Space travel at the speed of light

It is my understanding that time progression slows the closer you get to the speed of light and when reaching the speed of light, time doesn’t progress at all. Is this right?

In a hypothetical situation where humans discovered a way to travel at the speed of light and manage to deal with the other hazards of space travel at that speed, would, for the inhabitants of the spacecraft, time progress at all if they were traveling at light speed for any distance? So they could travel from one point in the universe to any other point in the universe without aging even a second. Is this true?

Yes - objects travelling at the speed of light do not experience time. However, external elapsed time still passes, so if you travelled at lightspeed to Alpha Centauri (~4 lightyears) and back, it would take no time at all from your perspective, but 8 years from the perspective of an observer on earth.

Also, from inside the spaceship, how do you know when to stop?
You cannot just set a timer…


Time would not progress for the passengers - the passengers would experience the journey as instantaneous.

It’s not possible though.

Let’s note though that it’s impossible to increase the speed of any object to the speed of light. It would take an infinite amount of energy.

Just to nitpick. It’s impossible to raise the speed of any object with mass to the speed of light.

Conversely, all massless particles must travel at the speed of light and can’t be slowed down.

Aha, you say. Got you. Light is slowed when it travels through a medium. Wrongo. The light ray as a whole propagates at a slower speed, because the photons are captured and re-emitted as they progress through the medium. That takes an instant of time. But each leg of the journey between is at light speed.

Of all the things in relativity that are difficult to imagine based on our everyday lives, I find that trying to imagine the perspective of a photon without time is the hardest. It’s created, it’s here, it travels and travels and travels, it’s there, it’s destroyed, but all of that happens simultaneously, one atop the other, a universe-spanning possibility of existence squashed into a nothingness of duration. My head hurts.

From the perspective of the photon here and there are the same location. It “sees” the rest of the universe collapsed into a 2-D plane via Loretnz contraction. So from the photon’s perspective its not taking a trip. It’s emission and absorption are a single momentary event at a single point in space.

Time slows down RELATIVE TO whatever you’re basing your speed on. It’s not like everything on your ship will be in slow motion - you won’t be able to tell any change in time.

So, from the photon point of view, it never exists, since it’s emitted and absorbed at the same time…

At the speed of light, time stops relative to every other time frame. For a photon emitted in the big bang and part of the CMB, the entire lifespan of the universe happens at the same instant of existence. When you achieve light speed (impossible, as noted above due to mass, but playing the hypothetical), there is only now - and that now is the same when you leave Sol as when you pass Alpha Centauri. Because it is still now, you don’t know to stop, even though to an outside observer it has taken you 4 years to get there.

It certainly is a head-hurter.


A stationary observer sees time flowing more slowly for objects moving relative to him. Since your ship is stationary relative to you, you see time flowing normally within it. And since Earth is moving relative to you, time flows more slowly there.

Earth sees the ship moving in slow motion.
The ship sees the Earth moving in slow motion.

It’s the acceleration the ship experiences at the beginning, turn-around, and end of the trip that puts it permanently behind the Earth, not the trip itself … .

Non Spoiler For Left Hand Of Darkness

In this (Excellent!) sf book, the protagonist is from a group of humans that have perfected faster than light travel. At one point, the king of a nation he’s negotiating with has this conversation with him-

‘If I banish you, you’ll just get in your ship and go?’

‘Yes, your majesty. I’ll just set the controls for home and leave. When I get there, I’ll set them for Winter and return.’

‘And I’ll be dead of old age and you will be no older?’

‘That is correct your highness’

LeGuinn has another story of the same planet in The Wind’s Twelve Quarters (I think it’s The King Of Winter. I can’t remember or remember where my copy is) that touches on the same issue

‘You left twenty two years ago. The reasons they sent you are irrelevant. Things have changed here.’

The hazards of travelling at light speed are somewhat different to the hazards of travelling at other speeds. Since the only things that can travel at light speed are massless particles, to travel at that speed you need to convert yourself into a massless particle.

In effect, you need to convert yourself into an information-carrying beam of particles, for example a radio or laser message of some kind.

In such a form you would experience exactly no passage of time between being transmitted and being received.

What would be the hazards of travelling in such a form- as a beam of information? Well, the worst one would be data loss. Even a laser beam or highly collimated radio beam would lose strength over interstellar distance; important parts of the message might be absorbed by the interstellar medium. So it seems advisable that the message would need to be transmitted at very high power and with a lot of repetition/redundancy.

Finally the last hazard such a message would face is the fact that there might be no suitable receiver at the far end. Such a receiver would need to be capable of picking up a fantastically weak interstellar information beam, and would also need to be capable of converting the beam back from information into a human space traveller.

I can’t see any way of guaranteeing that over interstellar distances and timescale. If such a method of travel ever becomes possible, it seems that a voyager will have to travel in hope of a successful arrival rather than in certain knowledge of the same.

I may be wrong, but the acceleration determines WHO’S slower. The length of the trip determines HOW MUCH.

It’s already been brought up, but I just want to reiterate the point about length contraction. Objects traveling at the speed of light experience infinite length contraction, so that essentially there is only one point of existence to that object. There is only one physical point in existence, and also, because of infinite time dilation, there is only one point in time.

So yeah. Every photon that has ever existed has believed that they were the entire universe, and that they existed the entire time-length of the universe. Self-centered pieces of crap!

I was just about to post something similar.

Due to length transversed shrinking to zero, and the time interval shrinking to zero, any lightspeed particle percieves it’s origin and destination as tangent points.

At lightspeed, Everything is touching each other! :dubious: