# What happens when you go near the speed of light and there's stuff in the way?

Normally when people talk about going near the speed of light they talk about Lorenz contraction and Time dilation, but to keep things simple they make one implicit assumption; there's nothing in the way.

What if you are going 99.99% the speed of light and you crash into a planet? What if you go through an atmosphere at that speed; heck what happens if you are just going through cosmic background radiation at that kind of speed? By the way I would love to see an equation for how hot a rocket would get if were moving through cosmic background radiation at near light speed.

PAllen
To interstellar medium, I would add the question of debris. Rare in interstellar space but if it happens... hitting 1 gram at just 90% the speed of light would expose you to about a Hiroshima bomb worth of kinetic energy.

FtlIsAwesome
Gold Member
What if you are going 99.99% the speed of light
what happens if you are just going through cosmic background radiation at that kind of speed?
If the craft was traveling through the interstellar medium: The radiation would be strongly blueshifted. It would kill all biological life on that craft, unless it had protective armor. But the armor would have to be prohibitively thick.
What if you go through an atmosphere at that speed
The atmosphere might tear through the craft, and the craft might make some kind of shockwave that would severely damage the area. Just my guess.
...and you crash into a planet?
:rofl:

KABOOM

There goes that planet.

Actually, you wouldn't need to go nearly as fast to destroy a planet. At 99.99% c you could take out something much larger than a planet, but I'd have to find the equations and go through the math.

bcrowell
Staff Emeritus
Gold Member
The moral of the story is first you raise the deflector shields, then you engage the warp drive.

KABOOM

There goes that planet.

Ah HA HA HA! I love it!

I am very pleased with all of these replies thank you all so much, but I absolutely must know more!

Please show me the equations that show how a gram of matter can explode like an atom bomb.

Please show me the equations that show the destruction of a planet through use of a very fast rocket

Please teach how hot a rocket would get moving through an interstellar medium of 10^-4 cm^-3 density of ionized particles.

Oh and bcrowell...judging from that link to the interstellar ramjet it seems you figured out I am only on this forum to find out how to travel in space really, really fast. Is that true?

PAllen
Ah HA HA HA! I love it!

Please show me the equations that show how a gram of matter can explode like an atom bomb.

Please show me the equations that show the destruction of a planet through use of a very fast rocket

Same formula. Kinetic energy = m*c^2( 1/sqrt(1 - v^2/c^2) -1)

Then you need to look up what a kiloton is: 4*10^19 ergs; and look up what the yield of the Hiroshima bomb was (estimates vary; I used 20 kilotons for simplicity).

Actually, I took a shortcut. Long ago I calculated that the energy equivalent of 1 gram was ballpark the same as the Hiroshima bomb. So all I need to know is what speed produces gamma of 2 to know that that would make 1 gm of matter into a Hiroshima bomb. (gamma is 1/sqrt(1 - v^2/c^2) )

Matterwave
Gold Member
The moral of the story is first you raise the deflector shields, then you engage the warp drive.

The warp drive actually creates a space-time bubble around the spaceship, so locally you're not actually traveling all that fast...I don't know what happens if you go through debris with that mechanism! ;)

Actually I think the CMB wouldn't bother you too much unless you were going very very very close to the speed of light. CMB radiation is at 3K, so to get it to blue shift into the x-ray or gamma ray range would take quite a bit of speed! By my calculations, to get to gamma ray frequencies (~10^19Hz) from CMB frequencies (~10^9Hz) would take speeds around 99.999999999999% the speed of light.

Same formula. Kinetic energy = m*c^2( 1/sqrt(1 - v^2/c^2) -1)

Then you need to look up what a kiloton is: 4*10^19 ergs; and look up what the yield of the Hiroshima bomb was (estimates vary; I used 20 kilotons for simplicity).

Actually, I took a shortcut. Long ago I calculated that the energy equivalent of 1 gram was ballpark the same as the Hiroshima bomb. So all I need to know is what speed produces gamma of 2 to know that that would make 1 gm of matter into a Hiroshima bomb. (gamma is 1/sqrt(1 - v^2/c^2) )

I think I am getting my units mixed up because when I use the kinetic energy formula for one gram I get 1.16313*10^17 ergs assuming it's moving at 90% the speed of light. I am getting confused somehow. Also what does the variable "Gamma" symbolize in this context.

