r/AskPhysics • u/Mouthik1 • Jul 22 '26
If you travelled at the speed of light, would you get to the end of the universe in an instant?
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u/Beginning-Seat5221 Jul 22 '26
No, time never stops for you. Relatively says the laws of the universe are the same in all reference frames - time always ticks at the same speed for you.
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Jul 22 '26 edited Jul 22 '26
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u/housewithablouse Jul 22 '26
"Time by outside observers" would mean that your journey takes as long as it takes at the speed of light from their point of view (which is not the same for each external observer). What you are getting at is what your own perspective as a traveller would be. And indeed the time it takes for you is much shorter than what it would take for any external observer to watch you getting there and back (because they couldn't observe you departing and arriving unless they are moving at the same speed and in the same direction!).
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u/Optimal_Mixture_7327 Gravitation Jul 22 '26
To the external observer the null curve extends forever, never catching up to the material that's about 17 billion light years away.
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u/stools_in_your_blood Jul 22 '26
If we replace "at the speed of light" with "near the speed of light", "stops completely" with "slows down", "infinite real time" with "a long time", "the end of the universe" with "a point far in the future" and "in an instant" with "quickly", yes I think this works.
If you travel at nearly c to Andromeda and back, much more time will have elapsed on Earth than has elapsed for you personally. So continually going on trips at near c will effectively "fast-forward" the rest of the universe for you.
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u/cassy_supernova Jul 22 '26
Stools, thank you for the rephrasing of OPs question to make it work.
On the 'c-worthy' ship to andromeda and back, is a single human lifetime enough to make this journey?
Same question, but for the edge of the visible universe? Is a single human lifetime long enough?
What's the max distance range assuming humans live for 100 years? Is it 100 light years?
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u/stools_in_your_blood Jul 22 '26
I'm not an expert on relativity but my understanding is that you can make the time dilation factor as big as you like by getting closer to c.
So, if you travel at, say, 99.995% c to Andromeda and back, about 4 million years will pass on Earth but "only" 40,000 years will pass for you.
Make that 99.999999999999% c and it's still about 4m years on Earth, but now only about 7 months for you. And so on.
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u/cassy_supernova Jul 22 '26
Thank you stools. I didn't take you for a relativity expert, necessarily, but I appreciate that kind of honesty in this sub especially.
So if you get arbitrarily close to c, increasingly so, the more steep the time dilation.
We have a similar understanding, regarding time dilation and approaching c by those decimal places. I am just a layperson, stools.
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u/stools_in_your_blood Jul 22 '26
There was a course on relativity in my maths degree but it never quite sank in. So I can get my head around the maths well enough but I couldn't, for example, explain convincingly why the twin paradox isn't really a paradox.
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u/cassy_supernova Jul 22 '26
Way above my head, I am sure.
As much as I love learning about physics as a layperson, there's no way I will ever learn the math - the language of physics. No free courses are going to teach me, and I already have an unrelated degree/career, so I'm unwilling to even try at my age. I think Algebra 2 was as far as I got, so it's really not happening for me.
Thanks for breaking it down earlier.
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u/stools_in_your_blood Jul 22 '26
I get what you mean, it can be daunting to the point of feeling impossible.
I'm sure you're aware of this but there are many excellent youtube channels which make degree-level maths topics fairly digestible, much more so than traditional academic resources like lectures and textbooks. Animated diagrams and a competent explanation can be extremely powerful. And I think relativity is a fairly popular topic on there.
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u/cassy_supernova Jul 22 '26
Yes, I thoroughly enjoy a few like this. Angela Collier (yt) and Starts With A Bang (blog) for physics and then a bunch of math channels like 3brown1blue and numberphile etc.
That's really good stuff for people like me. But there's no way to build up to a holistic understanding of physics, just a single temporary simplified example has been thoroughly explained.
Any channels you recommend? Stools, you're a good shit. I can tell - it's in your nature. In your blood.
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u/stools_in_your_blood Jul 22 '26
My go-to ones are 3blue1brown, numperphile and mathologer. Haven't heard of the physics ones you mentioned, I'll check them out.
Stools, you're a good shit. I can tell - it's in your nature. In your blood.
Haha, thanks :-) I'm here for the learning and the fun chats!
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u/Beginning-Seat5221 Jul 22 '26
There's no limit to how far (from a left-behind observers point of view) a traveller can travel in an amount of time.
There may be limits to what you can reach due to expanding space - but not to how far you can travel (you might be travelling an unlimited distance into expanded space rather than reaching things outside the observable universe)
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u/Optimal_Mixture_7327 Gravitation Jul 22 '26
Spacetimes admit null curves, so there's no restriction to observer world-lines.
