal. That is, I might measure the distance between two points to be some quantity X while another observer might measure that same distance as a different quantity Y. The same is true for time intervals.

3. If 2 is also 'false' due to the velocity being relative, or something to that effect, what frame of reference was decided for the maximum speed of things?

None. ALL inertial reference frames (frames that aren't under acceleration) will measure light to move at c. None of them are 'special' or 'privileged' in any way.

Wouldn't that imply that we know how fast the Earth is traveling in reference to the universe and therefore calculate the origin of the big bang (I don't know if we've calculated this, and this was more of an afterthought which is why I haven't just searched it up yet)?

There is no single origin point for the big bang. It happened everywhere, not in a single location. That is, you can run the clock backwards only to a certain point before the density everywhere in the universe (which is still infinite in size) is infinity. This point in time is what we call the big bang and the subsequent expansion, if it occurred at all, occurred everywhere.

 

[Ibix]

therefore assuming that we have a velocity, and somewhat substantial at that, as opposed to 0

Nothing "has a velocity". Things can only have velocity relative to something else. That there isno such thing as an absolute velocity has been known since the days of Newton and Galileo. This is one aspect of the principle of relativity.

why doesn't two beams of light traveling in opposite directions (we'll assume that Earth's atmosphere is a vacuum here) have a different velocity from one another relative to the point of emission?

As I noted above, there's no absolute sense in which the Earth is moving. The idea that there was some special sense of "at rest" for electromagnetism was called ether theory. Attempts to detect this frame failed in various ways on the latter part of the nineteenth century, leading eventually to Einstein's proposition that the speed of light is the same in all inertial frames of reference. He showed that this was consistent with the principle of relativity, resolved all the apparent problems with electromagnetism, and could be used to derive Newtonian physics for slow relative velocities.

So the short answer to "why don't light beams travel at different speeds" is that we don't know. The longer answer is that when we assume that there is a finite speed that is the same in all frames then our physical models make correct predictions. They also require all massless things (such as light) to travel at that invariant speed. So, given that there is an invariant speedn we know why light travels at it. Why there is an invariant speed, we don't know as yet.

 

[Inertia_Squared]

Thanks for all the great replies, I don't have any more conflicting concepts in my head, so I'd say its mission accomplished. I already knew about the concept of space and time being linked, but my brain thought that they were 'linked' and that was that; I never considered that the values of time and space themselves could also be relative, which fascinates me. Thanks again for all the help!

 

[Drakkith]

I already knew about the concept of space and time being linked, but my brain thought that they were 'linked' and that was that; I never considered that the values of time and space themselves could also be relative, which fascinates me.

Indeed. An observer in spaceship moving at 0.99999c towards Alpha Centauri would actually spend much less than 4.37 years traveling to the star system from their reference frame. How can they travel 4.37 light years in less than 4.37 years if they aren't moving superluminal? They don't actually travel 4.37 light years from their frame of reference. They travel much less than that.

An observer on Earth would see them take just over 4.37 years to travel the 4.37 light years, but both the time and the distance are shorter when measured from the moving frame of the spaceship.

 

[Ibix]

They don't actually travel 4.37 light years from their frame of reference. They travel much less than that.

They don't travel at all in their own inertial rest frame - the star is the one moving, so it comes to them. It does move a shorter distance than the measured distance in the Earth/star rest frame, as you say.