The warp drive actually creates a space-time bubble around the spaceship, so locally you're not actually traveling all that fast...I don't know what happens if you go through debris with that mechanism! ;)

Actually I think the CMB wouldn't bother you too much unless you were going very very very close to the speed of light. CMB radiation is at 3K, so to get it to blue shift into the x-ray or gamma ray range would take quite a bit of speed! By my calculations, to get to gamma ray frequencies (~10^19Hz) from CMB frequencies (~10^9Hz) would take speeds around 99.999999999999% the speed of light.

I found this quote form a wiki article on a warp drive:

"A more recent paper by Carlos Barceló, Stefano Finazzi, and Stefano Liberati makes use of quantum theory to show that the Alcubierre Drive at FTL velocities is impossible; mostly due to extremely high temperatures caused by Hawking radiation destroying anything inside the bubble at superluminal velocities and leading to instability of the bubble itself. These problems do not arise if the bubble velocity is kept subluminal, but it is still necessary to provide exotic matter for the drive to work"

Also I would love to see the equation for how radiation gets blueshifted as ship speeds through it.

PAllen
I think I am getting my units mixed up because when I use the kinetic energy formula for one gram I get 1.16313*10^17 ergs assuming it's moving at 90% the speed of light. I am getting confused somehow. Also what does the variable "Gamma" symbolize in this context.

You presumably 3*10^8 as speed of light. But for ergs, you need speed of light in centimeters/second, which is 3*10^10. Then you get a bit over 10^21 ergs.

I defined gamma in the post you replied to. Since this factor appears all over the place in relativity, it just allows you state, e.g. kinetic energy as: m*c^2 (gamma -1). Thus gamma of 2 means energy equal to rest mass of body is available kinetic energy. Gamma of 2 is produce by speed of about .86c. Thus, you know that at this speed a gram will have Hiroshima yield in K.E., a kilogram will be 20 modern H-bombs, etc.

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You presumably 3*10^8 as speed of light. But for ergs, you need speed of light in centimeters/second, which is 3*10^10. Then you get a bit over 10^21 ergs.

I defined gamma in the post you replied to. Since this factor appears all over the place in relativity, it just allows you state, e.g. kinetic energy as: m*c^2 (gamma -1). Thus gamma of 2 means energy equal to rest mass of body is available kinetic energy. Gamma of 2 is produce by speed of about .86c. Thus, you know that at this speed a gram will have Hiroshima yield in K.E., a kilogram will be 20 modern H-bombs, etc.

I redid the math and got 10^21ergs and I am sorrey for misunderstanding what Gamma was. So I can assume that at speeds like .86c and .90c that all of the mass and kinetic energy in an object turns into an explostion when it collides with something correct?

PAllen
I redid the math and got 10^21ergs and I am sorrey for misunderstanding what Gamma was. So I can assume that at speeds like .86c and .90c that all of the mass and kinetic energy in an object turns into an explostion when it collides with something correct?

Not quite. The rest mass of the object doesn't convert to energy. It's just that at that speed it has KE equal to the energy equivalent of its rest mass (at higher speeds it would have higher KE). If the object collides with a sufficiently massive stationary object, its KE will be will be release explosively.

In the case of the 1 gram fragment hitting a rocket, it may just puncture the rocket. But that would be pretty bad. The energy amount goes to show that no shield could absorb the force. What you would want is some active shield that deflects debris rather than trying to stop it.

In the case of the 1 gram fragment hitting a rocket, it may just puncture the rocket. But that would be pretty bad. The energy amount goes to show that no shield could absorb the force. What you would want is some active shield that deflects debris rather than trying to stop it.

I should learn about deflecting at some point, but for now lets assume I want the ship to get hot or at least produce a hot explosion. Is there any medium I can send the ship through to get to above 10^28 degrees K? If not what's the best stuff to send the ship through to get some heat?

PAllen
I should learn about deflecting at some point, but for now lets assume I want the ship to get hot or at least produce a hot explosion. Is there any medium I can send the ship through to get to above 10^28 degrees K? If not what's the best stuff to send the ship through to get some heat?

The idea is to use the ship as a bomb? Just head for a planet.

The idea is to use the ship as a bomb? Just head for a planet.

The idea is to use the ship to dulplicate conditions that occured just after the big bang. Prehaps even to recreate a Cosmic superforce (if only for the smallest fraction of a second). That is why tempetures should be above 10^28K. I was hoping that just sending a ship through the right intersteler medium at high speeds would do the trick, but it doesn't look like it would be that easy.

All that said I think it would be a good idea to talk about sheilding after all, while this topic of crashing into things is still fresh. Do you have an idea for a deflecting debris?

PAllen