Why is it impossible to go faster than the speed of light?

  • #1
davidortenn79
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Ok, I took like a class in physics in college. It was a class to learn physics for majors that we really didn't care. I remember him saying, if you remember anything from this class, remember inertia. That's about all I remember from the class... lol.

But, I don't understand why it's impossible to go faster than the speed of light. I was just reading. Einsteins theory that the closer something gets to the speed of light, the more energy it takes and as it gets close to the speed of light, it takes an infinite amount of energy to do it.

But, say you were deep out in space. Nothing near you at all. Isn't your speed a reference to something else? I mean, the way you'd measure your speed is in reference to something else in space. So, if there was a nearby planet, you'd be measuring your speed in reference to that planet. But, if there's another solar system in the planet that is moving closer to you, your speed could be measured to one of those planets too in the other solar system. Since that solar system is moving towards you and the other planet you were just measuring yourself against, if you were to forget about the other planet and now just measure towards the one that is moving towards you, you could just say instead that you're moving towards that planet at X mph. But, you didn't change your speed at all... you were sitting still next to the original planet. But, since the other solar system is coming at you, now you measure towards that. you didn't change your speed at all, but theoretically you're moving towards it, so you have a speed now. So, say that planet was moving towards you at half the speed of light. Then you had boosters that got you going half the speed of light towards it in reference to the old planet. Wouldnt' you be moving the speed of light now in reference to the new solar system you're now measuring against?

Also, I don't understand, in space why it takes any energy to maintain a speed. Seems like if you're going 0. Then you have thrusters, you will then accelerate to a speed. You're in space. You will continue that speed forever. You're not using any more energy to maintain that speed. So, then you just use another thruster, you'll speed up again. Seems like you could just keep doing that until you hit the speed of light. Maybe too hard to hold enough boosters on board a ship unless you're measuring yourself's speed against a comet flying in space because it's the only object that'e even near you while you're out in space.
 
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  • #2
Speeds don’t add the way you’re thinking.
If A is moving at speed .5c relative to B, and B is moving at speed .5c relative to C, then A’s speed relative to C is not .5c+.5c, it is .8c. The general formula is $$w=\frac{u+v}{1+\frac{uv}{c^2}}$$ where ##u## is A’s speed relative to B, ##v## is B’s speed relative to C, and ##w## is A’s speed relative to C, not ##w=u+v## as you’re expecting. So if you have a thruster that will bump your speed from rest by .1c and you’re moving at .9c relative to me, firing that thruster won’t bring your speed relative to me up to c - no matter how often you fire it, your speed relative to me will be less than c.

This effect is completely unnoticeable at any speed that we might encounter in daily life (which is why so many people are surprised to learn that ##w=u+v## is not exactly correct) but becomes significant with speeds that are an appreciable fraction of ##c##.
 
  • #3
I can't answer your question as I don't know much about this myself. However as far as I know, this is Einstein's second postulate for special relativity, and you don't really prove postulates (I might be wrong). Also take caution of any explanation that uses the Lorentz transformations, since Einstein derived those from his second postulate (although, there's a whole wikipedia page on how to derive those, so the argument might not be as circular as I'm thinking).

For other thought experiments, take two flashlights and point them towards eachother. Then the light from one flashlight should (but won't) appear coming at 2c speed relative to the light of the other.
 
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  • #4
davidortenn79 said:
Also, I don't understand, in space why it takes any energy to maintain a speed.
You are correct, it doesn't take any energy (well, there's friction against one hydrogen atom per cubic meter or so, but that's pretty negligible). The energy argument comes in when you try to explain why a rocket you see doing 1m/s less than the speed of light can't just boost for a moment and exceed the speed of light - it leads to "because however much reaction mass it has it needs more".

Fundamentally, though, "you can't exceed the speed of light" is a direct consequence of one of the postulates of relativity. So we don't really have a more fundamental explanation than "that's the way it is". All explanations based on energy or time dilation or anything else are based on formulae that derive from that postulate.
 
