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I'm laying in my bed right now. Am I traveling 0mph? Am I traveling 850-ish mph (south texas) as the earth rotates? Am I traveling 67,000mph as my planet orbits the sun? Am I traveling 560,000mph as our sun orbits the galaxy? What speed is our galaxy moving at as it travels through the universe? Point is, velocity requires relativity to a point in space, or relativity to an object which has a location.

I'm not saying that Einstein's theory of general or special relativity is flawed. I'm not claiming to be smarter than Einstein. I'm claiming that nobody has succeeded in clarifying this point to me and it keeps me up. I've spent so much time in my life trying to conceptualize this

How can we determine that it is impossible to travel faster than the speed of light if there is no universal point of origin in our universe from which all velocity is measured relative to? Wouldn't general relativity be more about observer perception?

I'm orbiting the center of the galaxy at 560k mph. If I shine a light in the same direction that I am orbiting why wouldn't the speed of that ray of light be c + 560,000 mph relative to the center of the galaxy?

Imagine hypothetically that I am on a spacecraft in the middle of open space, in between galaxies where no bodies of mass can affect my spacecraft with gravitationalpull. If I begin traveling from a certain location at 99.9% of c, why couldn't I fire the rocket up again to accelerate to 110% of c from that original location? What... is space a medium that prohibits this, or is my spacecraft just aware of Einstein's theory and doesn't want to break any rules?... my old physics professor who i refer to as an "information regurgitator" basically would tell me to just accept it and don't ask questions in a nutshell. He said that my spacecraft would not experience time if it was traveling at the speed of light. That does not pass through my mental filter. In my mind the spacecraft would experience time no different than anything else, it only wouldn't experience time RELATIVE to it's original location that it left from in the first place. The location that the spacecraft's velocity is relative to

I'm not looking for somebody to point out the mathematical demonstrations to me. Even when I do them myself they still don't clearly reveal the intuition behind this concept. I think most people do not totally understand general relativity because none of them have been able to decisively explain this to me

Qmechanic
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TAShannon
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    This is special relativity, not general. But anyway, (and this is not an answer) no one can really understand it intuitively. We use math because it lets us describe phenomena such as time dilation or the relativistic addition of velocities even when we don't have intuition. – Javier Dec 20 '21 at 15:58
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    Physics is a science. Hence first you measure then you explain. As Feynman said, a beautiful theory that does not fit the data is simply wrong. Michelson and Morley, trying to detect the existence of aether, measured the speed of light using earth's orbit at directions perpendicular and along the supposed aether. Lo and behold, they found no difference. They measured that the speed of light is independent of the referential. In special relativity, Einstein postulated that the speed of light was an upper limit, and this fits the observations. Until further notice $c$ us the limit. – rpsml Dec 20 '21 at 16:26
  • Recently, I gave up with a knowledge why photon(s) inertial mass should be $0$.$m\gamma=m_0$ $\gamma = 0$ for $v=c$. and if you want to travel faster than light speed than Your inertial mass must be imaginary. (Not saying that gamma factor is since an answerer have already mentioned it – Billy Istiak Dec 20 '21 at 16:38
  • TLDR: You can never observe anything travelling faster than the speed of light with respect to yourself. If You see Alice heading North from your location at 3/4 c, and she sees Bob heading North at 3/4 c, then from your point of view, Bob will appear to be heading North at 0.96 c, and not at 1.5 c as you might naively expect. (https://en.wikipedia.org/wiki/Velocity-addition_formula#Special_relativity) Even if Bob sees Charline heading North at 3/4 c, and Charline sees ... sees Zoey heading North at 3/4 c; from your point of view, Zoey will be heading North at somewhat less than c. – Solomon Slow Dec 20 '21 at 17:10
  • Thank you sir. You're giving me some more daydreaming ammunition. The disconnect for me lies with the difference between observation/perception and reality. Also I appreciate your help with my understanding. Most people scoff at my attempts to reasons through this – TAShannon Dec 20 '21 at 17:18
  • @Javier's remark that "nobody can understand it intuitively" makes me wish (not for the first time) that we had a way to downvote comments. – WillO Dec 20 '21 at 17:30
  • @WillO perhaps I was too harsh - I was trying to get OP to not dismiss mathematical explanations in search of a purely intuitive explanation which would be (at the very least) difficult, though I admit my phrasing wasn't the best. I also think you could have phrased your comment differently. – Javier Dec 20 '21 at 18:20
  • Wanted to point out that your thought why wouldn't the speed of that ray of light be c + 560,000 mph relative to the center of the galaxy points to part of the problem. Velocities $u$ and $v$ do not add together into $u+v$. Einstein & co. proved they add like $\frac{u+v}{1+uv/c^2}$. At low speeds ($u,v<<c$), the denominator is approx 1 and so the former expression is approximately true, which is the behavior we've lived with all our lives. – RC_23 Dec 23 '21 at 18:51
  • As a bonus exercise, plug in $u=560000$ and $v=c$ into the correct expression and see what happens – RC_23 Dec 23 '21 at 18:53

