# Viewing an object with the naked high at near light speeds

## Main Question or Discussion Point

Viewing an object with the naked eye at near light speeds

Suppose there was a hypothetical alien craft whizzing around the earth at near light speeds. Would we be able to see a blur of it wooshing across the sky? Perhaps if it was rather large, like city size (think independence day)? Or would it just be invisible?

Last edited:

Related Special and General Relativity News on Phys.org
ghwellsjr
Gold Member
If it was in the sky, meaning in the atmosphere, it would burn up like any high speed object would. If it were large enough, pieces of it, maybe very large chunks, would make it to the ground and create a crater, just like any other very large piece of matter.

If it was above the atmosphere, it would have to be firing some very large rockets in order to maintain its course at a constant altitude in order to continue "whizzing around the earth at near light speeds". Normally objects in earth orbit are just coasting at a speed determined by their altitude, the higher the altitude, the slower they go, the fastest speed just above the atmosphere where they won't burn up is less than 20 thousand miles per hour, no where near close to the speed of light.

So, assuming that this craft is just above the atmosphere and firing rockets to maintain its near light speed coarse, it would make about seven "orbits" per second and unless it was changing its altitude or position of its "orbit", it would likely be caught in the afterburn of its own rockets and be destroyed.

But assuming that the aliens on board are just smart enough to avoid this problem and to steer their craft on a course where they don't get caught in their own afterburn, then we might be able to see the exhaust of their rockets as it pollutes the space surrounding the earth. You must not think that the exhaust from a rocket in space looks anything like it does from a rocket in air. In air, the exhaust takes time to dissipate so it forms a well defined trail behind the rocket. In space, it spreads out in all directions very quickly so it will be very difficult for the aliens to avoid. These aliens have to be very smart in order to construct a craft to get all the way to earth and to steer it to avoid the above problems, and yet they have to be rather stupid in order to carry out such a mission, just so that we could see their exhaust polluting space around earth.

But aside from all that, if there were any high speed object traveling across your field of view, you would never be able to see it with the naked eye. At best, you would need extremely high speed photographic equipment to capture any details of the object. This is true even for an object traveling much less than the speed of light.

I like Serena
Homework Helper
Indeed, we would not be able to see the craft itself or hear it.

We would be able to see their exhaust trail.
And perhaps see the effect of the gravity their craft would exert at those speeds at nearby objects.

If they circle the earth and use say antimatter for fuel, they may leave an exhaust trail (expelled radially) we may not be able to see.

russ_watters
Mentor
If it reflected sunlight, we could see it.

ghwells:

...unless it was changing its altitude or position of its "orbit", it would likely be caught in the afterburn of its own rockets and be destroyed...
Interesting observation!!!!! On the other hand, if they regularly traverse atmospheres on their travels likely they have solved the 'burn' problem....In space, no conduction, no convection, just radiation via heat...right??....so maybe it would not be so 'hot' after all???

ghwellsjr
Gold Member
My main point was that no aliens would ever carry out such a pointless exercise so details don't matter. I get the impression that 007craft is writing a script for a movie.

I like Serena
Homework Helper
Why not? We might.

That is, if we had energy in abundance (like access to raw antimatter), the technology to harness it (a matter of time IMO), and an alien civilization to study.

Well maybe not with speeds near the speed of light, but as much as we could manage. Who knows how fast that will be.

ghwellsjr
Gold Member
But we would never use that energy to circle the earth or any other heavenly body at near light speed. Why would anyone do such at thing?

I like Serena
Homework Helper
Because they could?
Because they didn't really think about it (because they could)?
Because they want to impress and/or intimidate?
Or perhaps, because it takes too much time to decelerate and accelerate for the return journey?

What would the romans have thought if we came along with a jet fighter?
Would we go unnecessarily slow for some reason?

ghwellsjr
Gold Member
Because they could?
Because they didn't really think about it (because they could)?
Because they want to impress and/or intimidate?
Or perhaps, because it takes too much time to decelerate and accelerate for the return journey?

What would the romans have thought if we came along with a jet fighter?
Would we go unnecessarily slow for some reason?
Circling around the earth at near light speed or any speed greater than the natural orbital speed requires a lot of continual acceleration and therefore energy and also matter. There are other ways to impress and/or intimidate. Their very presence would do that just fine.

ghwellsjr
Gold Member
ghwells:

Interesting observation!!!!! On the other hand, if they regularly traverse atmospheres on their travels likely they have solved the 'burn' problem....In space, no conduction, no convection, just radiation via heat...right??....so maybe it would not be so 'hot' after all???
It may not be so "hot" from the standpoint of earth but it's going to be very hot to the craft the next time around.

I like Serena
Homework Helper
Circling around the earth at near light speed or any speed greater than the natural orbital speed requires a lot of continual acceleration and therefore energy and also matter. There are other ways to impress and/or intimidate. Their very presence would do that just fine.
Of course I do not know what reason they could have.
I can only indicate that there may well be reasons that we cannot quite grasp just yet.
I can find analogues within our current technology.
The fact that those exist, implies to me that there will be more reasons beyond my knowledge.

Consider for instance that when a plane has to make an emergency landing on an airport, it may well be required to make a number of circles to burn off excess fuel, before it is allowed to land.
Would the romans have thought of that (to extend my example)?

It may not be so "hot" from the standpoint of earth but it's going to be very hot to the craft the next time around.
Since they would have to expel their reaction mass/energy radially away from earth at high velocity, it might be out of the way the next time round.

ghwellsjr
Gold Member
OK, let's say all the technical and economic problems are solved. Let's consider just the g force the aliens would experience in order to circle the earth at near light speed. Do you have any idea how much that would be?

I like Serena
Homework Helper
Yeah, a bit much depending on the distance to earth.
Perhaps it's just a robotic ship with sensors? ;)
(Or perhaps they have gravitational dampers! :O).

ghwellsjr
Gold Member
Yeah, a bit much depending on the distance to earth.
Perhaps it's just a robotic ship with sensors? ;)
(Or perhaps they have gravitational dampers! :O).
You should be a technical consultant for sci-fi movies. I admit I would be no good at it.

I like Serena
Homework Helper
You should be a technical consultant for sci-fi movies. I admit I would be no good at it.
Oh, but you raise good points!
They really need to be addressed, or else we'd get a lousy movie (from a tech point of view).

Serena:
What would the romans have thought if we came along with a jet fighter?
Would we go unnecessarily slow for some reason?
Bravo!!! made me LOL...best comment of the year!!

I like Serena
Homework Helper
serena:

Bravo!!! Made me lol...best comment of the year!!

Last edited:
A.T.

Suppose there was a hypothetical alien craft whizzing around the earth at near light speeds. Would we be able to see a blur of it wooshing across the sky? Perhaps if it was rather large, like city size (think independence day)? Or would it just be invisible?
If it reflected light or even emitted light from some position lights we would see it. However due to signal delay we would see it at the wrong position, and the combination of signal delay and length contraction would distort its appearance:

http://www.spacetimetravel.org/fussball/fussball.html

You could even see the backside of the ship that was pointing away from you when it emitted/reflected the light:

http://www.spacetimetravel.org/bewegung/bewegung5.html