The universe is large, and even very large. Traveling to its limits, and coming back to tell everyone what is lying there (a return ticket is always better than a single ride) is a dream that probably many of us had had.
But can a human really do that (and come back before being a dead body)?
[image credits: IPAC / Caltech]
This is the very serious question that I will try to answer in this post, using only physics arguments.
In other words, I will assess what it takes to send a human to the end of the universe, without killing him/her, and to bring him/her alive, back on Earth.
Disclaimer: this topic originates from the comments of this post, written by @sauravrungta.
LIMITS OF THE UNIVERSE: HOW FAR SHOULD WE GO?
Before starting the journey, it is good to know how far we should go.
We know that the universe is slightly less than 14 billions years old. Therefore, as nothing can go faster than light, we may naively assumed that the end of the universe is located at a distance of 14 billions light-years. Of course, I am only considering the observable universe here.
This very naive estimation is however wrong: this ignores the fact the the universe is in expansion. Putting all things together, calculations show that the size of the universe is more or less 45 billions of light years, or exactly 425.441.472.996.960.000.000.000 kilometers, using a distance unit easier to figure out.
[image credits: Pixabay]
We must thus organize a trip to a location distant from 45 billions of light year, and come back. This means, traveling 850.882.945.993.920.000.000.000 kilometers.
Assuming we can travel at the speed of light, this consists of a trip of 90.000.000.000 years. Therefore, the naive answer to the question is: we cannot go to the end of the universe and come back with one single human.
Even worse, we are starting at rest (i.e. with a vanishing speed). We must hence accelerate from zero to the speed of light. Then, at mid-way, we need to decelerate (in short, the spaceship will rotate and accelerate in the reverse direction) so that we could stop at the end of the trip.
And finally, the same thing holds a second time for the return trip.
This trip could be way longer than expected…
RESCUED BY SPECIAL RELATIVITY?
But wait. We forgot one important point in our calculations: special relativity. Einstein can possibly save us.
[image credits: Wikipedia]
When traveling at high speed, two phenomena are predicted by special relativity: time dilation and length contraction.
This means that time is flying more slowly for someone moving at very high velocity. Similarly, distances are smaller.
Is this enough? There is one way to know: performing the actual calculations.
First, we said above that we start on Earth with a zero velocity. Then, we accelerate. But we cannot kill the human on board so that we cannot impose him/her too many G’s in his/her face.
As we may accelerate during a very long time, we impose that the acceleration cannot exceed 1G. This implies that gravity on board is the same as gravity on Earth.
Now we have everything to calculate the exact time taken for the trip.
COMPUTATIONS FOR ALL FRIENDS WAITING ON EARTH
As said above, the one-way trip is a trip of 425.441.472.996.960.000.000.000 kilometers. We will thus accelerate until half-way, and the decelerate in order to stop at the end of the universe.
In practice, half-way means a trip of 212.720.736.498.480.000.000.000 kilometers, starting with a zero velocity and accelerating constantly with an acceleration equal to 1G.
[image credits: homemade]
For someone on Earth, the time (t) it will take is given by solving the equation on the right.
The l on the left-hand side of the equal sign is the distance to travel, and g = 9.81 m/s2 is jsut the value of 1G.
It is not complicated to solve this equation, and the time needed for the total trip to the end of the universe (starting at rest from Earth and ending at rest at the end of the universe) is 45.000.000.000 years.
And we need to multiply this time by two, as we still need to account for the return trip. Okay, our space traveler will be dead, dead, dead and again dead…
This conclusion is however incorrect. The friends of our space traveler waiting on Earth will be dead, dead, dead and again dead. Not the traveler (we actually did not perform the right calculation for him/her).
COMPUTATIONS FOR THE TRAVELER
In the relativistic spaceship, time is flying in a slower way. The formula above needs thus to account for this.
[image credits: homemade]
Using this the formula shown on the left, we can calculate the time (denoted by τ this time) needed, but for the space traveler standpoint. In other words, how he or she will feel the trip.
And here is the funny part.
We obtain that the one-way trip to the end of the universe will last slightly more than 47.5 years!
Accounting for the way back to Earth, the total trip will only last 95 years for in total. Our traveler may well be very alive after that.
DISCUSSION AND REFERENCES
It is in principle possible to go to the end of the universe, and come back, provided that one can build a ship capable of accelerating to velocities very close to the speed of light.
In this context, it is sufficient to send a baby human, and an elder of 95 years will come back to Earth ‘slightly’ later… Okay, Earth will be 90.000.000.000 years older, but that is a small detail…
What I have also not said is that to maintain a 1G acceleration during 95 years, we need, give or take, an infinite amount of fuel. Conclusion: there is no hope to go to the end of the universe without inventing a very clever piece of technology.