5 Answers5

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How can we determine that it is impossible to travel faster than the speed of light if there is no universal point of origin in our universe from which all velocity is measured relative to? Wouldn't general relativity be more about observer perception?

You're thinking in terms of classical mechanics and missing the relativity part here. What you are saying is true in terms of classical mechanics, but classical mechanics does not set a speed limit so the model is inapplicable in relativistic speed regimes. A speed limit in classical mechanics is like redlining your car: You just can't go faster. That's it. Nothing else changes with speed. There are no additional effects added to the classical model. Most notably, if the lamp was moving away from you, you still would measure a slower speed just like every other thing because that still adheres to not breaking the maximum speed limit. Your question would hold if the speed of light in empty space could be measured lower, but never higher.

But relativity it isn't just about a speed limit. It's about an invariant speed. Of course, it also includes "other effects" like time dilation. Remember, light has been measured to travel the same speed in empty space regardless of relative motion. It cannot be sped up or slowed down. If you measure the speed of light and it is always the same regardless of relative motion then you do not need an absolute spatial reference because all measurements read the same anyways. Think less about about max speed and more about the speed being measured always being the same.

DKNguyen
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The relativistic "speed limit" is true in every intertial reference frame. No matter whether measured with respect to the planet, sun, galaxy, or any other valid (*; see footnote) frame, no matter can move at a speed greater than c. This is reflected in the relativistic addition of velocities principle: if you shoot a flashlight at .999c, you still see the beam it emits traveling at c.

(*) Obviously, you can design an inertial frame that travels at a relative speed of greater than c with respect to Earth -- and by extension with respect to all matter in the universe -- but that's not a valid inertial frame. If I recall, such frames do pop up as a mathematical convenience in astronomy (related to projecting 3D paths onto the sphere).

jwimberley
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Apparently Einstein recieved some angry letters from upset people who accused him of setting a speed limit to the universe and so exploding their dreams of interstellar space travel fed by SF.

The 'speed limit' is a limit set by nature and not by Einstein - he merely discovered it. It's been verified experimentally, and is a foundation stone in constructing physical theories.

It was pointed out by Maxwell's equations for electromagnetism that showed that the speed of light was absolute. At the time this wasn't taken seriously. But Einstein did.

Mozibur Ullah
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  • Just a quirky comment. Einstein did not discover it. Michelson and Morley measured, hence discovered, that the speed of light did not depend on the referential. Einstein postulated that the speed of light was an upper limit. And yes, I am splitting hairs here. – rpsml Dec 20 '21 at 16:20
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Because in special relativity there's this expression $\gamma$, known as the Lorentz factor, that breaks once you get close to the speed of light.

$\gamma \equiv \frac{1}{\sqrt{1-v^2/c^2}}$

Here $c$ is the speed of light and $v$ is your current speed.

$\gamma$ shows up everywhere in special relativity (in fact if you're doing a physics exam and you see it, you know you need to use special relativity on the question). You can see that once your speed $v$ approaches $c$, then $\gamma$ becomes arbitrarily large. This breaks a lot of things, such as the expression for kinetic energy ($KE = (\gamma -1) mc^2$).

Allure
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The basic reason that you cannot travel faster than light is that you have mass. The relationship between mass, energy, and momentum is $$m^2 c^2 = E^2/c^2 - p^2$$ This can be expressed in terms of velocity with the substitution $v/c=cp/E$. This gives the velocity in terms of energy as $$v=\frac{\sqrt{E^2-m^2c^4}}{E}c$$ which goes to $c$ as $E$ goes to infinity for any $m\ne0$

Velocity Vs Energy

For $m=0$ the expression simplifies to $v=c$ regardless of $E$.

Dale
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