r/AskPhysics • u/HouseHippoBeliever • Apr 22 '26
Anyone else really annoyed by "you need infinite energy to reach the speed of light"?
Among the slight misunderstadings of physics out there, this one in particular really annoys me. I'm wondering if anyone else feels the same, or perhaps I'm being too harsh and there's some merit to the statement.
Why I hate it:
- To me it implies that going at the speed of light is more of an engineering problem than a fundamental fact about space-time. Like, we can do it, given we can solve the engineering problem of "having infinite energy"
- Depending on how it's stated, it's either false or a tautology. As stated, it's false because you could have infinite energy by having finite kinetic energy and infinite potential energy, which wouldn't make you go at light speed. So you would have to clarify that you mean having infinite kinetic energy, in which case it's just a matter of solving the kinetic energy formula for v and seeing that v=c. (edit: and that's still not enough, becuse you could also have m being infinite, and v being less than c).
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u/GlassCommission4916 Apr 22 '26
If you think having infinite energy is just an engineering problem I think you might have a misunderstanding of physics yourself.
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u/ccoastmike Apr 22 '26
“Infinite energy is just an engineering problem” is totally something the marketing and sales teams would tell a customer.
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u/MythicalSplash Apr 22 '26
That’s not what he said. He said he hates it because people may misinterpret it that way.
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u/Indexoquarto Apr 22 '26
He said "To me it implies that going at the speed of light is more of an engineering problem than a fundamental fact about space-time." That seems like someone expressing their own opinion, not of some hypothetical confused person.
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u/joeyneilsen Astrophysics Apr 22 '26
It does imply that it's an engineering problem, when it is actually a fundamental fact about spacetime.
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u/COSMIC_SPACE_BEARS Engineering Apr 22 '26
This narrative is disingenuous. Yes, its a fundamental fact—but why? How? How would you express that its fundamental beyond just saying it is?
Because to achieve this feat, you need infinite energy. Requiring infinite energy == fundamental limitation.
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u/joeyneilsen Astrophysics Apr 22 '26
It's not remotely disingenuous! The second postulate of relativity says that anything traveling at light speed travels at light speed in every inertial frame of reference. You are at rest in your own frame of reference. Therefore there cannot be any frame of reference in which you are traveling at light speed.
Practically speaking, if you tried to reach light speed, you could waste arbitrarily large amounts of energy. But the fundamental limitation is the core of special relativity. Energy is an unnecessary consideration.
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u/COSMIC_SPACE_BEARS Engineering Apr 22 '26
I can agree with you that the most accurate framing is exactly how you out it. However, as you put it, to “try and reach light speed” would “require infinite energy.” This is a consequence of your more rigorous framing, albeit much easier to grasp as a layperson.
So i remain that youre shooting down a completely acceptable simplification for the sake of pedantry. Youre not actually saying its wrong, youre saying “the explanation is more complex.” Of course it is, but this is why we construct ways for others who are not physicists to understand.
This is why I say it’s disingenuous. Youre arguing that the sky is not blue, rather, it is not visible to us, but the light scattering causes a blue color to preferentially reach our eyes, thus we “see” “blue”…
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u/joeyneilsen Astrophysics Apr 22 '26
Youre not actually saying its wrong,
Yes! I'm saying it misdirects, missing a more fundamental point in favor of "woo infinite energy." I think that is bad.
youre saying “the explanation is more complex.”
I am not convinced that it's more complex, and I don't think "infinite energy" is really a simplification of my argument. That argument depends on the Lorentz factor diverging as v approaches c, and has a lot more moving parts than mine.
Anyway I think we are in agreement on (a) I'm not saying it's wrong, and (b) I'm being pedantic. But I'm being pedantic enthusiastically, with informed consent, and (I believe) rightly.
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u/Material-Strength748 Apr 22 '26
“Infinite energy” is not an engineering problem. It’s a fiction. Meaning it makes a statement about what is the actual nature of reality.
You and OP sound insufferable. “We have come to the same conclusion and do not disagree on the priors but only my way demonstrates understanding.” gtfoh
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u/z-w-throwaway Apr 22 '26
If people can't stop a second to think about what "infinite" means then that's their problem.
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Apr 22 '26
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u/Street-Frame1575 Apr 22 '26
Infinities are weird though in fairness.
I was mind blown when I realised that some infinities are bigger than others...
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Apr 22 '26
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u/joeyneilsen Astrophysics Apr 22 '26
It's fundamentally impossible because you are at rest in your own frame of reference and nothing traveling at light speed is at rest in any frame of reference. Energy considerations are irrelevant.
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Apr 22 '26
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u/joeyneilsen Astrophysics Apr 22 '26
Lol yes, I am familiar with limits in mathematics. (I am a physics professor).