There is a small flaw in the above calculations (mostly an unspoken approximation), which does not change the conclusions of course. A reward for the one who finds it! :)
More information can be found following the links below:
- About the age of the universe and its size.
- There are many textbooks about special relativity. My favorite is this one.
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I thought to myself "Well, if there was a spaceship which could do the job i would certainly go to see the edge of the universe" - That would be cool.
But there is something that isn't clear to me, if the universe is expanding in every direction (given any point) faster than light, how does the spaceship arrive at the edge of the universe? Or am I wrong?
You got the imprecision with the calculation. Congratulations! We should say, the limit of the universe today ;)
Actually, saying that the expansion of the universe is faster than light is a bit misleading, as velocities as such have no meaning in this context of very distant objects (in short, the entire concept of velocity is local).
Yay! Even if that wasn't the intention :-)
Absolutely true. But my doubt is:
Given that the universe is "stretching" at about 68 km/s per megaparsec, it means to me that, considering the earth as the center of the universe (or at least, of the observable universe), for each megaparsec in a given direction i should add ~68km/s until i reach a distance where there is a point which is "moving faster" than light (from the earth prospective) and that i can't reach at the speed of light.
Isn't that right?
Or is the movement of that point just apparent given my position on earth?
P.S. I know Imay sound petulant asking these questions, but hey... free physics lessons :-)
I always dreamt about time travel. I am sure there is a parallel universe with different laws of physics and science overall.
Maybe, maybe not. :)
i'll give you everything I have on this earth, just give me a time machine! lol
Lol I would do the same! Hahaha this topic always fascinates me! Astronomy should have been may main career hahaha
Are you sure about the calcul ? From many billions to 47.5 years... its maybe a little bit optimist.
Are you taking in consideration that the universe is in a contant expansion ?
However, this article is very interesting
Special relativity is what it is. I am 100% sure (and this can be easily verified by anyone as I give all details). What is not said is that this implies to reach a speed equal to the speed of light up to many many digits. That's the trick behind.
I said there was an imprecision on the reasoning. You found it. I should have said: the limits of the universe today.
Great article. I haven't calculated this myself yet, but how was the speed of time traveler close to speed to light in your calculations (as nothing with mass can achieve the speed of light)? Maybe here is this "unspoken approximation"? ;)
or: you didn't take into account expansion of the universe (the distance will increase in time)?
It is basically equal to the speed of light (up to more than 25 digits).
I didn't. This was the imprecision in the calculation that I actually mentioned. I should have instead said: the limits of the universe today.
More decimals that in Crypto!!!
Much much more :p
Would the travel in this case be linear? I would expect slight deviation in regards to curvature...
Time travel has always been a guilty pleasure of mine. My ears perk when I see or hear anything of relevance :)
Note: Time travel could possibly utilize another form of phenomena other than just the sol, correct? I've always been fascinated with the ability of perhaps warping space-time with some kind of distortion apparatus, along with a GPS peripheral to pick & choose your position in the cosmos :-)
I would say non-linear. We may indeed follow the path taken by light.
Indeed we need another way to travel. Otherwise, as I said, we need an infinite amount of energy to accelerate the ship. Warp drives are for the moment hypothetical. They may or may not be ever built. But it is cool to dream about options :D
You blew my mind professor. I don't know if the calculations are correct, but I would never expect the answer to be anything close to that!
lol, hehe
Special relativity... is special :)
Still deep in thought over this......like, what if????? The universe has no end???? So many what if's???? This is my problem in life.....sigh
The universe could be infinite. We just don't know. But the observable universe is in contrast finite. This is the number I used: the size of the observable universe. I should have mentioned that in the post. Thanks for letting me know (I will edit the post now :p ).
Oh, I wasn't counting it as a mistake....it just put me into deep thought....which is where I am most of the time ;) @lemouth
Not a mistake but instead an unspoken assumption ;)
This is so damn Interesting!
I enjoyed reading this post a lot! 95 years, that is very manageable, and soon men and women average age of dead will increase by a factor of two, I'm pretty sure about that. It is possible to get even quicker to the end of the universe, just check in the second book of Douglas Adams series Hitchhikers Guide to the Galaxy :) "The Restaurant at the End of the Universe". Off topic: I liked the film "Interstellar" touching this topic as well.
Thanks for passing by!
I am less sur that our life expectation will get a factor of 2 increase, in particular with the antibiotic resistance issues and stuff like that. But who knows. Only the future will tell us, as for many things ;)
Interstellar was a good movie. There were actually a bunch of general public events on the physics behind the movie, and people were very interested in these.
(I am not talking about Douglas Adams, but I could have mentioned it in the post... I haven't thought about it at all... Bad for me.)
What about the work is done in eg the cell aging? I'm not knowledgable enough to determine at what stage this could lead to a halt to cell aging for humans, but apparently some initial success are presented with a halt to cell aging.