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  • #5
Under some fairly basic and desirable assumptions, there are only two possibilities for relativity: Galilean and Einsteinian. This is a mathematical / geometrical fact. The Michelson-Morley experiment (among others) showed that even very careful measurement do not detect a different speed of light regardless of velocity of the observer. That is only possible in the Einsteinian relativity model. There is no such thing as a relative speed greater than ##c## in that space-time geometry.
One observer can see an object going in one direction at ##0.75 \cdot c## (relative to him) and another object going in the opposite direction at ##0.75 \cdot c## (relative to him) and think that they are traveling faster than ##c## relative to each other in their space-time geometries, but he would be wrong. In the space-time geometry of either object, the other object is still traveling at a relative speed less than ##c##
 
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  • #6
If it is possible to send a signal faster than light relative to a chosen reference frame then the postulates of special relativity require that it is possible to construct a "tachyonic antitelephone".

Such a device can send a signal into the past. This leads to such things as the Grandfather Paradox. Such paradoxes are one reason not to take faster than light travel seriously. At least within the realm of special relativity.

The speed of light limit applies to material objects and to light signals. Things that can carry information. There are "things" that can move faster than light without having any material object exceed the speed limit. For instance, if one were to flick a (suitably powerful and well collimated) laser beam across the face of the moon in less than one hundredth of a second, the illuminated spot would be travelling faster than light speed. Or if you prepared two massive scissor blades that were nearly parallel and set them moving toward each other then the point of intersection could sweep along the blades faster than light speed.
 
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  • #7
davidortenn79 said:
So, say that planet was moving towards you at half the speed of light.
That would be same thing as saying you're moving at half the speed of light towards that planet.
davidortenn79 said:
Then you had boosters that got you going half the speed of light towards it in reference to the old planet.
Before firing your boosters both you and the old planet are moving at half the speed of light towards that new planet.
davidortenn79 said:
Wouldnt' you be moving the speed of light now in reference to the new solar system you're now measuring against?
No. You can't reach the speed of light no matter how many times you fire the boosters. This is demonstrated every day by thousands of particle accelerators located all across the world.

This is consistent with your memory of the energy requirements. Each time you fire your boosters you expend energy. But no matter how many times you fire your boosters (expending energy) you never reach the speed of light.

You ask why, but you could just as easily ask why it should be possible to reach or exceed the speed of light. Either it is possible or it isn't. Every experiment performed or observation made demonstrates that it isn't possible. Thus a belief that it makes sense that it is possible must be mistaken.
 
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  • #8
DrBanana said:
However as far as I know, this is Einstein's second postulate for special relativity, and you don't really prove postulates (I might be wrong). Also take caution of any explanation that uses the Lorentz transformations, since Einstein derived those from his second postulate (although, there's a whole wikipedia page on how to derive those, so the argument might not be as circular as I'm thinking).
There exists much experimental evidence for a finite invariant speed:
https://math.ucr.edu/home/baez/physics/Relativity/SR/experiments.html

DrBanana said:
For other thought experiments, take two flashlights and point them towards eachother. Then the light from one flashlight should (but won't) appear coming at 2c speed relative to the light of the other.
Two light pulses can have a closing speed of ##2c##. This is consistent with special relativity.

The relativistic composition law for velocities is used for transforming a velocity from one inertial reference-frame to another. This does not apply to your scenario.
 
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  • #9
DrBanana said:
However as far as I know, this is Einstein's second postulate for special relativity, and you don't really prove postulates (I might be wrong).
You explore the logical consequences of your postulates and make predictions of the outcomes of experiments. You then do the experiments and see if your predictions match. If they do, the postulates you started with have gained support. If even one experiment doesn't match the results then you do some mix of rechecking your experiment, rechecking your maths, rejecting the original postulates, or noting that the theory is only valid in certain regimes.

So no, you don't prove postulates. You prove results based on those postulates, then test the predictions.
DrBanana said:
Also take caution of any explanation that uses the Lorentz transformations, since Einstein derived those from his second postulate (although, there's a whole wikipedia page on how to derive those, so the argument might not be as circular as I'm thinking).
It's not circular. But with any explanation based on anything (in relativity or any other field) you can ask "why" and "why" again and eventually you get back to the postulates from which the theory is derived. The "why" answers can be illuminating. For example the energy explanation tells you why a rocket can't exceed light speed - any finite amount of fuel corresponds to a final speed slower than light because of how the kinetic energy works in relativity. But if you ask why the kinetic energy formula is what it is you will always track back through a chain of reasoning to the postulates. And the only justification for them is that they work.
 