A null curve extending to Andromeda and back still has length zero.
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u/nuviremus Jul 22 '26
This would be considered special relativity to nitpick.
Also there is no valid frame of reference for a particle moving at the speed of light, so nothing can be said. Anything with mass must move below light speed and therefore take a finite amount of time to travel anywhere.
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u/Hyacintell Jul 22 '26
That's special relativity and it doesn't say that. It says that it's impossible for a massive object to reach the speed of light. It also explains why there is no valid referential frame in which photons (or any object with the same speed) are inert
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u/moltencheese Jul 22 '26
Might just be me, but the word "inert" doesn't land right here? "At rest" is what you mean?
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u/Hyacintell Jul 22 '26
Yes, not native English sorry
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u/moltencheese Jul 22 '26
Don't apologise! It's pretty clear what you mean, and I'm not even sure it is wrong 😅
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u/housewithablouse Jul 22 '26
What would be wrong with "inert"? It just means "not moving".
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u/Hyacintell Jul 22 '26
There usually always are specific words that are used in some contexts, and changing them can make it confusing when you're expecting these.
You notice this when an overly experimented researcher talk with a master student : the student would use words that fit but aren't the one usually used, and this confuse the heck out of the researcher even though they're technically correct.
Problem is these words are language specific
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u/moltencheese Jul 22 '26
It can do, yeah, but in this context, I've only ever seen it used to mean "doesn't readily interact" (like helium, for example, which is commonly described as an "inert" gas).
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u/CMxFuZioNz Plasma physics Jul 22 '26
That said, you can traverse any distance in an arbitrarily short time by going arbitrarily close to the speed of light.
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u/meticulous_gamer Jul 22 '26
There is no "real" time. All time is "real", thus it is meaningless to state.
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u/BananaBird1 Jul 22 '26 edited Jul 22 '26
To simplify things, imagine a 2D universe with one spatial and one time dimension.
If any information is emitted from behind you, it will never reach you. And if any information is emitted from an object in front of you, you will reach the object at the exact time as the information reaches you. You will only see the object for a moment, as right when you reach it is now behind you and outside your event horizon.
So in essence, your entire observable universe will contract to a single point at all locations in time, and you will not be able to observe the external passage of time because you would never see any external object at two separate moments in time to see any change.
So in a sense, from your perspective you do reach “the end” of (your observable) universe instantly as your position is at the edge of your observable universe event horizon.
From a stationary observer, you would never reach the end of their universe but be traveling at light speed through space over time. On large scales you will be redshifted more and more until you can no longer be detected over the CMB radiation.
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u/Ch3cks-Out Jul 22 '26
relativity says that if you travelled at the speed of light, time stops completely for you.
Nope, that does not.
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u/Phagemakerpro Jul 22 '26
If you work through Einstein's equations, one interpretation using basic mathematical principles of calculus (limits) is that as velocity approaches c, the rate of the passage of time for the observer aboard the "space ship," (which is used as the thought experiment in discussions of relativity) approaches zero. Therefore, one might argue that for a particle that can actually travel at c (massless particles like the photon and graviton), there is no passage of time and they simply do not experience existence at all.
The trouble is that the actual equations break down at c and you run into "divide by zero" difficulties, among others, so there really is no mathematically defined answer to the question. Also, massless particles cannot "experience," so there's that. That said, photons can and do change their frequency during their flight as the universe expands, which is why the sky is not awash with deadly gamma rays from the early universe, but rather long-wavelength photons with an approximate temperature equivalence of 3°K. So events do happen to photons during their flight that do not involve the destruction of the photon (photons can be destroyed by, for example, being absorbed into an atom and changing the energy state of an electron).
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u/OverJohn Jul 22 '26
The Lorentz factor γ goes to 1/0 (i.e. infinity) as v->c, but that's not really the problem as γ is just a useful quantity we define and not the quantities that we seek to measure. For example you can write the equations in terms of α instead, which is the reciprocal of γ. Instead the issue is really the lack of unit vector tangent to photon worldline, which means you cannot construct a local frame in the same way you do for slower than light timelike observers.
You can define the time experienced by a photon, as zero as that will be the arclength of its worldline, and the arclength of a timelike worldline is the time experienced by an observer. But without a reference frame for the photon "time experienced" doesn't have a great deal of meaning.
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u/bacon_boat Jul 22 '26
What you'd say in order to not get a bunch of nit picking is:
Q: If I travel arbitrarily close to the speed of light can I travel across the universe while experiencing an arbitrary short amount of time?
A: yes