What are your thoughts about some of the scientists who believe we will be able to download our brains in not so many decades from now? AI integration with the human brain through initiatives like Neuralink? And what they try to do in China, transplant a head onto another body (apparently successfully done with dogs). I maybe a little bit a dreamer, but I for one believe we will be able to extend our lives tremendously in one or multiple ways.
I know, and you shouldn't have mentioned it, but I couldn't resist :) I truly find the Hitchhikers Guide to the Galaxy hilarious, with a grand opening in the first couple of pages of the first book.
I agree with what you said concerning the improvements. The thing I am more afraid is the lack of improvements regarding diseases. I hope you are right and I am wrong. :)
Oh I can see the things as well that can harm humanity. It could be the anti biotics, it could be a disease we don't know yet, it could simply be the stupidity of humans to kill each other en mass, it could be AI that doesn't like humans. So many possibilities even that we will not make it long anymore as a whole, so many possibilities we will die younger in the future. But I like to think positive :)
Me too. In some way, it is better to make the best use of the time we have than being depressed because our time may be counted. That is what I am living for: enjoying every single moment :D
I master that better and better; learn from the past, view the future as something that will be there when getting there, and enjoy today to the max.
Very interesting how about the alcubierre proposal in order to warp the space-time? this could lead to speeds faster than light! maybe this travel could be feasible in less time!
For the moment, this proposal is only fiction. However, note that the Alcubierre drive does not allow to go faster than light. Your effective speed is faster than the speed of light. The word 'effective' is important. The speed will look faster than the speed of light because the structure of space is contracted.
47.5 years? Hopefully we can get bodies that can survive more than 1g by then, and cut it down by a few.
This will help, but we need to find a solution to two other problems:
This is a very interesting thought exercising , but I think I may have found "the flaw" you mentioned, specifically to do with the age of the travel.
Time is relative to gravity, so as our pilot travels the universe, time would constantly fluctuate do to traveling through a multitude of gravitation fields, all of which would have varying strengths.
Of course this is something that would be impossible to calculate until we get the entirety of the universe mapped. At least the parts the pilot would be traveling through.
I didn't think about gravitational effects, as I set up the problem in the special relativity context. For this exercise, we may imagine those to be negligible (this strongly depends on the trajectory and we may imagine the pilot to be clever enough in setting it up).
Theoretically, there's an easier and faster way to travel to-and-fro multiverse, and even inter-dimensional travel. This is through portal warmhole. Also, "teleportation" is not a bad idea. But all these exist only in theory
In some unproven theories, to be precise. In contrast, relativity is very well proved :)
Whatever we have in books is pure theory and no one has practical experienced and that makes humans as toddlers when it comes to space exploration. We can discuss theory upon theory and we are very limited to non-practicality of this concepts.
Special relativity is not really just a theory. Data tells which theory is right and which one is not. For the moment, special relativity seems to be right. And if not, the replacement must be very close to it.
Now, I agree with you on the fact that the 'experiment' described in this post is purely theoretical. Can't we dream? :)
Only time can tell. Knowledge is exponentially increasing through time though, and by that time, we're all dead already lol. Thanks I enjoyed it.
I am reading this in many places. I am afraid that to me, this has not really any meaning, as there is to my knowledge any quantitative way to estimate knowledge. We can only say we know more and more with time.
This is what I really meant by saying that hehehehe
Okay, so no quantitative statements. Then it is fine :)
interesting concept and one only limited by the unknown. traveling at the speed of light, all time is frozen,thus it can really happen in an instant, we will see one day. I am now following you for more
An instant of a human life :)
That's why I always admired people who're good at maths and physics, they can technically find a solution to every problem!
Très bien, monsieur le professeur! :)
Pwweeeet? :D
nice scientific topic, thanks for sharing.
Damn this is next level stuff.
The faster you go, the shorter you are.
- Albert Einstein
very interesting article
solid journey of space
I need to read it five times at least to understand everything :(
Oh :( Which part exactly? I can help.
You blew my mind professor.
I want to go with my girlfriend until the end of the universe!
Yest its posible and its called "Space Tourism"
Yeah, but I think it's too expensive
It depends if you have time to get the money ;)
Yes, yes :)
Great! Inspiring and Educative post.
I just followed and upvoted you Prof.
Kindly follow and upvote my post. I post articles on climate change and astronomy too...
Check my most recent post here
https://steemit.com/steem/@mickyscofield/this-is-how-money-is-created-on-steem
I am all caught up UV your posts!!!!!
Have you sent this math and post over to Elon Musk man?
lol
Not really. This is standard textbook stuff ;)
But it's on the right D:
And that spaceship is faaast :P
When you go fast enough, left and right are inverted... :D
More seriously, thanks for pointing this out. I just fixed it ^^
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