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  • #10
I still wonder if, the reason why we believe we can't go faster than light is because we cant imagine anything faster than light. Also, it's the "theory of relativity" that says that right? It's a theory. How long does something need to remain a theory and proven over and over before you make it the "fact of relativity." Theory makes me think the people that govern if this is a fact still have doubts... lol. IDK. I know all of you are super smart and darned if you'll get out smarted on the speed of light by a guy that took a 101 level physics class. (But how high of physics did einstein take? I don't think he took any physics classes... we're all learning from someone that never took one physics class. So my one class is one more than he took.)
 
  • #11
davidortenn79 said:
how high of physics did einstein take? I don't think he took any physics classes.
I have no idea why you would think this. When Einstein published his classic 1905 papers, he was in the final stages of getting his Ph.D. in physics.
 
  • #12
davidortenn79 said:
I still wonder if, the reason why we believe we can't go faster than light is because we cant imagine anything faster than light.
Huh? Seriously? Newtonian physics assumes gravity propagates at infinite speed. Tachyonic particle theories got a brief airing during the "faster than light neutrino" mess up about ten years ago. We can imagine them easily.
davidortenn79 said:
Also, it's the "theory of relativity" that says that right? It's a theory. How long does something need to remain a theory and proven over and over before you make it the "fact of relativity."
It remains a theory forever. Theory here means a coherent system of thought based on careful reason and clear postulates, not "some half assed idea I came up with", which its colloquial meaning.

There are a lot of experimental facts, however, that we cannot explain or predict without that theory.
davidortenn79 said:
Theory makes me think the people that govern if this is a fact still have doubts...
Yes. The underlying philosophy is called "positivism". All theoretical knowledge is conditional, and we may one day replace our best theories with better ones. In fact, we have strong reasons to believe that relativity is an approximation to a more general theory. The idea of having final perfect knowledge of The Laws Of Nature really went out of fashion when Einstein showed that Newton's work, that had stood gor two and a half centuries, was subtly wrong.
davidortenn79 said:
But how high of physics did einstein take? I don't think he took any physics classes...
He had a degree and a doctorate in physics.
 
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  • #13
davidortenn79 said:
I still wonder if, the reason why we believe we can't go faster than light is because we cant imagine anything faster than light.
This is getting into personal speculation, which is off limits here.

davidortenn79 said:
it's the "theory of relativity" that says that right? It's a theory. How long does something need to remain a theory and proven over and over before you make it the "fact of relativity."
Whatever the relevant time period is, the theory of relativity has passed it. So you might as well think of it as the fact of relativity as far as this discussion is concerned.

davidortenn79 said:
Theory makes me think the people that govern if this is a fact still have doubts... lol
Your lol here is highly inappropriate. You are talking about one of the most confirmed theories in all of physics. Even someone who has only taken a 101 level physics class should know better.
 
  • #14
Ibix said:
All theoretical knowledge is conditional, and we may one day replace our best theories with better ones.
I would put this differently. I would put it that we may one day extend our best theories with ones that cover a wider domain, and have our best current theories as valid approximations in the domains where our best current theories have already been thoroughly tested.

I say this because a phrase like "replace our best theories with better ones" carries the same sort of dismissive connotation as the word "theory" itself does to people who don't have a good understanding of what "theory" actually means to a scientist--as in, "replace our current half-assed guesses with slightly less half-assed guesses". That's not at all what any kind of future discoveries will look like as far as our current theory of relativity is concerned.
 
  • #15
Ibix said:
Huh? Seriously? Newtonian physics assumes gravity propagates at infinite speed. Tachyonic particle theories got a brief airing during the "faster than light neutrino" mess up about ten years ago. We can imagine them easily.

It remains a theory forever. Theory here means a coherent system of thought based on careful reason and clear postulates, not "some half assed idea I came up with", which its colloquial meaning.

There are a lot of experimental facts, however, that we cannot explain or predict without that theory.

Yes. The underlying philosophy is called "positivism". All theoretical knowledge is conditional, and we may one day replace our best theories with better ones. In fact, we have string reasons to believe that relativity is an approximation to a more general theory. The idea of having final perfect knowledge of The Laws Of Nature really went out of fashion when Einstein showed that Newton's work, that had stood gor two and a half centuries, was subtly wrong.
He had a degree and a doctorate in physics.
Did Einstein really have a PhD in physics? I'm being serious. Assumed at that time, PhDs in physics didn't exist. I know Einstein is the definition of a PhD in physics... but also assumed the degree wouldn't exist at the time to get a PhD in. He would first have to write his book so he could study it and past the tests.
 