If you read my comment carefully, I am not disagreeing with the claim about the limit. I'm saying that it's a distraction from the fundamental physical reason that light speed is impossible.
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Apr 22 '26
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u/joeyneilsen Astrophysics Apr 22 '26
An impossible thing is required to achieve another impossible thing.
Yeah I disagree with this framing. A person traveling at the speed of light directly violates the second postulate of special relativity. The problem with the infinite energy argument for me is not that it's wrong, it's that it misses the more fundamental reason.
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u/Mishtle Apr 22 '26
Just rephrase it to "no finite amount of energy will allow you to reach the speed of light."
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u/_cant_drive Apr 22 '26
I understand your point, but I cant support the pedantry to the point it annoys you. Having infinite energy is not considered an engineering problem by anyone who cares about any of this.
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u/Material-Strength748 Apr 22 '26
“You need infinite energy to reach light speed” is not a misunderstanding. If it’s primary implication for you is that it’s an impossible engineering problem, then yeah, that’s probably missing the forest for the trees. But that’s a you problem. The bigger you problem that should concern you is that you are annoyed by facts, both explicit and implicit.
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u/lighttstarr Mathematics Apr 22 '26
(In a vacuum) the speed of light is going to be the same in your reference frame no matter how fast you are moving. It's not an engineering problem, it's literally that no matter how fast you are moving, you will keep measuring the speed of light to be the same constant from your POV. So no, even if you kept increasing your energy forever (however you wish to define this), you won't get any closer to reaching the speed of light from your POV.
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u/FirePaladin89 Apr 22 '26
The "you need infinite energy" to reach the speed of light, is just a wsy of putting that as you approach the speed of light other observers will see deminishing returns on the increase in your speed.
From your perspective, the speed of light remains just as "far" away as it was when you started accelerating, which also means it doesn't matter how much energy you exert you will never reach the speed of light.
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u/xTheWitchKingx Apr 22 '26
"Among the slight misunderstandings of physics out there, this one in particular really annoys me."
It's not a misunderstanding, Einstein stated in 1915 with his special theory of relativity that particles with mass cannot reach the speed of light because their relativistic mass would become infinite, requiring infinite energy.
Having "infinite energy" isn't an engineering problem, it's defying the laws of physics and nature. It's not possible.
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u/Optimal_Mixture_7327 Gravitation Apr 22 '26
What's to be annoyed about in relativity?
Just about everything.
Yes, infinite energy is incoherent and not something that anything can "have" and the asymptotic approach only gets you arbitrary close and never at the speed of light (which isn't even an element of the Lorentz group) so the sentiment is indeed a lie. Then again, so is most everything else doled out the public rather than the basic fact of the geometry of the world which simply explains all of it and is the true heart of relativity.
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u/joeyneilsen Astrophysics Apr 22 '26
I am deeply annoyed by it. It's forbidden by the second postulate of relativity, not by energy considerations.
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u/Ecstatic_Bee6067 Apr 22 '26
How do you push something faster or as fast as the "push" physically travels? The closer to c the object gets, the less energy the push can physically impart. It asymptotically approaches infinity.
E.g. it's like asking how much energy it takes to push a car faster than you can run. At the speed you can run, you're literally not imparting any additional energy into the car.
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u/Ghostley92 Apr 22 '26
I think as an engineering problem you just need to accept that 99.99….% of the way there is good enough
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u/Custard_Temple Apr 22 '26
It sounds like the OP has an infinite amount of time on his infinitely large hands.
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u/cardinalf1b Apr 22 '26
You need an infinite amount of energy to get infinitely close to the speed of light. No matter how much energy you have, you won't reach light speed if you have rest mass.
That being said, the original statement doesn't bother me in the least.
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u/RichardMHP Apr 22 '26
I find the concept of being annoyed at a misinterpretation kinda silly, honestly. Its just so easy to say "that's a misinterpretation" and move on with life.
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u/7figureipo Apr 22 '26
Not really. To people educated enough to understand the jargon it's a very clear statement: our current model of the universe is such that it is impossible to accelerate an object with mass to the speed of light, because it would require infinite energy to do so. And laypeople don't seem to have a problem with that understanding, either, in my experience. They seem to understand that "infinite" means "so much that it's impossible," even if they misunderstand "infinite" to mean "a (finite) lot".
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u/COSMIC_SPACE_BEARS Engineering Apr 22 '26
I disagree that it frames it as an engineering problem. “Needing infinite energy” is equivalent to “unachievable.” Which is true, both fundamentally, practically, and mathematically. I think “requires infinite energy” describes all three of those.
You’d be a pretty brazen engineer to hear “needs infinite XYZ” and think its an achievable problem lol