  • #16

davidortenn79 said:
It's a theory.
It is a fundamental truth that Physics can only prove a theory wrong. That is the way it works, much to the consternation of those who do not undrstand the scientific method. But you only need one verified wrong to negate the whole house of cards. A million correct predictions makes you comfortable but is not "proof" of truth. "Only a theory" is a description of science , not a value judgment.
 
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  • #17
davidortenn79 said:
Did Einstein really have a PhD in physics? I'm being serious. Assumed at that time, PhDs in physics didn't exist. I know Einstein is the definition of a PhD in physics... but also assumed the degree wouldn't exist at the time to get a PhD in. He would first have to write his book so he could study it and past the tests.
This is painful to watch. Please, PLEASE! stop digging this hole and at least read his wikipedia page:
https://en.wikipedia.org/wiki/Albert_Einstein#1900–1905:_First_scientific_papers

And then maybe consider that professional physicists today probably know a bit about the Rock Star of their profession.....and, maybe also consider they know a little bit about how science itself works. The lack of even basic understanding of what science is, is -- wow.

Sheesh.
 
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  • #18
russ_watters said:
This is painful to watch. Please, PLEASE! stop digging this hole and at least read his wikipedia page:
https://en.wikipedia.org/wiki/Albert_Einstein#1900–1905:_First_scientific_papers
what he said (small).jpg
 
  • #19
PeterDonis said:
I would put this differently. I would put it that we may one day extend our best theories with ones that cover a wider domain, and have our best current theories as valid approximations in the domains where our best current theories have already been thoroughly tested.

I say this because a phrase like "replace our best theories with better ones" carries the same sort of dismissive connotation as the word "theory" itself does colloquially--as in, "replace our current half-assed guesses with slightly less half-assed guesses". That's not at all what any kind of future discoveries will look like as far as our current theory of relativity is concerned.
I see your point. However, I'd say that GR is an outright replacement for Newtonian gravity. Even in the domain where the predictions are near enough identical, the underlying reason for why objects move as they do is completely different - geometry versus force. And the sources can be so much more complex. It isn't just Newtonian gravity plus a few extra terms - it allows things like FLRW solutions that you simply can't imagine in Newtonian gravity.

I would not be surprised if the next theory of gravity produces a similar scale of re-evaluation of what gravity and spacetime actually is and that's why I used "replace". But I agree extend isn't unreasonable.
 
  • #20
davidortenn79 said:
Did Einstein really have a PhD in physics? I'm being serious. Assumed at that time, PhDs in physics didn't exist. I know Einstein is the definition of a PhD in physics... but also assumed the degree wouldn't exist at the time to get a PhD in. He would first have to write his book so he could study it and past the tests.
A PhD is awarded for making a substantial contribution to the body of human knowledge. It isn't a test you study for - it's your first independent piece of original research at the start of a research career. The viva at the end is experts interviewing you to confirm that you actually understand it and didn't get it ghostwritten.

I don't actually know when the practice of awarding formal doctorates began (edit: apparently 1150, again according to Wikipedia), but as an academic title Wikipedia says it dates back to the thirteenth century.
 
  • #21
davidortenn79 said:
Did Einstein really have a PhD in physics?
Yes. As you could have verified for yourself by just looking at his Wikipedia page.

davidortenn79 said:
I'm being serious.
No, you're not. A serious person would have done some basic checking online before asking such a question.

As for the rest of your post...

russ_watters said:
This is painful to watch.
It is indeed. And enough is enough. Thread closed.
 
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  • #22
One further note for future readers:

Ibix said:
I'd say that GR is an outright replacement for Newtonian gravity.
And there is certainly a viewpoint which makes that statement true; you give some aspects of that viewpoint in your post.

But the set of people who understand those issues and interpret statements like the one quoted above properly does not include the OP of this thread. And the OP asked about experimental testing. So for this thread, I wanted to emphasize the experimental testing part, and how thoroughly our current theory of relativity has been confirmed experimentally within the domain we can currently test, and how that will remain true no matter what future theories we discover.
 
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